The unusual inhibition of a dirhodium tetraphosphine-based bimetallic hydroformylation catalyst by PPh3

The addition of even small amounts of PPh 3 to the dirhodium tetraphosphine hydroformylation catalyst generated from 〚 rac -Rh 2 (nbd) 2 (et,ph-P4)〛(BF 4 ) 2 (nbd = norbornadiene; et,ph-P4 = Et 2 CH 2 CH 2 (Ph)PCH 2 P(Ph)CH 2 CH 2 PEt 2 ) causes a dramatic drop in the aldehyde linear-to-branched regioselectivity (25:1 to 3:1) in acetone solvent (90 °C, 6.1 bar, 1-hexene). Catalytic results are presented for differing amounts of added PPh 3 along with comparisons to the monometallic Rh catalyst family, HRh(CO) x (PPh 3 ) y ( x = 1–3; y = 3 – x ), generated from PPh 3 and Rh(acac)(CO) 2 (acac = acetylacetonate). The results point to the extremely effective inhibition of the regioselective bimetallic hydroformylation mechanism and the formation of an inefficient monometallic catalyst cycle, but not fragmentation to generate free HRh(CO)(PPh 3 ) 2 catalysts.