The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combined theoretical and experimental study. A gradient corrected density functional theory (DFT) method has been scaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE) calculations. The BP86 method using density fitting, and a basis set of triple-zeta quality emerged as a highly reliable tool for studying titanocene mediated radical reactions. The computational results revealed important kinetic and thermodynamic features of cyclopropane formation. Surprisingly, the beta-titanoxy radicals, the first intermediates of our investigations, were demonstrated to possess essentially the same thermodynamic stabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast to suggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorable in agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governed by the stability of the intermediates and thus the trans disubstituted products are formed preferentially. The observed ratios of products are in good to excellent agreement with the DFT results. By a combination of computational and experimental results, it was also shown that for the completion of the overall cyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive.

A catalyzed synthesis of cyclopropanes and cyclobutanes via radical chemistry is described. The method that generally proceeds in high yields uses epoxides as radical precursors and titanocene(III) complexes as the electron transfer catalysts (see scheme). The key to the success of the transformation is constituted by the chemoselectivity of radical reduction. Electrophilic enol radicals generated through cyclization are reduced rapidly whereas their precursors, nucleophilic alkyl radicals, remain unaffected.

The performance of tert-alkylations, alkoxyalkylations, and aldehyde enolate allylations proceeding with low catalyst loading (0.1 mol % ± 5 mol %) is described. The reactions are complete within short times and can even be performed without solvent and under ambient conditions. The mechanism of the reaction was investigated by deuterium labeling and cross-over studies.

Ketones Ketones P 0200Catalyzed Reactions of Enol Ethers with S N 1 Active Groups: A Novel Method for the Preparation of α-Alkylated Ketones. -Lewis acid catalyzed cleavage of enol ethers provides an efficient method for alkylation and allylation of enolates. The present protocol is characterized by mild reaction conditions, insensitivity to exposure to ambient conditions, low catalyst loadings and high turnover frequencies.-(GANSAEUER*, A.; FIELENBACH, D.; STOCK, C.; GEICH-GIMBEL, D.; Adv.