Carbenes dimethoxycarbene, stabilization

Indeed, such donation is calculated to stabilize singlet dimethoxycarbene by 76 kcal/mol relative to the corresponding triplet.93 The electron donation also modulates carbenic reactivity 78 a strong electron donor on Q raises both the carbene s HOMO and LUMO energies, thereby increasing the carbene s nucle-ophilicity while rendering its LUMO less accessible to nucleophiles (decreasing its electrophilicity).94 These consequences are illustrated by 69 and the related structures in Scheme 6. 

For example, dimethoxycarbene is devoid of electrophihcity toward alkenes because of electron donation by the methoxy groups. A methoxy group also can be expected to stabilize a carbene, by interaction of the p orbital of the central carbon atom with a lone pair of electrons on the oxygen atom (Scheme 5.2). 

In contrast to the electrophilic nature of most carbenes, highly resonance-stabilized carbenes can be nucleophilic, as suggested by the right-hand resonance structure in the margin. Dimethoxycarbene is a good example of a nucleophilic carbene. Intermediate cases such as methoxychlorocarbene can be ambiphilic, showing increased reactivity to both electron rich and electron deficient olefins. 

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