Catalytic Cycle for Cyclopropanation

Two mechanisms have been considered for metal-assisted cyclopropanation of alkenes by diazo compounds. These are shown by reactions 7.34 and 7.35. 

Reaction 7.34 involves a metal-carbene intermediate, while reaction 7.35 involves nucleophilic attack by the diazo compound to the coordinated alkene. With a rhodium-porphyrin catalyst direct spectroscopic evidence has been obtained for the carbene pathway (see Section 2.5.2). 

The catalytic cycle proposed for the rhodium-porphyrin-based catalyst is shown in Fig. 7.18. In the presence of alkene the rhodium-porphyrin precatalyst is converted to 7.69. Formations of 7.70 and 7.71 are inferred on the basis of NMR and other spectroscopic data. Reaction of alkene with 7.71 gives the cyclopropanated product and regenerates 7.69. As in metathesis reactions, the last step probably involves a metallocyclobutane intermediate that collapses to give the cyclopropane ring and free rhodium-porphyrin complex. This is assumed to be the case for all metal-catalyzed diazo compound-based cyclo-propanation reactions. 

In Figs. 7.1 and 7.2 identify oxidative addition, insertion, and reductive elimination steps. Show the formation of 7.7 from 7.6 by an electron pair (curly arrow) pushing formalism. 

The mechanism in Fig. 7.3 is applicable without modification for the hydrogenation of c-hexene, a-acetamidocinamic acid, and styrene by Wilkinson s catalyst. Discuss the validity of this statement. 

---