Electronic effects in asymmetric catalysis: Structural studies of precatalysts and intermediates in Rh-catalyzed mydrogenation of dimethyl itaconate and acetamidocinnamic acid derivatives using C-2-symmetric diarylphosphinite ligands

Enantioselectivity of Rh(I)-catalyzed asymmetric hydrogenation of dehydroam ino acid derivatives and dimethyl itaconate can be enhanced by the appropri ate choice of substituents on the aromatic rings of vicinal diarylphosphini tes derived from carbohydrates as well as trans-cyclohexane-1,2-diol. For e xample, the use of phosphinites with electron-donating bis(3,5-dimethylphen yl) groups at phosphorus provide high ee's in these reactions whereas elect ron-withdrawing aryl substituents decrease the enantioselectivity. In this paper, an attempt is made to clarify the origin of these remarkable electro nic effects at two levels. First, crystal structures of a number of precata lysts ([phosphinite](2)Rh+[diolefin]X-) were determined and their structure s were studied in detail to examine the electronic effects, if any, on the ground-state conformations of these molecules. A study of six of these comp lexes reveals that the gross conformational features of these precatalysts are largely unaffected by electronic effects, which suggests that other exp lanations have to be sought for the electronic amplification of enantiosele ctivity. One possibility is a change in the diastereomeric equilibrium betw een the initially formed [substrate]Rh+[phosphinite] complexes as a functio n of electronic effect of the ligand. In the Rh-catalyzed hydrogenation of dimethyl itaconate, we have examined this equilibrium between the major and minor complexes by P-31 NMR. There is a clear difference in the ratio of t hese two diastereomers when 3,5-dimethylphenylphosphinite vis-a-vis the uns ubstituted diphenylphosphinite is used. Electron-deficient ligands such as 1,2-bis-3,5-diflurophenylphosphinite and 1,2-bis-3, 5-bis-trifluromethylphe nylphosphinite appear to form these diastereomers more readily at room temp erature, even though the exact ratio of the diastereomers could not be esta blished with any certainty.