Vinylcarbene-cyclopropene isomerization

In principle, the vinylcarbene-to-cyclopropene isomerization is reversible. While this has not been reported for 167 (where ring-opening could produce not only alkenyl(sulfonyl)carbene 166, but also the isomeric l-alkenyl(silyl)carbene), it was found that 1-trimethylsilylcyclopropenone acetal 168 (equation 49) by thermal ring-opening yields the (trimethylsilyl)vinylcarbene 169 besides traces of the isomeric vinylcarbene 170. Both carbenes are obviously nucleophilic since they are able to cyclopropanate the... 

Alkinyloxy)diazoacetic esters 11 give rise to product mixtures that could be separated only partially. The isolated products result from a tandem intramolecular cyclopropenation/cyclopropene —> vinylcarbene isomerization (12, 14) and from a twofold intermolecular (3+2)-cycloaddition of the intact diazo compound (13). 

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

Interestingly, copper(I) salts also catalyze the cyclopropene-vinylcarbene isomerization [681]. In this case the transient carbene complexes again show electrophilic behavior, behavior similar to that of the complexes formed from copper(I) salts and diazoalkanes or sulfonium ylides. 

As discussed in Section 3.1.6, cyclopropenes can react with rhodium complexes [38,585,587-589,1061,1063] or other transition metal derivatives to yield vinylcarbene complexes (see Section 3.1.6). This reaction will proceed particularly smoothly with strained cyclopropenes, because these can already isomerize thermally to vinylcarbenes [1064]. Hence the formation of vinylcarbene complexes from alkynes can proceed by initial cyclopropanation, followed by reaction of the resulting cyclopropene with the complex L,M. 

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 photochemical isomerization of the parent system 1 to cyclopropene (2) was first detected in an argon matrix at 8 K. The allene photoisomerizations most likely involve singlet intermediates employing benzene as a triplet sensitizer leads to complex mixtures containing structurally completely different isomers. A two-step mechanism whereby hydrogen migrates first to the central allene carbon atom, followed by either cyclization or a second 1,2-hydrogen shift from a vinylcarbene intermediate, accounts for the overall features of the allene cyclization. ... 

In this case, all the products are primary and the disappearance of cyclopropene follows first-order kinetics with = 29.8 kcal mol, and an entropy of activation of — 17.8 eu. The reaction is thought to occur via ring opening to isomeric vinylcarbenes, but the large negative entropy of activation is not consistent with simple ring opening. 

The kinetics of the isomerization in tetrachloroethene at 413-433 K show an E/ of 25 1 kcal mol an enthalpy of activation of 24 + 1 kcal moP and an entropy of activation of — 22.4 eu. Changing the solvent to benzonitrile does not increase the rate and is consistent with a nonpolar transition state such as a vinylcarbene, but the entropy value indicates a more concerted mechanism. Indenes were also produced when a solution of the cyclopropene in cyclohexane was heated with copper stearate at 60 C for 30 minutes or when a solution in dichloromethane was heated with trifluoracetic acid, or when 1,3,3-triphenylcyclopropene was treated with [(C2H4)PtCl2]2- Reactions catalyzed by rhodium(I) heptafluorobutanoate have also been reported. ... 

While the cyclopropene complexes mentioned so far were prepared by reaction of a cyclopropene with a reactive metal reagent, complexes 17 a and b were prepared by reaction of the carbene complexes 15 a and b, respectively, with phenylethyne (16) at — 80°C. Compound 17a isomerizes easily to give a vinylcarbene complex above — 40 C, whereas 17b is reported to be slightly more stable. 

Finally there are a number of papers dealing with photoreactions of diaryl-cyclopropenes and arylindenes, which formally involve migration of a carbon group from the side-chain to the ortho-position of an aromatic ring. An example of the basic reaction is the formation of indenes when 1,3-diphenylcyclopropenes (199) are irradiated directly. Now it is shown that isomeric indenes are formed when the reaction is carried out in the presence of 9,10-dicyanoanthracene, and an electron-transfer mechanism is invoked to account for the change of product. In a separate paper concerned largely with intramolecular photocycloaddition reactions, cyclopropenes (199 R = allyl) are reported to give both types of indene on direct irradiation, via the normally postulated vinylcarbene intermediate. The... 

Cyclopropene undergoes a thermal isomerization to propyne with log k = 13.25 - 37 400/2.37 r and to allene with log k = 13.25-43 400/23RT The simplest reaction pathway would appear to be Cl-C3 cleavage to a biradical, which is also vinylcarbene, followed by a vicinal hydrogen shift from C2 to either C3 or Cl, respectively (Scheme 4.2). 

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