[ALPHA-(DIMETHYLAMINO)BENZYLLITHIUM DIETH YL ETHER]2, -(METHYLPIVALOYLAMINO)BENZYLLITHIUM-(-)-SPARTEINE, AND [3-IODO-2-LITHIO-1-METHYLINDOLE-2 TETRAHYDROFURAN]2 - CRYSTAL-STRUCTURE INVESTIGATIONS OF ALPHA-LITHIATED AMINES

In order to determine whether alpha-metalated (lithiated) amines are c arbenoids, as are alpha-lithiated halides and ethers, we have determin ed the crystal structures of the title compounds. In [alpha-(dimethyla mino)benzyllithium-diethyl ether]2 ([9.OEt2]2) lithium bridges the ani onic carbon and the nitrogen atom which corresponds to the most stable calculated model structure of LiCH2NH2. The C-N bond is only slightly (almost-equal-to 3 pm) elongated (compared to the neutral species). I f one of the N-methyl groups in [9.OEt2]2 is replaced by a pivaloyl gr oup as in a-(methylpivaloylamino)benzyllithium-(-)-sparteine (10 . spa rteine), a five-membered ring chelate is formed with Li being bound to the benzylic carbon atom and the oxygen atom of the pivaloyl group. I n agreement with model calculations avoidance of the bridged structure as in [9 . OEt2]2 is compensated energetically by the chelate. Again a very small C-N bond elongation (almost-equal-to 3 pm) is observed. I n [3-iodo-2-lithio-1-methylindole-2 THF]2 ([12 . 2 THF]2) lithium agai n does not bridge to the indole nitrogen atom because it is a weak don or. The C-N bond length to the anionic C atom is slightly longer (2-3 pm) than in the non-lithiated compound. Other alpha-lithiated amines, which have been published, although not analyzed so far under this par ticular aspect, show also a marginal C-N bond lengthening. However, si nce alpha-lithiated amines - in contrast to alpha-lithiated halides an d ethers until now have not been reported in the literature to react w ith nucleophiles RLi, there is no need to include them into the group of the above mentioned carbenoids - at least not to date.