Reactivity of 1,3-Disubstituted Indoles with Lithium Compounds: Substituents and Solvents Effects on Coordination and Reactivity of Resulting 1,3-Disubstituted-2-Indolyl Lithium Complexes.

Reactivity of 1,3-disubstituted indolyl compounds with lithium reagents was studied to reveal the substituents and solvent effects on coordination modes and reactivities resulting in different indolyl lithium complexes. Treatment of 1-alkyl-3-imino functionalized compounds 1-R-3-(R'N═CH)CHN [R = Bn, R' = Dipp (HL); R = Bn, R' = Bu (HL); R = CHOCH, R' = Dipp (HL); Dipp = PrCH] with MeSiCHLi or BuLi in hydrocarbon solvents (toluene or n-hexane) produced 1,3-disubstituted-2-indolyl lithium complexes [η:(μ-η:η)-1-Bn-3-(DippN═CH)CHNLi] (1), {[η:(μ-η:η:η)-1-Bn-3-(BuN═CH)CHN][η:η:(μ-η:η)-1-Bn-3-(BuN═CH)CHN][η:(μ-η:η)-1-Bn-3-(BuN═CH)CHN]Li} (2), and [η:η:(μ-η:η)-1-CHOCH-3-(DippN═CH)CHNLi] (3), respectively. The bonding modes of the indolyl ligand were kept in 1 by coordination with donor solvent, affording [η:(μ-η:η)-1-Bn-3-(DippN═CH)CHNLi(THF)] (4). The trinuclear complex 2 was converted to dinuclear form with a change of bonding modes of the indolyl ligand by treatment of 2 with donor solvent THF, producing [η:(μ-η:η)-1-Bn-3-(BuN═CH)CHNLi(THF)] (5). X-ray diffraction established that compounds 1, 3, 4, and 5 crystallized as dinuclear structures with the carbanionic sp carbon atoms of the indolyl ligands coordinated to lithium ions in a μ-η:η manner, while compound 2 crystallized as a trinuclear structure and the carbanionic atoms of the indolyl moieties coordinated to lithium ions in μ-η:η and μ-η:η:η manners. When the lithiation reaction of HL with 1 equiv of BuLi was carried out in THF, the monomeric lithium complex {η:η-1-Bn-3-(DippN═CH)-2-[1'-Bn-3'-(DippNCH)CHN]CHNLi(THF)} (6) having coupled indolyl moieties was obtained. The compound 6 can also be prepared by the reaction of 1 with 0.5 equiv of HL with a higher isolated yield. Accordingly, the lithium complexes [η:η-1-Bn-3-BuN═CH-2-(1'-Bn-3'-BuNCHCHN)CHNLi(L)] (L = THF, 7a; L = EtO, 7b) with the coupled indolyl moieties in η mode were isolated by treatment of HL with 2 in THF or EtO. All complexes were characterized by spectroscopic methods, and their structures were determined by X-ray diffraction study.