X-RAY CRYSTAL-STRUCTURE ANALYSIS OF AN OCTAMERIC LITHIUM N-ISOPROPYLBENZAMIDE AZA ENOLATE COMPLEX

Lithiated N-isopropylbenzamide crystallized from a THF solution containing equimolar amounts of TMEDA. The X-ray crystal structure shows the unique solvent-free octameric aggregate [C10H12NOLi]8 (1) of E aza enolate units (Figure 2). Crystal data: monoclinic, space group P21/c, a = 16.242 (7) Å, b = 22.02 (2) Å, c = 23.61 (2) Å, β = 103.98 (4)°, Z = 4. Four of these aza enolate units form four-membered Li, O, C, N, chelate rings along top and bottom edges of (LiO)4 cubes; the other four act as Li, N, C, O bridges between two such cubes, with the Li and O atoms being part of the cubes and the N atom an “external” ligand (Figure 3). NMR measurements (NOE, temperature dependence) and cryoscopy in THF indicate that there are different species in solution. The structure is compared with that of a previously published hexameric aggregate containing three THF molecules. The results are discussed in view of peptide alkylations through polylithiated and LiX-solvated derivatives. A comparison is made between dimeric subunits in these crystal structures and hydrogen-bonded dimers formed from simple amides and occurring in proteins and in DNA. The four-ring chelates present in these complex structures of a lithiated amide, which can also be viewed as an imino carboxylate, are quite different from the polymeric structures of Li carboxylates. © 1990, American Chemical Society. All rights reserved.