SYNTHESIS AND STRUCTURE OF BENZYLIC CARBANIONS

SCOTT THORNBURGH, Purdue University

Abstract

Solutions of the 2-phenyladamantyl, 9-phenylbicyclo{3.3.1}nonyl, and phenylcyclopropyl lithium and potassium compounds were prepared in THF and their structures investigated by ('13)C and ('1)H NMR spectroscopy. A new technique for the benchtop generation of carbanions for NMR study was developed. The 1-phenylcyclopropyllithium compound was found to be pyramidal at Cl with a barrier to inversion (DELTA)G(,298)('(NOT=)) = 14.0 (+OR-) 0.3 kcal/mole. In contrast, all other potassium and lithium compounds studied were found to be effectively planar on the NMR timescale. Differences in structure are attributed to strong Coulombic interaction of the carbanion with the lithium cation, which favors a pyramidal geometry. A planar geometry is assigned to the potassium compounds with the exception of 1-phenylcyclopropylpotassium, which is thought to be bent. The barrier to inversion of pyramidal benzyllithium compounds increases with increasing bond angle strain at the benzylic position. Mixtures of the 7-phenyl-norbornyl potassium and lithium compounds in THF were found to undergo cation exchange at rates which are slow on the NMR timescale. The rate of inversion of 7-phenylnorbornyllithium was found to be comparable to the rate of cation exchange in the mixed solution. It is concluded that dissociation of the lithium cation is the rate determining step in both processes. It is proposed that the inversion properties of amines are best compared to the isoelectronic organopotassium compounds, where coordination effects are less important. The synthesis of 7-aza-7-phenylnorbornane is described and a bent structure established at the nitrogen atom. An upper limit to its inversion barrier is estimated at (DELTA)G('(NOT=)) < 6.1 kcal/mole.

Degree

Ph.D.

Subject Area

Organic chemistry

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