- Allylation reactions of aldehydes with allylboronates in aqueous media: Unique reactivity and selectivity that are only observed in the presence of water
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Zn(OH)2-catalyzed allylation reactions of aldehydes with allylboronates in aqueous media have been developed. In contrast to conventional allylboration reactions of aldehydes in organic solvents, the α-addition products were obtained exclusively. A catalytic cycle in which the allylzinc species was generated through a B-to-Zn exchange process is proposed and kinetic studies were performed. The key intermediate, an allylzinc species, was detected by HRMS (ESI) analysis and by online continuous MS (ESI) analysis. This analysis revealed that, in aqueous media, the allylzinc species competitively reacted with the aldehydes and water. An investigation of the reactivity and selectivity of the allylzinc species by using several typical allylboronates (6a, 6b, 6c, 6d) clarified several important roles of water in this allylation reaction. The allylation reactions of aldehydes with allylboronic acid 2,2-dimethyl-1,3-propanediol esters proceeded smoothly in the presence of catalytic amounts of Zn(OH)2 and achiral ligand 4d in aqueous media to afford the corresponding syn-adducts in high yields with high diastereoselectivities. In all cases, the α-addition products were obtained and a wide substrate scope was tolerated. Furthermore, this reaction was applied to asymmetric catalysis by using chiral ligand 9. Based on the X-ray structure of the Zn-9 complex, several nonsymmetrical chiral ligands were also found to be effective. This reaction was further applied to catalytic asymmetric alkylallylation, chloroallylation, and alkoxyallylation processes and the synthetic utility of these reactions has been demonstrated. Still waters run deep: The Zn(OH)2-catalyzed allylation of aldehydes with allylboronates in aqueous media exclusively afford the α-addition products. This reaction was also applied to alkylallylation, chloroallylation, and alkoxyallylation reactions. The role of water is discussed. Copyright
- Kobayashi, Shu,Endo, Toshimitsu,Yoshino, Takumi,Schneider, Uwe,Ueno, Masaharu
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p. 2033 - 2045
(2013/09/23)
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- Chiral zinc-catalyzed asymmetric α-alkylallylation and α-chloroallylation of aldehydes
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Two birds with one stone: In the presence of Zn(OH)2 and a chiral bipyridine ligand, racemic α-substituted allylboronates 2 reacted with aldehydes 1 (see scheme) exclusively in an α-addition fashion to afford various homoallylic alcohols 3 bear
- Kobayashi, Sha,Endo, Toshimitsu,Ueno, Masaharu
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supporting information; scheme or table
p. 12262 - 12265
(2012/02/01)
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- Sulfur-ylide-mediated synthesis of functionalized and trisubstituted epoxides with high enantioselectivity; Application to the synthesis of CDP-840
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Benzyl and substituted allyl sulfonium salts react with a broad range of simple and functionalized aldehydes and ketones to give epoxides with high diastereoselectivity and high enantioselectivity (see scheme). The process has been applied to a short synthesis of the phosphodiesterase-IV inhibitor CDP-840. R1, R2 = hydrogen, alkyl, alkenyl, alkynyl, aryl, or pyridyl.
- Aggarwal, Varinder K.,Bae, Imhyuck,Lee, Hee-Yoon,Richardson, Jeffery,Williams, David T.
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p. 3274 - 3278
(2007/10/03)
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- Cr(Salen)-catalyzed addition of 1,3-dichloropropene to aromatic aldehydes. A simple access to optically active vinyl epoxides.
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[reaction: see text]. Chiral Cr(Salen) complex (1) prepared in situ from CrCl3 promotes the enantioselective addition of 1,3-dichloropropene to aromatic aldehydes in the presence of Mn as the stoichiometric reductant and Me3SiCl as a scavenger. The result
- Bandini,Cozzi,Melchiorre,Morganti,Umani-Ronchi
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p. 1153 - 1155
(2007/10/03)
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- Diastereo- and enantioselective synthesis of syn-α-vinylchlorohydrins and cis-vinylepoxides
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A new method to generate chiral syn-vinylchlorohydrins and cis-vinyloxiranes is reported. Reaction of (α-haloallyl)lithiums with methoxy-9-BBN or Ipc2BOMe followed by treatment with BF3·OEt2 leads to (Z)-(γ-haloallyl)boranes which react with aldehydes to yield cis-vinylepoxides (de ≥ 90%) upon oxidative workup. Alternatively, addition of ethanolamine to the allylboration product yields syn-α-halohydrins (de ≥ 90%) that are also easily cyclized to cis-vinylepoxides. Extension of this protocol using [(Z)-γ-chloroallyl]BIpc2 leads to chiral syn-α-chlorohydrins and cis-vinylepoxides in high de (≥90%) and ee (90-99%). Enantioselectivity of reactions of chiral (Z)-(γ-chloroallyl)boranes with aldehydes are more sensitive to reaction conditions than enantioselectivity of reactions of other α-or γ-substituted allylboranes. The effects of proportion of BP3·OEt2 and the relative efficacies of LiNR2 bases on diastereo- and enantioselectivity of the chloroallylation are reported.
- Hu, Shaojing,Jayaraman, Seetharaman,Oehlschlager, Allan C.
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p. 7513 - 7520
(2007/10/03)
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