Organic Syntheses Utilizing Titanium Carbene Complexes

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Abstract Synthetic reactions utilizing titanium carbene complexes prepared by the desulfurizative titanation of thioacetals and related organosulfur compounds with the titanocene(II) species [Cp2Ti{P(OEt)3}2] are described. The Wittig type olefination of carbonyl compounds including carboxylic acid derivatives with the titanium carbene complexes affords a variety of olefinic compounds. The reaction is successfully applied to the intramolecular carbonyl olefination of carboxylic acid derivatives, giving various carbo- and hetero-cycles. The organotitanium species generated by the treatment of gem-dihalides and certain alkyl halides with the titanocene(II) species are also useful for the transformation of carbonyl compounds into highly substituted olefins or into olefins bearing a bulky substituent. The alkylidenetitanocenes generated from thioacetals react with olefins in both inter- and intramolecular fashions to give olefin metathesis products. The ring-closing olefin metathesis of the carbene complexes formed from thioacetals having a terminal olefin provides a variety of unsaturated heterocyclic compounds. On the other hand, cyclopropanation proceeds when terminal olefins are treated with alkenyl- and alkynyl-carbene complexes. Cyclopropanes are also obtained by treatment of gem-dihalides having a terminal carbon–carbon double bond. The carbene complexes are also highly reactive toward triple bonds. Their reactions with nitriles afford the acylation products after hydrolysis of intermediary titanium vinylimido complexes. The reactions of the carbene complexes with internal alkynes via the formation of titanacyclobutene intermediates afford conjugated dienes with high stereoselectivity, whereas metathesis polymerization proceeds to give trans-polyacetylene when the carbene complexes are treated with excess acetylene. The carbene complexes also react with t-alkyl halides and group 14 metal hydrides.