Studies on the structure and reactivity of bicyclic systems

Abstract

Factors controlling the regio- and stereo selectivity in small polycyclic molecules have been investigated by X-ray diffraction, reactions of model compounds and molecular mechanics calculations. The bicyclo[ 3.2.0]heptene system was investigated in detail. The products on addition of HOBr were isolated and characterised. Similarly, the analogous exo- and endo-epoxides were synthesised and reacted with hydrobromic acid. Product ratios were determined by g.l.c. and confirmed by isolated yields. The compounds displaying high selectivity have the possibility of a transannular 0...C=0 interaction which could stabilise the transition state leading to the major product. Investigations of these interactions showed them to be a consequence of the molecular conformation. The energies involved are not sufficient to affect the product ratios. One compound which displayed an enhanced 0...C=0 inter action due to the presence of two chlorines vicinal to the carbonyl crystallised as a 1:1 mixture of ring and chain forms in a single crystal. This is the first example of ring-chain tautomers (or reactant and product of a chemical reaction) being isolated in a crystal lattice. The ground state structures of seven compounds were determined by X-ray diffraction and the transition states were modelled by molecular-mechanics calculations. The stereoselectivity was explained by steric congestion around the double bond. X I . Regioselectivity depends on the preferred conforma tion. From the available data (n.m.r., X-ray structures, molecular-mechanics) the endo-envelope conformer of the bicyclo[ 3.2.0]heptane system is preferred to the exo-twist conformer. Assuming an antiperiplanar arrangement of the reacting species, this preferred conformation explains the observed regioselectivity. The exo-bromonium ions and epoxides display higher selectivity than their endo analogues. This has been explained by conformational and steric factors.