Titanacyclobutanes preparation

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). 

The reactions of titanium-alkylidenes prepared from thioacetals with unsymmetrical olefins generally produce complex mixtures of olefins. This complexity arises, at least in part, from the concomitant formation of the two isomeric titanacyclobutane intermediates. However, the regiochemistry of the titanacyclobutane formation is controlled when an olefin bearing a specific substituent is employed. Reactions of titanocene-alkylidenes generated from thioacetals with trialkylallylsilanes 30 afford y-substituted allylsilanes 31, along with small amounts of homoallylsilanes 32 (Scheme 14.16) [28]. 

Titanacyclobutanes also serve as useful synthetic intermediates the titanacycle 43, prepared by the intramolecular reaction of the alkenylidene complex 44, affords the a-dike-tone 45 and the other functionalized cyclic compounds by further transformations (Scheme 14.20) [35]. 

An unusual reductive elimination can ensue from titanacyclobutanes possessing an alkenyl group at the carbon a to the titanium atom. Thus, alkenylcarbene complexes 48, prepared by the desulfurization of (fy-unsaturated thioacetals 49 or l,3-bis(phe-nylthio)propene derivatives 50 with a titanocene(II) reagent, react with terminal olefins to produce alkenylcyclopropanes 51 (Scheme 14.22, Table 14.4) [37]. This facile reductive... 

In the formation of block copolymers by sequential addition of monomers it generally does not matter which monomer is polymerized first, and diblock or multiblock copolymers of narrow MWD and of any desired sequence length are readily prepared. Termination is usually effected by reaction of the living ends with aldehydes ketones can be used for terminating titanacyclobutane ends, while unsaturated ethers are used for terminating ruthenium carbene complexes. 

The unsaturated cationic titanium allyl compound [Gp 2Ti(allyl)]+BPh4- is prepared by oxidation of the titanium(m) allyl complex Cp 2Ti(allyl) with a ferrocenium cation. Nucleophilic addition to the cationic unit proceeds regioselectively to the central allylic position. Reaction with benzyl Grignard or the enolate of propiophe-none affords the corresponding titanacyclobutane complexes (Scheme 513).1305... 

Reaction of the Tebbe reagent with 3,3-dimethylcyclopropeneand4-(dimethylamino)pyridine (DMAP) affords the titanacycle 1 in 33% isolated yield. Alternatively, 1 can be prepared by alkene exchange with other titanacyclobutanes. Acidolysis of 1 with hydrochloric acid gives a 89% yield of 1,1,2-trimethylcyclopropane. ... 

Titanacyclobutane d° complexes with two >/ -cyclopentadienyl ligands (Cp), of the kind shown in eqn. (15), were first prepared by Howard (1980a). 

It was found that titanacyclobutanes could be prepared by removing the aluminum from 3 with a strong Lewis base in the presence of an olefin, as shown in Eq. 9. These metallacycles were stable with respect to rearrangement to an olefin via a 3 hydride process, and showed a tendency to generate intermediate alkylidene complexes in situ. They ultimately proved to be useful as catalysts for the ring opening metathesis polymerization of a variety of strained olefins. [47,48] It should be noted that the masked titanium methylene complex 3 (Eq. 

Living polymerization using titanacyclobutane initiators enabled also the preparation of block copolmers by sequential addition of different monomers [114-116] and synthesis of highly conjugated polymers and block copolymers of 3,4-diisopropylidene-cyclobutene [116]. 

Alienation Another olefin metathesis-carbonyl olefination sequence utilizing titanacyclobutanes 11 is employed for the preparation of tri- or tetrasubstituted allenes [56]. Titanocene-methylidene 4 generated from 11 reacts with 1,1-disubsti-tuted allenes to produce the a-alkylidenetitanacyclobutanes 25 with the hberation of an olefin. Simple treatment of 25 with ketones and aldehydes at room temperature affords substituted allenes. The vinylidene complexes 26 are formed as the active species in these transformations (Scheme 4.20). 

Tab. 4.7. Carbonyl olefination utilizing titanacyclobutanes 11 prepared from titanocene-methylidene and strained olefins.

Attempts to prepare Cp2Ti(CFs)2 invariably afford titanocene difluoride perhaps by an a-elimination pathway (11). Electron transfer is proposed to trigger C-F activation in tetrakis(trifluoromethyl)cyclopentadienone upon reaction with bis(cyclopentadienyl)-titanacyclobutanes (12). 

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