Carbenes titanium-zinc complexes

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

Philip Kocienski published an elegant synthesis of racemic olean. The starting material is the THP ether of 4,4-dibromobutanol. In spite of the acid-sensitivity of the acetal, the formation of a carbene complex with titanium tetrachloride and zinc can be achieved. Its reaction with a corresponding ester leads to an enol ether, which cyclises to olean under acidic conditions. [217]... 

Takai reported a simple, general, and stereoselective method for the alkylidenation of ester to give Z-enol ethers The titanium carbene complex is easily prepared in situ by the reaction of RCHBr2 with a low-valent titanium species generated by treatment of TiCU with zinc and tetramethylenediamine (TMEDA) in THE. Without isolation, the complex is used for carbonyl alkenation. It was reported that the presence of a small amount of lead in the zinc was crucial to the reaction ... 

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