The conformation of 2-phenylpropionaldehyde and alkyl 1-phenylethyl ketones as evidenced by ab initio calculations.: Relevance of the CH/π and CH/O interactions in stereochemistry

Ab initio MO calculations were carried out for the conformation of 2-phenylpropionaldehyde 1 and related ketones CH3CH(C6H5)-CO-R (R = CH3 2, C2H5 3, i-C3H7 4, and t-C4H9 5) at the MP2/6-311G(d,p)//MP2/6-31G(d) level. The conformation whereby the alkyl group R is synclinal to the phenyl group (rotamer a, C6H5-C-CO-R torsion angle phi 64-93degrees) has been found to be the most stable. The second most favorable one has been shown to have R flanked by the benzylic methyl group and C6H5 (rotamer b: phi 282-297). The difference in the enthalpy, between a and b has been calculated to be 1.58, 2.16, 2.19, 2.08, and 4.89 kcal mol(-1), respectively, for 1, 2, 3, 4, and 5. The C6H5/R antiperiplanar conformation (rotamer c) has been shown to be the least stable for 1 (phi 171degrees) and is not at an energy minimum for 2-5. In rotamers a and b, short interatomic distances have been shown between one of the alkyl hydrogens and the phenyl group. In rotamer a, a short distance has been calculated between one of the hydrogens of the benzylic methyl group and the carbonyl oxygen. The ab initio results are compatible with those obtained by NMR measurements. Contributions of the CH/pi and CH/O interactions to the conformational equilibria have been invoked to accommodate the above results. Instability of rotamer c may be due to the unfavorable electrostatic interaction of the C=O dipole vs the quadrupole of the phenyl group. Conformational energies of methyl formate 6, N-methylformamide 7, and propionaldehyde 8 were also calculated to examine the effect of the CH/O interaction in carbonyl compounds. The results are consistent with the notion that the CH/O interaction is important in stabilizing the CH3/O eclipsed conformation.