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Nickel-Catalyzed Cross-Coupling of Photoredox-Generated Radicals:

Uncovering a General Manifold for Stereoconvergence in Nickel-Catalyzed Cross-Couplings

 

Department of Chemistry, Roy and Diana Vagelos Laboratories, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
J. Am. Chem. Soc., 2015, 137 (15), pp 4896–4899
DOI: 10.1021/ja513079r
Publication Date (Web): April 2, 2015
Copyright © 2015 American Chemical Society
 ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Cite this:J. Am. Chem. Soc.  137, 15, 4896-4899

Abstract

Abstract Image
The cross-coupling of sp3-hybridized organoboron reagents via photoredox/nickel dual catalysis represents a new paradigm of reactivity for engaging alkylmetallic reagents in transition-metal-catalyzed processes. Reported here is an investigation into the mechanistic details of this important transformation using density functional theory. Calculations bring to light a new reaction pathway involving an alkylnickel(I) complex generated by addition of an alkyl radical to Ni(0) that is likely to operate simultaneously with the previously proposed mechanism. Analysis of the enantioselective variant of the transformation reveals an unexpected manifold for stereoinduction involving dynamic kinetic resolution (DKR) of a Ni(III) intermediate wherein the stereodetermining step is reductive elimination. Furthermore, calculations suggest that the DKR-based stereoinduction manifold may be responsible for stereoselectivity observed in numerous other stereoconvergent Ni-catalyzed cross-couplings and reductive couplings.
In the decades following their inception, transition-metal-catalyzed cross-coupling reactions (CCRs) have assumed a privileged role among methods for the construction of C–C bonds.(1) Although highly reliable for C(sp2)–C(sp2) couplings, significant limitations are often encountered in the application of sp3 hybridized reagents, particularly poorly nucleophilic secondary alkylborons. Here, slower rates of transmetalation often necessitate forcing conditions and/or harsh reagents (high temperatures, excess boronic acid, aqueous base), thereby limiting functional group tolerance while augmenting undesired side reactions, including protodeboronation, β-hydride elimination, and subsequent isomerization.(2)
In an effort to circumvent the challenges of transmetalation within the conventional catalytic regime, we recently reported a novel dual catalytic CCR in which the cooperative functions of an Ir photoredox catalyst and a Ni catalyst effect the cross-coupling of electronically activated potassium alkyltrifluoroborates with a variety of aryl bromides under exceptionally mild conditions (eq 1).(3) Most notably, the cross-coupling of a secondary benzylic trifluoroborate occurs stereoconvergently in the presence of a chiral ligand (eq 2), a stereochemical outcome that is unprecedented with boron reagents.(4)
We initially hypothesized a mechanistic scenario in which the Ni(0) catalyst 1 first engages the aryl bromide in oxidative addition to afford arylnickel(II) complex 3 (Figure 1, blue). In parallel, oxidative fragmentation of an alkyltrifluoroborate 6 by the excited state of Ir photoredox catalyst 4 yields a C-centered radical that is rapidly captured by this Ni(II) complex. Reductive elimination from the resultant Ni(III) species 10 yields the cross-coupled product and Ni(I) complex 12. Finally, single-electron reduction of Ni(I) by iridium complex 8 simultaneously regenerates the Ni(0) catalyst and the ground state photocatalyst. MacMillan, Doyle, and co-workers hypothesized a similar mechanistic scenario for the related cross-coupling of α-amino acids and N,N-dialkyl-N-arylamines with aryl halides.(5)
figure
Figure 1. Initially proposed catalytic cycles (blue) and possible alternative indicated by computation (red) for photoredox/nickel dual catalytic CCR of potassium benzyltrifluoroborate and aryl bromides. Ir = Ir[dFCF3ppy]2(bpy)PF6.
To understand more fully the mechanistic intricacies of this novel class of CCRs, we undertook a computational analysis of the Ni catalytic cycle. We were particularly interested in addressing two key questions: (1) To which oxidation state of Ni does the radical add? (2) Which step in the catalytic cycle is enantiodetermining? Importantly, although there have been numerous computational and experimental studies of traditional transition-metal-catalyzed CCRs,(6) there are limited computational analyses of Ni-catalyzed CCRs in which C-centered radicals and paramagnetic Ni species are invoked.(7) Herein, we report a detailed density functional theory (DFT) study of the catalytic cross-coupling of alkyltrifluoroborates and aryl bromides via single-electron transmetalation. Results reveal that the final reductive elimination accounts for the origin of stereoinduction for this important transformation.(8) A stereochemical model is proposed and, for the first time, supported by experiments with a series of substituted aryl bromides. These mechanistic findings are proposed to have far-reaching implications related to other stereoconvergent CCRs.
