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Radical Anti-Markovnikov Addition to Alkenes: Mechanism01:17

Radical Anti-Markovnikov Addition to Alkenes: Mechanism

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The reaction of hydrogen bromide with alkenes in the presence of hydroperoxides or peroxides proceeds via anti-Markovnikov addition. The radical chain reaction comprises initiation, propagation, and termination steps.
The mechanism starts with chain initiation, which involves two steps. In the first chain initiation step, a weak peroxide bond is homolytically cleaved upon mild heating to form two alkoxy radicals. In the second initiation step, a hydrogen atom is abstracted by the alkoxy...
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Radical Anti-Markovnikov Addition to Alkenes: Overview01:25

Radical Anti-Markovnikov Addition to Alkenes: Overview

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The addition of hydrogen bromide to alkenes in the presence of hydroperoxides or peroxides proceeds via an anti-Markovnikov pathway and yields alkyl bromides.
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Regioselectivity of Electrophilic Additions-Peroxide Effect02:35

Regioselectivity of Electrophilic Additions-Peroxide Effect

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In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
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Hydroboration-Oxidation of Alkenes03:08

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In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
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Oxidative Cleavage of Alkenes: Ozonolysis01:46

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In ozonolysis, ozone is used to cleave a carbon–carbon double bond to form aldehydes and ketones, or carboxylic acids, depending on the work-up.
Ozone is a symmetrical bent molecule stabilized by a resonance structure.
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Oxidation of Alkenes: Anti Dihydroxylation with Peroxy Acids02:04

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Diols are compounds with two hydroxyl groups. In addition to syn dihydroxylation, diols can also be synthesized through the process of anti dihydroxylation. The process involves treating an alkene with a peroxycarboxylic acid to form an epoxide. Epoxides are highly strained three-membered rings with oxygen and two carbons occupying the corners of an equilateral triangle. This step is followed by ring-opening of the epoxide in the presence of an aqueous acid to give a trans diol.
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通过金属氧介导酶催化抗马尔科夫尼科夫氧化

Stephan C Hammer1, Grzegorz Kubik1, Ella Watkins1

  • 1Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, MC 210-41, Pasadena, CA 91125, USA.

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|October 14, 2017
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概括

研究人员设计了一种用于有效抗马尔科夫尼科夫氧化的P450酶. 这一突破简化了合成途径,使得使用二氧化物选择性氧化原料.

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科学领域:

  • 生物催化
  • 有机化学
  • 酶工程

背景情况:

  • 催化抗马尔科夫尼科夫氧化具有重要的合成挑战.
  • 细胞P450酶是多功能催化剂,但通常有利于其他氧化途径.

研究的目的:

  • 设计一种能够催化抗马尔科夫尼科夫氧化的P450酶.
  • 在这个具有挑战性的化学转化中实现高效率和选择性.

主要方法:

  • 使用定向进化来修改细胞P450酶.
  • 该工程酶使用二氧化物作为终端氧化剂.
  • 该机制涉及捕获高能中间体和氧转移,包括化物迁移.

主要成果:

  • 该工程酶在催化金属氧介导的抗马尔科夫尼科夫氧化中表现出高效率.
  • 在动力偏好的烯环氧化过程中实现了抗马尔科夫尼科夫氧化的选择性.
  • 该过程包括一个对1,2-化物选择性的迁移步骤.

结论:

  • 工程P450细胞染色体可以克服长期存在的选择性氧化挑战.
  • 这种抗马尔科夫尼科夫氧酶提供了一种简化合成途径,
  • 该酶可以被整合到合成代谢途径中进行多种功能化反应.