Synthesis and application of the first chiral and highly Lewis acidic silyl cationic catalyst
Synthesis and application of the first chiral and highly Lewis acidic silyl cationic catalyst
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DOI：
10.1021/ja981021k
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发表时间：
1998-08-05
影响因子：
15
通讯作者：
Helmchen, G
中科院分区：
文献类型：
--
作者：
Johannsen, M;Jorgensen, KA;Helmchen, G
近年来，在硅化学领域取得了很大的进展，在缩合相中制备期望已久的硅阳离子似乎成为可能。1-15尽管硅基路易斯酸在有机化学中得到了广泛的应用，但对活性更强的阳离子类的利用却很少受到关注。据我们所知，还没有人试图合成手性硅基路易斯酸，也没有人研究过这些手性阳离子作为催化剂的高反应性。手性硅催化剂的化学性质对合成化学家来说是非常重要的，因为硅配合物是少数几种能催化重要反应的金属配合物，如亚胺加成反应和弗里德尔-克拉兹反应。17-21另一个重要方面是这些催化剂作为手性阳离子聚合引发剂的应用。根据硅基阳离子化学的最新发展，现在可以合理地假设，只要仔细选择阴离子和溶剂，就可以得到反应性很强的路易斯酸催化剂。本文介绍了硅基手性路易斯酸（(S)-1a, b）的首次制备、部分表征和催化反应。从(S)-1a， b中可以看出，手性锚定在与BINOL配体相关的c2对称的2,2 ' -二甲基- 1,1 ' -联萘基主链上。手性二萘骨架(S)-2的合成从2-甲基萘开始，涉及到二萘配体的分解步骤。(S)-2的硅基化是通过二甲基化配体与甲基三甲氧基硅烷的亲核加成而进行的，得到甲氧基取代硅烷，26通过还原可转化为(S)-3（方案1）。手性硅烷(S)-3是一种空气和水分稳定的固体，无需任何特殊的预防措施即可处理。已经进行了几次尝试将(S)-3转化为相应的硅基阳离子配合物。各种Brønsted酸的质子溶解作用过于剧烈，导致Si-H和Si-C键断裂。还尝试用CuCl2加热将硅烷转化为硅氯。手性硅酰氯对水分的敏感性高于母体硅烷，用三氟化银处理得到三氟化硅。根据核磁共振光谱，这种方法更成功，尽管催化剂仍然有50%以上的不纯。制备硅基阳离子物种的关键方法是在硅基氢化物(S)-3和三烷基四（五氟苯基）硼酸盐（TrTPFPB）(4) 30,31或三烷基四（3,5 -二（三氟甲基）苯基）硼酸盐（TrTFPB）(5)之间进行Corey氢化物转移28,29。32反应5min后，三烷基试剂的黄色消失，核磁共振光谱显示形成(S)-1a， b为唯一产物[图1]。
Recently, great progress has been made in the field of silylium chemistry and it seems possible to prepare the long-desired silicon cation in condensed phase. 1-15 Although there is a widespread use of silicon-based Lewis acids in organic chemistry, only very little attention has been paid to the utilization of the more reactive cationic species. 16 To our knowledge, no attempts have ever been made to synthesize chiral silicon-based Lewis acids, nor has the expected high reactivity of these chiral cationic species as catalysts ever been investigated. The chemistry of chiral silicon catalysts can be of great importance to the synthetic chemist, as silicon complexes are some of the few metal complexes which catalyzes important reactions, such as addition to imines and Friedel-Crafts reactions. 17-21 Another important aspect is the application of these catalysts as chiral cationic polymerization initiators. 22 On the basis of the recent developments in silyl cation chemistry, it is now reasonable to assume that a very reactive Lewis acid catalyst can be obtained by a careful choice of anion and solvent. 3-5, 14, 16 This paper presents the first preparation, partial characterization, and catalytic reactions using a silylium-based chiral Lewis acid ((S)-1a, b). It appears from (S)-1a, b that the chirality is anchored in a C2-symmetric 2, 2′-dimethyl-1, 1′-binaphthyl backbone, related to the BINOL ligand.The synthesis of the chiral binaphthylic skeleton (S)-2 starts from 2-methylnaphthalene and involves a resolution step of the binaphthyl ligand. 23-25 The silylation of (S)-2 was carried out by a nucleophilic addition of the dilithiated ligand to methyltrimethoxysilane giving the methoxy substituted silane, 26 which could be converted to (S)-3 by reduction (Scheme 1). The chiral silane (S)-3 is an air and moisture stable solid which can be handled without any special precautions. Several attempts have been performed to convert (S)-3 into the corresponding silyl cationic complex. Protonolysis with various Brønsted acids were too harsh, cleaving both the Si-H and Si-C bonds. Attempts were also performed to convert the silane to the silyl chloride by heating with CuCl2. 27 The chiral silyl chloride was more moisture sensitive than the parent silane, and treatment with silver triflate gave the silyl triflate. According to NMR spectroscopy, this approach was more successful, although the catalyst was still more than 50% impure. The key method for the preparation of the silyl cationic species was a Corey hydride transfer28, 29 between the silyl hydride (S)-3 and trityl tetrakis (pentafluorophenyl) borate (TrTPFPB)(4) 30, 31 or trityl tetrakis (3, 5-bis (trifluoromethyl)-phenyl) borate (TrTFPB)(5). 32 After 5 min of reaction time, the yellow color of the trityl reagent had vanished and (S)-1a, b was formed as the only product according to NMR spectroscopy [Figure 1].