Chemistry - A European Journal p. 17904 - 17920 (2015)
Update date:2022-07-30
Topics:
Gerhards, Frank
Griebel, Nicole
Runsink, Jan
Raabe, Gerhard
Gais, Hans-Joachim
X-ray crystal structure analysis of the lithiated allylic α-sulfonyl carbanions [CH2=CHC(Me)SO2Ph]Li-diglyme, [cC6H8SO2tBu]Li-PMDETA and [cC7H10SO2tBu]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)SO2tBu]Li in THF at 164K 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 6Li,1H NOE experiments of [EtCH=CHC(Et)SO2tBu]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)SO2Me]Li-(Me2O)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. 1H DNMR spectroscopy of racemic [EtCH=CHC(Et)SO2tBu]Li, [iPrCH=CHC(iPr)SO2tBu]Li and [EtCH=C(Me)C(Et)SO2tBu]Li in [D8]THF gave estimated barriers of enantiomerization of ΔG≠=13.2kcal mol-1 (270K), 14.2kcal mol-1 (291K) and 14.2kcal mol-1 (295K), respectively. Deprotonation of sulfone (R)-EtCH=CHCH(Et)SO2tBu (94 % ee) with nBuLi in THF at -105 °C occurred with a calculated enantioselectivity of 93 % ee and gave carbanion (M)-[EtCH=CHC(Et)SO2tBu]Li, the deuteration and alkylation of which with CF3CO2D and MeOCH2I, respectively, proceeded with high enantioselectivities. Time-dependent deuteration of the enantioenriched carbanion (M)-[EtCH=CHC(Et)SO2tBu]Li in THF gave a racemization barrier of ΔG≠=12.5kcal mol-1 (168K), which translates to a calculated half-time of racemization of t1/2=12min at -105 °C.
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