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jiangchen077木虫 (小有名气)
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Amines and their derivatives are important functionalities invarious natural products and important synthetic targets.Theyare widely used as basic intermediates to prepare solvents,fine chemicals,pharmaceuticals,agrochemicals and catalysts for polymerization.1 As a result,the development of new methods to prepare these compounds has become the goal of many research groups over the ears.Traditionally,amines are alkylated in solution phase using inorganic acids as the catalysts and alkyl halides or dimethyl sulfate as the alkylating agents,2 besides methanol3 and dimethyl carbonate.4 However, yields or products selectivity(mono-or bis-alkylations)are,with few exceptions,low and depend on the nature of the catalysts and on the reaction conditions.From a methodological viewpoint,direct nucleophilic attack of amines to alkyl halides is the most straightforward procedure for the alkylation of amines.5 The major synthetic problem for direct alkylation procedure is the competing over-alkylation,which leads to mixtures of secondary and tertiary amines and even quaternary ammonium salts,and the prevention of over-alkylation becomes more difficult when highly reactive alkylating agents such as methyl,ethyl,benzyl,and allyl halides are used as alkylating agents.6 Thermal reaction between alkyl halides and amines in the presence of a base requires longer reaction time period and affords lower yields of desired product.7 Inorganic bases like NaH,8 K2CO3,9 CsOH·H2O,10 Cs2CO311 have been employed in N-alkylation of amines.Recently,Varma and his co-workers studied the formation of tertiary amines via N-alkylation in aqueous alkaline medium under microwave irradiation to evolve a greener procedure.12 Use of strong inorganic bases,however, can lead to undesired products in the presence of base-sensitive functional groups and hydrolysis of the corresponding alkyl halides may also result.Copper-catalyzed Ullmann and Gold-berg reactions13 or Pd-catalyzed Buchwald–Hartwig reaction14 have been established as versatile C–N bond-forming reactions.However,transition metal-free amination of electron-rich benzyl halides has remained largely unexplored.15 Although most of the methods are apparently useful,many of them are limited byharsh reaction conditions,low yields,long reaction times and use of toxic solvents or catalysts.N-alkylations promoted on solid phase are often associated with strong basicity of the surface limiting the scopes of recyclability and selectivity.16 Further-more,the possibility of the concomitant over-alkylations,when the amine is employed as the limiting substrate,often reduces the application of these methods.17 Therefore,development of mild,efficient,selective and environmentally benign method of N-alkylation of amines including diamines and heterocyclic compounds bearing acidic hydrogen atom attached to nitrogen has been a major challenge in organic synthesis. |
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云和风清
金虫 (著名写手)
- 翻译EPI: 4
- 应助: 0 (幼儿园)
- 金币: 3407.1
- 散金: 28
- 红花: 2
- 帖子: 2061
- 在线: 201.5小时
- 虫号: 430842
- 注册: 2007-08-11
- 性别: GG
- 专业: 传递过程
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wypward(金币+1):欢迎新虫 2010-07-01 20:25:57
wypward(金币+1):欢迎新虫 2010-07-01 20:25:57
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胺类及其衍生物是在许多的天然产物和重要的合成目标产物中的重要的功能化合物。 它们被广泛用作制备溶剂、精细化学品、药物、农业化学品及聚合反应的催化剂的基础中间体[1]。因此，制备这类产物的新方法的进展已经成为众多研究机构的目标。通常意胺类是在液相中烷基化的，使用无机酸作催化剂，卤代烷或硫酸二甲酯作烷基化试剂[2]，此外烷基化试剂还有甲醇[3] 和碳酸二甲酯[4]。但是，除少数例外外，产率或产物的选择性(单-或重烷基化)不太好，且依赖于催化剂的性质和反应条件。 从方法论角度上讲，如胺类直接亲核攻击卤代烷则将是胺类烷基化最直接的方式[5]， 直接烷基化面临的最重大的合成困难是如何应对过度烷基化，其可能会导致仲铵盐、叔胺盐甚至季铵盐产生，特别使用甲基、乙基、苄基和烯丙基卤化物等高活性试剂作为烷基化试剂时，避免过度的烷基化就会变得更加困难[6]。烯丙基卤化物与胺类在碱存在条件下的热反应需要较长的反应时间且理想产物的产率较低[7]，无机碱如NaH[8]，K2CO3[9]，CsOH•H2O[10]，Cs2CO3[11]也已被用于胺类的N-烷基化。最近，Varma及其合作者研究了微波辐射条件下经由N-烷基化合成叔胺的形成过程，给我们提供了一条绿色的合成途径[12]。但是由于使用强的无机碱，在碱敏感基团存在条件下可能会产生非理想产物，也可能会导致相应的卤代烷水解。铜催化的Ullmann和Gold-berg 反应[13]或钯催化的Buchwald–Hartwig反应[14]被公认为通用的C–N键合成反应，但是，富电荷的苄基卤代物的在无过渡金属条件下的胺化则尚未充分研究[15]，尽管很明显这些方法中大多数是很有用的，但其中很多由于反应条件严苛，产率低、反应时间长和使用的溶剂或催化剂有毒性而使其应用受到限制。固相上的改进N-烷基化通常是与表面的强碱性相联系，这就制约了再生性和选择性[16]。进一步讲，但胺类充当限制性物质时，过度烷基化伴生的可能性通常会降低应用这些方法的可行性[17]。因此，开发出一种温和、高效、选择性好、环境友好的胺类(包含二胺类和具有与氮原子相连的酸性H原子的杂环化合物)N-烷基化的方法已经成为有机合成中重要的研究课题。 翻译的不太好，有些翻译很不尽人意，有点难以理解 |
2楼2010-07-01 20:03:46
