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dem verehrten Kollegen und Freund, zum 60. Geburtstag gewidmet (12.VI.85)On the Interaction between Lithium Enolates and Secondary Amines in Solution and in the Crystal When lithium derivatives generated by lithium diisopropylamide (LDA) are quenched with a D+ source (large excess of ROD), there is frequently only partial deuterium incorporation (non-stoichiometric effect, Scheme I ) . Experiments with tert-butyl 2-methylpropionate (1) and its deuterated derivative (2-D)-1 and LDA indicate that the lack of deuteration is not caused by an isotope effect (Scheme 2). A H-bonded complex between the amine (R,NH) and the Li-derivative was envisioned as being responsible for the observed effects. Some crystalline Li-enolate solvates with secondary amines were isolated, and gave rise to only partially deuterated products upon quenching with AcOD/CD30D ( Table I ) . Enolates containing the Li-chelating diamine N,N,N -trimethylethylenediamine (TriMEDA) turned out to be especially suitable for the isolation of single crystals. Thus, two X-ray crystal-structure analyses were undertaken to obtain detailed information about the geometry and bonding in such complexes. The structures of the dimeric N,N-dimethyl-propionaniide lithium (2)-enolate ([2 (2)-4.2 TriMEDA]; R value 0,049, see Eqn. 2, Figs. 2.4 and 5 ) and of the dimeric pinacoloue lithiumenolate ([2 6.2 TriMEDA]; R value 0,082, see Eqn.3 and Figs.3, and 6) were determined. In both centrosymmetric dimeric structures there is a Li-0-Li-0 four-membered ring anda H-bridge between TriMEDA and the enolate moiety. In the case of the amide enolate (2)-4 .TriMEDA the bridge points from the amine N-atom to the pyramidalized, enatnine-type N-atom of

The crystal structure of the cumyl cation (1; 2-phenyl-2-propyl cation) has been determined at -124°C (as hexafluoroantimonate, 1‚SbF 6 ; R 1 ) 0.0502, wR 2 ) 0.1054). The cation 1 is nearly planar and has a short C + -C ipso bond (1.41(2) Å) and bond lengths in the phenyl ring which agree with strong benzylic delocalization. The weak but distinct shortening of the C + -CH 3 bonds by 0.025(12) Å indicates weak C-H hyperconjugation. Nearly all H atoms are involved in H‚‚‚F contacts to SbF 6 anions, and one close C + ‚‚‚F contact (3.11(2) Å) is observed. The phenyl rings form infinite stacks and are shifted against each other in the stack.

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Phenylthiomethyllithium/N,N,N′,N′‐tetramethylethylenediamine (TMEDA) (1:1) is shown by X‐ray analysis to be dimeric in the crystalline state. The structural unit contains a six‐membered ring in which each Li is bonded to the terminal C atom of one thioanisole and to the S atom of the other, as well as to the two N atoms of a TMEDA. Methylthiomethyllithium/TMEDA (1:1) is also dimeric, but the structural unit contains a four‐membered ring in which each Li is bonded to two C atoms and to the two TMEDA N atoms. An error has been detected in an earlier published electron‐density difference map for 2‐lithio‐2‐phenyl‐1, 3‐dithiane/TMEDA/THF (1:1:1). The conclusion drawn then that this complex exists as a tight ion‐pair in the crystal is now retracted.