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首页> 外文学位 >Advances in Hydroboration: Metal-Free Oxygen-Directed Hydroboration and Asymmetric Hydroboration with N-tosyl-(R,R)-2,6-diisopropyl-1,4,borazinane.
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Advances in Hydroboration: Metal-Free Oxygen-Directed Hydroboration and Asymmetric Hydroboration with N-tosyl-(R,R)-2,6-diisopropyl-1,4,borazinane.

机译：硼氢化研究的进展：无金属氧导向的硼氢化和与N-甲苯磺酰基-（R，R）-2,6-二异丙基-1,4，硼氮烷的不对称硼氢化。

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摘要

The work described herein addresses two unresolved issues in the hydroboration literature: 1) The regioselective hydroboration of 1,2-disubstituted olefins, and 2) the asymmetric hydroboration of prochiral olefins.;Metal-free oxygen-directed hydroboration of 1,2-disubstituted olefins in homoallylic alcohols, lithium alkoxides, and ethers has been achieved using triflic acid-activated dimethylsulfide·borane conditions building on previous work in the Vedejs group involving homoallylic amine-directed hydroboration. Optimization has led to a pre-activation approach to generate TfOBH2 that provides 1,3-diols from alcohols and alkoxides with excellent regioselectivity (>20:1) upon oxidative workup. An in situ approach to TfOBH 2 provides optimal results in the case of homoallylic ethers, including allyl ethers, which afford monoprotected 1,3-diols upon oxidative workup. The originally envisioned mechanistic pathway for these transformations, in which the generation of a TfOBH2-oxygen complex enables intramolecular SN2-like boron-alkene complexation, can account for the regioselectivity, and the observation that homoallylic alcohols are preferentially hydroborated in the presence of excess cyclohexene is consistent with this mechanism. However, diastereoselectivity remains elusive, indicating that the mechanism of oxygen-directed hydroboration (ODHB) is not straightforward. Mechanistic studies have provided little insight into the pathway(s) leading to the regioselective, yet non-diastereoselective transformations. Oxygen-directed hydroboration remains an enigmatic phenomenon, in general.;N-Tosyl-(R,R)-2,6-diisopropy1-1,4-borazinane, the first C2-symmetric borane incorporated into a 2,6-disubstituted-six-membered boracycle, has been generated in pursuit of an asymmetric hydroborating reagent that is capable of achieving excellent enantioselectivity on all four types of prochiral alkenes. Masamune's (R,R)-2,5-dimethylborolane is the single most versatile asymmetric hydroborating agent in the literature, and serves as both inspiration and measuring stick for this work. The synthesis of N-tosyl-(R,R)-2,6-diisopropy1-1,4-borazinane is streamlined compared with the preparation of Masamune's borolane, thanks to diastereoselective boracycle formation, successful resolution of trans-borazinane enantiomers via alanine complexation/crystallization, and a one-pot hydroboration procedure from the diastereo-enriched alanine complex. On the other hand, N-tosyl-(R,R)-2,6-diisopropy1-1,4-borazinane only achieves good enantioselectivity (86% ee) with cis-1,2-disubstituted (Type II) alkenes. This is more reminiscent of Brown's diisopinocampheylborane than of Masamune's borolane, which provides superior enantioselectivity with cis- and trans-1,2-disubstituted (Type II and III) and trisubstituted (Type IV) alkenes.
机译：本文所述的工作解决了加氢硼化文献中两个未解决的问题：1）1,2-二取代烯烃的区域选择性加氢硼化，以及2）前手性烯烃的不对称加氢硼化;; 1,2-二取代的无金属氧直接加氢硼化基于三氟甲基磺酸活化的二甲基硫醚·硼烷条件，已经在Vedejs组以前的工作中涉及到均烯丙基胺定向的氢硼化反应，已实现了均烯丙基醇，醇锂和醚中的烯烃。优化导致了一种预活化方法来生成TfOBH2，该TfOBH2在氧化后处理后从醇和醇盐中提供1,3-二醇，具有出色的区域选择性（> 20：1）。对于TfOBH 2的原位处理方法在均烯丙基醚（包括烯丙基醚）的情况下提供了最佳结果，该烯丙基醚在氧化处理后可得到单保护的1,3-二醇。最初设想的这些转化的机理途径，其中生成TfOBH2-氧配合物使得分子内SN2样硼-烯烃络合成为可能，这是区域选择性的结果，并且观察到在过量的环己烯存在下均丙醇优先被硼氢化。与此机制是一致的。但是，非对映选择性仍然难以捉摸，这表明氧指导的硼氢化（ODHB）的机理并不直接。机理研究对导致区域选择性但非非对映选择性转化的途径了解甚少。一般而言，氧气导向的硼氢化反应仍然是一个谜。N-甲苯磺酰基-（R，R）-2,6-二异丙基1-1,4-硼氮烷，第一个C2对称硼烷并入2,6-二取代的-为了寻求一种不对称的硼氢化试剂，已经生成了六元硼环，该试剂能够对所有四种类型的前手性烯烃实现出色的对映选择性。 Masamune的（R，R）-2,5-二甲基硼烷是文献中最通用的单一不对称硼氢化剂，并且是这项工作的灵感和量尺。与制备Masamune的硼烷相比，N-甲苯磺酰基-（R，R）-2,6-二异丙基1-1,4-硼氮烷的合成过程得到了简化，这得益于非对映选择性硼环的形成，通过丙氨酸络合成功拆分了反式硼氮烷对映体结晶，以及从非对映异构体富集的丙氨酸络合物中进行一锅硼氢化处理。另一方面，N-甲苯磺酰基-（R，R）-2,6-二异丙基1-1,4-硼氮烷仅与顺式1,2-二取代的（II型）烯烃达到良好的对映选择性（86％ee）。与Masamune的硼烷相比，这更让人联想到Brown的diisopinocampheylborane，后者具有顺式和反式1,2-双取代（II和III型）和三取代（IV型）烯烃的优异对映选择性。

著录项

  • 作者

    Rarig, Robert-Andre F.;

  • 作者单位

    University of Michigan.;

  • 授予单位 University of Michigan.;
  • 学科 Chemistry Organic.
  • 学位 Ph.D.
  • 年度 2010
  • 页码 198 p.
  • 总页数 198
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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