Some aspects of the chemistry of haloboranes

The reaction between polyhalomethanes and the borane-dimethyl sulphide complex (BMS) was investigated. cx4+ bh3.s(ch3)2 xbh2.s(ch3)2 + hcx3The reaction between BMS and tetrachloromethane (or tribro-mochloromethane) followed the kinetic form, -d(BMS) / dt = k(BMS) (XCCl3). On the basis of solvent effects and inhibition experiments the following mechanism was proposed; (1) (CH3)2S.BH3 + Initiator (CH3)2S.BH2 + product(2)(CH3)2S.BH2. + XCY3(CH3)2S.BH2X +CY3(3) (CH3)2S.BH3 + CY3(CH3)2S.BH2 + HCY3. The observed relative ease of displacement for the halogen-hydrogen transfer are I>Br>Cl"F. This parallels the bond strengths of the Carbon-Halogen bond and indicates that step (2) is rate-determining. The halomethanes and halo-triphenyl methanes, in their reaction with BMS, have led to convenient methods for producing halogenoboranes for use as hydroborating agents. The reaction between BMS and tetrachloromethane was used to generate dialkylchloroboranes. Reaction of these with some substituted benzaldehydes were investigated and found to follow the kinetic formed (Pent-l-ene) /dt = k (Ar.CHO) ((n-C5H11)2BC1) for the reaction, 9°CAr.CHO + (n-C5H11) 2BCl -Ar. CH2O. B(Cl) + pent-l-ene. The rate of the reaction increases with electron withdrawing substituents in the para-position and decreases with electron donating substituents. The rate of the reaction correlates best with q+ giving a value of p of 2.1. 11B n.m.r studies suggest initial complex formation. The proposed mechanism is k3 (hydride transfer) being the rate-determining step.