Computational prediction of the regio- and diastereoselectivity in a rhodium-catalyzed hydroformylation/cyclization...

The regioselectivity of the hydroformylation reaction of 2-methyl-3-(3-acetylpyrrol-1-yl)prop-1-ene catalyzed by an unmodified Rh catalyst has been investigated at the B3LYP/6-31G* level with Rh described by effective core potentials in the LANL2DZ valence basis set. Considering the population of all the H-Rh(CO)3-olefin transition state complexes, a regioselectivity ratio (B:L) of 12:88 has been obtained, in satisfactory agreement with the experiment producing the chiral linear aldehyde as the only product. The aldehyde, after complete diastereoselective cyclization, yields a 1:1 mixture of 1-acetyl-6R(S)-methyl-8R(S)-hydroxy-5,6,7,8-tetrahydroindolizine (having the same configuration on both stereogenic carbon atoms) and 2-acetyl-6-methyl-5,6-dihydroindolizine [Lett Org Chem (2006) 3:10-12]. The reason for such a high degree of diastereoselectivity has been elucidated examining the B3LYP/6-31G* potential energy surface for the reactions leading to the RR and RS diastereomers on a model system (without the acetyl substituent) and the actual compound. In the absence of a catalyst, a very high barrier is found along the reaction pathway, whereas spontaneous annulation occurs to a protonated pentahydroindolizine in the presence of H+. When a counterion (F-) is added, the proton on the newly formed tetrahedral carbon is abstracted, obtaining a structure closer to the final product (tetrahydroindolizine). Replacing H+ with Rh+, an initial adduct along the RS path much more favorable than any of those computed along the RR one is located because of the presence of the acetyl group. Tentative approaching paths obtained using [Rh(CO)3]+, bound to the aldehyde O, feature a higher barrier along the RS one, and offer a convincing explanation for the observed diastereoselectivity.