Oxidative Addition of α-Bromo Esters to Tetrakis(isocyanide) Rhodium(I) Complexes. Evidence for a Chain Mechanism

The oxidative additions of chiral alkyl halides, (S)-(−)-ethyl α-bromopropionate [α]d−18.9° (c 3.3, CHCl3) and (S)-(+)-ethyl α-phenylbromoacetate [α]d+56.4° (c 1.6, C2H5OH) to [RH(R′NC)4]+ (R′=t-Bu, p-CH3C6H4) produced the corresponding adducts, trans-[RhBr(R)(R′NC)4]+ (R=CH3CHCO2Et, PhCHCO2Et), which were well characterized by elemental analysis, IR and 1H NMR spectra. These adducts were found optically inactive, a fact which precludes a concerted mechanism. The reaction of C6H5CHBrCO2C2H5 with [Rh(t-BuNC)4]+ commenced immediately in the absence of light. Taking this system as a typical example, the rate was studied by the stopped-flow method to find a rate equation, R=k[Rh(I)]2[RX]. The relative rate of addition of p-XC6H4CHBrCO2C2H5 to [Rh(t-BuNC)4]+ decreases in an order of Cl>H>CH3 for X. The addition of C6H5CHBrCO2C2H5 to [Rh(p-CH3C6H4NC)4]+ in the dark proceeds only slowly, but rapidly under a low energy photo-irradiation (440 nm) with a large quantum yield (φ 4.8). The addition of CH3CHBrCO2C2H5 requires photo-initiation. The optical activity of (S)-(−)-CH3CHBrCO2C2H5 was completely lost before the oxidative addition commenced. The rate of decrease in optical activity of the system (S)-(−)-C6H5CHBrCO2C2H5/[Rh(t-BuNC)4]+ exhibited approximate second order kinetic behavior, and quantitative racemization took place prior to the completion of the oxidative addition. A chain mechanism appears to be consistent with all these results. Alternate mechanisms are also discussed in the light of the present results.