Carbanions 730 VII. Structure and Reactivity of Lewis Acid Complexed Amino Carbanions 731 VIII. Amine Activation by BH 3 and Other Lewis Acids 732 IX. Diastereo-and Enantioselective Transformation 733 X. Future Prospects and Challenges 735 XI. Acknowledgments 735 XII. References and Footnotes 735 Scheme 27 Scheme 28

BF3 complexes of typical benzylic, allylic and saturated N-methyl tertiary amines were a-lithiated, with lithium tetramethylpiperidide (LTMP) or sec-butyllithium, and were treated with electrophiles.This Communication describes the concept of facilitating a-deprotonation of heteroatom compounds by complexation with a Lewis acid, and shows its utility for regioselective formation of C-C bonds in some tertiary amines (Scheme 1). $ Tertiary amines offer a real test of this idea since nitrogen, unlike phosphorus and sulphur, does not provide sufficient t Part of this work was presented at the 17th IUPAC International

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In dimethyl sulphoxide the dianions derived from 2,3or 3,4-dihydroxybenzaldehydes and 4-methylesculetin afford products corresponding to alkylation at the less acidic site while the monoanions give the isomeric phenols.

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Metallation of BF3-pyridine complex with lithium tetramethylpiperidide (LTMP) in ether at -78 "C, followed by reaction with carbonyl compounds, affords a-pyridyl alcohols in good yields. a-Pyridyl alcohols, which are intermediates of some pharmacological interest ,132 are often synthesised from halogen derivatives or N-oxides of pyridine because direct metallation of this heterocycle is deemed impractical.3-5 It was, therefore, of interest to see if Lewis acid complexation can be used for this purpose (Scheme 1).6Our attempts to obtain the alcohol 3a through deprotonation of pyridine with the highly hindered base lithium tetramethylpiperidide (LTMP) furnished only 2,2'-bipyridy17 at -78"C, while no reaction was observed at -120°C. However, treatment of an ether solution? of the BF3-complex 1 with LTMP at -78 "C, followed by addition of benzaldehyde afforded 3a8 in 85% yields.$ The alcohol 3b9 (87%) was t Use of tetrahydrofuran (THF) as solvent gave inferior yields. Also reactions with methyl iodide and benzyl chloride proceeded poorly.

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Reaction of BF3 coordinated N-ethylpyrrolidine with benzophenone, using a preformed complex of s-BuLi and (−)-sparteine for lithiation at −78 °C, affords enantio-enriched R-product (er 85:15). With a warm−cool cycle (−78° → 0°, 2 h, → −78°) prior to electrophile addition the enantioselectivity is reversed. Similarly, in the reaction of BF3-indolizidine complex syn substitution (dr 97:3) occurs but with a warm−cool cycle the syn−anti ratio gets changed to 41:59. These results indicate initial syn lithiation and formation of excess of the anti intermediate on equilibration. DFT calculations at the B3LYP/6-31+G* level show the syn intermediate to be stable in the gas phase, but with the solvent continuum treatment the relative stability shifts toward the anti arrangement and it become more stable in highly polar solvents. These computations also reveal a close contact between Li and boranato fluorine, and its implications for superior effectiveness of BF3 in promoting lithiation of amines are discussed.