Suzuki–Miyaura Cross-Couplings of Alkylboranes: Transmetalation Studies and Synthesis of Model Organopalladium Pretransmetalation Species

This report investigates the transmetalation of B-alkyl-9-borabicyclo[3.3.1]nonane (B-alkyl-9-BBN)
coupling partners in Csp2–Csp3 Suzuki–Miyaura (SM) reactions. Kinetically stable,
model organopalladium(II) SM pretransmetalation intermediates were
synthesized, which enabled comprehensive X-ray crystallographic and
NMR spectroscopic characterization of these elusive species at last. Computational studies of kinetically
relevant systems were consistent with transmetalation proceeding with
retention of configuration with respect to the carbon atom via a frontside
SE2 (coordination) mechanism. This is proposed to involve
a four-membered transition state with electron transfer into the vacant
palladium dx2–y2 orbital.
Furthermore, integrated theoretical and experimental results indicated
that in the presence of hydroxide in THF/water mixtures, catalytic
turnover is kinetically favored via anionic transmetalation path A,
which involves the initial reaction of [Pd(Ph)(PPh3)2Br] with [B-alkyl-9-BBN(OH)]−, over neutral path B, which requires initial formation of [Pd(Ph)(PPh3)2OH] then reaction with B-alkyl-9-BBN.
It is notable that in path A, the conversion of [Pd(Ph)(PPh3)2Br] to the phenylpalladium(II) boronate pretransmetalation
species only appears to be feasible if phosphine ligand exchange with
[B-alkyl-9-BBN(OH)]− precedes the
loss of bromide. It is likely that these nuances derive, at least
partly, from the very high Lewis acidity and steric bulk of the boron
center within B-alkyl-9-BBN and its derivatives.