We initiated our studies by calculating the Gibbs free energy profile with 2,2′-bipyridine as a model ligand for the 4,4′-dtbbpy ligand used experimentally (Figure 2). Because of the presence of radicals and low-spin Ni intermediates, all optimizations were performed using a spin-unrestricted broken-symmetry UB3LYP functional with both the LANL2DZ and 6-31G(d) basis sets (with the Guess=mix keyword as implemented in Gaussian09).(9) Multiple spin states were considered for all intermediates and transition states. This method has been used before to rationalize selectivities accurately,(10) model radical Ni systems,(7a, 7b) and account for changes associated with ligands.(11) Single-point energy calculations of optimized structures were carried out in water (SMD solvation model) at the (U)M06/6-311+G(d,p) level of theory. For comparison, we computed the energetic profile by varying the basis set [6-311+G(d,p) for C, N, O, Br, H and SDD for Ni] and solvent (SMD in acetone), which showed similar energetics (see Supporting Information). Exhaustive conformational searches were performed for all intermediates to map out the lowest energy profile, and intrinsic reaction coordinate (IRC) calculations were undertaken to ensure transitions states connected the illustrated ground states.
figure
Figure 2. Reaction coordinate for the competing pathways using 2,2′-bipyridine. Relative Gibbs free energy values calculated with SMD-water-(U)M06/6-311+G(d,p)//UB3LYP/6-31G(d) and SMD-water-(U)M06/6-311+G(d,p)//UB3LYP/LANL2DZ (in parentheses).(12)
Beginning from square planar Ni(bpy)(COD) A, dissociation of 1,5,-cyclooctadiene (COD) and complexation to bromobenzene is energetically disfavored by 6–8 kcal/mol (Figure 2). However, oxidative addition is energetically feasible (15–18 kcal/mol) leading to square planar Ni(II) intermediate A2, which is ∼26 kcal/mol downhill in energy. The Ni(II)-to-Ni(III) process, occurring via addition of a benzyl radical (presumably generated in the concomitant photocatalytic cycle(3, 5) from Figure 1), is found to proceed via a low barrier (∼4 kcal/mol) transition state A2-TS and is reversible. Significantly, the reductive elimination transition state (C-TS) leading to the CCR product and Ni(bpy)Br intermediate is ∼6 kcal/mol higher in energy than the radical addition/dissociation.
In an alternative mechanistic pathway, the Ni catalytic cycle can proceed via an alkylnickel(I) intermediate preceding oxidative addition (Figure 2 red). Ligand dissociation and radical η2-complexation to Ni(0) leads to intermediate B1, which proceeds via a ∼5 kcal/mol energy barrier to form benzylnickel(I) intermediate B2, a process that is favorable by ∼10–15 kcal/mol. This Ni(I) intermediate can undergo facile and irreversible oxidative addition (via B2-TS) to merge the two energetically feasible pathways via the pentacoordinated Ni(III) intermediate C. This result implies that, depending on the concentration of Ni(0) or Ni(II), both pathways can occur. Irrespective of the specific pathway, the dual photoredox/cross-coupling cycle converges onto a Ni(III) intermediate that can dissociate the stabilized radical to form Ni(II) more rapidly than undergoing reductive elimination! Subsequent reduction by the photoredox cycle will generate the Ni(0) intermediate to restart the catalytic cycle (Figure 1).
In our recent report, we observed modest enantioselectivity (75:25 er) with the use of chiral 4,4′-dibenzyl-2,2′-bis(2-oxazoline) ligand, L1 (eq 2). We had previously suggested that the origin of enantioselectivity in the single-electron transmetalation of secondary alkyltrifluoroborates arises from facial selectivity in the addition of the prochiral radical to the ligated Ni(II) center, followed by stereoretentive reductive elimination. However, if homolytic equilibration of the Ni(III)/Ni(II) pair is faster than reductive elimination, as these calculations indicate, then the origin of stereoselectivity should be found in the reductive elimination step.(7a) Thus, we propose that enantioselectivity arises from a process best described as a dynamic kinetic resolution (DKR)(13) of Ni(III) complex C′.(14) In other words, addition of the secondary radical to the Ni center operates under Curtin-Hammett conditions(15) furnishing two equilibrating diastereomeric Ni(III) complexes, one of which reductively eliminates at a faster rate, leading to the major enantiomer. Stereoconvergence then results via stereochemical scrambling of the secondary alkyl subunit through dissociation and recombination. Indeed, computations of the diastereomeric transition states C′ corresponding to eq 2 correlate well with experiment;(16) specifically, a Boltzmann distribution from calculated free energies of the eight lowest energy diastereomeric transition states predicts a 68% ee vs the experimental 50% ee. Examination of the structures reveals that the α-methylbenzyl group rotates to avoid gauche-like interactions along the forming C–C bond (Figure 3). In the lower energy diastereomeric transition state these interactions are minimized.
figure
Figure 3. Competing diastereomeric transition states in the reductive elimination. Relative free energies (kcal/mol) are computed using SMD-water-(U)M06/6-311+G(d,p)//UB3LYP/6-31G(d).
