STEREODEPENDENCE OF THE EFFECT OF ALPHA-SUBSTITUENTS ON THE FACE REACTIVITY OF CONFORMATIONALLY RIGID KETONES TOWARD METAL-HYDRIDES - IS THERE AN ANTIPERIPLANAR (CIEPLAK) EFFECT

The relative rates of hydride addition to the carbonyl group of the co nformationally fixed bridged biaryl ketone, 1, and its alpha derivativ es bearing methyl, methylthio, methoxy, chloro, and fluoro substituent s in axial-like and equatorial-like orientations have been measured fo r three different reactions. The derived rate constants were partition ed, on the basis of diastereoselectivity, to provide reactivities for each face antiperiplanar to the axial substituents and anticlinal to t he equatorial substituents. These face reactivities gave reasonably li near correlations with the inductive substituent parameter, sigma(I). The strength of the inductive effects as indicated by the slopes of th e Taft plots were large and positive, small and positive, and negative for the reductions involving lithium aluminum hydride, sodium borohyd ride, and triethylsilane, respectively. The substituent effects on fac e reactivity and their stereodependencies failed to follow either the theory of Cieplak or that of Anh. The major influences of substituents can be explained on the basis of through-bond interactions with the t ransition state and the differences between axial and equatorial effec ts on electrostatic (through-space) interactions with the transition s tate.