STRUCTURAL STUDY OF METHYL AND TERT-BUTYL PHENYLACETATE ENOLATES IN SOLUTION - SPECTROSCOPIC DETERMINATION OF THEIR E-CONFIGURATION OR Z-CONFIGURATION

The structures of Li and K methyl and tert-butyl phenylacetate enolates A,M and B,M have been examined by IR and C-13 NMR spectroscopy in different solvents and solvent mixtures. In the IR, coupling of the 8a (or 8b) aromatic ring mode with the nu(C-O) stretching vibration allows assignment of the E (or Z) configuration to the corresponding enolate. The IR mode assignments are secured by specific deuteration of the phenyl or methyl moieties in methyl phenylacetate. Free anions 1A and 1B and solvent- and cryptand-separated pairs 1A,[M] and 1B,[M] exhibit the Z geometry. In THF, when M = Li, two species 2A,Li and 3A,Li or 2B,Li and 3B,Li in slow equilibrium on the NMR time scale are observed, both having the E configuration. Monomeric ion pairs 2A,Li or 2B,Li, which are primarily generated, are slowly associated into dimeric, tight ion pairs 3A,Li or 3B,Li, the position of the equilibrium depending upon the nature of the enolate (Me or t-Bu) and its concentration. The addition of HMPA to 2A,Li and 2B,Li or 3A,Li and 3B,Li THF solutions induces an E --> Z isomerization, leading to Li-HMPA solvates 2'A,Li and 2'B,Li which have been characterized by low-temperature P-31 NMR. In this system, lithium enolate trapping in THF by Me3SiCl is unreliable as a method for assigning their E geometry.