Having established reductive elimination as the enantiodetermining step in these systems, other potential substrates were probed with the aim of establishing a correlation between the calculated and experimental selectivities. Calculations of the diastereomeric transition states for several substrates suggested that substituents at the para-position of the aryl bromide could enhance the enantioselectivity. In particular, larger para-substituents encounter steric interactions with the ligand benzyl group in the transition state leading to the minor enantiomeric product (see bottom structure in Figure 3). Notably, the stereochemical influence of these substituents distal from the bond-forming site would not be evident in the absence of this computational model. Gratifyingly, these predictions correlated well with experiment and afforded improved enantioselectivity in generating 1,1-diarylethane 15 (Figure 4).
figure
Figure 4. Predicted and experimental reaction enantioselectivities.(17)
Moving forward, we became curious whether this DKR-controlled enantioselectivity operates in other asymmetric Ni-catalyzed cross-coupling processes. Of particular interest are reports documenting Ni-catalyzed asymmetric cross-couplings (Suzuki, Negishi, Hiyama, and Kumada)(18) and reductive cross-couplings.(19) Importantly, it can be argued that the “black box” nature of these transformations have limited their widespread development and adaptation, as no general model for stereoinduction has yet been proposed despite the large number of processes reported to date. Although a number of these asymmetric cross-couplings employ alkyl groups that would be precursors to stabilized radicals (i.e., benzylic, allylic, α-carbonyl, etc.), several examples of asymmetric cross-couplings of electronically unactivated alkyl subunits have been reported.(20) Although the analogy of the former examples to that reported here is readily apparent, it was less clear whether the proposed Ni(III) DKR manifold would be viable for systems in which less stable (e.g., unstabilized secondary alkyl) radicals were generated via homolysis of the Ni(III) intermediate. In an effort to address this question, the stereoconvergent cross-coupling of unactivated secondary alkyl bromides and primary alkylboranes reported by Fu and co-workers (eq 3)(20e) was examined computationally.
Beginning from the putative Ni(III) complex, the transition states for homolysis of the secondary alkyl substituent and C–C bond-forming reductive elimination were computed. As shown in Figure 5, these calculations convincingly support a scenario analogous to that described above; that is, Ni(III) complex 10a exists in homolytic equilibrium with Ni(II) complex 3a and the free alkyl radical in a process that is much faster than the subsequent reductive elimination leading to Ni(I) complex 12a and cross-coupled alkane product. As such, we propose that stereoconvergence in these processes occurs by the same Ni(III) DKR process that we have elucidated for photoredox/nickel dual catalytic organoboron cross-coupling. This newfound knowledge regarding the fundamental origin of enantioinduction in Ni-catalyzed stereoconvergent processes can be used to augment stereoselectivity in known transformations through rational design and may be helpful in identifying new substrate classes that can participate via this manifold. These results are in agreement with the lack of products with long-lived radical intermediates. Specifically, radicals that quickly and favorably complex to the Ni center as proposed in Figure 2 avoid radical pathways such as cyclization by a pendant alkene. We are currently investigating the full scope of this proposal for various Ni-catalyzed C–C bond-forming processes involving alkyl radical intermediates, including the factors that might change the enantiodetermining step.
figure
Figure 5. Energy barriers for the competing unstabilized alkyl radical dissociation and reductive elimination transition states with chiral diamine ligand L2. Relative free energies (kcal/mol) are computed using SMD-water-(U)M06/6-311+G(d,p)//B3LYP/6-31G(d) in SMD (water) level of theory.
In summary, we have employed DFT calculations to investigate the reaction pathway of the nickel/photoredox dual catalytic cross-coupling of aryl bromides with C-centered radicals derived from alkyltrifluoroborates. These computations suggest a mechanistic scenario wherein the radical can enter the cross-coupling cycle by addition to either Ni(0) or Ni(II).(21) The two pathways converge upon a common Ni(III) intermediate that is able to release the stabilized alkyl radical via Ni–C bond homolysis, thus establishing an unexpected equilibrium between this high valent Ni(III) and the Ni(II)/radical pair. The cross-coupled product is then generated via irreversible reductive elimination. The reductive elimination barrier was computed to be significantly higher in energy than the barrier associated with the reversible homolysis process. Calculations show that the stereoinduction occurs through DKR of the Ni(III) intermediate according to the Curtin–Hammett principle. Experimental results have offered support for the proposed stereochemical model. Most importantly, the Curtin–Hammett DKR stereoinduction model appears to be broadly operative in various related stereoconvergent Ni-catalyzed processes,(7, 18) offering a rationalization for the mechanism of stereoselectivity in these transformations for the first time.

Supporting Information

Computational and experimental details; complete ref 9. This material is available free of charge via the Internet at http://pubs.acs.org.

Nickel-Catalyzed Cross-Coupling of Photoredox-Generated Radicals: Uncovering a General Manifold for Stereoconvergence in Nickel-Catalyzed Cross-Couplings

The authors declare no competing financial interest.

Acknowledgment

We are grateful to the National Institutes of Health (GM-087605 to M.C.K. and GM-081376 to G.A.M.) and the National Science Foundation (CHE1213230 to M.C.K.) for financial support of this research. Computational support was provided by XSEDE on SDSC Gordon (TG-CHE120052). Simon Berritt and members of the UPenn-Merck High Throughput Experimentation Center at the University of Pennsylvania are acknowledged for purification of reaction mixtures and for access to chiral stationary phase supercritical fluid chromatography.

