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

Abstract

The reactions of enantiopure S-tert-butyl sulfones of the type R 1 CH(R 2 )SO 2 tBu (Ն99 % ee) with lithiumorganyl compounds gave the corresponding chiral α-sulfonyl carbanion salts [R 1 C(R 2 )SO 2 tBu]Li with Ն94 % 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 [R 1 C(R 2 )-SO 2 tBu]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 C α -S bond rotation as the rate-determining step. The salts [R 1 C(R 2 )SO 2 tBu]Li have half-lives of racemization in the order of several hours at -105°C. The deuteriation of the salts at -105°C with

Chiral Fluorinated α‐Sulfonyl Carbanions: Enantioselective Synthesis and Electrophilic Capture, Racemization Dynamics, and Structure

“…According to natural bond orbital (NBO) calculations of [R 1 C(R 2 )SO 2 R] À (R = tBu, CF 3 ) the n C !s* SÀR interaction is much stronger for R = CF 3 3 ]-A C H T U N G T R E N N U N G NBu 4 predominantly form monomers in tetrahydrofuran (THF) at À108 8C. [14] Results and Discussion Synthesis of achiral and racemic (S)-trifluoromethyl-and (S)-nonafluorobutylsulfones: Because of our previous studies of the (S)-tert-butylsulfonyl carbanion salts Ia-d, [3,4] we selected the (S)-trifluoromethylsulfonyl carbanion salts 1, 2, 3 a-d, 4, and 5 ( Figure 2) for the determination of 1) the configurational stability of salts of type II and its dependence on the counterion and substituents at the C a atom, and 2) the study of the enantioselective synthesis and electrophilic capture of II. Keywords: ab initio calculations · carbanions · chirality · racemization · sulfone meric O À Li contact ion pairs (CIPs) in solution in tetrahydrofuran (THF) and in the crystal phase.…”

“…For the nonafluorobutylsulfonamide (PhCH 2 ) 2 NSO 2 C 4 F 9 and heptadecafluorooctylsulfonamide Et 2 NSO 2 C 8 F 17 , DNMR spectroscopy had yielded activation barriers for N À S rotation of DG°r ot = (14.9 AE 0.8) kcal mol À1 (298 K, CDCl 3 ) [36c] and DG°r ot = 15.3 kcal mol À1 (309 K, (CD 3 ) 2 CO), [36d] respectively. [4] Salt (M)-4 was selected for the polarimetric experiments at low temperatures. Unfortunately, information about the N À S rotational barriers of the corresponding (S)-alkyl-A C H T U N G T R E N N U N G (aryl)sulfonamides and (S)-tert-butylsulfonamides are lacking.…”

“…[4] Salt (M)-4 was prepared from triflone (R)-12 (! [4] Salt (M)-4 was prepared from triflone (R)-12 (!…”

“…A comparison of the kinetic parameters of both salts ( Table 2) shows that the half-life, Racemization of (M)-4 in THF at c = 0.05 m was polarimetrically monitored (l = 546 nm) [4] at five different temperatures as a function of time. Salt (M)-4 was prepared from triflone (R)-12 (!…”

“…[1] In particular, the fac-tors that determine the configurational stability of chiral carbanions are topics of key interest. [3,4] The salts predominantly form monomeric and di-Abstract: Enantiomerically pure triflones R 1 CH(R 2 )SO 2 CF 3 have been synthesized starting from the corresponding chiral alcohols via thiols and trifluoromethylsulfanes. [3,4] The salts predominantly form monomeric and di-Abstract: Enantiomerically pure triflones R 1 CH(R 2 )SO 2 CF 3 have been synthesized starting from the corresponding chiral alcohols via thiols and trifluoromethylsulfanes.…”