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Experimental and Theoretical Investigation of the Enantiomerization of Lithium α‐ tert ‐Butylsulfonyl Carbanion Salts and the Determination of Their Structures in Solution and in the Crystal
Author(s) -
Scholz Roland,
Hellmann Gunther,
Rohs Susanne,
Raabe Gerhard,
Runsink Jan,
Özdemir Diana,
Luche Olaf,
Heß Thomas,
Giesen Alexander W.,
Atodiresei Juliana,
Lindner Hans J.,
Gais HansJoachim
Publication year - 2010
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201000409
Subject(s) - chemistry , steric effects , carbanion , lone pair , crystal structure , stereochemistry , crystallography , nuclear magnetic resonance spectroscopy , lithium (medication) , monomer , medicinal chemistry , molecule , organic chemistry , medicine , endocrinology , polymer
Dynamic NMR (DNMR) spectroscopy of [R 1 C(R 2 )SO 2 R 3 ]Li (R 1 , R 2 = alkyl, phenyl; R 3 = Ph, t Bu, adamantyl, CEt 3 ) in [D 8 ]THF has shown that the S ‐ t Bu, S ‐adamantyl, and S ‐CEt 3 derivatives have a significantly higher enantiomerization barrier than their S ‐Ph analogues. C α –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 α atom. Ab initio calculations on [Me(Ph)SO 2 t Bu] – gave information about the two C α –S rotational barriers, which are dominated by steric effects. Cryoscopy of [R 1 C(R 2 )SO 2 t Bu]Li in THF at –108 °C revealed the existence of monomers and dimers. X‐ray crystal structure analysis of the monomers and dimers of [R 1 C(R 2 )SO 2 t Bu]Li · L n (R 1 = Me, Et, t BuCH 2 , PhCH 2 , t Bu; R 2 = Ph, L = THF, 12‐crown‐4, PMDTA) and [R 1 C(R 2 )SO 2 Ph]Li · 2diglyme [R 1 = R 2 = Me, Et; R 1 –R 2 = (CH 2 ) 5 ] showed them to be O–Li contact ion pairs (CIPs). The monomers and dimers have a C α –S conformation in which the lone‐pair orbital at the C α atom bisects the O–S–O angle and a significantly shortened C α –S bond. The C α atom of [R 1 C(R 2 )SO 2 R 3 ]Li · L n (R 1 = Ph; R 3 = Ph, t Bu) is planar, whereas the C α atom of [R 1 C(R 2 )SO 2 R 3 ]Li · L n (R 1 = R 2 = alkyl) is strongly pyramidalized in the case of R 3 = Ph and most likely planar for R 3 = t Bu. Ab initio calculations on [MeC(Me)SO 2 R] – gave a pyramidalized C α atom for R = Me and a nearly planar one for R = CF 3 and t Bu. The [R 1 C(R 2 )SO 2 t Bu]Li salts were characterized by 1 H, 13 C, and 6 Li NMR spectroscopy. 1 H{ 1 H} and 6 Li{ 1 H} NOE experiments are in accordance with the existence of O–Li CIPs. 1 H and 13 C NMR spectroscopy of [R 1 C(R 2 )SO 2 t Bu]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 7 Li NMR spectroscopy, the addition of HMPA to [MeC(Ph)SO 2 t Bu]Li in [D 8 ]THF at low temperatures causes the formation of the separated ion pair [MeC(Ph)SO 2 t Bu]Li(HMPA) 4 .