Latent, thermally activated organic catalysts for the on-demand living polymerization of lactide
Latent, thermally activated organic catalysts for the on-demand living polymerization of lactide
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DOI：
10.1002/anie.200500723
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发表时间：
2005-01-01
影响因子：
16.6
通讯作者：
Hedrick, JL
中科院分区：
文献类型：
--
作者：
Coulembier, O;Dove, AP;Hedrick, JL
过去十年，卡宾化学取得了重大进展，包括分离第一个杂原子取代的单线态卡宾和持久性三线态二芳基卡宾。[1, 2] 这些进展激发了人们新的兴趣，并扩大了这些活性物质产生的可能性范围。继 Wanzlick 等人[3] 的开创性工作之后，Bertrand [4] 和 Arduengo [5] 的研究小组分离并表征了稳定的 N-杂环卡宾，并在过去十年中描述了稳定的咪唑啉-2-亚基和咪唑-2-亚基卡宾的空间和电子要求。 [6]因此，对 N-杂环卡宾 (NHC) 的性质和用途的研究已成为一个主要研究领域。 [2, 7] 例如，使用 NHC 代替膦配体产生的过渡金属络合物具有增强的催化性能和稳定性。 [8] N-杂环卡宾也被证明是用于安息香缩合、Stetter反应、酰化/酯交换反应和环酯聚合的有效有机催化剂。[9]尽管在结构修饰方面取得了进展以增强N-杂环卡宾的稳定性[10]并且开发了原位生成方法[11, 12]，NHC的使用由于其极端的空气和湿度敏感性而变得复杂，这需要使用手套箱技术。我们的注意力集中在受保护的 NHC 的制备和应用上，特别是那些可以热激活的 NHC。许多研究小组在早期研究中意识到，强亲核卡宾与含有酸性 CÀH 键的化合物发生 CÀH 插入反应，形成高产率的加合物。 [13]饱和卡宾的氯仿和五氟苯加合物[12]是过渡金属卡宾配合物的有用前体；它们还在高温下催化丙交酯的开环聚合 (ROP)（方案 1）。[12h] Grubbs
The last decade has seen significant advances in carbene chemistry, including the isolation of the first heteroatomsubsituted singlet carbene and persistent triplet diarylcarbenes.[1, 2] These advances have stimulated renewed interest and expanded the scope of possibilities engendered by these reactive species. Following the pioneering work by Wanzlick et al.,[3] the research groups of Bertrand [4] and Arduengo [5] isolated and characterized stable N-heterocyclic carbenes, and over the last decade the steric and electronic requirements for stable imidazolin-2-ylidene and imidazol-2-ylidene carbenes have been described.[6] Consequently, investigation into the properties and uses of N-heterocyclic carbenes (NHCs) has become a major area of research.[2, 7] For example, the use of NHCs in place of phosphine ligands produces transition-metal complexes that exhibit enhanced catalytic performance and stability.[8] N-Heterocyclic carbenes have also proven to be effective organic catalysts for benzoin condensations, Stetter reactions, acylation/transesterification reactions, and the polymerization of cyclic esters.[9]Despite advances in structural modifications to enhance the stability of N-heterocyclic carbenes [10] and the development of in situ methods for their generation,[11, 12] the use of NHCs is complicated by their extreme air and moisture sensitivity, which necessitates the use of glove-box techniques. Our attention has been drawn towards the preparation and application of protected NHCs, particularly those that could be activated thermally. Many research groups realized in early studies that strongly nucleophilic carbenes underwent CÀH insertion reactions with compounds containing acidic CÀH bonds to form adducts in high yields.[13] Chloroform and pentafluorobenzene adducts of saturated carbenes [12] are useful precursors to transition-metal–carbene complexes; they also catalyze the ring-opening polymerization (ROP) of lactide (Scheme 1) at elevated temperatures.[12h] Grubbs