Desymmetrization of Prochiral Phosphanes Using Derivatives of (−)‐Cytisine

Abstract

Historically, (−)‐sparteine·sec‐BuLi has been used to desymmetrize prochiral phosphanes. In this report, derivatives of an alkaloid extracted from the seeds of Laburnum anagyroides have been utilized to mimic (+)‐sparteine, which is not readily available. In several cases, the enantioselectivities achieved with the (+)‐sparteine surrogates outperformed (−)‐sparteine itself in the deprotonation of alkyl‐substituted (as well as aryl‐substituted) prochiral phosphane derivatives. In addition, use of these surrogates allows a new methodology for a chiral switch in phosphorus chemistry. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Phosphorus‐Bearing Lithium Compounds in Modern Synthesis

“…Kann et al solved this problem by employing (−)-cytisine derivatives (+)-24 as surrogates of (+)-sparteine (O'Brien's method) [5]. The enantioselective deprotonation of phosphine-boranes 13b,c with [s-BuLi⋅(−)-15] proceeds with similar enantioselectivity to the reaction with (−)-14 but in the opposite sense, leading to the complementary enantiomers (R P )-16 [6] after quench with benzophenone. For the bulkiest substrate 13a, the asymmetric induction achieved by (−)-15a (ee 92%) was considerably higher than with ligands (−)-15b (ee 75%) and (−)-14 (ee 76%).…”

Chiral Bispidines

“…The first homoaldol reactions, such as that of 13 → 14, that proceed under excellent stereocontrol via configurationally stable 17 allyllithium intermediates have been realized recently. 18 The concept found further extension in the stereoselective deprotonation of enantiotopic groups; namely, of methyne groups in meso-epoxides such as 15, 19,20 of methyl groups in prochiral dimethylphosphine boranes like 17, 21,22 and of aromatic protons as in the ferrocene 19. 23 The enantioselective silylation of the orthoethyl benzamide 21 is an example of a (-)-sparteine-mediated dynamic kinetic resolution of a configurationally labile carbanion 24 and does, as such, not rely on a stereoselective deprotonation step.…”

Synthesis of optically active polymers using P-chiral bisphosphines as anionic initiators

“…The phosphorus atom, similar to the carbon atom, can act as a chiral center because the inversion energy of tri-or pentavalent phosphorus compounds is generally much higher than that of the nitrogen analogues [36,37]. The tri-and pentavalent phosphorus atoms are known to adopt the pyramidal structure, and thus, several P-chiral phosphine compounds have been reported so far [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54]. Among such compounds, as an example of stable chiral phosphine compounds, (S,S)-1,2-ethanebis(t-butylmethylphosphineborane) (S,S)-1 [55][56][57] was reported by Imamoto and co-workers as a chiral ligand precursor for asymmetric hydrogenation reactions; this compound can be easily prepared with high enantiomer excess (ee > 99%).…”