DIASTEREOSELECTIVE HYDROFORMYLATION OF CHIRAL ALKENES IN ORGANIC SYNTHESIS

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Taking advantage of hydroformylation of alkenes in organic synthesis, the rhodium complex-catalyzed hydroformylation of chiral allylic alcohols was examined. Despite of related hydrogenation with excellent diastereoselectivity in the product based on a potential hydroxyl group-directed assistance, hydroformylation exhibited an inverse selectivity in the major diastereoisomer. Accordingly, hydroformylation of a non-functionalized substrate, 4-t-butylmethylenecyclohexane (1), using [Rh(acac)(CO)_2] as a catalyst precursor was examined in detail in terms of diastereoselectivity in the products, trans- (2) and cis-4-t-butyl-cyclohexylacetaldehyde (3), as a function of applied initial pressure of syn-gas (H_2-CO = 1 : 1). It was clearly demonstrated that the conversion to aldehydes was found to be satisfactory under a higher pressure of syn-gas (5 - 9.5 MPa), the distereoselectivity 2 : 3 being deteriorated uniformly as the initial pressure increased, while under a lower pressure of syn-gas (2 - 0.5 MPa), diastereoselecivity 2 : 3 became satisfactorily high (up to 92 : 8) at the expense of the formation of a regioisomeric aldehyde, cis-4-t-butyl-1-methylcyclohexane-carboxaldehyde (4) as well as much isomerization of 1 into 4-t-butyl-1-methylcyclohexene (5) These results may well be interpreted in terms of a kinetic controlled insertion of the substrate 1 into [HRh(CO)_n], under a higher pressure of syn-gas, resulting in lower diastereoselectivity of 2 : 3, whereas a thermodynamic controlled step of substrate insertion into [HRh(CO)_n] under a lower pressure may give rise to the observed isomerization to 4 or inverse insertion to form 5, at the same time, affording the highest diastereoselectivity 2 : 3 = 92.8.Use of [Rh(acac)(CO)_2]-[Ru_3(CO)_<12>] as a co-catalyst was found to depress any isomerization of 1, keeping the diastereoselectivity 2 : 3 intact. Thus, the co-catalyst system has averted cleanly a dilemma observed above.