Study of the effects of alkali metal salts on styrene hydroformylation reactions catalyzed by rhodium(I) complexes of bis(phosphite) ligands

摘要

A functionalized alpha,omega-bis(phosphite-donor) ligand containing two amido groups, 8, has been prepared from tartaric acid. An in situ formed rhodium(l) metallacrown ether complex of 8 has been compared to an in situ formed rhodium(l) complex of (R,R)-Chiraphite for the hydroformylation of styrene. Both Rh(l) complexes are active catalysts, but the complex of (R,R)-Chiraphite exhibits a higher regioselectivity than does the metallacrown ether catalyst (iso of 10.1 versus 3.9) as expected. The additions of alkali metal salts to the hydroformylation reactions cause increases in the regioselectivities (approximately 2-fold for both LiBPh4 center dot 3dme and NaBPh4 for ligand 8; 3-fold for both LiBPh4 center dot 3dme and NaBPh4 for (R,R)-Chiraphite) with virtually no effect on the activity. The facts that (I) no change in the iso ratio is observed upon addition of the noncoordinating salt, TBA center dot BPn(4), (2) LiBPh4 center dot 3dme has little effect on the regioselectivity of an in situ formed rhodium(l) complex of diphos, and (3) the increase in regioselectivity levels out at high salt:Rh ratios suggest the increase in regioselectivity is due to coordination of the alkali metal salt to the oxygen of a carbonyl ligand and to the phosphite oxygens during the regioselectivity-determining step. This proposal is consistent with the observation of only weak 1:1 coordination of LiBPh4 center dot 3dme with a model metallacrown ether, cis-Mo(CO)(4)(8) in acetonitrile-d(3).