Journal Article

PreJuSER-9771

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Experimental and Theoretical Study of the Enantiomerization Barrier of a-Sulfonyl Carbanions and Determination of the Structure of Lithium a-tert-blutylsulfonyl Carbanion Salts in Solution and the Crystal

Scholz, R. ; Hellmann, G. ; Rohs, S.FZJ* ; Raabe, G. ; Raabe, G. ; Runsink, J. ; Özdemir, D. ; Luche, O. ; Heß, T. ; Giesen, A. W. ; Atodiresei, J. ; Lindner, H. J. ; Gais, H.-J.

2010
Wiley-VCH Verl. Weinheim

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Please use a persistent id in citations: doi:10.1002/ejoc.201000409

Abstract: Dynamic NMR (DNMR) spectroscopy of [(RC)-C-1(R-2)SO2R3]Li (R-1, R-2 = alkyl, phenyl; R-3 = Ph, tBu, adamantyl, CEt3) in [D-8]THF has shown that the S-tBu, S-adamantyl, and S-CEt3 derivatives have a significantly higher enantiomerization barrier than their S-Ph analogues. C-alpha-S bond rotation is most likely the rate-determining step of the enantiomerization of the salts bearing a bulky group at the S atom and two substituents at the C-alpha, atom. Ab initio calculations on [Me(Ph)SO(2)tBu](-) gave information about the two C-alpha-S rotational barriers, which are dominated by steric effects. Cryoscopy of [(RC)-C-1(R-2)SO(2)tBu]Li in THF at -108 degrees C revealed the existence of monomers and dimers. X-ray crystal structure analysis of the monomers and dimers of [(RC)-C-1(R-2)SO(2)tBu]Li center dot L-n (R-1 = Me, Et, tBuCH(2), PhCH2, tBu; R-2 = Ph, L = THF, 12-crown-4, PMDTA) and [(RC)-C-1(R-2)SO2Ph]Li center dot 2diglyme [R-1 = R-2 = Me, Et; R-1-R-2 = (CH2)(5)] showed them to be O-Li contact ion pairs (CIPs). The monomers and dimers have a C-alpha-S conformation in which the lone-pair orbital at the C-alpha atom bisects the O-S-O angle and a significantly shortened C -S bond. The C-alpha atom of [(RC)-C-1(R-2)SO2R3]center dot L-n (R-1 = Ph; R-3 = Ph, tBu) is planar, whereas the C-alpha atom of [(RC)-C-1(R-2)SO2R3]Li center dot I., (R-1 = R-2 = alkyl) is strongly pyramidalized in the case of R-3 = Ph and most likely planar for R-3 = tBu. Ab initio calculations on [MeC(Me)SO2R](-) gave a pyramidalized C-alpha atom for R = Me and a nearly planar one for R = CF3 and tBu. The [(RC)-C-1(R-2)SO(2)tBu]Li salts were characterized by H-1, C-13, and Li-6 NMR spectroscopy.H-1{H-1} and Li-6{H-1} NOE experiments are in accordance with the existence of O-Li CIPs. H-1 and C-13 NMR spectroscopy of [(RC)-C-1(R-2)SO(2)tBu]Li in [D-8]THF at low temperatures showed equilibrium mixtures of up to five different species being most likely monomeric and dimeric O-Li CIPs with different configurations. According to Li-7 NMR spectroscopy, the addition of HMPA to [MeC(Ph)SO(2)tBu]Li in [D8ITHF at low temperatures causes the formation of the separated ion pair [MeC(Ph)SO(2)tBu]Li(HMPA)(4).

Keyword(s): J ; Carbanions (auto) ; Chirality (auto) ; Enantiomerization (auto) ; Ab initio calculations (auto) ; Cryoscopy (auto)

Note: This research was supported by the Deutsche Forschungsgemeinschaft (DFG), the Volkswagen Foundation, and Fonds der Chem'schen Industrie. We thank Professor Dr. H. Fritz and Dr. D. Hunkler for NMR measurements, Professor Dr. W. Bauer for cryoscopy measurements and valuable experimental advice, Dr. E. Keller for a version of SCHAKAL, and C. Vermeeren and D. Wolters for the preparation of the graphics. We are grateful to Professor Dr. H. J. Reich for his assistance in the analysis of the NMR spectra of the salt rac-9 in the presence of HMPA and for information concerning his unpublished results of a NMR spectroscopic study on the behaviour of lithium S-phenylsulfonyl carbanion salts towards H M PA.

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Research Program(s):

1. Atmosphäre und Klima (P23)

Appears in the scientific report 2010

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