CONFORMATIONAL-ANALYSIS OF CHIRAL ALKENES AND OXONIUM IONS - ABINITIO MOLECULAR-ORBITAL CALCULATIONS AND AN IMPROVED MM2 FORCE-FIELD

Calculations on the conformations of isopropylbenzene, 3-methyl-1-butene, 4-methyl-cis-2-pentene, and 2,3-di-methyl-1-butene, were performed at the MP2/6-31G*//3-21G level. Calculations on the 0-alkylated aldehyde cations, O-isopropylacetaldehyde (36), cis-O-isopropylaldehyde (37), O-methylformaldehyde (38), cis- and trans-protonated acetaldehyde (39), cis- and trans-O-methylacetaldehyde (40), and O-ethylformaldehyde (41) as well as cis- and trans-protonated propionaldehyde (42), were performed at the MP2/6-31G*//6-31G* level. 3-Methyl-1-butene has little 1,3-allylic strain in any conformation; consequently, the two conformational minima, the double-skew and skew-eclipsed conformers, differ in energy by only 0.7 kcal/mol, with the skew-eclipsed conformer being the global minimum. The oxonium ion, O-isopropylacetaldehyde, 36, exhibited increased 1,3-strain in the crowded configurations so that the double-skew conformer is 1.2 kcal/mol more stable than the skew-eclipsed conformer. Both 4-methyl-cis-2-pentene and cis-0-isopropylacetaldehyde, 37, which have methyl groups cis to isopropyl groups, have a 3.2-3.5 kcal/mol preference for the skew conformer due to strong 1,3-allylic strain in the crowded conformations. That 1,2-allylic strain is small relative to 1,3-allylic strain in alkenes is demonstrated by 2,3-dimethyl-1-butene, which has a methyl group geminal to an isopropyl group on one terminus of the double bond and a double-skew conformer which is only 0.5 kcal/mol lower in energy than the skew-eclipsed conformer. The normal MM2 parameters for alkenes were found to give conformational energies for 3-methyl-1-butene, 4-methyl-cis-2-pentene, and 3,3-dimethyl-1-butene which were inconsistent with thc ab initio results. New MM2 parameters were developed for alkenes and oxonium salts. MM2 calculations, using these new parameters, for rotation about the allylic bond of a series of alkenes were consistent with the same potential energy surfaces determined by ab initio calculations. Allinger's new force field, MM3, also improves the results for alkenes to some extent. The new force fields plus the existing MM2 force field for aromatic hydrocarbons were used to calculate the conformations of relatively large chiral molecules for aromatic, olefinic, and oxonium ion systems. The selectivities of these compounds with electrophilic and nucleophilic reagents are related to the conformational equilibria of the ground-state molecules.