Regioselective hydroformylation of cyclic vinyl and allyl ethers with rhodium catalysts. Crucial influence of the size of the phosphorus cocatalyst

Article abstract of DOI:10.1021/om00059a015

In the course of studies aimed at developing new catalytic systems, we have explored the possibilities offered by the modification of thiolato bridges in dinuclear rhodium complexes, together with the influence of the cocatalyst, on the selective hydroformylation of cyclic vinyl and allyl ethers. The dinuclear complex [Rh₂(μ-S(CH₂)₃NMe₂) ₂(cod)₂] (cod = 1,5-cyclooctadiene) has been prepared, and its reactivity with CO, phosphines, and phosphites has been investigated. The complex crystallizes in the monoclinic space group C2/c with Z = 8, a = 22.543 (4) A?, b = 12.040 (2) A?, c = 21.547 (3) A?, and β = 98.77 (1)°. For the determination of the structure 4091 unique reflections were used, and the final refinement gave R = 4.1% and R_w = 4.4%. The molecular structure reveals that the two rhodium atoms are bridged by the two thiolato ligands, and the cyclooctadiene completes the coordination of the metal atoms. The amine groups are not bonded to the rhodium. The dinuclear complex has been used in the hydroformylation of 2,3-dihydrofuran, 2,5-dihydrofuran, 3,4-dihydro-2H-pyran, and 3,6-dihydro-2H-pyran. Hydroformylation reactions of dihydrofurans required conditions milder than those for dihydropyrans. The major product in the hydroformylation of 3,4-dihydro-2H-pyran or 3,6-dihydro-2H-pyran was tetrahydropyran-2-carbaldehyde. A systematic study of the influence of the reaction parameters on the selectivity of the hydroformylation of 2,3-dihydrofuran and 2,5-dihydrofuran was undertaken. The study allowed the rationalization of the observed selectivity and the optimization of the yields and regioselectivities. Thus, by modification of the reaction parameters, tetrahydrofuran-3-carbaldehyde was obtained in quantitative yields from 2,5-dihydrofuran and tetrahydrofuran-2-carbaldehyde can be prepared from either 2,3-dihydrofuran or 2,5-dihydrofuran in approximately 75% yield.