Diarylcarbene cyclopropanations with

Heteroatom-substituted carbene complexes are less electrophilic than the corresponding methylene, dialkylcarbene, or diarylcarbene complexes. For this reason cyclopropanation of electron-rich alkenes with the former does not proceed as readily as with the latter. Usually high reaction temperatures are necessary, with radical scavengers being used to supress side-reactions (Table 2.16). Also acceptor-substituted alkenes can be cyclopropanated by Fischer-type carbene complexes, but with this type of substrate also heating is generally required. 

In the following sections the methods used to prepare cyclopropanes by addition of aryl-and diarylcarbenes to alkenes are presented. Most mechanistic aspects of the methods have been dealt with in detail by Wentrup reaction mechanisms will, therefore, be not discussed in detail. 

Decomposition of diaryldiazomethanes results in formation of diarylcarbenes or diaryl-carbenoid species, which are capable of reacting with C-C double bonds forming gcw-diaryl-cyclopropanes. The decomposition can be carried out by three different methods photolysis, thermolysis and salt catalysis. 

Diarylcarbene complexes of transition metals are considerably less reactive toward alkenes than the analogous monoarylcarbene complexes and, therefore, have been little used for preparation of gem-diarylcyclopropanes. Some examples, however, have been reported. When a solution of pentacarbonyldiphenylmethylidenetungsten in ethyl vinyl ether was heated to 37 °C for 3 hours, l-ethoxy-2,2-diphenylcyclopropane (2) was obtained in 61% yield together with 1,1-diphenylethene (3) and 2-ethoxy-l,l-diphenylethene (4).114,115 If the reaction is carried in less polar solvents, such as benzene or pentane, the yield of cyclopropane drops considerably.115... 

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