52208-98-7Relevant articles and documents
Chiral fluorinated α-sulfonyl carbanions: Enantioselective synthesis and electrophilic capture, racemization dynamics, and structure
Hellmann, Gunther,Hack, Achim,Thiemermann, Eric,Luche, Olaf,Raabe, Gerhard,Gais, Hans-Joachim
, p. 3869 - 3897 (2013/04/10)
Enantiomerically pure triflones R1CH(R2)SO 2CF3 have been synthesized starting from the corresponding chiral alcohols via thiols and trifluoromethylsulfanes. Key steps of the syntheses of the sulfanes are the photochemical trifluoromethylation of the thiols with CF3Hal (Hal=halide) or substitution of alkoxyphosphinediamines with CF3SSCF3. The deprotonation of RCH(Me)SO2CF3 (R=CH2Ph, iHex) with nBuLi with the formation of salts [RC(Me)-SO2CF3]Li and their electrophilic capture both occurred with high enantioselectivities. Displacement of the SO2CF3 group of (S)-MeOCH2C(Me)(CH 2Ph)SO2CF3 (95 % ee) by an ethyl group through the reaction with AlEt3 gave alkane MeOCH2C(Me)(CH 2Ph)Et of 96 % ee. Racemization of salts [R1C(R 2)SO2CF3]Li follows first-order kinetics and is mainly an enthalpic process with small negative activation entropy as revealed by polarimetry and dynamic NMR (DNMR) spectroscopy. This is in accordance with a Cα-S bond rotation as the rate-determining step. Lithium α-(S)-trifluoromethyl- and α-(S)-nonafluorobutylsulfonyl carbanion salts have a much higher racemization barrier than the corresponding α-(S)-tert-butylsulfonyl carbanion salts. Whereas [PhCH 2C(Me)SO2tBu]Li/DMPU (DMPU = dimethylpropylurea) has a half-life of racemization at -105 °C of 2.4 h, that of [PhCH 2C(Me)SO2CF3]Li at -78 °C is 30 d. DNMR spectroscopy of amides (PhCH2)2NSO2CF 3 and (PhCH2)N(Ph)SO2CF3 gave N-S rotational barriers that seem to be distinctly higher than those of nonfluorinated sulfonamides. NMR spectroscopy of [PhCH2C(Ph)SO 2R]M (M=Li, K, NBu4; R=CF3, tBu) shows for both salts a confinement of the negative charge mainly to the Cα atom and a significant benzylic stabilization that is weaker in the trifluoromethylsulfonyl carbanion. According to crystal structure analyses, the carbanions of salts {[PhCH2C(Ph)SO2CF3] Li×L}2 (L=2 THF, tetramethylethylenediamine (TMEDA)) and [PhCH2C(Ph)SO2CF3]NBu4 have the typical chiral Cα-S conformation of α-sulfonyl carbanions, planar Cα atoms, and short Cα-S bonds. Ab initio calculations of [MeC(Ph)SO2tBu]- and [MeC(Ph)SO2CF3]- showed for the fluorinated carbanion stronger nC→σ* S-CF 3 and n O→σ* S-CF 3 interactions and a weaker benzylic stabilization. According to natural bond orbital (NBO) calculations of [R 1C(R2)SO2R]- (R=tBu, CF3) the nC→σS-R interaction is much stronger for R=CF3. Ab initio calculations gave for [MeC(Ph)SO2tBu] Li×2 Me2O an O,Li,Cα contact ion pair (CIP) and for [MeC(Ph)SO2CF3]Li×2 Me2O an O,Li,O CIP. According to cryoscopy, [PhCH2C(Ph)SO2CF 3]Li, [iHexC(Me)SO2CF3]Li, and [PhCH 2C(Ph)SO2CF3]NBu4 predominantly form monomers in tetrahydrofuran (THF) at -108 °C. The NMR spectroscopic data of salts [R1(R2)SO2R3]Li (R 3=tBu, CF3) indicate that the dominating monomeric CIPs are devoid of Cα-Li bonds. Worth their salt: Chiral Li α-(S)-trifluoromethylsulfonyl carbanion salts with ≥98 % enantiomeric excess (ee) and high configurational stability are accessible through the reaction of chiral triflones (≥98 % ee) with RLi (see figure). Their electrophilic capture occurs with high enantioselectivity. Studies of the structure and dynamics of chiral Li α-(S)-trifluoromethylsulfonyl carbanion salts revealed monomeric contact ion pairs. Copyright
