Chiral Lithiated Allylic α-Sulfonyl Carbanions: Experimental and Computational Study of Their Structure, Configurational Stability, and Enantioselective Synthesis.
X-ray crystal structure analysis of the lithiated allylic α-sulfonyl carbanions [CH2 CHC(Me)SO2 Ph]Li⋅diglyme, [cC6 H8 SO2 tBu]Li⋅PMDETA and [cC7 H10 SO2 tBu]Li⋅PMDETA showed dimeric and monomeric CIPs, having nearly planar anionic C atoms, only OLi bonds, almost planar allylic units with strong CC bond length alternation and the s-trans conformation around C1C2. They adopt a C1S conformation, which is similar to the one generally found for alkyl and aryl substituted α-sulfonyl carbanions. Cryoscopy of [EtCHCHC(Et)SO2 tBu]Li in THF at 164 K revealed an equilibrium between monomers and dimers in a ratio of 83:17, which is similar to the one found by low temperature NMR spectroscopy. According to NMR spectroscopy the lone-pair orbital at C1 strongly interacts with the CC double bond. Low temperature (6) Li,(1) H NOE experiments of [EtCHCHC(Et)SO2 tBu]Li in THF point to an equilibrium between monomeric CIPs having only OLi bonds and CIPs having both OLi and C1Li bonds. Ab initio calculation of [MeCHCHC(Me)SO2 Me]Li⋅(Me2 O)2 gave three isomeric CIPs having the s-trans conformation and three isomeric CIPs having the s-cis conformation around the C1C2 bond. All s-trans isomers are more stable than the s-cis isomers. At all levels of theory the s-trans isomer having OLi and C1Li bonds is the most stable one followed by the isomer which has two OLi bonds. The allylic unit of the C,O,Li isomer shows strong bond length alternation and the C1 atom is in contrast to the O,Li isomer significantly pyramidalized. According to NBO analysis of the s-trans and s-cis isomers, the interaction of the lone pair at C1 with the π* orbital of the CC double bond is energetically much more favorable than that with the "empty" orbitals at the Li atom. The C1S and C1C2 conformations are determined by the stereoelectronic effects nC -σSR * interaction and allylic conjugation. (1) H DNMR spectroscopy of racemic [EtCHCHC(Et)SO2 tBu]Li, [iPrCHCHC(iPr)SO2 tBu]Li and [EtCHC(Me)C(Et)SO2 tBu]Li in [D8 ]THF gave estimated barriers of enantiomerization of ΔG(≠) =13.2 kcal mol(-1) (270 K), 14.2 kcal mol(-1) (291 K) and 14.2 kcal mol(-1) (295 K), respectively. Deprotonation of sulfone (R)-EtCHCHCH(Et)SO2 tBu (94 % ee) with nBuLi in THF at -105 °C occurred with a calculated enantioselectivity of 93 % ee and gave carbanion (M)-[EtCHCHC(Et)SO2 tBu]Li, the deuteration and alkylation of which with CF3 CO2 D and MeOCH2 I, respectively, proceeded with high enantioselectivities. Time-dependent deuteration of the enantioenriched carbanion (M)-[EtCHCHC(Et)SO2 tBu]Li in THF gave a racemization barrier of ΔG(≠) =12.5 kcal mol(-1) (168 K), which translates to a calculated half-time of racemization of t1/2 =12 min at -105 °C.
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen (Germany).|Present address: Philipp Reis Strasse 12, 40215 Düsseldorf (Germany).
Frank
F
Gerhards
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen (Germany).|Present address: Kaiserstrasse 66, 52080 Aachen (Germany).
Nicole
N
Griebel
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen (Germany).
Jan
J
Runsink
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen (Germany).
Gerhard
G
Raabe
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen (Germany). gais@rwth-aachen.de.
Hans-Joachim
HJ
Gais
allyl anion; alpha-sulfonyl carbanion; chiral compounds; lithium; reaction mechanisms; structure elucidation