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Enantioselective deprotometalation of alkyl ferrocenecarboxylates using bimetallic bases

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NEW JOURNAL OF CHEMISTRY
卷 45, 期 48, 页码 22579-22590

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ROYAL SOC CHEMISTRY
DOI: 10.1039/d1nj04526a

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  1. Agence Nationale de la Recherche (ACTIVATE program)

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The enantioselective deprotometalation of alkyl ferrocenecarboxylates using mixed lithium-zinc or lithium-cadmium bases was investigated in this study. Different chiral lithium alkyl-amidozincates showed varied performance in producing enantiomerically enriched derivatives. The reaction with cadmium instead of zinc did not take place, suggesting the different reactivity of the two metal species.
The enantioselective deprotometalation of alkyl ferrocenecarboxylates (FcCO(2)R) using mixed lithium-zinc or lithium-cadmium bases is described. By using FcCO(2)Me as the substrate, chiral lithium alkyl-amidozincates prepared from exo-(alpha R)- or endo-(alpha S)-N-(alpha-phenylethyl)bornylamine (H-exo-born-R or H-endo-born-S) were tested; the best results (27% yield and 62% ee in favor of the R-P enantiomer) were obtained by using Bu-2(endo-born-S)ZnLi in tetrahydrofuran (THF) at -30 degrees C before iodolysis. Due to the low compatibility of FcCO(2)Me with alkyl-containing lithium zincates, 1 : 1 mixtures of lithium and zinc amides were tested. Chiral (H-exo-born-R or H-endo-born-S) or/and achiral (lithium 2,2,6,6-tetramethylpiperidide or lithium diisopropylamide) secondary amines gave good results, the best (81% yield and 44% ee in favor of the R-P enantiomer) being obtained by using (endo-born-S)(3)ZnLi in THF at room temperature. Among other secondary amines also prepared and/or tested, commercial (S,S)-bis(alpha-phenylethyl)amine (H-PEA-S) proved promising. After optimization of the reaction conditions, the best enantioselectivity (26% yield and 80% ee in favor of the RP enantiomer) was observed by treating a THF solution of FcCO(2)Me and Zn(PEA-S)(2) with Li-PEA-S at -80 degrees C before iodolysis. That no reaction took place with cadmium instead of zinc suggests the formation of 'ate complexes upon treatment of Cd(PEA-S)(2) by Li-PEA-S while Zn(PEA-S)(2) and Li-PEA-S would rather work in tandem (Li-PEA-S as the base and Zn(PEA-S)(2) as the in situ trap for the formed ferrocenyllithium). While FcCO(2)Me, FcCO(2)tBu and FcCO(2)iPr could be converted into their racemic 2-iodinated derivatives with a yield of 84 to 87% by employing LiTMP (2 equiv.) in the presence of ZnCl2.TMEDA (1 equiv.) as an in situ trap, their enantioselective deprotometalation rather required Li-PEA-S together with Zn(PEA-S)(2) to produce the enantio-enriched derivatives with yields of 45-82% and 71% ee.

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