1,1-Dimethylcyclopropane, singlet carbenes

In the case of the photochemical reaction, irradiation of diazomethane in the presence of cw-2-butene provides cw-l,2-di-methylcyclopropane with no detectable amount of the tram isomer (eq 45). This reaction is thought to proceed via a singlet carbene. However, if the same reaction is carried out via a triplet carbene, generated via triplet sensitization, then a 1.3 1 mixture of tram to cis dimethylcyclopropane is observed (eq 46). The yields in the photochemical reaction are typically lower than the metal-mediated processes, and are usually accoii5)anied by more side products. 

An important example of cheletropic reactions is the reversible insertion of a singlet carbene into a carbon—carbon double bond to give a cyclopropane derivative. Only singlet carbenes will be considered here, as the pericyclic selection rules cannot be applied to triplet states. Singlet carbenes add thermally to alkenes in a concerted manner, therefore, the geometry of the alkene is preserved in the product, i.e., the reaction is stereospecific. Hence, in the present context, the reaction is described as suprafacial on the olefin. Therefore, the reaction of -but-2-ene and Z-but-2-ene with singlet carbene gives trans- and di-l,2-dimethylcyclopropane, respectively (Scheme 5.1). 

Sol 1. (a) For singlet carbenes, the alkene insertion reaction is stereospecific. Therefore, the reaction of Z-but-2-ene with singlet carbene gives cis-1,2-dimethylcyclopropane. 

Sol 3. (d) Diazo compounds liberate the free carbenes under photochemical conditions that are in singlet states. This process can be directed toward the formation of a triplet carbene using a triplet sensitizer like benzophenone. For triplet carbenes, the aUcene insertion reaction is nonstereospecific. Therefore, the reaction of Z-but-2-ene with triplet carbene gives a mixture of cisHrans-1,2-dimethylcyclopropane. 

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