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    Ren, Qinghua; Jiang, Feng; Gong, Hegui
    Journal of Organometallic Chemistry (2014), 770 (), 130-135CODEN: JORCAI; ISSN:0022-328X. (Elsevier B.V.)
    Ni-catalyzed reductive cross-coupling reactions of electrophilic regents provide an important method to form C-C bonds. The present study explored several single electron transfer mechanisms for Ni-catalyzed reductive cross-coupling of aryl bromide and secondary alkyl bromide using D. Functional Theory (DFT) calcns. The results showed that two of the proposed mechanisms were feasible. One was a six-step catalytic cycle including oxidative addn., redn., radical prodn., radical addn., reductive elimination and catalyst regeneration. The other was a five-step mechanism involving radical prodn., redn., oxidative addn., radical addn., and reductive elimination. The rate-limiting step for both mechanisms was the radical addn. step with the energy barrier of 10.42 kcal/mol. All DFT calcns. were implemented in the gas phase.
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  8. 8.
    (a) Lloyd-Jones, G. C.; Ball, L. T. Science 2014, 345, 381
    [Crossref], [PubMed], [CAS]
    8.
    Self-control tames the coupling of reactive radicals
    Lloyd-Jones, Guy C.; Ball, Liam T.
    Science (Washington, DC, United States) (2014), 345 (6195), 381-382CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    There is no expanded citation for this reference.
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    (b) Leonori, D.; Varinder K; Aggarwal, V. K. Angew. Chem., Int. Ed. 2014, 54, 1082
    [Crossref]
    There is no corresponding record for this reference.
  9. 9.
    Frisch, M. J.; Gaussian 09, rev C.01; Gaussian, Inc.: Wallingford, CT, 2009.
    There is no corresponding record for this reference.
  10. 10.
    Um, J. M.; Gutierrez, O.; Schoenebeck, F.; Houk, K. N.; MacMillan, D. W. C. J. Am. Chem. Soc. 2010, 132, 6001
    [ACS Full Text ACS Full Text], [CAS]
    10.
    Nature of Intermediates in Organo-SOMO Catalysis of α-Arylation of Aldehydes
    Um, Joann M.; Gutierrez, Osvaldo; Schoenebeck, Franziska; Houk, K. N.; MacMillan, David W. C.
    Journal of the American Chemical Society (2010), 132 (17), 6001-6005CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The intramol. α-arylation of aldehydes via organo-SOMO catalysis was studied using d. functional theory (B3LYP and M06-2X functionals). The geometries, spin densities, Mulliken charges, and MOs of the reacting enamine radical cations were analyzed, and the nature of the resulting cyclized radical cation intermediates was characterized. In agreement with exptl. observations, the calcd. 1,3-disubstituted arom. system shows ortho selectivity, while the 1,3,4-trisubstituted systems show para, meta (instead of ortho, meta) selectivity. The selectivity change for the trisubstituted rings is attributed to a distortion of the ortho substituents in the ortho, meta cyclization transition structures that causes a destabilization of these isomers and therefore results in selectivity for the para, meta product.
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  11. 11.
    (a) Uyeda, C.; Peters, J. C. Chem. Sci. 2013, 4, 157
    [Crossref], [PubMed], [CAS]
    11.
    Access to formally Ni(I) states in a heterobimetallic NiZn system
    Uyeda, Christopher; Peters, Jonas C.
    Chemical Science (2013), 4 (1), 157-163CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Heterobimetallic NiZn complexes featuring metal centers in distinct coordination environments were synthesized using diimine-dioxime ligands as binucleating scaffolds. A tetramethylfuran-contg. ligand deriv. enables a stable 1-electron-reduced S = 1/2 species to be accessed using Cp2Co as a chem. reductant. The resulting pseudo-square planar complex exhibits spectroscopic and crystallog. characteristics of a ligand-centered radical bound to a Ni(II) center. Upon coordination of a π-acidic ligand such as PPh3, however, a five-coordinate Ni(I) metalloradical is formed. The electronic structures of these reduced species provide insight into the subtle effects of ligand structure on the potential and reversibility of the NiII/I couple for complexes of redox-active tetraazamacrocycles.
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    (b) Nomura, M.; Cauchy, T.; Geoffroy, M.; Adkine, P.; Fourmigué, M. Inorg. Chem. 2006, 45, 8194
    [ACS Full Text ACS Full Text], [CAS]
    11.
