Metallacyclobutanes and Metallacyclobutenes

Four-membered titanacycle intermediates have been often involved in titanium carbene mediated olefin metathesis and related reactions (Eqs. 12-13). Both ti-tanacyclobutanes and titanacyclobutenes have been reported and are useful intermediates for the synthesis of various organic compounds. Since many reviews have appeared [13], no details will be described in the paper. 

Except the well-known olefin-metathesis protocol, generally, there is lack of preparative methods for four-membered metallacycles. Takahashi and coworkers reported formation of zirconacyclobutene-silacyclobutene fused ring compound via a novel skeletal rearrangement of zirconacycles (Eq. 14) [14]. The structure of this compound has been characterized by X-ray analysis [14b]. 

Rosenthal and coworkers reported a series of interesting reactions of bis(trimethylsilyl)acetylene-zirconocene and -titanocene complexes. A variety of metallacyclobutene derivatives were prepared via metallacycocumulenes from 1,3-diynes, linear tetraynes, and tetraalkynylsilanes (Fig. 2). Cleavage of C-C single bonds in conjugated diynes was observed [5]. 

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Among more recent innovations, the alkylative metallacyclobutane and metallacyclobutene synthesis, involving central carbon addition and addition/elimination reactivity patterns, holds considerable promise for near-term synthetic developments (Section 2.12.12). This is equally true in both the nucleophilic and free radical versions of this process, for both catalytic and stoichiometric transformations and multistep reaction cascades. 

In addition to the stoichiometric preparative and reactivity manifolds discussed throughout this chapter, metallacyclobutane and metallacyclobutene intermediates play an essential role in a range of synthetically valuable, or potentially valuable, synthetic transformations, both catalytic and stoichiometric. The tremendous range of... 

Similarly, treatment of 17 with 1-alkenes and alkynes affords the corresponding metallacyclobutane and metallacyclobutene compounds, respectively [57,59]. Heating the dimethyl complex of titanocene generates the methylene species [44-46,71-73]. 

Metallacyclobutenes have also been isolated from the reaction of titanocene metallacyclobutanes and alkynes however, these products are stable, and do not go on to give polyenes. See E N. Ibbbe, R. L. Harlow, J. Am. Chem. Soc. 1980, 102, 6149-6151. 

Tebbe s complex [cp2Ti(/i-CH2)(/i-Cl) AlMcj] readily reacts with alkenes and alkynes to form metallacyclobutanes (unstable) and metallacyclobutenes. It was found that there is a CH2 exchange between metallacyclobutene and CH2=CMe2 [equation (5.155)]. The /r-methylene compounds of the first series of the transition... 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

Titanocene methylidene phosphine complexes Cp2Ti= CH2L (L = PMes, PMe2Ph, PEts) have been generated by the thermolysis of titanocene metallacyclobutanes in the presence of excess phosphine (equation 9). These titanocene methylidene phosphine complexes have been found to react with alkenes, alkynes, and CO to form metallacyclobutanes, metallacyclobutenes, and a ketene complex, respectively. The phosphine ligand is labile, and an equilibrium mixture is rapidly established if a second phosphine is added. ... 

Aratani has rationalized the stereoselectivity of his catalysts by a model based on a metallacyclobutene intermediate (cf. Scheme 4, reaction b) [18]. For semi-corrin and related ligands, formation of a metallacyclobutane seems less attractive because of steric hindrance, and a different pathway, analogous to reaction a in Scheme 4, has been proposed [36,40,53b]. Fig. 2 shows the postulated ster-... 

A reaction closely related to olefin metathesis is alkyne polymerization, which occurs between a metal carbene and an alkyne [64]. After initial alkyne coordination to the metal center, [2 -I- 2] cycloaddition leads to a metallacyclobutene complex instead of the metallacyclobutane formed in olefin metathesis (Fig. 4.20)... 

Subsequently, other Ti-cyclobutane species were shown to be the reaction products of the Tebbe complex with olefins the analogous reaction with acetylenes gives metallacyclobutenes." Utilization of Ti-metaUacycles as initiators in metathesis provides the first example of a living metathesis polymerization system. Clear evidence of the intervention of metaUacarbenes and metallacyclobutanes in olefin metathesis was later furnished by Kress et al. through minute nuclear magnetic resonance (NMR) studies on norbomene polymerization with tungsten alkylidenes. 

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