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

Rocco I. Papasergio, Brian W. Skelton, Paul Twiss, Allan H. White, Colin L. Raston

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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(SiMe 3)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.
Original languageEnglish
Pages (from-to)1161-1172
Number of pages12
JournalJournal of the Chemical Society, Dalton Transactions
Issue number4
DOIs
Publication statusPublished - Apr 1990
Externally publishedYes

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