Titanocyclobutane

Titanocene(n) species promote the conversion of unsaturated thioacetals to cyclic compounds. This cyclization proceeds with the loss of the terminal alkene carbon. Treatment of the thioacetal 83 with the low-valent titanium species Cp2Ti[P(OEt)3]2 (3 equiv) in refluxing THF afforded benzoxocines 86 and 87 (by isomerization of 86) in 61% yield (Scheme 14) <1999SL354>. Using 4 equiv of the titanocene(n), the yield is higher (70%) but the selectivity is lower (the ratio 86 87 becomes 82 18). The mechanism or the reaction probably involves the formation of the titanium carbene complex 84, its intramolecular reaction with the double bond to form titanocyclobutane 85, and the subsequent elimination of methylidenetitanocene <1999SL354>. 

In contrast to 106, the electron-rich compounds 107-110 (all containing substituted allyl ligands) reacted with free radicals to form titanocyclobutanes. The reactions were all regioselective with alkylation only occurring at the central carbon of the allyl ligand. Thus, /rradical alkylation of compounds 107 and 108. 

The development of novel titanium carbene complexes by Grubbs has opened up a route to living polymer systems, using coordinating polymerizations as opposed to those derived from ionic initiators, which can be used to form block copolymers or produce chains with a functionalized end group. The initiating species are formed by the reaction of norbomene with a titanocyclobutane derived from 3,3-dimethyl cycloprene... 

Block copolymers of the AB and ABA types, containing segments of narrow molecular weight distribution, are obtained from norbornene, benzonorbornene, 6-methylbenzonorbornene, and endo- or cxu-dicyclopen-tadiene in the catalytic presence of titanocyclobutanes [56]. Illustrative examples are (82) and (83), the diblock and triblock copolymers from norbornene and dicyclo-pentadiene, having polydispersity indices of 1.08 and 1.14, respectively ... 

Carbene Chemistry. Isobutene reacts readily with car-benes and metallocarbenes to form substituted cyclopropanes (eqs 25-27). Reaction of Ethyl Diazoacetate with isobutene in the presence of a chiral copper catalyst affords the cyclopropane in high optical purity (eq 28). Isobutene has also been used to form titanocyclobutanes that are more stable forms of the Tebbe reagent (see L-Chlorobis(cyclopentadienyl)(dimethylaluminum)- L-methylenetitanium) (eq 29). ... 

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