Eclipsing effect

However, eclipsing effects are not the only factors that affect the cis/trans ratio in anti E2 eliminations. Other factors are the nature of the leaving group, the base, the solvent, and the substrate. Not all these effects are completely understood. ... 

Clear evidence that the geometry of the transition state is not constant has been provided by studies of eclipsing effects . Assuming anti stereospecificity, the threo and erythro isomers of the 1,2-diphenyl-l-propyl-X systems undergo bimolecular elimination to give the trans- and cw-a-methyl stilbenes respectively (75). 

The term eclipsing effects incorporates non-bonded interactions of substituents at C and Cg steric inhibition of resonance and differences in degrees of solvation. 

The principal factors affecting orientation in acetate decompositions have been adequately summarised by DePuy and King Essentially three influences were recognised, these being termed statistical, steric and thermodynamic effects. Statistical control is observed in pyrolysis of simple aliphatic esters which under the elevated reaction temperatures experience little resistance to conformational rotation and the number of beta hydrogen atoms in each branch determines the direction of elimination (147)= 37o distortion in statistical control is imposed by the steric influence of a t-butyl substituent (148), and is also illustrated by the predominance of trans- over m-olefin formation (148, 149) due to eclipsing effects . The latter example, however, may also arise from thermodynamic influences which are more certainly demonstrated by preferential elimination towards a phenyl rather than an alkyl substituent (150) . The influence of substituents on olefin stability rather than beta hydrogen acidity seems more critical as elimination occurs more often towards a p-methoxyphenyl rather than a phenyl substituent (151... 

The orientational behaviour of amine oxide pyrolyses has been adequately summarised by Cope and TrumbulE . As for the acetate decompositions, orientation in the simple alkyl systems is controlled primarily by statistical factors but departure from this influence is noted with the bulky r-butyl substituent and the acid strengthening beta phenyl substituents Eclipsing effects are greater in the planar five-membered transition states than in the puckered six systems and this is borne out by the greater preference for trans-olefin formation from amine oxides than esters and xanthates (152, cf. 149). 

The factors which may influence the transjcis olefin ratios arising from base-induced elimination reactions have been discussed and are summarised in a recent paper by Feit and Gitlin". These factors include, in addition to eclipsing effects, steric effects of unsymmetrical leaving groups and bases, and the incursion of 5y/i-elimination (c/. Section 2.2.2.(d)). 

That the elimination is syn has been established by use of deuterium labels. Deuterium was introduced stereospedfically by LiAlD4 reduction of cis- and frans-stilbene oxide. The product of the subsequent ester pyrolysis is frans-stilbene because of eclipsing effects in the transition state. Tlie syn elimination is demonstrated by retention of deuterium in the olefin from frans-stilbene oxide and its absence in the olefin from c -stilbene oxide. " ... 

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