Structure lithium carbenoids

The particular substitution pattern of lithium carbenoids, the fact that both an electropositive metal and an electronegative substituent X are bound to the same carbon atom, causes the ambiphilic character of this species. The chameleon-like reactivity becomes evident from the resonance formulas of the carbenoid lb (equation 1) Whereas the carbanionic character is expressed by the resonance formula la, the electrophilic character is represented by Ic. In an analogous way, the reactivity of vinylidene carbenoids 2b is expressed by the mesomeric structures 2a and 2c. 

The deshielding of the resonance in lithium carbenoids is explained by the polarization of the elongated C—X bond and shows that the carbocationic structures Ic and 2c have to be taken into account. The postulated high s character of the C—Li bond is also confirmed by the NMR-spectroscopic investigations insofar as enhanced C- Li coupling constants have been observed in lithium carbenoids . Thus, the effect of the metalation that leads to the formation of lithium carbenoids can be summarized as follows (Scheme 3) . 

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... 

The chemistry of lithium carbenoids, organohthium compounds with an electronegative heteroatom in the a-position, has been developed in two respects during the last two decades. First, the ambiphilic character of these partly short-lived and mostly thermally sensitive compounds is well understood today, due to structure determinations, spectroscopic investigations and theoretical calculations. Second, many of the lithium carbenoids, formerly considered as exotic species, have developed into useful reagents that proved to be particularly fruitful in stereoselective syntheses. 

The stereochemically defined lithium carbe-noid 11 forms the bicyclus 13 by intramolecular cyclopropanation even at — 110 In contrast, no conversion of the epimeric lithium carbenoid 14 into diastereomeric bicyclus 16 is observed under similar conditions. These results are explained on the basis of the transition state structures 12 and 15a/b. Structure 12 allows complexation of the lithium atom by ether oxygen. This leads to activation of the carbenoid and accelerates the cyclopropanation (11 - 12— 13) by a transition state choreography of type 10. In structure 15a the ether group is in equatorial position, which... 

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. 

Recent calculations on unsolvated a-methoxyvinyllithium (8) confirmed the structural characteristics of C(Li)0 carbenoid organolithium compounds the Ca-0 bond, bridged by lithium, was computed to be elongated and agrees well with the X-ray structural features of polymeric, unsolvated Qf-ethoxyvinyllithium. ... 

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