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首页> 外文期刊>Chemistry: A European journal >Chiral fluorinated α-sulfonyl carbanions: Enantioselective synthesis and electrophilic capture, racemization dynamics, and structure
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Chiral fluorinated α-sulfonyl carbanions: Enantioselective synthesis and electrophilic capture, racemization dynamics, and structure

机译：手性氟化α-磺酰基碳负离子：对映选择性合成和亲电捕获，外消旋动力学和结构

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摘要

Enantiomerically pure triflones R~1CH(R~`2)SO _2CF_3 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 CF_3Hal (Hal=halide) or substitution _of alkoxyphosphinediamines with CF_3SSCF_3. The deprotonation _of RCH(Me)SO_2CF_3 (R=CH_2Ph, iHex) with nBuLi with the formation _of salts [RC(Me)-SO_2CF_3]Li and their electrophilic capture both occurred with high enantioselectivities. Displacement of the SO_2CF_3 group of (S)-MeOCH_2C(Me)(CH 2Ph)SO_2CF_3 (95 % ee) by an ethyl group through the reaction with AlEt3 gave alkane MeOCH_2C(Me)(CH 2Ph)Et of 96 % ee. Racemization _of salts [R~1C(R 2)SO_2CF_3]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 accordan_ce with a C_α-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)SO_2tBu]Li/DMPU (DMPU = dimethylpropylurea) has a half-life of racemization at -105 °C of 2.4 h, that of [PhCH 2C(Me)SO_2CF_3]Li at -78 °C is 30 d. DNMR spectroscopy of amides (PhCH_2)_2NSO_2CF _3 and (PhCH_2)N(Ph)SO_2CF_3 gave N-S rotational barriers that seem to be distin_ctly higher than those of nonfluorinated sulfonamides. NMR spectroscopy of [PhCH_2C(Ph)SO _2R]M (M=Li, K, NBu_4; R=CF_3, tBu) shows for both salts a confinement of the negative charge mainly to the C_α atom and a significant benzylic stabilization that is weaker in the trifluoromethylsulfonyl carbanion. According to crystal structure analyses, the carbanions of salts {[PhCH_2C(Ph)SO_2CF_3] Li×L}2 (L=2 THF, tetramethylethylenediamine (TMEDA)) and [PhCH_2C(Ph)SO_2CF_3]NBu_4 have the typical chiral C_α-S conformation _of α-sulfonyl carbanions, planar C_α atoms, and short C_α-S bonds. Ab initio calculations of [MeC(Ph)SO_2tBu]- and [MeC(Ph)SO_2CF_3]- 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(R~`2)SO_2R]- (R=tBu, CF_3) the nC→σS-R interaction is much stronger for R=CF_3. Ab initio calculations gave for [MeC(Ph)SO_2tBu] Li×2 Me_2O an O,Li,C_α contact ion pair (CIP) and for [MeC(Ph)SO_2CF_3]Li×2 Me_2O an O,Li,O CIP. According to cryoscopy, [PhCH_2C(Ph)SO_2CF _3]Li, [iHexC(Me)SO_2CF_3]Li, and [PhCH 2C(Ph)SO_2CF_3]NBu_4 predominantly form monomers in tetrahydrofuran (THF) at -108 °C. The NMR spectroscopic data of salts [R~1(R~`2)SO_2R~3]Li (R ~3=tBu, CF_3) indicate that the dominating monomeric CIPs are devoid of C_α-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.
机译：从相应的手性醇经硫醇和三氟甲基硫烷开始合成对映体纯的三氟甲酮R-1CH（R-2）SO _2CF_3。合成硫醚的关键步骤是硫醇的光化学三氟甲基化-用CF_3Hal（Hal =卤化物）或烷氧基膦二胺用CF_3SSCF_3取代。 RCH（Me）SO_2CF_3（R = CH_2Ph，iHex）与nBuLi脱质子化，形成盐[RC（Me）-SO_2CF_3] Li，并且它们的亲电捕获都具有高对映选择性。通过与AlEt 3的反应，乙基取代了（S）-MeOCH_2C（Me）（CH 2Ph）SO_2CF_3（95％ee）的SO_2CF_3基团，得到烷烃MeOCH_2C（Me）（CH 2Ph）Et为96％ee。盐的外消旋体[R〜1C（R 2）SO_2CF_3] Li遵循一级动力学，主要是焓法，负极化熵和动态NMR（DNMR）光谱表明其负激活熵较小。这与作为速率确定步骤的C_α-S键旋转一致。 α-（S）-三氟甲基锂和α-（S）-九氟丁基磺酰基碳负离子锂比相应的α-（S）-叔丁基磺酰基碳负离子具有更高的外消旋能垒。 [PhCH 2C（Me）SO_2tBu] Li / DMPU（DMPU =二甲基丙基脲）在-105°C的消旋半衰期为2.4 h，而[PhCH 2C（Me）SO_2CF_3] Li的消旋半衰期为-78°C。 30天酰胺（PhCH_2）_2NSO_2CF_3和（PhCH_2）N（Ph）SO_2CF_3的DNMR光谱显示N-S旋转势垒似乎明显高于非氟化磺酰胺。 [PhCH_2C（Ph）SO _2R] M（M = Li，K，NBu_4; R = CF_3，tBu）的NMR光谱表明，这两种盐的负电荷主要集中在C_α原子上，而苄基稳定度则弱得多在三氟甲基磺酰基碳负离子中。根据晶体结构分析，盐{[PhCH_2C（Ph）SO_2CF_3] Li×L} 2（L = 2 THF，四甲基乙二胺（TMEDA））和[PhCH_2C（Ph）SO_2CF_3] NBu_4的碳负离子具有典型的手性C_α-S α-磺酰基碳负离子，平面C_α原子和短C_α-S键的构象。 [MeC（Ph）SO_2tBu]-和[MeC（Ph）SO_2CF_3]-的从头算计算表明，对于氟化碳负离子而言，较强的nC→σ* S-CF _3和n O→σ* S-CF _3相互作用更弱，且苄基稳定。根据[R 1C（R〜`2）SO_2R]-（R = tBu，CF_3）的自然键轨道（NBO）计算，对于R = CF_3，nC→σS-R相互作用要强得多。从头算计算得出[MeC（Ph）SO_2tBu] Li×2 Me_2O为O，Li，C_α接触离子对（CIP），而[MeC（Ph）SO_2CF_3] Li×2 Me_2O为O，Li，O CIP。根据低温镜检查，[PhCH_2C（Ph）SO_2CF _3] Li，[iHexC（Me）SO_2CF_3] Li和[PhCH 2C（Ph）SO_2CF_3] NBu_4主要在-108°C下在四氢呋喃（THF）中形成单体。盐[R〜1（R〜`2）SO_2R〜3] Li（R〜3 = tBu，CF_3）的NMR光谱数据表明，主要的单体CIP不含C_α-Li键。值得购买的盐：手性三氟酮（≥98％ee）与RLi的反应可得到对映体过量（ee）≥98％的手性Liα-（S）-三氟甲基磺酰基碳负离子盐（参见图）。它们的亲电子捕获以高对映选择性发生。手性Liα-（S）-三氟甲基磺酰基碳负离子盐的结构和动力学研究表明单体接触离子对。

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