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)­C8H5N [R = Bn, R′ = Dipp (HL1); R = Bn, R′ = tBu (HL2); R = CH3OCH2, R′ = Dipp (HL3); Dipp = iPr2C6H3] with Me3SiCH2Li or nBuLi in hydrocarbon solvents (toluene or n-hexane) produced 1,3-disubstituted-2-indolyl lithium complexes [η1:(μ2-η1:η1)-1-Bn-3-(DippNCH)­C8H4NLi]2 (1), {[η1:(μ3-η1:η1:η1)-1-Bn-3-(tBuNCH)­C8H4N]­[η2:η1:(μ2-η1:η1)-1-Bn-3-(tBuNCH)­C8H4N]­[η1:(μ2-η1:η1)-1-Bn-3-(tBuNCH)­C8H4N]­Li3} (2), and [η1:η1:(μ2-η1:η1)-1-CH3OCH2-3-(DippNCH)­C8H4NLi]2 (3), respectively. The bonding modes of the indolyl ligand were kept in 1 by coordination with donor solvent, affording [η1:(μ2-η1:η1)-1-Bn-3-(DippNCH)­C8H4NLi­(THF)]2 (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 [η1:(μ2-η1:η1)-1-Bn-3-(tBuNCH)­C8H4NLi­(THF)]2 (5). X-ray diffraction established that compounds 1, 3, 4, and 5 crystallized as dinuclear structures with the carbanionic sp2 carbon atoms of the indolyl ligands coordinated to lithium ions in a μ2-η1:η1 manner, while compound 2 crystallized as a trinuclear structure and the carbanionic atoms of the indolyl moieties coordinated to lithium ions in μ2-η1:η1 and μ3-η1:η1:η1 manners. When the lithiation reaction of HL1 with 1 equiv of nBuLi was carried out in THF, the monomeric lithium complex {η1:η1-1-Bn-3-(DippNCH)-2-[1′-Bn-3′-(DippNCH)­C8H5N]­C8H4NLi­(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 HL1 with a higher isolated yield. Accordingly, the lithium complexes [η1:η4-1-Bn-3-tBuNCH-2-(1′-Bn-3′-tBuNCHC8H5N)­C8H4NLi­(L)] (L = THF, 7a; L = Et2O, 7b) with the coupled indolyl moieties in η4 mode were isolated by treatment of HL2 with 2 in THF or Et2O. All complexes were characterized by spectroscopic methods, and their structures were determined by X-ray diffraction study.