The Stereochemistry of Alkanes and Non-Conjugated Alkenes

Molecular force fields, which were developed before 1973 to deal with calculations on alkanes and/or non-conjugated alkenes, were each known to have certain defects in that they could not adequately reproduce all the experimental data. The parametrization of a new and improved force field for calculations on alkanes and non-conjugated alkenes was therefore undertaken. The force field, which was subsequently-developed (referred to as the White-Bovill force field or WBFF, for short), incorporated into its parametrization large amounts of recently published experimental data concerning the thermodynamic and structural properties of a carefully selected group of 75 compounds and was found to be more general and reliable than the previously developed force fields. The final form of the WBFF consisted of 22 parameters for aliphatic functions and 18 parameters for double bonds and its surroundings. The mean deviation between the calculated and experimental enthalpies of 60 compounds was 0.55 kcal./mole and. the corresponding figures for geometric properties were 0.009, 0.6, and 0.9 in bond lengths, bond angles and torsion angles. Molecular mechanics(MM) calculations, utilising the WBFF, were then undertaken on a series of cyclenes which had. one double bond in the ring, ranging from C6 - C10 and with two double bonds in the ring, ranging from C8 - C10, so that detailed knowledge of the relationship between conformational structure and energy would be obtained. The WBFF produced extremely reliable results for geometries, heats of formation and heats of hydrogenation of a number of medium ring compounds so that it was possible to place a certain degree of confidence in the results from this investigation of the plethora of conformations and transannular interactions exhibited by medium rings. In addition, the minimum energy conformation (MEC) of cis,cis,cis-1,5,9-cyclododecatriene was assigned on the basis of MM calculations and an NMR analysis. A series of KM calculations was performed on a carefully selected group of sixteen bicyclic bridgehead olefins with the aim of assessing the limits of the application of the rules, proposed by Bredt, Fawcett and. Wiseman, regarding the stability of bridgehead olefins and providing guidelines or rules for situations not covered by the previous rules. The experimental data, which were mostly of a qualitative nature, were satisfactorily reproduced by the WBFF and it was found that, within a given skeleton, systems with a double bond placed in the second largest bridge will be less strained than those with a double bond located in the largest bridge, which in turn are less strained than those with a double bond situated in the smallest bridge. The recent publication of results of a series of hydroboration/oxidation experiments on hindered 1,3,5-tri- and tetra-substituted alkylcyclohexenes provided reference data for testing the application of summed atom centred congestion ratios (based on the Wipke and Gund congestion algorithm) in the prediction of the outcome of stereoselective additions to hindered double bonds since hydroboration is known to be sensitive to the balance of steric hindrance to the two faces of the double bond. The agreement between the predicted and experimentally observed stereoselectivity was favourable (within 15% of each other) when steric control dominates the reaction. It has been suggested that Raman optical activity in methyl torsion modes of vibration in chiral molecules can be correlated with the chiral part of the electronic potential barrier hindering the rotation of the methyl group. In a perturbed harmonic oscillator approximation, the chiral part is proportional to the displacement of the equilibrium position of the methyl group from the symmetric position. MM calculations of this displacement in (+)-3-methylcyclohexanone, (+)-3-methyl-cyclopentanone, (-)-limonene, (+)-carvone and (-)-menthol was found to correlate in sign, and approximately in magnitude, with the observed Raman optical activity in bands assigned to methyl torsion modes. A structure analysis of 1,8-dimethyl-2-naphthyl acetate was carried out as part of a series of investigations, by Dr. J. Carnduff, into the autoxidation of 2-naphthols. On the basis of the rates of autoxidation reactions, the 1,8-dimethyl derivative was expected to be more strained than the 1-isopropyl derivative, which,in turn, should be more strained than the 1-methyl derivative. This hypothesis was supported by the correspondind structure analyses and, from a comparison of structural data for eight Cl and/or C8 substituted naphthalenes, the only consistent indicators of steric crowding are the C1-C9-C8 valence angle and C1...C8 nonbonded distance