Alkenylidenes complexes

The ruthenium compounds described above show a distinctly lower metathetic activity than the molybdenum alkenylidene complex 24 developed by Schrock et al. (Fig. 4, see also the chapter by R.R. Schrock, this volume) [18], which is another standard catalyst for any type of olefin metathesis reaction. However, they... 

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]. 

As noted above, titanocene-alkylidenes can be prepared using various methods and starting materials. Like the methylidene complex, higher alkylidene complexes are useful for the transformation of carbonyl compounds to highly substituted olefins. Ketones and aldehydes are converted into substituted allenes by treatment with titanocene-alkenylidenes prepared by olefin metathesis between titanocene-methylidene and substituted allenes (see Scheme 14.7) [17]. Titanocene-alkenylidene complexes can also be prepared from... 

The formation of highly substituted titanacyclobutenes utilizing titanocene-alkylidenes has been investigated (Scheme 14.33). Alkylidenetitanacyclobutenes 76 are produced by the reaction of titanocene-alkenylidene complexes with alkynes [76]. The alkenylcydopro-pane 77 can be synthesized by thermolysis of dicydopropyltitanocene in the presence of diphenylacetylene, which is assumed to proceed through formation of the titanacyclobutene 78 [25c],... 

Double cyclization of iodoenynes is proposed to occur through a Rh(I)-acetylide intermediate 106, which is in equilibrium with vinylidene lOS (Scheme 9.18). Organic base deprotonates the metal center in the course of nucleophilic displacement and removes HI from the reaction medium. Once alkenylidene complex 107 is generated, it undergoes [2 + 2]-cycloaddition and subsequent breakdown to release cycloisomerized product 110 in the same fashion as that discussed previously (Scheme 9.4). Deuterium labeling studies support this mechanism. 

The luminescence properties of a series of dinuclear platinum alkenylidene complexes [Pt2(/x-C=CHPh)(C=CPh)... 

Several routes that lead to formation of alkenylidene type complexes are known [130]. A route involving the side-on coordination of an alkyne to a low valent transition metal complex (10) may lead to a hydrido alkynyl complex (12) through the activation of the C-H bond in the coordinated alkyne (11) as shown in Scheme 7.23. Proton 1,3-migration in the alkynyl-hydrido complex to the alkynyl carbon in 12, or the direct 1,2-transfer of the proton in 11 gives the alkenylidene complex 13 [131]. It is also possible that the alkenylidene complex is produced by protonation of the alkynyl complex. 

The photochemical step likely involves loss of a CO ligand from manganese, followed by coordination of diphenylacetylene and insertion into the manganese-aryl bond. A similar photoinduced coupling occurs with diphenylacetylene and a diplatinum //-alkenylidene complex ... 

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