Structure and reactivity of carbenes

Depending upon the mode of generation, a carbene may initially be formed in either the singlet or triplet state, no matter which is lower in energy. These two 

The presence of more complex substituent groups complicates the description of carbene structure. Furthermore, since carbenes are high-energy species, structural entities that would be unrealistic for more stable species must be considered. As an example, one set of MO calculations arrives at structure A as a better description of carbomethoxycarbene than the conventional structure B. 

The radical versus electrophilic character of triplet and singlet carbenes also shows up in relative reactivity patterns, shown in Table 10.1. The relative reactivity of singlet dibromocarbene toward the three alkenes in Table 10.1 is more similar to that of electrophiles (bromination, epoxidation) than to that of radicals ( CCI3). 

Transition state for concerted singlet carbene addition 

7T Delocalization involving divalent carbon in conjugated cyclic systems has been studied in the interesting species cyclopropenylidene (C) and cyclohep-tatrienylidene (D). In these molecules, the empty p orbital on the carbene carbon can be part of the aromatic tt system and is therefore delocalized over the entire 

Substituents perturb the relative energies of the singlet and triplet state. In general, alkyl groups resemble hydrogen as a substituent, and dialkylcarbenes have triplet ground-states. Substituents that act as electron-pair donors stabilize the singlet state more than the triplet state by delocalization of an electron pair into the empty p orbital.55,56 

From the point of view of both synthetic and mechanistic interest, much attention has 

CHAPTER 10 REACTIONS INVOLVING CARBOCATIONS, CARBENES, AND RADICALS AS REACTIVE INTERMEDIATES 

---

Structure and reactivity of carbenes having aryl substituents, 22, 311... 

Structure and Reactivity of Carbenes having Aryl Substituents... 

---