    [CpNi(dithiolene)] (and diselenolene) neutral radical complexes
    Nomura, Mitsushiro; Cauchy, Thomas; Geoffroy, Michel; Adkine, Prashant; Fourmigue, Marc
    Inorganic Chemistry (2006), 45 (20), 8194-8204CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Nickel(III) cyclopentadienyl dithiolene and diselenolene complexes CpNiE2C2Q (E2C2Q = dddt, E = S, Q = SCH2CH2; ddds, E = Se, Q = SCH2CH2S; dsdt, E = S, Q = SeCH2CH2Se; bdt, E = S, Q = 1,2-C6H4; bds, E = Se, Q = 1,2-C6H4) were prepd. by reacting of nickelocene or CpNiLn with dithiolene transfer source and oxidant. Various prepns. of the neutral radical [CpNi(dddt)] complex (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) were investigated with CpNi sources, [Cp2Ni], [Cp2Ni](BF4), [CpNi(CO)]2, and [CpNi(cod)](BF4), and dithiolene transfer sources, O:C(dddt), the naked dithiolate (dddt2-), the monoanion of square-planar Ni dithiolene complex (NBu4)[Ni(dddt)2], and the neutral complex [Ni(dddt)2]. The reaction of [CpNi(cod)](BF4) with (NBu4)[Ni(dddt)2] gave the highest yield for the prepn. of [CpNi(dddt)] (86%). 5,6-Dihydro-1,4-dithiin-2,3-diselenolate [CpNi(ddds)], 5,6-dihydro-1,4-diselenin-2,3-dithiolate [CpNi(dsdt)], 1,2-benzenedithiolate [CpNi(bdt)] and 1,2-benzenediselenolate [CpNi(bds)] were prepd. by the reactions of [Cp2Ni] with the corresponding neutral Ni dithiolene complexes [Ni(ddds)2]2, [Ni(dsdt)2], [Ni(bdt)2], and [Ni(bds)2], resp. The prepd. formally Ni(III) cyclopentadienyl radical complexes oxidize and reduce reversibly. They exhibit, in the neutral state, a strong absorption in the NIR region, from 1000 nm in the dddt/ddds/dsdt series to 720 nm in the bdt/bds series with ε values between 2500 and 5000 M-1 cm-1. The mol. and solid state structures of the five complexes were detd. by x-ray structure analyses. Complexes [CpNi(dddt)] and [CpNi(ddds)] are isostructural, while [CpNi(dsdt)] exhibits a closely related structure. Similarly, [CpNi(bdt)] and [CpNi(bds)] are also isostructural. Correlations between structural data and magnetic measurements show the presence of alternated spin chains in [CpNi(dddt)], [CpNi(ddds)] and [CpNi(dsdt)], while a remarkably strong antiferromagnetic interaction in [CpNi(bdt)] and [CpNi(bds)] is attributed to a Cp···Cp face-to-face σ overlap, an original feature in organometallic radical complexes.
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    (c) Jones, G. D.; Martin, J. L.; McFarland, C.; Allen, O. R.; Hall, R. E.; Haley, A. D.; Brandon, R. J.; Konovalova, T.; Desrochers, P. J.; Pulay, P.; Vicic, D. A. J. Am. Chem. Soc. 2006, 128, 13175
    [ACS Full Text ACS Full Text], [CAS]
    11.
    Ligand Redox Effects in the Synthesis, Electronic Structure, and Reactivity of an Alkyl-Alkyl Cross-Coupling Catalyst
    Jones, Gavin D.; Martin, Jason L.; McFarland, Chris; Allen, Olivia R.; Hall, Ryan E.; Haley, Aireal D.; Brandon, R. Jacob; Kanovalova, Tatyana; Desrochers, Patrick J.; Pulay, Peter; Vicic, David A.
    Journal of the American Chemical Society (2006), 128 (40), 13175-13183CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The ability of the terpyridine ligand to stabilize alkyl complexes of nickel has been central in obtaining a fundamental understanding of the key processes involved in alkyl-alkyl cross-coupling reactions. Here, mechanistic studies using isotopically labeled (TMEDA)NiMe2 (TMEDA = N,N,N',N'-tetramethylethylenediamine) have shown that an important catalyst in alkyl-alkyl cross-coupling reactions, (tpy')NiMe (2b, tpy' = 4,4',4''-tri-tert-butylterpyridine), is not produced via a mechanism that involves the formation of Me radicals. Instead, it is proposed that (terpyridine)NiMe complexes arise via a comproportionation reaction between a Ni(II)-di-Me species and a Ni(0) fragment in soln. upon addn. of a terpyridine ligand to (TMEDA)NiMe2. EPR and DFT studies on the paramagnetic (terpyridine)NiMe (2a) both suggest that the unpaired electron resides heavily on the terpyridine ligand and that the proper electronic description of this nickel complex is a Ni(II)-Me cation bound to a reduced terpyridine ligand. Thus, an important consequence of these results is that alkyl halide redn. by (terpyridine)NiRalkyl complexes appears to be substantially ligand based. A comprehensive survey investigating the catalytic reactivity of related ligand derivs. suggests that electronic factors only moderately influence reactivity in the terpyridine-based catalysis and that the most dramatic effects arise from steric and soly. factors.
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  12. 12.
    M06-2X/6-311+G(d,p)-solvated single-point calculations using B3LYP geometries are more effective than B3LYP alone in estimating absolute reaction barriers. See:
    Krenske, E. H.; Agopcan, S.; Aviyente, V.; Houk, K. N.; Johnson, B. A.; Holmes, A. B. J. Am. Chem. Soc. 2012, 134, 12010
    [ACS Full Text ACS Full Text], [CAS]
    12.
