Antibonding interactions, singlet carbenes

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). 

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