Isomerization of cyclopropenes

Fig. 3.29. Isomerization of cyclopropene complexes into vinylcarbene complexes [5831.

Isomerization of cyclopropenes. Irradiation of the cyclopropcne 1 results in the alkcncs 2 and 3, formed by cleavage of the h bond. Exposure of 1 to silver ion results in two different alkenes, 4 and 5, formed by cleavage of the a bond. 

Vinylcarbene is also regarded as an intermediate in the catalytic isomerization of cyclopropene in the presence of [CuCl P(OPh)3 ]. 3-Methyl-3-cyclopropylcyclopropene (83a) and 3,3 dicyclo-propylcyclopropene (83b) react with cyclopentene derivatives under die influence of CuCl/P(OPh)3 to yield cntfo-vinylcyclopropane derivatives (84 equation 37) in 55-72% yields. 3-Methoxycarbonyl-l-propylcyclopropene also reacts with noibomadiene in the presence of CuCl to give the corresponding vinylcarbene adducts in 60% yield. 

The previous section and Houben-Weyl, E19b. p708ff give details of a range of thermal and photochemical isomerizations of cyclopropenes leading to dienes, often as one of a number of products. An example is the thermal isomerization of l-t Tt-butyl-3,3-dimethylcyclopropcne... 

Table 3. Activation parameters for thermal isomerization of cyclopropenes...

A computational study into PtCl2-catalysed isomerization of cyclopropenes to allenes supports a mechanism that involves an Sj 2 -type C-C bond activation of the cyclo-propene by PtCl2 fixed on a cyclopropene molecule. ... 

The Diels-Alder reaction of cyclopropenes with 1,2,4,5-tetrazines (see Vol.E9c, p 904), a reaction with inverse electron demand, gives isolable 3,4-diazanorcaradienes 1, which are converted into 4H-1,2-diazepines 2 on heating. The transformation involves a symmetry allowed [1,5] sigmatropic shift of one of the bonds of the three-membered ring, a so-called walk rearrangement , followed by valence isomerization.106,107... 

Analogous reactivity was observed by Jones303 with methylene cyclopropene 554, which on treatment with diazomethane yielded the pyrazole 556, thought to arise from thermal isomerization of pyrazolenine 555 formed initially ... 

In another study the kinetics and mechanism of an unprecedented T/2-vinyl isomerization of a highly fluorinated tungsten(II) metalla-cyclopropene complex was studied (92). Photolysis of a tungsten(II) tetrafluoroaryl metallacycle 1 and perfluoro-2-butyne results in the formation of the kinetic rf -vinyl complex 2 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. Upon heating 2 is converted to the thermodynamic rf -vinyl complex 3 in which the fluoride ligand is now cis to the inserted alkyne and trans to one CO and cis to the second CO ligand as shown in Scheme 1. 

For the thermal isomerization of 2 to 1 [6] on the basis of kinetic data, it was suggested that at least 50% of 1 is formed via cyclopropene 3 as an intermediate [7] rather than a direct 1,3-H-shift as proposed before [8, 9]. Later theoretical calculations provided further support for that suggestion [10], A number of other publications, some of them with basic catalysts such as sodium hydroxide, deal with this reaction [11-17]. Furthermore, 2 can be isomerized to 1 by a stoichiometric reacion with [RuHCl(CO)(PPh3)3] and subsequent treatment with NaS2CNMe2 [18]. The 1,1-bisdeuterated 2 has also been investigated [19]. 

In 1987, Nitta reported the formation of an unexpected vinylketene complex from the reaction of an azido-substituted cyclopropene with diiron nonacarbonyl.104 They had previously investigated the chemical behavior of the complexed nitrene intermediates that result from the reaction of organic azides and iron carbonyls113 and were interested in replicating the thermal isomerization of 3-azido-l,2,3-triphenylcyclopropene (163) into 4,5,6-triphenyl-l,2,3-triazine using a metal carbonyl-promoted re-... 

Additional methods for preparing non-heteroatom-substituted carbene complexes include nucleophilic or electrophilic additions to carbyne complexes (Section 3.1.4), electrophilic additions to alkenyl or alkynyl complexes (Section 3.1.5), and the isomerization of alkyne or cyclopropene complexes (Section 3.1.6). 

The formation of the cyclopropene derivative Z-36 is very similar to the isomerization of p-phenylenebisnitrene (13) to 14 (Scheme 2) and both processes are likely to be photochemical, rather than thermal. Further irradiation of Z-36 causes its isomerization to another compound, which according to DFT simulations, is compatible with the structure of its E-isomer (E-36, Scheme 7). 

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

The interesting reaction of cyclopropenes 20 with nitriles mediated by gallium(III) or indium(III) salts offers a new route to isomerically pure pyrroles 21 in low to moderate yields. A mechanistie rationale for this transformation was also provided <030BC4025>. Likewise, pyrroles 21 have been obtained via a cascade comprising [3 -i- 2] dipolar cycloaddition of a wide variety of nitriles to the donor-acceptor containing cyclopropanes 22, following dehydration, and isomerization <030L5099>. 

The rapid and quantitative isomerization of 1-methylcyclopropene , and related methyl substituted cyclopropenes to the corresponding methylenecyclopropanes is indicative of relief of ring strain in the transition state which is predominantly carbanionic... 

The preparation of cyclopropene has been achieved in the reaction of allyl chloride with sodium amide 1-Alkylcyclopropenes have analogously been synthesized from 2- and 3-alkylallyl chlorides but the products are liable to isomerize under the strongly basic conditions to alkylidenecyclopropanes and 1-alkenylcyclopropanes. The use of potassium amide gives the rearranged products in a higher extent than that with sodium amide (equation 24). Methylenecyclopropane has been prepared by the successive treatments... 

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