Adamantyl, singlet carbenes

The stability of azole carbenes can be attributed to electronic factors which operate in both the Tran d CT-frameworks (92JA5530). In the TT-framework, electron donation into the carbene out-of-plane p-orbital by the electron-rich system moderates the typical electrophilic reactivity of carbenes. In the o-framework, additional stability for the carbene electron pair may be gained from the o-electron-withdrawal effects on the carbene center by the more electronegative nitrogens, which moderates the carbene nucleophilic reactivity. The combination of these a- and TT-effects serves to increase the singlet-triplet gap and stabilize the singlet carbene over the more reactive triplet state. For carbenes with bulky substituents (tert-butyl, 1-adamantyl, etc.) steric effects provide additional stabilization. 

In some cases donation of nonbonded electrons by atoms )3 to the carbene carbon can stabilize singlet carbenes dramatically. Arduengo and co-workers reported that treating 1,3-di-l-adamantylimidazolium chloride (34) with the anion CH3SOCH2 in THF produced the singlet carbene 1,3-di-l-adamantyl-inudazol-2-ylidene (35, equation 5.29). This carbene is stabilized thermodynamically by electron donation from the two amino groups, and it is stabilized kinetically by bulky adamantyl substituents that make it less likely to react intermolecularly. As a result, 35 is stable in the presence of air and moisture, and the crystals of 35 melt at 240-241°C. 

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