[α‐(Dimethylamino)benzyllithium–Diethyl Ether]₂, (<i>S</i>)‐α‐(Methylpivaloylamino)benzyllithium – (–)‐Sparteine, and [3‐Iodo‐2‐lithio‐1‐methylindole – 2 Tetrahydrofuran]₂: Crystal Structure Investigations of α‐Lithiated Amines

Abstract

Abstract In order to determine whether α‐metalated (lithiated) amines are carbenoids, as are α‐lithiated halides and ethers, we have determined the crystal structures of the title compounds. In [α‐(dimethylamino)benzyllithium–diethyl ether] 2 ([ 9 · OEt 2 ] 2 ) lithium bridges the anionic carbon and the nitrogen atom which corresponds to the most stable calculated model structure of LiCH 2 NH 2 . The C–N bond is only slightly (≈ 3 pm) elongated (compared to the neutral species). If one of the N ‐methyl groups in [ 9 7dot; OEt 2 ] 2 is replaced by a pivaloyl group as in ( S )‐α‐(methylpivaloylamino)benzyllithium – (–)‐sparteine ( 10 dot; sparteine), a five‐membered ring chelate is formed with Li being bound to the benzylic carbon atom and the oxygen atom of the pivaloyl group. In agreement with model calculations avoidance of the bridged structure as in [ 9 · OEt 2 ] 2 is compensated energetically by the chelate. Again a very small C–N bond elongation (≈ 3 pm) is observed. In [3‐iodo‐2‐lithio‐1‐methylindole – 2 THF] 2 ([ 12 · 2 THF] 2 ) lithium again does not bridge to the indole nitrogen atom because it is a weak donor. The C–N bond length to the anionic C atom is slightly longer (2–3 pm) than in the non‐lithiated compound. Other α‐lithiated amines, which have been published, although not analyzed so far under this particular aspect, show also a marginal C–N bond lengthening. However, since α‐lithiated amines – in contrast to α‐lithiated halides and ethers – until now have not been reported in the literature to react with nucleophiles RLi, there is no need to include them into the group of the above mentioned carbenoids – at least not to date.