Organic Synthesis via Formation of Unsaturated Titanacycles

New synthetic reactions which involve the formation of unsaturated titanacycles are described. The reaction of carbene complexes of titanium, generated from thioacetals or gem-dichlorides, with alkynes produced titanacyclobutenes. Hydrolysis or β-hydride elimination of these titanacycles gave a variety of alkenes and dienes. When alkynyl sulfones were employed, β-elimination of the initially formed 3-sulfonyltitanacyclobutenes produced allenyltitanium species which gave homopropargylic alcohols on treatment with carbonyl compounds. Titanacyclobutenes with different substitution patterns were also produced by the reductive titanation of γ-chloroallylic sulfides with titanocene-triethylphosphite complex. The formation of titanacyclopentenes by the reaction of alkenes bearing a leaving group such as vinyl pivalate and (Z)-alkenyl sulfones, alkynes, and the titanocene(II) reagent, and the subsequent β-elimination constitute useful methods for the vinylation and E-selective alkenylation of alkynes. Carbonyl compounds were also transformed into allylic alcohols when treated with the same reagent system. The alkenylation of alkynyl sulfones and subsequent reductive titanation of the resulting α-sulfonyldienyltitanium species generated titanium-alkenylalkenylidene complexes which reacted with carbonyl compounds to produce various alkenylallenes with complete stereoselectivity.