Stable carbenes, silylenes, germylenes

The nitrogen heteroatoms in imidazole and some closely related heterocycles can stabilize a carbene center at the 2-position (97AG(E)2162). Thus, 1,3-disubstituted imidazole-2-ylidenes (163)-(170), l,3-dimesitylimidazoline-2-ylidene (171), 1,3,4-triphenyl-1H-1,2,4-triazole-5-ylidene (172), and their silylene (173) and germylene (174) analogues are stable (in the absence of oxygen and moisture) solids with definite melting points, which can be recrystallized from appropriate hydrocarbon solvents. The exception is carbene (163) which is an unstable liquid however, it is stable in solution. 

Many crystallographic structures of these carbenes and carbenoids has been solved, indicating (1) they are true carbenes with little ylidic character thus the contribution of ionic structures (175b) is relatively small in comparison with (175a) (2) small N-C-N angles at the carbene center (101-102°) in comparison with typical values (108.5-109.7°) for the corresponding angle in imidazolium salts  

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. 

Saturated five-membered heterocyclic compounds are non-planar, existing in half-chair or envelope conformations. The far-IR spectra of THF and 1,3-dioxolane (176) show both to have barriers of ca. 0.42 kJ mol-1. 

3-Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-1,3-dioxolane (177) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-1,3-dioxolane (178) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by 13C NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane derivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (179) display anti and gauche forms about the exocyclic C(2)-0 bond. 

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Electronic structure of stable carbenes, silylenes and germylenes... 

Whether carbene, germylene, and silylene are justifiable terms for stabilized versions of the reactive species is a very debatable question [28], However, for synthetic chemists, the most important issue is to know whether these stable species feature the reactivity of transient carbenes. Formation of azaphospholidines upon thermolysis (intramolecular carbene insertion into a carbon-hydrogen bond), cyclopropanation reactions, and [l + l]-addition to isocyanides giving keteneimines... 

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