Carbon-centered radical capture at nickel(II) complexes: Spectroscopic evidence, rates, and selectivity

•Rates of radical capture by nickel(II) are on the scale 106∼107 M−1s−1•Nickel(III) intermediates derived from trapping observed EPR spectroscopy•There different rate-determining steps for C(sp3)–C(sp3) and C(sp3)–C(sp2) bond formation•Stereoselectivity is conferred with evident ligand control First-row transition-metal complexes represent sustainable affordable catalysts organic synthesis. Because their stable states, first-row metal can mediate pathways in catalysis initiating formation via redox processes capturing a at center. Nickel(II) proposed to trap carbon radicals engage subsequent C–C nickel(III) intermediate. This fundamental step has broadened scope cross-coupling partners, enabled stereoconvergent synthesis, controlled stereo- chemoselectivity. Yet, there lack experimental characterization this critical step. report provides spectroscopic evidence capture, calibrates rates, demonstrates ligand-controlled stereoselectivity. These results help form hypotheses reaction design as an intermediate inform optimization achieve broad scope. The carbon-centered center commonly featured recent metallaphotoredox catalytic reactions. Despite its widespread application catalysis, lacks characterization. portrays catalytically relevant centers several aspects, including structure-activity relationships ligands, mechanism, kinetics, Spectroscopic data provide Strikingly reactivity between nickel-aryl nickel-alkyl implies C(sp2)–C(sp3) formation. Kinetic benchmark rates 107 M−1s−1 106 primary secondary radicals, respectively. Overall, activation energy higher than that previous computational estimations. Finally, stoichiometric experiments well-defined chiral nickel demonstrate confer diastereoselectivity enantioselectivity drastic effect. Recent advancements reactions exploit initiate propagate reactions.1Tasker S.Z. Standley E.A. Jamison T.F. advances homogeneous catalysis.Nature. 2014; 509: 299-309Crossref PubMed Scopus (1491) Google Scholar,2Choi J. Fu G.C. Transition metal–catalyzed alkyl-alkyl formation: another dimension chemistry.Science. 2017; 356: eaaf7230Crossref (396) Scholar,3Fu Transition-metal nucleophilic substitution reactions: alternative SN1 SN2 processes.ACS Cent. Sci. 3: 692-700Crossref (332) Scholar,4Diccianni J.B. Diao T. Mechanisms nickel-catalyzed reactions.J. Trends Chem. 2019; 1: 830-844Abstract Full Text PDF (224) Scholar,5Chan A.Y. Perry I.B. Bissonnette N.B. Buksh B.F. Edwards G.A. Frye L.I. Garry O.L. Lavagnino M.N. Li B.X. Liang Y. et al.Metallaphotoredox: merger photoredox transition catalysis.Chem. Rev. 2022; 122: 1485-1542Crossref (333) Scholar A involved these cycles features center, generating readily undergo reductive elimination (Figure 1A).6Zhu S. Zhao X. H. Chu L. Catalytic three-component dicarbofunctionalization involving nickel.Chem. Soc. 2021; 50: 10836-10856Crossref pathway differs traditional two-electron processes, frequently mediated palladium other noble metals, novel synthetic opportunities forming bonds. For example, catalyzed allow stereo-convergent synthesis.7Schley N.D. Nickel-catalyzed Negishi arylations propargylic bromides: mechanistic investigation.J. Am. 136: 16588-16593Crossref (292) ability them partners. Although be generated halogen-atom abstraction low-valent species,8Lin Q. Liu P. Monovalent nickel-mediated concerted dissociation determined electroanalytical studies.J. 143: 14196-14206Crossref (32) abundant electrocatalytic approaches have been developed wide range precursors, carboxylic acids, C–H bonds, boronates, auxiliaries.5Chan Scholar,9Crespi Fagnoni M. Generation alkyl radicals: tyranny tin photon democracy.Chem. 2020; 120: 9790-9833Crossref (163) Scholar,10Kvasovs N. Gevorgyan V. Contemporary methods generation aryl radicals.Chem. 2244-2259Crossref Scholar,11Sumida Ohmiya Direct excitation strategy synthesis.Chem. 6320-6332Crossref Moreover, high-valent upon could promote bond-forming both kinetically thermodynamically.12Zheng B. Tang F. Luo Schultz J.W. Rath N.P. Mirica L.M. Organometallic carbon–heteroatom 6499-6504Crossref (147) Scholar,13Bour J.R. Camasso N.M. Meucci Kampf Canty A.J. Sanford M.S. Carbon-carbon isolated complexes.J. 2016; 138: 16105-16111Crossref (87) determine chemoselectivity cross-electrophile coupling reactions14Lin Mechanism Ni-catalyzed 1,2-dicarbofunctionalization alkenes.J. 141: 17937-17948Crossref (132) stereoselectivity diastereoselective enantioselective reactions.15Lucas E.L. Jarvo E.R. Stereospecific cross-couplings electrophiles.Nat. 1Crossref Scholar,16Cherney A.H. Kadunce N.T. Reisman S.E. Enantioselective enantiospecific transition-metal-catalyzed organometallic reagents construct bonds.Chem. 2015; 115: 9587-9652Crossref (570) Radical complex formally increases oxidation state +1 characterized systems, transformations Ni(I) Ni(II),17Bakac A. Espenson J.H. Kinetics mechanism alkylnickel one-electron reductions alkyl-halides hydroperoxides macrocyclic nickel(I) complex.J. 1986; 108: 713-719Crossref (126) Ni(II) Ni(III),18Vorobyev D.Y. Plyusnin V.F. Ivanov Y.V. Grivin V.P. Larionov S.V. Lemmetyinen Photochromic reversible coordination perfluorothionaphthyl flat dithiolate Photochem. Photobiol. 2002; 149: 101-109Crossref (6) Scholar,19Bakhoda A.G. Wiese Greene C. Figula B.C. Bertke J.A. Warren T.H. complexes: C–C, C–N, C–O formation.Organometallics. 39: 1710-1718Crossref (7) Scholar,20Shin Gwon Kim Lee Park K. Correlation activity C-to-Ni charge transfer Ni 142: 4173-4183Crossref (9) Ni(III) Ni(IV).20Shin Scholar,21Bour Ferguson D.M. McClain E.J. Connecting Ni(IV): organonickel 8914-8920Crossref (38) most system N-ligand coordinated Ni(II)-aryl complexes, such (bpy)Ni(Ar)(Br) (bpy = bipyridine), which investigated computationally. Two possible generally considered: first inner-sphere involves discrete intermediate, second outer-sphere bonds directly formed without interaction entering 1B). former radicals,22Gutierrez O. Tellis J.C. Primer D.N. Molander Kozlowski M.C. photoredox-generated uncovering general manifold stereoconvergence cross-couplings.J. 137: 4896-4899Crossref (406) Scholar,23Shu W. García-Domínguez Quirós M.T. Mondal R. Cárdenas D.J. Nevado nonactivated alkenes: insights.J. 13812-13821Crossref (129) Scholar,24Yuan Song Z. Badir S.O. Gutierrez On nature tertiary cross-couplings: case study Ni/photoredox halides.J. 7225-7234Crossref (103) Scholar,25Matsui J.K. Gutiérrez-Bonet Á. Rotella Alam Photoredox/nickel-catalyzed single-electron Tsuji-Trost reaction: development insights.Angew. Int. Ed. 2018; 57: 15847-15851Crossref (64) Scholar,26Campbell M.W. Yuan Polites V.C. Photochemical enables olefin dicarbofunctionalization.J. 3901-3910Crossref (61) Scholar,27Kolahdouzan Khalaf Grandner J.M. Chen Terrett Huestis M.P. Dual photoredox/nickel-catalyzed conversion halides aminooxetanes: substrate-dependent switch mechanism.ACS Catal. 10: 405-411Crossref (24) Scholar,28Lau S.H. Borden M.A. Steiman T.J. Wang L.S. Parasram Doyle Ni/photoredox-catalyzed styrene oxides iodides.J. 15873-15881Crossref (43) Scholar,29Yin Mechanistic investigation enantioconvergent Kumada racemic α-bromoketones nickel/bis(oxazoline) 15433-15440Crossref (88) Scholar,30Guo Zhang Zhu General method carboarylation alkenes visible-light dual photoredox/nickel catalysis.J. 20390-20399Crossref (93) Scholar,31Xu Yang D. DFT IrIII/NiII-metallaphotoredox-catalyzed difluoromethylation bromides.Inorg. 60: 8682-8691Crossref whereas latter proved plausible radicals.24Yuan Calculated kinetic barriers (ΔG‡) vary among studies 1C).32Yuan Mechanisms, challenges, cross-couplings.Wiley Interdiscip. Comput. Mol. 12: e1573Crossref (11) benzyl calculated almost barrierless,22Gutierrez slower spreads across values.23Shu estimated fast precluded consideration rate-limiting relative following studies. There competent results, seldom compared effect actor ligands capture.31Xu Herein, we address knowledge gap comprehensive nickel(II)-aryl nickel(II)-alkyl bearing applied reactions, bpy phen (phen 1,10-phenanthroline). sheds light key aspects reaches unforeseen conclusions: (1) distinguish intermediate; (2) rate constants reveals density functional theory (DFT) calculations underestimated barrier limiting relation elimination; (3) contrasting unveils steps, accounts challenges prompts reconsideration mechanism; (4) investigations stereochemistry elucidate impact substrates stereochemical outcome. We conducted survey various phen-, pyrox-, pybox-, bpy-nickel establish effects auxiliary capture. Benzyl dihydropyridine 1 (Bn–DHP) shown eject photoexcitation, driven resulting Hantzsch pyridine.33Buzzetti Prieto Roy S.R. Melchiorre Radical-based C−C photoexcitation 4-alkyl-1,4-dihydropyridines.Angew. 