| Summary: | In an attempt to expand the scope of titanacyclobutenes, malonate-derived ,-alkylpropargyl dibromides and ,-bis(bromopropargyl) malonates were prepared and examined for their reactivity with titanocene reagents. Unfortunately, all of the ,-alkylpropargyl dibromomalonates failed, presumably from chelation of the carbonyl oxygen followed by radical or nucleophilic attack. The ,-bis(bromopropargyl) malonate substrates allowed the successful synthesis of ester-functionalized allenyl or alkynyl-substituted bicyclic titanacyclobutene complexes. Allenyl-substitution is favoured when the being formed ring is small. When ring strain is minimal, cycloalkynes are obtained. An impressive example of a twelve-membered macrobicyclic titanacyclobutene was achieved, supported by extensive NMR spectroscopy and X-ray crystallography.
3-Propargyltitanium(III) complexes were synthesized and displayed equilibrium behaviour between the monomeric propargyl and dimeric di(titanacyclobutene) forms. Both steric and electronic effects are believed to be contributing factors for dimerization. Bimetallic titanacyclobutenes are obtained from the reaction of an epoxide and titanocene monochloride in the presence of a 3-propargyltitanium(III) complex.
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