Markedly Different Selectivity in the Rhodium Catalyzed Hydroformylation of Vinyl Olefins Containing a Chiral Alkoxy or Alkyl Group: Good Agreement between Theory and Experiment

A theoretical investigation on the regio- and stereoselective outcomes of the hydroformylation reaction with an unmodified rhodium catalyst (H—Rh(CO)3) was carried out at the B3LYP/SBK(d) level on related chiral olefins, namely (1-vinyloxy-ethyl)-benzene (1) and (1-methyl-but-3-enyl)-benzene (2). A thorough and computationally expensive examination of the various possible H—Rh(CO)3-olefin complex intermediates was performed, in order to determine, interpret, and eventually predict, the regio- and stereoselectivity of the aforementioned reactions, whose products are a mixture of the linear aldehyde and of two diastereomers of the branched aldehyde. Regio- and diastereoselectivity of the reaction have been experimentally determined via hydroformylation runs at 20° and 100° C for 1 and at 20° C for 2. The theoretical results obtained are in good agreement with the experimental ones, which put forward a high chiral discrimination for chiral vinyl ether substrates in contrast to the lack of regio- and diastereo-selectivity observed in the hydroformylation of 2. For the hydroformylation of 1, a regioselectivity ratio (B:L) of 72:28 and a diastereoselectivity ratio (b:b’) of 97:3 have been computed which compare well to the corresponding experimental results (85:15 and 88:12). Therefore, theoretical calculations can give reliable estimates of regio- and diastereoselectivity provided a careful and accurate surface scan is performed for the alkyl-rhodium intermediates and the reaction is carried out at room temperature and, hence, in the absence of branched to linear alkyl isomerization.