Science of Synthesis

Mikami, K.; Matsumoto, Y.; Shiono, T., Science of Synthesis, (2003) 2, 557.

[Ti(Cp)2(R1C≡CR1)(PMe3)] complexes ‌246‌ are also available from [Ti(Cp)2(PMe3)2] (‌116‌) by the replacement of trimethylphosphine with 1 equivalent of alkyne under mild conditions (‌Scheme 120‌).[‌309‌] In contrast, complex ‌116‌ reacts with 2 equivalents of ethene to form titanacyclopentane ‌170‌ (‌Scheme 121‌).[‌308‌,‌309‌] There is extensive backbonding to the ethene ligand; this is indicated by the low IR νC=C band at 1618 cm−1 and the absence of any evidence for the ethene ligand in the variable-temperature NMR spectrum.[‌308‌,‌309‌] Terminal and disubstituted alkenes give similar alkene- or metallacyclic products, whereas terminal alkynes are not polymerized.[‌308‌,‌309‌] Phosphine-free [Ti(Cp)2(R1C≡CR1)] complexes (e.g., ‌198‌) can also be prepared by reaction of [Ti(Cp)2(PMe3)2] (‌116‌) with alkynes. In solution, in the presence of trimethylphosphine, [Ti(Cp)2(MeC≡CMe)] is probably in equilibrium with [Ti(Cp)2(MeC≡CMe)(PMe3)].[‌325‌] The trimethylphosphine ligands in alkyne complexes ‌246‌ are labile and the complexes react readily with electrophiles, alkenes, alkynes, and carbonyl compounds to give metallacycles (‌Scheme 120‌).[‌307‌,‌363‌–‌365‌]

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References