Alkoxy-substituted group 6 allenylidene complexes – Synthesis and properties

Bis(alkoxy)allenylidene complexes, [(CO)5MCCC(OR′)OR], as well as mono(alkoxy)allenylidene complexes, [(CO)5MCCC(OR′)Ph], of chromium and tungsten are accessible from propynones [HCCC(O)Ph] or propynoic acid esters [HCCC(O)OR; R = Et, (−)-menthyl, endo-bornyl] by the following reaction sequence: (a) deprotonation of the alkynes, (b) reaction with [(CO)5M-THF] (M = Cr, W), and (c) alkylation of the resulting alkynyl metallate, [(CO)5MCCC(O)R], with Meerwein salts. Vinylidene complexes, [(CO)5MCC(R′)C(O)OR], are formed as a by-product by Cβ-alkylation of the alkynyl metallate. Dimethylamine displaces one alkoxy substituent of the bis(alkoxy)allenylidene complexes to give dimethylamino(alkoxy)allenylidene complexes, [(CO)5MCCC(OR)NMe2]. The analogous reaction of dimethylamine with a mono(alkoxy)-substituted allenylidene complex affords the aminoallenylidene complex [(CO)5CrCCC(NMe2)Ph]. When the amine is used in large excess, the α,β-unsaturated aminocarbene complex [(CO)5CrC(NMe2)C(H)C(NMe2)Ph] is additionally formed by addition of the amine across the CαCβ-bond of the allenylidene ligand. The reaction of [(CO)5MCCC(OEt)2] with dimethyl ethylenediamine offers access to bis(amino)allenylidene complexes, in which Cγ is part of a five-membered heterocycle. Photolysis of bis(alkoxy)allenylidene complexes in the presence of triphenylphosphine yields tetracarbonyl- and tricarbonyl{bis(phosphine)}allenylidene complexes. Diethylaminopropyne inserts into the CβCγ bond of [(CO)5MCCC(OEt)OMethyl] to give alkenylallenylidene complexes. Subsequent acid-catalyzed intramolecular cyclization affords a pyranylidene complex.

Graphical abstractAlkoxyallenylidene complexes, [(CO)5MCCC(R′)OR] (R′ = OR, Ph), of chromium and tungsten are accessible from deprotonated propynones or propynoic acid esters by reaction with [(CO)5M-THF] (M = Cr, W), followed by alkylation of the resulting alkynyl metallate. The alkoxy substituents are readily displaced by N-nucleophiles, thus offering easy access to various substitution patterns.Figure optionsDownload full-size imageDownload as PowerPoint slide