Lithiation of 2-Me3SiCHRC5H4N(R = H or SiMe3): influence of solvent on the nature of the product (from X-ray structure determinations) and asymmetric induction. A note on the lithiation of some analogous 3- and 4-methylpyridines

Abstract

Metallation of 2-Me₃SiCHRC₅H₄N (R = H or SiMe₃) using LiBuⁿ in hexane, and in the presence of diethyl ether, and the ditertiary amines tmen (N,N,N′,N′-tetramethylethylenediamine) or sp [(–)sparteine] yields crystalline lithium complexes. These have been characterized using X-ray diffraction data for [Li{2-C(SiMe₃)₂C₅H₄N}{2-CH(SiMe₃)₂C₅H₄N}](1), [{Li[2-C(SiMe₃)₂C₅H₄N]}₂](2), [Li(tmen){2-C(SiMe₃)₂C₅H₄N}](3), [{Li(OEt₂)[2-CH(SiMe₃)C₅H₄N]}₂](4), [{Li(tmen)[2-CH(SiMe₃)C₅H₄N]}₂](5), and [Li(sp){2-CH(SiMe₃)C₅H₄N}](6). In (1), (3), (4), and (6) the hydrocarbyl group acts as an η³-aza-allyl through the ring nitrogen [2.00(1), 1.96(2), 2.19(1), and 1.985(6)Å], ipso-carbon [2.22(1), 2.43(2), 2.356(9), and 2.323(6)Å], and adjacent ring carbon [2.32(1), 2.46(2), 2.34(1), and 2.353(6)Å]; in (4) the nitrogen centre also bridges to another metal centre [2.041(1)Å]. Complex (2) has two co-ordinated lithium centres, each lithium bound by the ipso-carbon [2.213(7)Å] and a nitrogen centre from a centrosymmetric ligand [1.936(6)Å] and is thus an alkylmetal species; it can be recrystallized from tetrahydrofuran even though the lithiums are co-ordinatively unsaturated. In complex (5), the anionic ligand acts as an amide bridging two metal centres through the ring N [Li–N 2.08(1), 2.25(1)Å, N–Li–N 103.6(7), 93.0(9)°]. Treating (6) with MeI in hexane at –78 °C yields 2-[(1-trimethylsilyl)ethyl]pyridine of 20% estimated optical purity. The isomeric complex [Li(sp){4-CH(SiMe₃)C₅H₄N}] under the same conditions gave no asymmetric induction; other metallations of 4-Me₃SiCH₂C₅H₄N, and some of 3-Me₃SiCH₂C₅H₄N are also reported.