Chiral synthesis via organoboranes. 34. Selective reductions. 47. Asymmetric reduction of hindered .alpha.,.beta.-acetylenic ketones with B-chlorodiisopinocampheylborane to propargylic alcohols of very high enantiomeric excess. Improved workup procedure for the isolation of product alcohols
摘要:
The Midland reagent, Alpine-Borane (2a), is excellent for the asymmetric reduction of many acetylenic ketones, but it fails with hindered derivatives. On the other hand, B-chlorodiisopinocampheylborane (DIP-Chloride, 4) reacts with hindered alpha,beta-acetylenic ketones to provide the corresponding propargylic alcohols in 96 to greater-than-or-equal-to 99% ee. The reaction is in accordance with the tentative mechanism proposed earlier. While 4-phenyl-3-butyn-2-one is reduced in only 21% ee, 4 reduces 4-methyl-1-phenyl-1-pentyn-3-one in 53% ee and 4,4-dimethyl-1-phenyl-1-pentyn-3-one in greater-than-or-equal-to 99% ee. The generality of this observation is demonstrated by reducing a series of hindered acetylenic ketones with increasing steric requirements and differing electronic environments. Thus, 2,2-dimethyl-4-tridecyn-3-one, 1-cyclopentyl-4,4-dimethyl-1-pentyn-3-one, and 3,3-dimethyl-5-tetradecyn-4-one are all reduced to the corresponding alcohols in greater-than-or-equal-to 99% ee and 97% ee, respectively. A modified and operationally simpler workup procedure for obtaining the alcohols in high isolated yields is described. Comparison of reagent 4 with 2a is also made, making clear the range of applicability of each reagent. This development makes it possible to reduce asymmetrically any acetylenic ketone by a judicious choice of either 2a or 4.
Chiral synthesis via organoboranes. 34. Selective reductions. 47. Asymmetric reduction of hindered .alpha.,.beta.-acetylenic ketones with B-chlorodiisopinocampheylborane to propargylic alcohols of very high enantiomeric excess. Improved workup procedure for the isolation of product alcohols
作者:P. Veeraraghavan Ramachandran、Aleksandar V. Teodorovic、Milind V. Rangaishenvi、Herbert C. Brown
DOI:10.1021/jo00034a034
日期:1992.4
The Midland reagent, Alpine-Borane (2a), is excellent for the asymmetric reduction of many acetylenic ketones, but it fails with hindered derivatives. On the other hand, B-chlorodiisopinocampheylborane (DIP-Chloride, 4) reacts with hindered alpha,beta-acetylenic ketones to provide the corresponding propargylic alcohols in 96 to greater-than-or-equal-to 99% ee. The reaction is in accordance with the tentative mechanism proposed earlier. While 4-phenyl-3-butyn-2-one is reduced in only 21% ee, 4 reduces 4-methyl-1-phenyl-1-pentyn-3-one in 53% ee and 4,4-dimethyl-1-phenyl-1-pentyn-3-one in greater-than-or-equal-to 99% ee. The generality of this observation is demonstrated by reducing a series of hindered acetylenic ketones with increasing steric requirements and differing electronic environments. Thus, 2,2-dimethyl-4-tridecyn-3-one, 1-cyclopentyl-4,4-dimethyl-1-pentyn-3-one, and 3,3-dimethyl-5-tetradecyn-4-one are all reduced to the corresponding alcohols in greater-than-or-equal-to 99% ee and 97% ee, respectively. A modified and operationally simpler workup procedure for obtaining the alcohols in high isolated yields is described. Comparison of reagent 4 with 2a is also made, making clear the range of applicability of each reagent. This development makes it possible to reduce asymmetrically any acetylenic ketone by a judicious choice of either 2a or 4.
Ru-Catalyzed Hydroboration of Ynones Leads to a Nontraditional Mode of Reactivity
作者:Qiang Feng、Shijia Li、Zhiyang Li、Qiaolin Yan、Xiangfeng Lin、Lijuan Song、Xinhao Zhang、Yun-Dong Wu、Jianwei Sun
DOI:10.1021/jacs.2c06024
日期:2022.8.17
initial rate-determining conjugateaddition of hydroborane to form the key boryl allenolate intermediate followed by a fast second hydroboration of the enolate motif of the allenolate. Notably, direct 1,4-addition of hydroborane to carbonyl-conjugated alkynes also represents a new mode of reactivity. Despite the overwhelming complexity of this process, which involves selectivitycontrol in almost every step