Carbene Addition to Indoles

The ring opening ionization is probably favored by relief of strain. Further addition of dichlorocarbene to the chloro-substituted carbon-carbon double bond yields adduct II. Finally, a third addition of dichlorocarbene to the exocyclic olefin yields adduct III [41]. 

The ratio of the three products depends on the molar ratio of chloroform used as well as on reaction time. Control experiments showed that generation of dichlorocarbene in the presence of I led to II and III, while generation of dichlorocarbene in the presence of II led to high yields of III. The lack of reactivity of the carbon-carbon double bond in the second cyclobutene ring may result from factors similar to those operating in the norbbrnadiene case. 

One further addition-rearrangement sequence which should be mentioned is the reaction of phase transfer generated dichlorocarbene with spirocyclopropylindene. The initial carbene adduct is unstable under the reaction conditions and undergoes ring expansion and dehydrochlorination to yield a mixture of l-(2-chloroethyl)-3-chloronaphthalene and l-vinyl-3-chloronaphthalene [23]. The reaction is formulated in equation 2.24. 

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