Vinylalkylidene complexes

The latter method was utilized in the synthesis of ruthenium vinylalkylidene complexes of the type RuCl2(=CHCH=CPh2)(PR3)2 (R = Ph [29] and R=Cy [30]. The phenyl carbene species was isolated when diazomethylbenzene was used [31], Figure 16.17. 

A Ta vinylalkylidene complex 6, confirmed by a single crystal X-ray analysis, was revealed to polymerize 2-butyne in a manner of living polymerization.The initiation efficiency is quantitative, and the living end can be end-capped with aromatic aldehydes. As polymers from symmetric acetylenes are generally insoluble, soluble poly(2-butyne) is accessible if the degree of polymerization is suppressed below 200. The NMR analysis of living oligomers of 2-butyne clearly indicates that both cis- and 7ra r-structures exist in the main chain. 

Gycloreversion to the vinylalkylidene is rarely observed directly, despite being a commonly proposed step in carbene or alkylidene/alkyne reaction cascades. Bridged titanacyclobutene complex 57, however, clearly resists the formation of a strained internal alkylidene from alkyne extrusion, equilibrating instead with vinylalkylidene complex 58, stabilized by the addition of trimethylphosphine (Equation 20) <2003ICA27>. This system provides a very rare simultaneous observation of interconverting metallacyclobutene and vinylalkylidene isomers. 

Routes to the important class of well-defined ruthenium initiators of the Grubbs type (20b-22b) are summarized in Eigure 4 for details, see Table 2. COMC (1995) described the first example of this family, vinylalkylidene 20a, prepared by reaction of RuCl2(PPh3)3 with 2,2-diphenylcyclopropene. Subsequent treatment with PCys yields 20b (path (a)). (The corresponding complex 21a was later prepared by reaction of RuHCl(PPh3)3 with propargyl chloride see below). Initiator 20a effected controlled ROMP of and bicyclo[3.2.0]heptene M6, but ROMP of less... 

Most successful of the diphosphine derivatives (e.g. 27, 28) are the cationic, edge-bridged dimers of type 28 developed by the Hofmann group, and described in a recent review.Routes to the neutral diphosphine complexes commonly utilize 22b as precursor (Scheme 4) 134-136 commercial availability, and consequent synthetic convenience, counterbalancing its slower rate of phosphine exchange, relative to 22a or 2 Vinylalkylidene... 

At 60 °C, bicyclic titanacyclopentene complex 179 rearranges to 2-vinyl-substituted titanacyclobutene complex 180 in indeterminate yield (Equation 91) <1993CB1541>. This transformation proceeds by bis(trimethylsilyl)ethyne extrusion, rearrangement of the residual coordinated cyclopropene to vinylalkylidene, and [2+2] cycloaddition to reincorporate alkyne. 

The reactions between an alkylidene and a terminal olefin are routinely employed to prepare new alkylldenes, usually from a neopentylidene or neophylidene. In all such reactions, the intermediate that leads to the new alkylidene, an a,a -disubstituted metallacyclobutane (Eq. (1.6)), must be formed, but it is rarely stable enough to be observed. However, a 14-electron Mo vinylalkylidene MAP complex, sy -Mo(NAr)(CHCH=CMe2)(Me2Pyr)(OHMT), was successfully prepared and isolated by treating Mo(NAr)(CH-f-Bu)(Me2Pyr)(OHMT) with excess4-methyl-l,3-pentadiene [14]. 

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