Crystal structure, lithium carbenoids

The hypothesis of a particular hybridization of the carbon center in lithium carbenoids has been confirmed by recent X-ray structure analyses performed by Boche and coworkers. To date, the crystal structures of two chloro-substituted and several oxygen-substituted... 

SCHEME 6. Crystal structures of a-oxygen-substituted lithium carbenoids Uthiated benzofuran 8 and carbamate 9... 

This stereochemical outcome of the Fritsch-Buttenberg-Wiechell rearrangement is well compatible with the crystal structure of the carbenoid 3 (Figure 1, Scheme 4). The aryl moiety trans to the vinylic chlorine atom is bent towards Cl (C1-C2-C9 116.5°). Thus, migration of the fraw -aryl group with simultaneous liberation of lithium chloride becomes evident. The free vinylidene carbene can be ruled out as the intermediate. 

The crystal structure of a dimer of 4-i-butyl-2-lithiothiazole incorporating two molecules of diglyme has been determined by X-ray analysis (95AG(E)487). The structure features the lithium atom positioned halfway between nitrogen and C-2, thus providing a carbenoid nature to this metallated thiazole. 

Three-membered ring carbenoids have been structurally characterized for species where X = OR or NR2. The crystal structure of the diethyl ether-solvated a-(dimethylamino)-benzyllithium dimer 2 shows crystallographically equivalent three-membered Li-C-N rings, with (Lij-C) = 2.475(6) A, and . As predicted by theory, the anionic carbon sits closer to the lithium in the other three-membered ring (,7(LiiA-C) = 2.230(7) A) than to Lij. This seems to present a means for the anionic carbon to delocalize its charge. 

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