    Causation in a Cascade: The Origins of Selectivities in Intramolecular Nitrone Cycloadditions
    Krenske, Elizabeth H.; Agopcan, Sesil; Aviyente, Viktorya; Houk, K. N.; Johnson, Brian A.; Holmes, Andrew B.
    Journal of the American Chemical Society (2012), 134 (29), 12010-12015CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The factors controlling chemo-, regio-, and stereoselectivity in a cascade of reactions starting from a bis(cyanoalkenyl)oxime and proceeding via nitrone cycloaddns. have been unraveled through a series of d. functional theory calcns. with several different functionals. Both kinetic and thermodn. control of the reaction cascade are important, depending upon the conditions. Kinetic control was analyzed by the distortion/interaction model and found to be dictated by differences in distortions of the cycloaddends in the transition states. A new mechanism competing with that originally proposed in the application of these reactions to the histrionicotoxin synthesis was discovered in these studies.
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  13. 13.
    Beak, P.; Basu, A.; Gallagher, D. J.; Park, Y.-S.; Thayumanavan, S. Acc. Chem. Res. 1996, 29, 552
    [ACS Full Text ACS Full Text], [CAS]
    13.
    Regioselective, Diastereoselective, and Enantioselective Lithiation-Substitution Sequences: Reaction Pathways and Synthetic Applications
    Beak, Peter; Basu, Amit; Gallagher, Donald J.; Park, Yong Sun; Thayumanavan, S.
    Accounts of Chemical Research (1996), 29 (11), 552-560CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review with 38 refs.
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  14. 14.
    Reaction of (R)-1-phenylethyltrifluoroborate using chiral ligand L1 resulted in enantioselectivity indistinguishable from that observed using racemic substrate. Reaction of the enantioenriched trifluoroborate with an achiral ligand produced racemic product (see Supporting Information). These results rule out a classical kinetic resolution for this process.
    There is no corresponding record for this reference.
  15. 15.
    (a) Seeman, J. I. J. Chem. Ed. 1986, 63, 42
    [ACS Full Text ACS Full Text], [CAS]
    15.
    The Curtin-Hammett Principle and the Winstein-Holness equation. New definition and recent extensions to classical concepts
    Seeman, Jeffrey I.
    Journal of Chemical Education (1986), 63 (1), 42-8CODEN: JCEDA8; ISSN:0021-9584.
    The title concepts are defined and discussed.
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    (b) Seeman, J. I. Chem. Rev. 1983, 83, 84
    [ACS Full Text ACS Full Text]
    There is no corresponding record for this reference.
  16. 16.
    Preliminary calculations of A2-TS′ corresponding to eq 2 reveal a conformer 2 kcal/mol lower in energy than the lowest energy C-TS′.
    There is no corresponding record for this reference.
  17. 17.
    Calculated ratios were computed using the lowest two diastereomeric transition states computed using SMD-water-UM06/6-311+G(d,p)//UB3LYP/6-31G(d).
    There is no corresponding record for this reference.
  18. 18.
    For reviews on secondary alkyl halides in Ni-catalyzed cross-coupling, including stereoconvergent examples, see:
    (a) Tasker, S. Z.; Standley, E. A.; Jamison, T. F. Nature 2014, 509, 299
    [Crossref], [PubMed], [CAS]
    18.
    Recent advances in homogeneous nickel catalysis
    Tasker, Sarah Z.; Standley, Eric A.; Jamison, Timothy F.
    Nature (London, United Kingdom) (2014), 509 (7500), 299-309CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    A review. Tremendous advances have been made in nickel catalysis over the past decade. Several key properties of nickel, such as facile oxidative addn. and ready access to multiple oxidn. states, have allowed the development of a broad range of innovative reactions. In recent years, these properties have been increasingly understood and used to perform transformations long considered exceptionally challenging. Here we discuss some of the most recent and significant developments in homogeneous nickel catalysis, with an emphasis on both synthetic outcome and mechanism.
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    (b) Rudolph, A.; Lautens, M. Angew. Chem., Int. Ed. 2009, 48, 2656
    [Crossref], [PubMed], [CAS]
    18.
    Secondary alkyl halides in transition-metal-catalyzed cross-coupling reactions
    Rudolph, Alena; Lautens, Mark
    Angewandte Chemie, International Edition (2009), 48 (15), 2656-2670CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Enormous effort has gone into the development of metal-catalyzed cross-coupling reactions with alkyl halides as electrophilic coupling partners. Whereas a wide array of primary alkyl halides can now be used effectively in cross-coupling reactions, the synthetic potential of secondary alkyl halides is just beginning to be revealed. This Mini-review summarizes selected examples of the use of secondary alkyl halides as electrophiles in cross-coupling reactions. Emphasis is placed on the transition metals employed, the mechanistic pathways involved, and implications in terms of the stereochem. outcome of reactions.
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  19. 19.
    (a) Cherney, A. H.; Reisman, S. E. J. Am. Chem. Soc. 2014, 136, 14365
    [ACS Full Text ACS Full Text], [CAS]
    19.
    Nickel-Catalyzed Asymmetric Reductive Cross-Coupling Between Vinyl and Benzyl Electrophiles
    Cherney, Alan H.; Reisman, Sarah E.
