Structures of lithium salts of 2,3,3-trimethylindolenine and its 5-methoxy derivative in solution and the solid state.

An X-ray crystal structure of lithium 2,3,3-trimethylindolenide therate shows it to be a disolvated dimer having a 7)-azaallyl-type structure. The structures of the salt in several solvents have been established by studies of I3C chemical shifts, 6Li,15N spinspin splitting, 7Li quadrupole splitting constants, and apparent degrees of aggregation determined by vapor pressure barometry. It is the ?3-azaallyl dimer disolvate in diethyl ether, a tetrasolvated dimer in dioxolane, a mixture of monomer and tetrasolvated dimer in tetrahydrofuran, and a monomer in pyridine. The species are characterized by ’Li quadrupole splitting constants of 230, 156, 180-190 (0.27-0.75 M), and 217 kHz, respectively. In diethyl ether with 4 equiv of hexamethylphosphoric triamide, the salt is a mixture of monomeric and triple ion species. Lithium 5-methoxy-2,3,3-trimethylindolenide forms similar species except that, in tetrahydrofuran, the tendency for dimer formation is enhanced, which leads to a higher proportion of Cto N-methylation in its reaction with methyl chloride in that solvent. In recent years there has been considerable interest in the potential of imine anions, particularly (1-azaallyl)lithium reagents, in organic ~ynthes is .~ Such reagents offer advantages over the corresponding enolates in that they generally react with electrophiles much faster than they undergo proton transfer with the resulting imines so that monoalkylation, for instance, is not complicated by concomitant dialkylation. Furthermore, certain (1 -azaallyl)lithium reagents,” lithioimino and hydrazones9 form the basis of one of the most useful classes of chiral auxiliaries employed in organic synthesis. Imine anions are ambident and undergo reactions a t both their Cand N-termini.lo Frequently, reactions are performed on the lithium salts in ether or tertiary amine solvents, and it is probable that in these solvents the salts, like those of aromatic secondary amines,” are monomeric or dimeric tight ion pairs, the structures of which determine reactivity and regioand stereochemistry. In order to extend our studies1* of the relation of the solution structure of lithium salts involving organic ambident anions to reactivity and regiochemistry to include imine anions, we have chosen the salts la and b since they are readily generated from the corresponding imines by treatment with alkyllithium reagents and have fixed (anti) stereochemi~try.’~ In this paper, we present the results of structural studies of the salts in several solvents as well as an X-ray crystal structure of the etherate of la. In addition, we report some preliminary results that establish that these salts can undergo both (1) The Pennsylvania State University. (2) Brown University. (3) Whitesell, J. K.; Whitesell, M. A. Synthesis 1983, 517 and references (4) Meyers, A. I.; Williams, D. R.; Druelinger, M. J . Am. Chem. SOC. (5) Whitesell, J. K.; Whitesell, M. A. J . Org. Chem. 1977, 42, 377. (6) Hashimoto, S.; Koga, K. Chem. Pharm. Bull. 1979, 27, 2760. (7) Meyers, A. I.; Knaus, G.; Kamata, K.; Ford, M. E. J . Am. Chem. SOC. (8) Schollkopf, U. Pure Appl. Chem. 1983, 55, 1799. (9) Enders, D.; Eichenauer, H. Angew. Chem., I n t . Ed. Engl. 1976, 15, 549. (IO) Heiszwolf, G. J.; Kloosterziel, H. Red . Trau. Chim. Pays-Bas 1970, 89, 1217; Ahlbrecht, H.; Liesching, D. Synfhesis 1976, 746. (1 1) Jackman, L. M.; Scarmoutzos, L. M. J . Am. Chem. SOC. 1987, 109, 5348. (12) (a) Jackman, L. M.; Lange, B. C. J . Am. Chem. Soc. 1981,103,4494. (b) Jackman, L. M.; Dunne, T. S. Ibid. 1985, 107, 2805. (13) In general, C-alkylation can give rise to syn and anti isomeric iminesl4-I6 and N-alkylation to (E)and (Z)-enamines.15J7 (14) Fraser, R.; Banville, J.; Dhawan, K. J . Am. Chem. SOC. 1978, 100, 7999. Frazer, R.; Banville, J. J . Chem. SOC., Chem. Commun. 1979, 47. (15) Knorr, R.; Low, P. J . Am. Chem. SOC. 1980, 102, 3241. (16) (a) Wanat, R. A,; Collum, D. B.; Van Duyne, G.; Clardy, J.; DePue, R. T. J . Am. Chem. SOC. 1986, 108, 3415. (b) Kallman, N.; Collum, D. B. Ibid. 1987, 109, 7466. (17) Lee, J. Y.; Lynch, T. J.; Mao, D. T.; Bergbreiter, D. E.; Newcomb, M. J . Am. Chem. SOC. 1981, 103, 6215, and references cited therein. cited therein.