Carbene Addition to Conjugated Olefins

The phase transfer catalyzed (BTEAC) addition of dichlorocarbene to 2-methylfuran (2.5%), 2-methyl thiophene (2.3%) and 2-methylbenzofuran (38%) yields products in accord with equation 2.26 [43]. 

Analogous additions of dichlorocarbene to the carbon-carbon double bonds of methyl substituted naphthalenes and benzenes also have been reported. The products appear to arise from the following reaction sequence. Initial addition of dichlorocarbene to a methyl substituted carbon-carbon double bond yields a 1,1-dichlorocyclopropane. Ionization of this intermediate with ring-opening yields a substituted tropylium ion. Proton loss from the methyl group yields 1-methylene-2,4,6-cycloheptatriene. Addition of dichlorocarbene to the exocyclic methylene group yields product (see Eq. 2.27). While this reaction is mechanistically interesting, it is of little preparative use since yields are poor [24]. Likewise, the addition of dichlorocarbene to naphthalene [30, 43], anthracene [30], and 1,4,5,8-tetramethoxynaphthalene [44] have all been studied. 

We have previously mentioned that the reaction of dichlorocarbene with diphenyl-acetylene yields diphenylcyclopropenone. Dichlorocarbene apparently reacts more readily with a carbon-carbon double bond than with a carbon-carbon triple bond. 

Several examples of reactions of dichlorocarbene with conjugated unsaturated systems possessing both C-C double and C-C triple bonds have been reported [47, 48]. 

Adducts of dichlorocarbene and allenes have also been prepared under phase transfer conditions. The product of dichlorocarbene addition to tetraphenylallene is particularly interesting because addition is reported to occur at the C=C double bonds of a benzene ring rather than to the C=C double bond of the allene system. Likewise, the reaction of dichlorocarbene with phenylallene yields an unusual product. In contrast, tetraphenylbutatriene reacts with dichlorocarbene to give a mixture of normal products [31 ]. 

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Carbene Addition to Conjugated Olefins Table 2.6. Addition of dichlorocarbene to conjugated enynes... 

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