Alkylboranes in the Suzuki-Miyaura coupling: Stereochemical and mechanistic studies

Both erythro and three isomers of B-(3,3-dimethyl-1,2-dideuterio-1-butyl)-9-BBN (6) were prepared from 3,3-dimethyl-1-butyne (4) through a hydroboration-deuteronolysis-hydroboration sequence employing first 9-BBN-H and then 9-BBN-D, or in reverse order, respectively. Employing the Whitesides protocol, the stereochemistry of B --> Pd alkyl group transfer in the Suzuki-Miyaura coupling of 6 to PhBr has been found to occur with complete retention of configuration with respect to carbon. For the coupling process, the Lewis acidity of the boron plays an important role with B-alkyl-9-BBN (10) forming [HO(R)-9-BBN](-1) (12) with the added base, in marked contrast to their B-alkyl-9-oxa-10-borabicyclo[3.3.2]decane counterparts (R-OBBD, 11) which do not. This behavior parallels their coupling rates with the exclusive reaction of 10 over 11 in competitive experiments. Five five possible roles were demonstrated for the added base in the coupling: (1) the formation of 12, (2) the hydrolysis of Ph(Ph3P)(2)PdBr (14) to provide monomeric Ph(Ph3P)(2)PdOH (15), (3) the complexation of HOBR2 byproducts which can compete with 10 for base, (4) accelerated coupling rates for 11, and (5) catalyst regeneration. Kinetic studies reveal that the couplings are zero-order in the borane but for 10 exhibit a first-order dependence on [PhBr] (i.e., oxidative addition), while for 11 exhibit a first-order dependence on [OH-1] (i.e., Pd(II)X hydrolysis). These data are interpreted in terms of attack of 14 by 12 to form a hydroxo mu(2)-bridged intermediate 8(a) [PhL2 Pd <-- (OH)BR(9-BBN)]. This provides the precursor to transmetalation through a four-centered transition state 9. Because the analogous hydroxyborate complex is absent for 11, 14 is hydrolyzed by OH-1 forming 15 in a slower process, with this ultimately reacting with 11 to form a related intermediate 8(b) [PhL2 Pd(OH) --> BR(OBBD)] which also collapses to products through 9.