Suzuki–Miyaura\nCross-Couplings of Alkylboranes:\nTransmetalation Studies and Synthesis of Model Organopalladium Pretransmetalation\nSpecies

This report investigates the transmetalation of <i>B</i>-alkyl-9-borabicyclo[3.3.1]nonane (<i>B</i>-alkyl-9-BBN)\ncoupling partners in C<sub>sp<sup>2</sup></sub>–C<sub>sp<sup>3</sup></sub> Suzuki–Miyaura (SM) reactions. Kinetically stable,\nmodel organopalladium(II) SM pretransmetalation intermediates were\nsynthesized, which enabled comprehensive X-ray crystallographic and\nNMR spectroscopic characterization of these elusive species at last. Computational studies of kinetically\nrelevant systems were consistent with transmetalation proceeding with\nretention of configuration with respect to the carbon atom via a frontside\nS<sub>E</sub>2 (coordination) mechanism. This is proposed to involve\na four-membered transition state with electron transfer into the vacant\npalladium d<sub>x<sup>2</sup>–y<sup>2</sup></sub> orbital.\nFurthermore, integrated theoretical and experimental results indicated\nthat in the presence of hydroxide in THF/water mixtures, catalytic\nturnover is kinetically favored via anionic transmetalation path A,\nwhich involves the initial reaction of [Pd(Ph)(PPh<sub>3</sub>)<sub>2</sub>Br] with [<i>B</i>-alkyl-9-BBN(OH)]<sup>−</sup>, over neutral path B, which requires initial formation of [Pd(Ph)(PPh<sub>3</sub>)<sub>2</sub>OH] then reaction with <i>B</i>-alkyl-9-BBN.\nIt is notable that in path A, the conversion of [Pd(Ph)(PPh<sub>3</sub>)<sub>2</sub>Br] to the phenylpalladium(II) boronate pretransmetalation\nspecies only appears to be feasible if phosphine ligand exchange with\n[<i>B</i>-alkyl-9-BBN(OH)]<sup>−</sup> precedes the\nloss of bromide. It is likely that these nuances derive, at least\npartly, from the very high Lewis acidity and steric bulk of the boron\ncenter within <i>B</i>-alkyl-9-BBN and its derivatives.