Stereoselective synthesis & application of enantioenriched main group α-haloalkyl organometal reagents Public

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  • Sulfoxide-ligand exchange (SLE) and asymmetric halogen-metal exchange (AHME) processes were separately examined for the enantioselective synthesis of functionalized alpha-haloalkylmetal (carbenoid) reagents. Carbenoids derived from SLE were used to effect stereospecific reagent-controlled homologation (StReCH) of boronic esters and those generated via AHME were engaged in Darzens-type chemistry with aldehydes. Abstract for Part 1. Scalemic syn alpha-chloroalkylsulfoxides p-TolS(O)CHClR [R = allyl, (1,3-dioxolan-2-yl)methyl, proparygyl, and 2-(benzyloxy)ethyl] were prepared from the corresponding thioethers by Jackson-Ellman-Bolm catalytic enantioselective sulfoxidation [cat. VO(acac)₂, tert-leucinol derived chiral Schiff base ligand, aq. H₂O₂, CHCl₃; 76-80% yield, >98% ee] followed by non-racemizing chlorination mediated by N-chlorosuccinimide in the presence of potassium carbonate (84-86% yield, syn:anti ≥ 20:1). The corresponding anti diastereoisomers were accessed from their syn epimers by sodium hexamethyldisilazide mediated deprotonation (THF, –78 °C) followed by treatment with either CH₃OH or CD₃OD to yield alpha-[¹H] or alpha-[²H] isotopomers, respectively (88% yield, anti:syn ≥ 17:1). Allyl and (1,3- dioxan-2-yl)methyl substituted chlorosulfoxides reacted with R'Li (t-BuLi or PhLi, THF, –78 °C) to give the expected products of SLE [p-TolS(O)R' and LiCHClR or LiCDClR]; however, neither the benzylether nor propargyl substituted substrates gave wholly satisfactory results under the same reaction conditions. The functionalized carbenoid reagents so obtained, 1-chloro-3-butenyllithium and 1-chloro-2-(1,3- dioxolan-2-yl)ethyllithium, were applied to the StReCH of B-(2-chloropyrid-5-yl) pinacol boronate but only the latter gave acceptable yields of chain extended products. The anti alpha-[²H]-chlorosulfoxide dioxolanyl bearing carbenoid precursor gave superior results to the analogous syn or anti alpha-[¹H]-chlorosulfoxides for StReCH of the B-pyridyl boronate [79% conversion, ≥ 89% ee (99% stereofidelity), vs. ≤ 68% conversion for non-deuterated chlorosulfoxides]. The origin of this isotope effect was traced to a deleterious proton transfer pathway between the alpha- chloroalkyllithium reagent and its chlorosulfoxide precursor. Sequential double iterative StReCH of B-(2-chloropyrid-5-yl) pinacol boronate with two separate portions of (S)-1-[²H]-1-chloro-2-(1,3-dioxolan-2-yl)ethyllithium (generated via SLE with phenyllithium) followed by oxidative work-up (with KOOH) gave (1R,2R)-1,2- [²H]₂-2-(2-chloropyrid-5-yl)-1,2-bis[(1,3-dioxolan-2-yl)methyl]ethanol (40% yield, ≥ 98% ee, dr = 85:15). Substitution of the (R)-configured carbenoid for its antipode in the second StReCH stage above gave the unlike (1S,2R)-isomer of the same pyridylethanol derivative (49% yield, ≥ 98% ee, dr = 79:21). The unlike diastereoisomer was advanced to the trifluoroacetamide of (1R,2R)-1,2-[2H]2-1- amino-2-(2-chloropyrid-5-yl)cyclohex-4-ene (6 steps, 5% overall yield); the non- deuterated isotopomer of this compound was previously advanced to the analgesic alkaloid (–)-epibatidine by Corey and co-workers. Abstract for Part 2. Scalemic planar chiral N,N-dialkyl 2-iodoferrocene carboxamides envisioned as recyclable precursors to ferrocenyl metal reagents for AHME, were prepared from ferrocene carboxylic acid by a three step sequence of: acid chloride formation [(COCl)₂ and cat. DMF)], aminolysis (with R₂NH, R = Me, Et, i-Pr; 65- 80% yield over 2 steps), and sec-butyllithium/(–)-sparteine mediated enantioselective directed ortho-metallation (DoM) followed by iodinolysis (87% yield, ≥ 96% ee). Attempts to access more elaborate 5-substituted 2-iodoferrocene carboxamides via DoM/iodinolysis of ortho-substituted ferrocene carboxamides (Me, Ph, or SiMe₃ substituents) mostly failed; however, analogous trisubstituted ferrocene oxazolines could be synthesized. Treatment of N,N-diisopropyl 2-iodoferrocene carboxamide (298, ≥ 96% ee) with n-BuLi (THF, –78 °C) resulted in complete conversion to the corresponding lithioferrocene (327) via I/Li interchange; subsequent iodinolysis initiated reverse Li/I exchange and returned iodoferrocene 298 without diminished enantiomeric excess, establishing configurational stability for the lithiated ferrocene intermediate. Prochiral (RCHI₂) and racemic (RCHICl) geminal dihalide substrates for AHME studies were prepared by electrophilic quench of dihalomethylsodiums with either Ph(CH₂)₃I or Me₃SiCl (50-78% yield). Of the four dihalides so produced, only prochiral substrate Me₃SiCHI₂ engaged in I/Li exchange with scalemic lithioferrocene 327 resulting in regeneration of its precursor iodoferrocene 298 and the formation of a putative chiral carbenoid Me₃SiCHLiI. Trapping of the carbenoid with aldehydes RCHO (R = Ph, 4-MeOC₆H₄, Ph(CH₂)₂, c-C₆H₁₁) in the presence of Me₂AlCl gave the expected epoxysilane products (35-40% yield, cis:trans ≥ 2:1) but without discernable enantiomeric excess. Hypotheses to account for the apparent lack of stereoinduction in this AHME cycle are presented. Comparable experiments using analogous magnesiated ferrocenes failed to produce putative carbenoid species from the same set of geminal dihalide substrates.
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