Biscarbenes, cyclic

Sometimes the reaction products depend on the deprotonation method used. For example, RajanBabu and coworkers have obtained chiral N-hetero-cyclic biscarbenes of Pd and Ni from a binaphtyl bisimidazolium salt. The in situ deprotonation of the bisimidazolium salt with KOfBu prior to the addition of Pd(OAc)2 gives exclusively the trans isomer, 24, while direct reaction of the salt with the metal complex in hot DMSO (see next section) gives a mixture of the cis-(25) and trans-isomers (Scheme 17) [81]. 

A polycyclic system with a l,3-dioxa-2-silacyclohepta-4,6-diene fragment was obtained in low yield from bis(diazocarbonyl)silane 671. In the first stage, a biscarbene or a biscarbenoid, generated in the presence of copper triflate or palladium acetate, forms 1,2-diacylcyclopropene, then rearranging into cyclic cumulene triene 672 by migration of a silicon atom to an oxygen atom. The latter dimerizes to [4]-radialen 673 or reacts with furan to afford 674 (Scheme 32) (89AG175()). 

The excellent coordination properties of alkynes with ruthenium catalysts led to their use as partners for the coupling with a large variety of unsaturated molecules. The first examples of dimerization of terminal alkynes involved acetylide or vinylidene intermediates. By contrast, with (CsR5)Ru catalysts, a completely different stoichiometric head-to-head coupling of alkynes has been discovered affording ruthenacyclopentatrienes, which are cyclic biscarbenes produced by the oxidative coupling of two alkynes [1-3] (Scheme 1). 

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