Rhodium(III)-to-rhodium(I) alkyl transfer: A rhodium macrocycle which is both nucleophile and leaving group

Article abstract of DOI:10.1021/om00133a007

The rhodium(I) macrocycles Rh^I(BPDOBF₂) (1a) and Rh^I(PPDOBF₂) (1b) are strong nucleophiles toward alkyl halides and may also be alkylated by their rhodium(III) alkyl halide adducts R′-Rh^III(BPDOBF₂)-X and R′-Rh^III(PPDOBF₂)-X (2a and 2b), respectively. In the latter process, 1a,b act as both nucleophile and leaving group. We have studied the kinetics of alkyl exchange between, e.g., 1a and 2b by ¹H NMR and the rate constants are (5.8 ± 0.5) × 10^-3 M^-1 s^-1 for methyl exchange (R′ = Me, X = I) and (6.4 ± 1.0) × 10^-2 M^-1 s^-1 for benzyl exchange (R′ = PhCH₂, X = Cl). Other alkyl groups do not participate in this reaction. These data are consistent with rhodium(I) attack at carbon; the process may be viewed as a carbon-bridged two-electron transfer. Halide-bridged electron transfer is implied in the much faster exchange (k > 0.5 M^-1 s^-1) between 1a,b and their rhodium(III) dichloride derivatives 3a and 3b.