Addition Reactions to form Ring Systems: (Cycloaddition Reactions)

             Cyclopropane formation:  Use of carbene intermediates

                   Metal initiated formation of the carbene intermediate

Earlier in this chapter, we learned how to form gem-dihalides.  Once formed, they have several uses, one of which is metal insertion which leads to carbene formation.  As the system above reveals, mixing a metal with a gem dihalide will result in a metal insertion just as we learned in Chapter 1 (Click HERE for review and mechansim ) and reviewed in Chapter 6 (Click HERE for further review) .  The metal inserted species is a carbon anion with a very good leaving group attached.  If the second halide leaves, a neutral molecule is formed.  This species, called a carbene, contains only 6 electrons, but is more stable than the carbon anion with a leaving group attached to the same carbon.  Another method for removing the first bromine to form the anion is use of an alkyl lithium base.  A transmetallation occurs, leading to the anion and subsequent formation of the carbene once again.

Diazoalkane and diazirine use is quite popular as the N₂ byproduct is an inert gas and will not react further with other species in solution.  When metal or base formation is used, the metal salt and alkyl halide may both interact within the solution, forming unwanted side products.   Diazirines are more difficult to form than are diazoalkanes due to the 3-membered ring strain, and are not found in literature to the same extent.

The carbene that forms may be either a singlet (both electrons in one orbital, leaving one orbital empty) or a triplet (each electron in a different orbital).  The names are derived from viewing the Electron Spin Resonance of carbenes.  For a singlet carbene, both electrons are in the same orbital, so they must be paired.  In ESR readings, they will give a single resonance, hence the name singlet.  For a triplet carbene, both electrons are in a separate orbital and are not spin paired.  As such they show “spin coupling” and give rise to a triplet signal.  This is similar to splitting you studied in NMR last semester.