MECHANISTIC STUDIES ON RHODIUM-CATALYZED CARBONYLATION REACTIONS - SPECTROSCOPIC DETECTION AND REACTIVITY OF A KEY INTERMEDIATE, [MERH(CO)2I3]-

The catalytic intermediate [MeRh(CO)2I3]- (B) (present to the extent of ca. 1% of A at steady state) has been spectroscopically detected in the reaction of [Rh(CO)2I2]- (A) with neat Mel giving [Rh(CO)(COMe)I3]- (C) (Scheme II). FTIR and low-temperature C-13 NMR spectroscopic measurements using both (CO-)-C-13 and CH3-(C-13)-labeled complexes, and the analogy to [MeIr(CO)2I3]- all favor a fac,cis stereochemistry for B. Measurement of the steady-state ratio [B]/[A] and the pseudo-first-order rate constant for conversion of A into C has allowed k2 to be estimated (5-35-degrees-C). Activation parameters for migratory insertion (B --> C) are DELTAH(double dagger) = 63 +/- 2 kJ mol-1 and DELTAS(double dagger) = -59 +/- 9 J mol-1 K-1. The ratio k2/k-1 is estimated to be ca. 9 in CH2Cl2 at 35-degrees-C from measurements of the solution behavior of C. Activation parameters for the overall reaction of A with neat MeI are DELTAH(double dagger) = 50 +/- 1 kJ mol-1 and DELTAS(double dagger) = -165 +/- 4 J mol-1 K-1. Kinetic measurements (15-35-degrees-C) for isotopic label scrambling in ([Rh((CO))(COMe)I3]-)-C-12-C-13 give activation parameters for deinsertion (C --> B) of DELTAH(double dagger) = 100 +/- 1 kJ mol-1 and DELTAS double dagger = -5 +/- 4 J mol-1 K-1. Complete rate data for the reaction shown in Scheme II at 35-degrees-C are k1 = 6.8 X 10(-5) M-1 s-1 (MeI), k-1 = 1.5 X 10(-2) s-1 (CH2Cl2), k2 = 1.33 X 10(-1) s-1 (MeI), and k-2 = 4.20 X 10(-5) s-1 (MeI). Derived equilibrium constants (K1 = 4.5 x 10(-3) M-1, K2 3.2 X 10(3) (35-degrees-C)) show that B is unstable with respect to both migratory insertion and reductive elimination of MeI. Rhodium methyl intermediates analogous to B, such as [MeRh(CO)2Cl2I]-, have been detected in the reaction between [Rh(CO)2Cl2]- and MeI to give [Rh(CO)2I2]- and MeCl, indicating that halide exchange occurs via an oxidative addition/reductive elimination sequence.