通过膦-铜催化的 1,3-烯炔硼氢化反应对三取代的阿伦基-B（pin）化合物的对映选择性合成。关于 Cu-Allenyl 中间体的立体化学完整性的见解,Journal of the American Chemical Society

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通过膦-铜催化的 1,3-烯炔硼氢化反应对三取代的阿伦基-B（pin）化合物的对映选择性合成。关于 Cu-Allenyl 中间体的立体化学完整性的见解
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2018-02-08 , DOI: 10.1021/jacs.7b13296
Youming Huang ₁ , Juan del Pozo ₁ , Sebastian Torker ₁ , Amir H. Hoveyda ₁

Affiliation

公开了催化对映选择性硼氢化物加成到 1,3-烯炔，其提供了丙二烯基-B(pin) (pin = pinacolato) 产品。易于获得的双膦-铜复合物促进了转化，并涉及市售的 HB(pin)。该方法适用于芳基和烷基取代的 1,3-烯炔。三取代的丙二烯基-B(pin) 产物的产率为 52-80%，在大多数情况下，丙二烯基：炔丙基和 92：8-99：1 的对映体比例大于 98:2。通过对醛的高度非对映选择性加成，以及提供具有三个碳基取代基的对映异构富集丙二烯的立体有择催化交叉偶联过程，突出了实用性。阐明了以下关键机械属性：(1) 光谱和计算研究表明低对映选择性可能是由于动力学立体选择性的丧失而引起的，正如实验证据所表明的那样，这可能是由于形成由异溶 Cu-C 裂解产生的炔丙基阴离子而发生的。当 Cu-allenyl 中间体的捕获缓慢时，这尤其成问题，即当缺电子 1,3-烯炔或反应性较低的硼氢化物试剂（例如，HB(dan) (dan = naphthalene-1,8 -diaminato)) 或在非最佳条件下使用（例如，较低的氢化硼浓度导致较慢的捕获）。(2) 对于含有空间要求的邻芳基取代基的烯炔，可能会产生相当数量的炔丙基-B(pin) 异构体 (25-96%)，因为对于 Cu/B 交换的空间要求较低的过渡态变得有利。(3)膦配体可促进对映体富集的二烯基-B(pin)产物的异构化；因此，较低的配体负载有时可能是最佳的。(4) 与富含对映异构体的二烯基-B(pin) 化合物的催化交叉偶联可能以高立体特异性进行（例如，膦-钯催化的交叉偶联）或导致相当大的外消旋化（例如，膦-铜催化的烯丙基取代））。

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Enantioselective Synthesis of Trisubstituted Allenyl–B(pin) Compounds by Phosphine–Cu-Catalyzed 1,3-Enyne Hydroboration. Insights Regarding Stereochemical Integrity of Cu–Allenyl Intermediates

Catalytic enantioselective boron-hydride additions to 1,3-enynes, which afford allenyl-B(pin) (pin = pinacolato) products, are disclosed. Transformations are promoted by a readily accessible bis-phosphine-Cu complex and involve commercially available HB(pin). The method is applicable to aryl- and alkyl-substituted 1,3-enynes. Trisubstituted allenyl-B(pin) products were generated in 52-80% yield and, in most cases, in >98:2 allenyl:propargyl and 92:8-99:1 enantiomeric ratio. Utility is highlighted through a highly diastereoselective addition to an aldehyde, and a stereospecific catalytic cross-coupling process that delivers an enantiomerically enriched allene with three carbon-based substituents. The following key mechanistic attributes are elucidated: (1) Spectroscopic and computational investigations indicate that low enantioselectivity can arise from loss of kinetic stereoselectivity, which, as suggested by experimental evidence, may occur by formation of a propargylic anion generated by heterolytic Cu-C cleavage. This is particularly a problem when trapping of the Cu-allenyl intermediate is slow, namely, when an electron deficient 1,3-enyne or a less reactive boron-hydride reagent (e.g., HB(dan) (dan = naphthalene-1,8-diaminato)) is used or under non-optimal conditions (e.g., lower boron-hydride concentration causing slower trapping). (2) With enynes that contain a sterically demanding o-aryl substituent considerable amounts of the propargyl-B(pin) isomer may be generated (25-96%) because a less sterically demanding transition state for Cu/B exchange becomes favorable. (3) The phosphine ligand can promote isomerization of the enantiomerically enriched allenyl-B(pin) product; accordingly, lower ligand loading might at times be optimal. (4) Catalytic cross-coupling with an enantiomerically enriched allenyl-B(pin) compound might proceed with high stereospecificity (e.g., phosphine-Pd-catalyzed cross-coupling) or lead to considerable racemization (e.g., phosphine-Cu-catalyzed allylic substitution).

更新日期：2018-02-08

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