Enantioselective Synthesis, Configurational Stability, and Reactivity of Lithium α-tert-Butylsulfonyl Carbanion Salts

The reactions of enantiopure S‐tert‐butyl sulfones of the type R1CH(R2)SO2tBu (≥99 % ee) with lithiumorganyl compounds gave the corresponding chiral α‐sulfonyl carbanion salts [R1C(R2)SO2tBu]Li with ≥94 % ee. The enantioselectivity of the deprotonation of the phenyl‐ but not dialkyl‐substituted sulfones is strongly dependent on the nature of the lithiumorganyl. Because of this observation and the strong decrease in enantioselectivity in the presence of TMEDA and HMPA, we propose an intramolecular proton transfer following complexation of the sulfone by RLi. Racemization of [R1C(R2)SO2tBu]Li follows first‐order kinetics and seems to be mainly an enthalpic process with a small negative activation entropy, as revealed by polarimetric measurements at low temperatures. This is in accordance with Cα–S bond rotation as the rate‐determining step. The salts [R1C(R2)SO2tBu]Li have half‐lives of racemization in the order of several hours at –105 °C. The deuteriation of the salts at –105 °C with CF3CO2D proceeded with enantioselectivities of ≥94 % ee, the magnitude of which was not significantly affected by the presence of TMEDA and HMPA. The salts also reacted with carbon‐based electrophiles at low temperatures with high enantioselectivity. The conversion of R1CH(R2)SO2tBu via [R1C(R2)SO2tBu]Li to R1C(R2,E)SO2tBu, which involves the loss of stereogenicity at the α‐stereogenic center and its re‐establishment upon reaction of the chiral carbanion with electrophiles, occurred with high overall enantioselectivity. Electrophiles attack the anionic C atom of [R1C(R2)SO2tBu]Li with high selectivity on the side syn to the O atoms and anti to the tert‐butyl group. The reactivity of the dialkyl‐substituted salts [R1C(R2)SO2tBu]Li (R1, R2 = alkyl) is significantly higher than that of the benzylic salts [RC(Ph)SO2tBu]Li (R = alkyl) and the HMPA‐coordinated SIPs of [MeC(Ph)SO2tBu]Li are significantly more reactive towards EtI than the corresponding O–Li contact ion pairs. Deprotonation of chiral S‐tert‐butyl sulfones with tBuLi gives enantiomerically highly enriched lithium α‐tert‐butylsulfonyl carbanion salts that are configurationally stable at –105 °C in the timescale of their synthesis and reactions with electrophiles. They react at low temperatures with electrophiles with high enantioselectivity.