Bettels, Bernd; (1993) Conformational analysis of some highly branched hydrocarbons. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This work comprises a study of conformational properties of three distinct classes of compound in which conformational exchange takes place by means of rotation about a carbon-carbon single bond. The parameter used for classification is the type of hybridisation of the two carbon centres at either end of the rotational axis, viz sp2 - sp2, sp2 - sp3, and sp3 - sp3. In the sp2 - sp2 class, symmetrical cis- and trans- 1,2-dialkyl-l,2-di(l-naphthyl)ethylenes are discussed. These compounds are nonplanar and exist as mixtures of syn and anti rotational isomers. Barriers to 1-naphthyl rotation through the double bond plane have been measured by NMR as has the syn/anti equilibrium. Molecular Mechanics calculations serve to illuminate the structure of the ground states as well as pathways to conformational interconversion. In the sp2 - sp3 class, 13,14-dialkylpicenes have been synthesised. The discussion emphasises compounds with two identical alkyl substituents of one, two, six, and ten carbon atoms. In the ground state conformation the ?-alkyl carbon atoms are located on opposite faces of the plane, the two substituents thus being anti. The barrier to interconversion of enantiomeric anti forms is reported. The ground state conformation of 2,4,4-trimethyl- 3-isopropyl-2-pentene has been elucidated by NOE experiments and Molecular Mechanics and comparisons are made with 2,2-dimethyl-1,1-diisopropylcyclopropane. Both compounds exist as an equilibrium between two different conformations which can be detected by NMR. In the sp3 - sp3 class, tri- and tetrasubstituted methanes have been investigated. The substituents are combinations of t-butyl, isopropyl, and methyl groups. The results are discussed in terms of barriers to t-butyl and isopropyl rotation, distortion of ground state conformations, and conformational exchange. Comparisons are made among compounds of similar substitution. The study underlines the importance of ground state conformations far removed from staggered.

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