Bond interactions singlet carbenes

Due the nature of the substituents, all the stable singlet carbenes exihibit some carbon-heteroatom multiple-bond character and for some time their carbene nature has been a subject of controversy. One has to keep in mind that apart from dialkyl-carbenes, all the transient singlet carbenes present similar electronic interactions. As early as 1956, Skell and Garner drew the transient dibromocarbene in its ylide form based on the overlap of the vacant p-orbital of carbon with the filled p orbitals of the bromine atoms (Scheme 8.31). 

An alkene bearing a heteroatom also shows multiplicity selectivity. Moderately electrophilic singlet carbenes interact with the lone-pair electrons on the heteroatom to generate an ylide by accepting an electron pair in the empty n orbital, and this interaction is more efficient than that with n electron of the double bond. Thus,... 

As a cycloaddition, singlet carbene addition to an alkene must obey the rules of orbital symmetry discussed in Chapters 35 and 36. We might consider the empty p orbital of the carbene (LUMO) interacting with the % bond (HOMO) of the alkene or the lone pair of the carbene in its hlled sp orbital (HOMO) interacting with the k antibonding orbital of the alkene (LUMO). 

The concerted cyclopropanation reaction of singlet carbenes can be classified as a symmetry allowed pericyclic process. It is sometimes referred to as a cheletropic reaction. The carbene is thought to adopt a sideways approach in order to develop a bonding interaction between the HOMO of the alkene and the LUMO of the carbene. 

Figure 11.6 Dicopper compiexes bridged by (a) nucleophilic and (b) electrophilic carbenes. (c) Bonding interactions between singlet carbene orbitals and (LCu)2 combinations.

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