    Journal of the American Chemical Society (2014), 136 (41), 14365-14368CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A Ni-catalyzed asym. reductive cross-coupling between vinyl bromides and benzyl chlorides has been developed. This method provides direct access to enantioenriched products bearing aryl-substituted tertiary allylic stereogenic centers from simple, stable starting materials. A broad substrate scope is achieved under mild reaction conditions that preclude the pregeneration of organometallic reagents and the regioselectivity issues commonly assocd. with asym. allylic arylation.
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    (b) Cherney, A. H.; Kadunce, N. T.; Reisman, S. E. J. Am. Chem. Soc. 2013, 135, 7442
    [ACS Full Text ACS Full Text], [CAS]
    19.
    Catalytic Asymmetric Reductive Acyl Cross-Coupling: Synthesis of Enantioenriched Acyclic α,α-Disubstituted Ketones
    Cherney, Alan H.; Kadunce, Nathaniel T.; Reisman, Sarah E.
    Journal of the American Chemical Society (2013), 135 (20), 7442-7445CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The first enantioselective Ni-catalyzed reductive acyl cross-coupling has been developed. Treatment of acid chlorides and racemic secondary benzyl chlorides with a NiII/bis(oxazoline) catalyst in the presence of Mn0 as a stoichiometric reductant generates acyclic α,α-disubstituted ketones in good yields and high enantioselectivity without requiring stoichiometric chiral auxiliaries or pregeneration of organometallic reagents. The mild, base-free reaction conditions are tolerant of a variety of functional groups on both coupling partners.
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  20. 20.
    (a) Wilsily, A.; Tramutola, F.; Owston, N. A.; Fu, G. C. J. Am. Chem. Soc. 2012, 134, 5794
    [ACS Full Text ACS Full Text], [CAS]
    20.
    New Directing Groups for Metal-Catalyzed Asymmetric Carbon-Carbon Bond-Forming Processes: Stereoconvergent Alkyl-Alkyl Suzuki Cross-Couplings of Unactivated Electrophiles
    Wilsily, Ashraf; Tramutola, Francesco; Owston, Nathan A.; Fu, Gregory C.
    Journal of the American Chemical Society (2012), 134 (13), 5794-5797CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The ability of two common protected forms of amines (carbamates and sulfonamides) to serve as directing groups in Ni-catalyzed Suzuki reactions has been exploited in the development of catalytic asym. methods for cross-coupling unactivated alkyl electrophiles. Racemic secondary bromides and chlorides undergo C-C bond formation in a stereoconvergent process in good ee at room temp. in the presence of a com. available Ni complex and chiral ligand. Structure-enantioselectivity studies designed to elucidate the site of binding to Ni (the oxygen of the carbamate and of the sulfonamide) led to the discovery that sulfones also serve as useful directing groups for asym. Suzuki cross-couplings of racemic alkyl halides. To our knowledge, this investigation provides the first examples of the use of sulfonamides or sulfones as effective directing groups in metal-catalyzed asym. C-C bond-forming reactions. A mechanistic study established that transmetalation occurs with retention of stereochem. and that the resulting Ni-C bond does not undergo homolysis in subsequent stages of the catalytic cycle.
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    (b) Zultanski, S. L.; Fu, G. C. J. Am. Chem. Soc. 2011, 133, 15362
    [ACS Full Text ACS Full Text], [CAS]
    20.
    Catalytic asymmetric γ-alkylation of carbonyl compounds via stereoconvergent Suzuki cross-couplings
    Zultanski, Susan L.; Fu, Gregory C.
    Journal of the American Chemical Society (2011), 133 (39), 15362-15364CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    With the aid of a chiral nickel catalyst, enantioselective γ- (and δ-) alkylations of carbonyl compds. can be achieved through the coupling of γ-haloamides with alkylboranes. In addn. to primary alkyl nucleophiles, for the first time for an asym. cross-coupling of an unactivated alkyl electrophile, an arylmetal, a boronate ester, and a secondary (cyclopropyl) alkylmetal compd. are shown to couple with significant enantioselectivity. A mechanistic study indicates that cleavage of the carbon-halogen bond of the electrophile is irreversible under the conditions for asym. carbon-carbon bond formation.
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    (c) Lu, Z.; Wilsily, A.; Fu, G. C. J. Am. Chem. Soc. 2011, 133, 8154
    [ACS Full Text ACS Full Text], [CAS]
    20.
    Stereoconvergent Amine-Directed Alkyl-Alkyl Suzuki Reactions of Unactivated Secondary Alkyl Chlorides
    Lu, Zhe; Wilsily, Ashraf; Fu, Gregory C.
    Journal of the American Chemical Society (2011), 133 (21), 8154-8157CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A new family of nickel-catalyzed stereoconvergent cross-couplings of unactivated secondary alkyl electrophiles has been developed, specifically, arylamine-directed alkyl-alkyl Suzuki reactions. This represents the first such investigation to be focused on the use of alkyl chlorides as substrates. Structure-enantioselectivity studies are consistent with the nitrogen, not the arom. ring, serving as the primary site of coordination of the arylamine to the catalyst. The rate law for this asym. cross-coupling is compatible with transmetalation being the turnover-limiting step of the catalytic cycle.
