Cyclopropenes, formation from vinylcarbene

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. 

The photochemical study of 3H-pyrazoles was carried out in the search for a route to cyclopropenyl tertiary alcohols. Irradiation of 63a in dry dichloromethane at 300 nm and at room temperature for 0.5 h led to the exclusive formation of the gem-dimethylcyclopropene 65 (Scheme 17). The formation of cyclopropene 65 arises from the loss of N2 and cycUzation of the vinylcarbene intermediate (III). 

The elusive diazoalkenes 6 and 14 are unlikely to react with methanol as their basicity should be comparable to that of diphenyldiazomethane. However, since the formation of diazonium ions cannot be rigorously excluded, the protonation of vinylcarbenes was to be confirmed with non-nitrogenous precursors. Vinyl-carbenes are presumedly involved in photorearrangements of cyclopropenes.21 In an attempt to trap the intermediate(s), 30 was irradiated in methanol. The ethers 32 and 35 (60 40) were obtained,22 pointing to the intervention of the al-lylic cation 34 (Scheme 10). Protonation of the vinylcarbene 31 is a likely route to 34. However, 34 could also arise from protonation of photoexcited 30, by way of the cyclopropyl cation 33. The photosolvolysis of alkenes is a well-known reaction which proceeds according to Markovnikov s rule and is, occasionally, associated with skeletal reorganizations.23 Therefore, cyclopropenes are not the substrates of choice for demonstrating the protonation of vinylcarbenes. 

The position of the alkyl substituent in the product indicates that cyclisation occurs with rearrangement of the double bond, ie., by 1,1-elimination and formal formation and cyclisation of a vinylcarbene. Although the overall yields are not always good, the reagents are readily available and large quantities of the simple alkylcyclopropenes can be produced. 1,2-Dimethylcyclopropene has been prepared in a similar process by treatment of methallyl chloride with two equivalents of phenyl lithium, followed by quenching with methyl iodide presumably, the initial reaction leads to 1-methylcyclopropene which is converted in situ to the 2-lithio-species 10). The elimination of HBr from brominated alkylidenemalonates also leads to cyclopropenes, though in low yield U) ... 

Further support for the intervention of vinylcarbenes in such reactions stems from the cyclopropenes 239 which provide diene products, the proportions of which reflect the facility of the aryl ring to stabilize the developing carbenic centre. The formation of... 

In the case where there is no hydrogen on C3 of the cyclopropene, products apparently formed by addition of a vinylcarbene, derived by ring opening of the cyclopropene, to the alkene are observed, e.g. formation of 2 from... 

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... 

---