Mechanistic Insights into Regioselective Arene Insertion Using Bis(phosphine) Cobalt(I) Hydrides to Form 1,3-Cyclohexadienes

Bis(phosphine) cobalt allyl complexes were used as isolable precursors to access transient, 14-electron cobalt(I) hydrides that promoted the insertion of a series of substituted benzenes. The selectivity of the insertion reaction was assayed by analysis of the resulting diamagnetic 18-electron bis(phosphine)Co(η⁵-cyclohexadienyl) complexes by multinuclear NMR spectroscopy. Monitoring the insertion of α,α,α-trifluorotoluene (PhCF₃) with [(R,R)-(^iPrDuPhos)CoH] by NMR spectroscopy revealed an initially aselective kinetic product mixture that underwent isomerization to the thermodynamically preferred ipso-CF₃ product. The site selectivity for ipso-CF₃ insertion over alkyl, boryl, and silyl substituents was observed across a series of disubstituted benzenes, motivating an investigation into the kinetic and thermodynamic selectivities of representative alkyl trifluoromethylbenzenes using NMR spectroscopy and X-ray crystallography. Protonation of various cobalt(I) cyclohexadienyl complexes afforded C(sp³)-CF₃-substituted cyclohexadienes in high yields, which were trapped by Diels–Alder reactions.