Stereoselectivity

Abstract

The Woodward-Hoffmann rules state that the thermal opening of cyclobutenes occurs by a conrotatory process. This ring opening gives two different and distinguishable products if the cyclobutene has substituents about the C 3 C 4 bond (cyclobutene numbering). They result from in or out opening:It does not depend on steric factors. 1 For example, Curry and Stevens have shown that compounds such as 1 may open to give products whose bulky group is found in the 'inside' position: 1 (a) Curry M. R H 3 4 out in H H H H Interaction of R with the cyclobutene LUMO Interaction of R with the cyclobutene HOMO Figure 6.1 Frontier overlaps between an R substituent and a cyclobutene ring. Stereoselectivity 132should rotate inward. However, other factors may also intervene. For example, neither cis,cis-1,3,5,7-octatetraene nor cis,cis,cis-1,3,5-cyclooctatriene is planar, so that the transition structure of their conrotatory interconversion takes a helical conformation. According to ab initio calculations, 5 steric effects then prevail over electronic effects and direct the torquoselectivity of cyclooctatetraene.A priori, we would expect disrotatory reactions to show poorer torquoselectivity than conrotatory reactions for two reasons. Consider, for example, the hexatriene ↔ cyclohexadiene interconversion. On the one hand, the overlap between R and the distal carbon C 6 is similar for the in and out pathways, as in the in mode, the major lobe at C 6 is oriented away from R: H H C 6 C 6 out mode in mode 13 + 11 : 1Answer From Exercise 7, we know that an ester group, although electron attracting, tends to rotate outwards. In the fi rst reaction, this preference is impeded by the β-methyl group.