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The involvement of a catalytic metal vinylidene species was proposed for the first time in 1986 to explain the regioselective formation of vinyl carbamates directly from terminal alkynes, carbon dioxide, and amines. Since this initial report, various metal vinylidenes and allenylidenes, which are key activation intermediates, have proved extremely useful for many alkyne transformations. They have contributed to the rational design of new catalytic reactions. This 20th anniversary is a suitable occasion to present the advancement of organometallic vinylidenes and allenylidenes in catalysis.

A variety of vinylidene−ruthenium complexes [trans-RuCCHR(Cl)(dppe)2]PF6 (2) are obtained by reaction of RuCl2(dppe)2 (1) with terminal alkynes and NaPF6. On treatment with a base, complexes 2 afford the alkynyl−ruthenium derivatives trans-Ru−C⋮C−R(Cl)(dppe)2 (3). trans-Ru(C⋮CR)2(dppe)2 4 are prepared under mild conditions via the reaction of RuCl2(dppe)2 with terminal alkynes HC⋮C−R (R = Ph, nBu, SiMe3, C10H21, CH2OSiMe3, CH2OMe) in the presence of an excess amount of NaPF6 and NEt3, whereas trans-Ru(C⋮CH)2(dppe)2 (5) was obtained directly from HC⋮C−SnBu3 and precursor 1. In contrast, unsymmetrically substituted complexes trans-Ru(C⋮CR1)(C⋮CR2)(dppe)2 (6) were built from the vinylidenes 2 in the presence of another alkyne, NaPF6, and NEt3. On protonation with NH4 +PF6 -, the bis(alkynyl) derivatives 4 lead to the release of RC⋮CH and the formation of trans-[Ru(NH3)(C⋮CR)(dppe)2]PF6 complexes 7. The structure of trans-[Ru(NH3)(C⋮CC6H5)(Ph2PCH2CH2PPh2)]PF6 (7a) has been determined by X-ray diffraction.

A series of well accessible cationic ruthenium allenylidene complexes of the general type [(eta6-arene)(R3P)RuCl(=C=CR'2)]+ X- is described which constitute a new class of pre-catalysts for ring closing olefin metathesis reactions (RCM) and provide an unprecedented example for the involvement of metal allenylidenes in catalysis. They effect the cyclization of various functionalized dienes and enynes with good to excellent yields and show a great tolerance towards an array of functional groups. Systematic variations of their basic structural motif have provided insights into the essential parameters responsible for catalytic activity which can be enhanced further by addition of Lewis or Bronsted acids, by irradiation with UV light, or by the adequate choice of the "non-coordinating" counterion X-. The latter turned out to play a particularly important role in determining the rate and selectivity of the reaction. A similarly pronounced influence is exerted by remote substituents on the allenylidene residue which indicates that this ligand (or a ligand derived thereof) may remain attached to the metal throughout the catalytic process. X-ray crystal structures of the catalytically active allenylidene complexes 3b.PF6 and 15.OTf as well as of the chelate complex 10 required for the preparation of the latter catalyst are reported.

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The ruthenium catalysed cross-metathesis of fatty-esters arising from plant oils with acrylonitrile is presented. The resulting linear nitrile ester products have potential as new intermediates for polyamides synthesis. A series of commercially available catalysts are able to promote the transformation of methyl 10-undecenoate 1, dimethyl octadec-9-en-1,18-dioate 5 and methyl ricinoleate 9 with acrylonitrile and a protocol based on the slow addition of catalyst allowed TONs as high as 1900 (92% yield) to be reached for cross-metathesis with acrylonitrile. These cross-metathesis conditions have been applied to methyl acrylate and TONs up to 7600 were obtained.

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