56: 15039-15043Crossref (165) When irradiated 390 nm presence (phen)Ni(Ar)(Br) 2, product Ar-Bn quantitative yields 2Α, entry 1). Monitoring UV-visible spectroscopy comparing spectra (phen)NiBr 3 verified concomitant S6). Control revealed 2 under photo-irradiation ruled out induced direct photoexcitation.34Shields B.J. Kudisch Scholes G.D. Long-lived charge-transfer states halide facilitate bimolecular photoinduced electron transfer.J. 140: 3035-3039Crossref (161) yield was substantially reduced 10% bulky 4 (entry 2). Ni-alkyl 5 6 lacked homo-coupling products low (entries 4).35Efforts synthesize (tBubpy)Ni-methyl ethyl were complicated instability. neosilyl group stabilize β-effect silicon, not believed make difference reactivity.Google fate cases toluene, remaining stayed intact. Complex 7 afforded biaryl major product, consistent reports complexes’ instability 5).36Yamano M.M. Kelleghan A.V. Shao Giroud Simmons Houk K.N. Garg N.K. Intercepting fleeting cyclic allenes asymmetric 586: 242-247Crossref (21) then examined N-ligands, bpy, tBubpy (tBubpy di-tert-butyl-bipyridine), CF3bpy (CF3bpy di-trifluoromethyl-bipyridine), pyrox (pyrox pyridine-oxazoline), pybox (pybox pyridine-bis-oxazoline)7Schley entries 6–12). Good classes. more primarily better solubilization solvents. Both gave similar 7). electron-deficient 10 8) led slightly lower 8 9. (iPrpybox)Ni 14 noticeably those bidentate ligands. attribute decrease steric bulk isopropyl groups pybox. subjected conditions Figure 2A, corresponding pyrox-ligated only yielded trace amounts 9 11). trend hybridization (phen)Ni analogs: (phen)Ni-aryl efficient mediating after accepting radical, but (phen)Ni-alkyl little products. strikingly prompted us probe C(sp2) C(sp3) electrophiles. carried DHP substrate 15 bromide electrophiles reported co-workers 2Β, equation 1).33Buzzetti underwent smooth 16, report,33Buzzetti n-propyl no 17. Subsequently, performed photo-electron paramagnetic resonance (EPR) verify species (tBubpy)Ni(II) complexes. Irradiation Bn-DHP any present giso value 2.0035 S7).33Buzzetti photo-excited slow (k 9.5 × 10−6 s−1) S5). gathered 9, 11 irradiation.37Complex unstable exhibited impurity spectrum before irradiation thus excluded study.Google With accumulation isotropic signal 77 K, assigned 2.0038 (Figures 3A S10). displayed species. After three hours, analysis mixture GC 28% yield. rest remained unreacted. Conducting photo-EPR experiment resulted observation K metal-centered 3B), suggesting temperature-dependent relaxation.38Hagen W.R. Biomolecular spectroscopy.in: Spectroscopy. CRC Press, 2009: 65Google signal, spanning 240 400 mT, gz turning points g 2.52 1.67, correspond m −1 0 z direction, respectively.39Roessler Salvadori E. Principles applications chemical sciences.Chem. 47: 2534-2553Crossref broader common S 1/2 complex, usually 300 340 mT.40Que Jr., Physical bioinorganic chemistry magnetism.in: Methods Bioinorganic Chemistry Spectroscopy Magnetism. University Science Books, 2000: 123Google relaxation multiple line shapes suggest species, zero-field splitting (ZFS) points. calculate magnitude D tensor 0.061 cm−1,41Abe Diradicals.Chem. 2013; 113: 7011-7088Crossref (890) ZFS parameter strong dipolar-dipolar short distance two spins.42Le Vaillant Reijerse Leutzsch Cornella Dialkyl ether nickel.J. 19540-19550Crossref (15) Simulation =1 values [2.2201, 2.1105, 2.0139] [2.2205, 2.1350, 2.0135], respectively S9). resemblance assign monomeric dimeric 18 19.12Zheng Scholar,42Le Scholar,43Xu Diccianni Katigbak Hu Bimetallic 4779-4786Crossref (62) 2.1148 small Δg 0.11 falls into expected ORCA package putative 20 nickel-centered S11). measurements (phen) (tBubpy)Ni(III)-aryl intermediates. To circumvent this, employed clock reduction 21 Bu3SnH (365 nm) 23 25. seven-membered 7-endo cyclization 27 also quantity (Scheme S1). proceeds through Bu3Sn⋅ Bu3SnH, abstracts bromine atom generate 27. Hydrogen-atom (HAT) (k1) 23. Competing pathway, 6-exo (k2) followed HAT 29 process depending [Bu3SnH], unimolecular. Thus, ratio [23]/[25] display linear dependence assumption afford 25 irreversible. parallel varying concentrations recorded ratios each conversion, when consumption negligible large excess initial [Bu3SnH] 4A). relationship slope corresponds k1/k2. Given k1 (2.7 ± 0.1) M−1s−1,44Newcomb Competition scales kinetics.Tetrahedron. 1993; 49: 1151-1176Crossref (607) solved unimolecular constant k2 (2.8 105 s−1 28 k3 (1.5 M−1s−1.44Newcomb Applying same described above analogous substrate, measured k4 (3.8 0.3) (k3) comparable, reflects minimal cyclization. k4, because governed reorganization molecule