Alkynes dihalocarbenes

Alkynes tend to be much less reactive than alkenes. For example, 1,2-diphenylethyne produces only 23% of the dichlorocyclopropene from its reaction with dichlorocarbene, compared with 96% of the dichlorocyclopropane obtained from rrans-stilbene under analogous conditions [4]. Conjugated eneynes react preferentially at the C=C bond with dihalocarbenes [18-20, 22, 38] and with dimethylvinylidene carbene [158],... 

Interest in the reaction of dihalocarbenes with alkynes has been limited. In comparison with alkenes, dihalocarbenes react sluggishly with alkynes (cf. equation 17) and the resulting dihalocyclopropenes are unstable (Scheme 3). These are, however, valuable precursors for cyclopropenones (equations 24 and... 

The addition of dihalocarbenes to alkynes is again a rather inefficient process and usually leads, to the isolation of the cyclopropenone rather than the 3,3-dichlorocyclo-propene. In a rather unusual example, however, 2-butyne is reported to be converted to (67). This product is apparently derived by addition of dichlorocarbene to the corresponding methylenecyclopropene, derived in turn by elimination of HC1 from the primary adduct (68). The cyclopropene (67) does not appear to ring open to a vinylcarbene, but can be trapped in Diels-Alder reactions with cyclopentadiene 60). A related addition of dichlorocarbene to ethyl 2-butynoate also leads to a low yield of the 3,3-dichlorocyclopropene, which may be hydrolysed to the cyclopropenone 6l). 

When the dihalocarbene is generated thermally or photochemically the product of cheletropic addition to the alkyne is usually isolable in adequate yield. For example, pyrolysis of (trichloromethyl)trifluorosilane at 140°C and 6.5 atm in the presence of perfluorobut-2-yne provides " 3,3-dichloro-l,2-bis(trifluoromethyl)cyclopropene in 93 % yield. The corresponding perfluorocyclopropene is available from addition of di-fluorocarbene which can be generated by thermolysis of perfluoropropene oxide ... 

The addition of dihalocarbenes to alkynes under many conditions results in hydrolysis of an intermediate 3,3-dihalocyclopropene to produce a cyclopropenonc. Such reactions are described in Section 5.A.1.1.1. Related reactions in which a halocyclopropene may be isolated are shown in Table 5. 

A final class of carbene complexes includes vinylidenes (E in Figure 2.14). Vinylidenes possess one mediylene substituent at the carbene carbon instead of two substituents connected by single bonds. Free vinylidene is electrophilic, and the singlet state is calculated to lie below the triplet state by 43.1 kcal/mol. The electrophilicity of free vinylidene is similar to that of a dihalocarbene. It is highly reactive toward rearrangement to the much more stable alkyne tautomer. 

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