Hydroformylation of olefins under mild conditions Part II: the Co4-nRhn(CO)12 + xPPh2H + yL (n = 2, 4) system

The hydroformylation of cyclohexene, 1-pentene and styrene under mild conditions (25 - 50 °C, 1 atm equimolar CO and H2) in toluene or benzene using the system Co4-nRhn(CO)12 + xPPh3-mHm + yL (n = 2, 4; m = 1,2; L = trisubstituted phosphine or phosphite) as catalyst precursor has been investigated. This system was designed to generate in situ clusters containing a variable number of μ-PPh3-m bridging ligands. A set of experiments, in which x and y were independently and systematically varied in the range 1 - 4, points out that the Co4-nRhn(CO)12 + xPPh2H + yL system is an active hydroformylation catalyst for x = 1 - 2 and y = 2 - 4. Furthermore, the combined addition of PPh2H and a trisubstituted ligand L to Rh4(CO)12 apparently brings about a 'synergetic effect' which results in hydroformylation rates of 1-pentene and styrene often greater than those observed in the absence of PPh2H. An increase of x to 3 or 4 causes a sharp drop in the catalytic activity. Accordingly, preformed Rh3(μ2-PPh2)3(CO) 5 and Rh4(μ2-PPh2)4(CO)6 , both in the presence or in the absence of additional trisubstituted ligands, are substantially inactive. The Co4-nRhn(CO)12 + xPPhH2 + yL system in contrast is barely active for all tested values of x and y, although an increase in the temperature brings about some activity in a few cases. Unlike 1-pentene and styrene, the catalytic hydroformylation of cyclohexene could not be induced by combined addition of PPl2H and additional trisubstituted ligand to Rh4(CO)12 in any ratio. It is therefore conceivable that the formation of clusters containing the μ2-PPh2 group could be at the origin of the previously reported deactivation of the Rh4(CO)12 + xPPh3 system in the hydroformylation of cyclohexene.