Titanacyclobutanes thermolysis

Aluminum-free titanocene-methylidene can be generated by thermolysis of titana-cyclobutanes 6, which are prepared by reaction of the Tebbe reagent with appropriate olefins in the presence of pyridine bases [9]. Alternatively, the titanacyclobutanes are accessible from titanocene dichloride and bis-Grignard reagents [10] or from 71-allyl titanocene precursors [11]. The a-elimination of methane from dimethyltitanocene 7 provides a convenient means of preparing titanocene-methylidene under almost neutral conditions [12] (Scheme 14.5). 

Titanacycle 10 ring-opens photochemically at temperatures well below 0°C, Eq. (27) [41]. The unstable titanium carbene 20 can be trapped by exo-trans-exodiene 21, derived from (2 + 2) cycloaddition of norbornadiene, to produce a mixture of two isomeric bis(titanacyclobutanes), 22a, b [42]. Bis(titanacycle) 22a, b could not be synthesized cleanly by thermolysis of 10 because formation of 22a, b competes with ring-opening and the addition of a second equivalent of diolefin. 

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