The Structure of a Chiral Lithium Amidocuprate in Solution Determined by Multinuclear NMR Spectroscopy

Structural Studies on a Lithium Organo‐Amidocuprate in the Solid State and in Solution

“…[4][5][6] More recently, Davidsson and co-workers confirmed, using NMR spectroscopy, the existence of such a dimer for the heterocuprate with R = nBu and L = (R)-N-methyl-1-phenyl-2-(1-pyrrolidinyl)ethanamine. [7] However, monomeric aggregates such as 2 (Scheme 1, M = Cu) have also been suggested to play a role in the reaction of heterocuprates with enones, particularly in polar solvents. [8] Calculations by Yamanaka and Nakamura have shown the importance of the trans effect and strong Li À L bonds for the nontransferability of the heteroatom groups in these reagents.…”

“…Although the solid-state structure of 3 provides some valuable insights into the bonding within organo-amidocuprates, virtually all cuprate addition reactions are carried out in solution, and hence elucidation of the solution structures is crucial to understanding the reaction mechanisms and selectivity of these reagents. 7 Li NMR spectroscopic studies on a [D 8 ]toluene solution of 3 revealed no less than five main lithium environments (corresponding to signals A-E in Figure 2), indicative of at least a partial break up of the dimeric form 3 and the presence of Schlenk-type equilibrium. Cyroscopic relative molecular mass (RMM) measurements on 3 in benzene (0.19 m solution) gave an association value of n = 1.8 for [CuLiMes{N(CH 2 Ph) 2 }] n , suggesting a predominance of dimeric aggregates.…”

“…[16] The largest peak in the 7 Li NMR spectrum, signal C (d = À5.79 ppm), shows strong NOE cross-peaks with mesityl CH 3 , dibenzylamido CH 2 , and aromatic ArH protons. Consequently, Li C must lie in close proximity to both mesityl and amido groups, and can therefore be assigned as the head-totail dimer 3 (Scheme 2), which is analogous to the solid-state structure.…”

“…We tentatively assign this peak to the mixed homocuprate species 5, in which, because of geometric restraints, the Li + ions are h 1 -coordinated to the mesityl group and hence more downfield than the h 6 -coordinated Li centers in 3. Although an alternative assignment would perhaps be a monomeric species similar to 2, this possibility can be [19] in either the 1 H or 7 Li NMR spectra, signifying that 3 does not disassemble back into its monometallic precursors.…”

“…In addition, DFT calculations have been carried out, which predict this head-to-tail dimer to be favored over other possible structural isomers for a simple model system. However, NMR studies on [Cu 2 Li 2 Mes 2 {N(CH 2 Ph) 2 } 2 ] have shown a mixture of structural isomers to be present in toluene solution as a result of Schlenk equilibrium, and these structural isomers have been identified with the aid of modified PFG inverse-detected 1 H- 7 Li HOESY NMR spectroscopy. To our knowledge, this is the first time such structural isomers have been identified in solution.…”