Vinylidenes cyclopropene

A detailed, extensive review of cyclopropene has appeared. Cyclopropene is the last of the small strained ring hydrocarbons to have its thermal decomposition subjected to intensive investigation.158 Cyclopropenes can decompose by a variety of mechanisms involving diradicals, vinylcarbenes, and vinylidenes. Cyclopropene itself has been shown to be an intermediate in the allene-propyne rearrangement. 

This stoichiometric reaction constitutes a new contribution to vinylidene chemistry and a novel method to generate alkenylcarbene ligand from simple propargyl alkyl ethers rather than via activation of cyclopropenes [4] or by stoichiometric activation of butadiene [6[. When linked to a suitable metal-ligand moiety this carbene constitutes an alkene metathesis initiator. 

Evidence for this pathway came first from the observation of a decreased rate of reaction with alkyl substitution at Cl rather than a rate increase due to increased substitution. This rate retardation results from preventing the 1,3-hydrogen shift to make the vinylidene, resulting in slower formation of acetylene via the pathway described in Scheme 4.2. Further, pyrolysis of carbon-labeled 1,3,3-trimethyl-cyclopropene gave one acetylene, which could be derived from either pathway (Scheme 4.2 or 4.3), and nearly equal amounts of another acetylene, which could only be derived by Scheme 4.3 (Scheme 4.4). ... 

This work also determined the activation parameters for the overall loss of 1,3-dimethylcyclopropene as log k = 13.36 — 39 300/2.3/ T and that for the formation of 1-ethylcyclopropene as log A = 12.29 — 39 800/2.3/ 7. The activation parameters for loss of the latter material were found to be very similar to that of the former cyclopropene. Still later work using 2-deuterium-labeled material not only demonstrated that the ethyl group migrated in preference to the methyl group in the vinylidene by a factor of roughly 3, but also found a deuterium isotope effect of 1.32 at 509 K. This isotope effect could result from a pathway like that depicted in Scheme 4.2, but the authors suggested that it arises from the 1,3-hydrogen shift to form the vinylidene. 

Three papers report thermally-generated carbenes. Vinylidenes have been generated from cyclopropenes, such as (82), by thermolysis. The vinylidene (83) rearranged to the corresponding alkyne (84). The proposed vinylidene intermediate was shown, by calculation, to be an energy minimum. The energy barriers are within reach of the substrate under the conditions needed to generate the vinylidenes and the barriers are lower than those for the alternative radical pathways. ... 

Metal vinylidenes easily form by isomerization of metal-alkynecomplexes, which is important for instance in alkyne polymerization by the metathesis mechanism. Synthesis of Grubbs ruthenium vinylidene complexes [Ru(PR3)2Cl2(=C=CHPh)] originally proceeded by reaction of the ruthenium precursor with diphenyl-cyclopropene, but a more modern method leading to the parent vinylidene complex involves benzylidene exchange for the vinylidene in the metathesis with 1,3-butadiene ... 

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