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    (d) Owston, N. A.; Fu, G. C. J. Am. Chem. Soc. 2010, 132, 11908
    [ACS Full Text ACS Full Text], [CAS]
    20.
    Asymmetric Alkyl-Alkyl Cross-Couplings of Unactivated Secondary Alkyl Electrophiles: Stereoconvergent Suzuki Reactions of Racemic Acylated Halohydrins
    Owston, Nathan A.; Fu, Gregory C.
    Journal of the American Chemical Society (2010), 132 (34), 11908-11909CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A method for asym. alkyl-alkyl Suzuki reactions of unactivated secondary alkyl electrophiles, specifically, cross-couplings of racemic acylated halohydrins with alkylborane reagents, has been developed. A range of protected bromohydrins, as well as a protected chlorohydrin and a homologated bromohydrin, are coupled in good ee by a catalyst derived from com. available components.
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    (e) Saito, B.; Fu, G. C. J. Am. Chem. Soc. 2008, 130, 6694
    [ACS Full Text ACS Full Text], [CAS]
    20.
    Enantioselective Alkyl-Alkyl Suzuki Cross-Couplings of Unactivated Homobenzylic Halides
    Saito, Bunnai; Fu, Gregory C.
    Journal of the American Chemical Society (2008), 130 (21), 6694-6695CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The first effective method for asym. cross-couplings of unactivated alkyl electrophiles has been developed, specifically, a nickel-based catalyst for stereoconvergent Suzuki reactions of homobenzylic bromides with alkylboranes. To the best of our knowledge, there are no previous examples of enantioselective Suzuki couplings of alkyl electrophiles (activated or unactivated). Both of the catalyst components are com. available.
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    (f) Jiang, X.; Sakthivel, S.; Kulbitski, K.; Nisnevich, G.; Gandelman, M. J. Am. Chem. Soc. 2014, 136, 9548
    [ACS Full Text ACS Full Text], [CAS]
    20.
    Efficient Synthesis of Secondary Alkyl Fluorides via Suzuki Cross-Coupling Reaction of 1-Halo-1-fluoroalkanes
    Jiang, Xiaojian; Sakthivel, Sekarpandi; Kulbitski, Kseniya; Nisnevich, Gennady; Gandelman, Mark
    Journal of the American Chemical Society (2014), 136 (27), 9548-9551CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Organofluorine compds. have found extensive applications in various areas of science. Consequently, the development of new efficient and selective methods for their synthesis is an important goal in org. chem. Here, we present the first Suzuki cross-coupling reaction which utilizes dihalo compds. for the prepn. of secondary alkyl fluorides. Namely, an unprecedented use of simple 1-halo-1-fluoroalkanes as electrophiles in Csp3-Csp3 and Csp3-Csp2 cross-couplings allows for the formal site-selective incorporation of the F-group in the alkyl chain with no adjacent activating functional groups. A highly effective approach to the electrophilic substrates, 1-halo-1-fluoroalkanes, via iododecarboxylation of the corresponding α-fluorocarboxylic acids is also presented. The conceptually new route to organofluorides was used for the facile prepn. of biomedically valuable compds. In addn., we demonstrated that an asym. version of the developed reaction for the stereoconvergent synthesis of chiral secondary alkyl fluorides is feasible.
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  21. 21.
    For a recent report on Ni(II) intermediates in the related Negishi coupling, see:
    Schley, N. D.; Fu, G. C. J. Am. Chem. Soc. 2014, 136, 16588
    [ACS Full Text ACS Full Text], [CAS]
    21.
    Nickel-Catalyzed Negishi Arylations of Propargylic Bromides: A Mechanistic Investigation
    Schley, Nathan D.; Fu, Gregory C.
    Journal of the American Chemical Society (2014), 136 (47), 16588-16593CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Although nickel-catalyzed stereoconvergent couplings of racemic alkyl electrophiles are emerging as a powerful tool in org. chem., to date there have been no systematic mechanistic studies of such processes. Herein, we examine the pathway for enantioselective Negishi arylations of secondary propargylic bromides, and we provide evidence for an unanticipated radical chain pathway wherein oxidative addn. of the C-Br bond occurs through a bimetallic mechanism. In particular, we have crystallog. characterized a diamagnetic arylnickel(II) complex, [(i-Pr-pybox)NiIIPh]BArF4, and furnished support for [(i-Pr-pybox)NiIIPh]+ being the predominant nickel-contg. species formed under the catalyzed conditions as well as a key player in the cross-coupling mechanism. On the other hand, our observations do not require a role for an organonickel(I) intermediate (e.g., (i-Pr-pybox)NiIPh), which has previously been suggested to be an intermediate in nickel-catalyzed cross-couplings, oxidatively adding alkyl electrophiles through a monometallic pathway.
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Article Views: 10,306 Times
Received 23 December 2014
Published online 2 April 2015
Published in print 22 April 2015
Learn more about these metrics Article Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.

The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated.
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