Helical Polyisocyanides with Fan-Shaped Pendants
Helical Polyisocyanides with Fan-Shaped Pendants
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DOI：
10.1021/ma802345g
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发表时间：
2009-01
期刊：
影响因子：
5.5
通讯作者：
Takashi Kajitani;Hongzhen Lin;Kanji Nagai;K. Okoshi;H. Onouchi;E. Yashima
中科院分区：
文献类型：
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作者：
Takashi Kajitani;Hongzhen Lin;Kanji Nagai;K. Okoshi;H. Onouchi;E. Yashima
具有单手螺旋构象的光学活性聚合物表现出有趣的功能，如液晶、对映选择性识别和催化剂，在手性材料科学领域引起了相当大的关注。为此，通过非外消旋单体的聚合和用手性催化剂或引发剂对非手性或前手性单体的螺旋选择性聚合制备了具有光学活性的螺旋聚合物。一般来说，强的吸引或排斥相互作用，如分子内氢键3和相邻垂基团之间的空间位阻2a-e， h，是产生具有优先手螺旋感的光学活性螺旋聚合物所必需的。然而，Veciana和他的同事报告说，在聚合过程中，立体中心远离聚合物主链的异氰酸酯可以形成具有过量左旋或右旋螺旋构象的螺旋型多异氰酸酯。这种手性从侧链转移到聚合物主链的长程效应归因于相邻的苯甲酸苯之间的位阻效应。4 .螺旋诱导过程对手性取代基的位置表现出奇偶效应（a-c, e, 5）。此外，Meijer et al. 6和Percec et al. 7分别制备了带有多个(S)- 3,7 -二甲基氧基链的树突状和扇形垂链的螺旋聚乙炔，并广泛研究了空间效应对其螺旋构象的影响。这些螺旋聚乙炔在热致性体系中形成液晶（LC）宏观结构。6,7最近，我们报道了一种前所未有的对映体纯苯基异氰酸酯的螺旋控制聚合，该聚合带有长n-癸基链的l -或d -丙氨酸悬垂。8与氨基酸或肽结合的螺旋聚（异氰酸肽）s, 1e, 3a, b， d形成鲜明对比，如果与非手性镍催化剂（NiCl2）的聚合是非对映选择性地进行，则产生右旋或左旋螺旋聚（苯基异氰酸酯），其螺旋感可由聚合溶剂和温度控制。在繁殖反应中，生长链端悬垂的酰胺残基与单体之间的分子间氢键似乎在聚异氰酸苯形成稳定的右或左螺旋结构的动力学或热力学过程中起着至关重要的作用。这一推测得到了以下事实的支持：具有与悬垂物相同的n-癸基链的l -乳酸残基的聚（苯基异氰化物）与聚合条件无关，表现出非常弱的棉花效应。9在这项研究中，我们合成了一系列新的光学活性苯基异氰化物，它们带有单、二或三(S)烷氧基链，其立体中心位置与垂链不同（1a-3a和1b-3b），它们与非手性NiCl2或µ-乙二基钯铂（Pd-Pt）配合物聚合，10已知这是苯基异氰化物活性聚合的有效催化剂。所得的聚（苯基异氰酸酯）s（聚1a-聚3a，聚1b-聚3b，聚3a ‘和聚3b ’）（图1）具有光学活性的烷氧基链，并且不能期望通过吸引相互作用（如氢键）形成优先手螺旋。然后我们利用圆二色性（CD）和吸收光谱研究了聚合物的垂基团的空间效应和立体中心位置（奇偶效应）对聚合物的旋光性和LC性质以及螺旋结构的影响。
Optically active polymers with a single-handed helical conformation, 1 exhibiting interesting functions, such as liquid crystals, enantioselective recognition, and catalysts, have drawn considerable attention in the fields of chiral materials science. To this end, optically active helical polymers have been prepared by the polymerization of nonracemic monomers and by the helix-sense selective polymerization of achiral or prochiral monomers with chiral catalysts or initiators. 1, 2 In general, strong attractive or repulsive interactions, such as intramolecular hydrogen bonds3 and steric hindrance2a-e, h between the neighboring pendant groups, respectively, are required for producing optically active helical polymers with a preferred-handed helix sense. However, Veciana and coworkers reported that helical polyisocyanides with an excess left-or right-handed helical conformation could be formed during the polymerization of isocyanides with a stereogenic center positioned far from the polymer backbone. This long-range effect of chirality transfer from the side chains to the polymer backbones was ascribed to the steric effect between the neighboring phenyl benzoates at the pendants. 4 The helicity induction process showed odd-even effects4a-c, e, 5 with respect to the position of the chiral substituent. In addition, the helical polyacetylenes bearing dendritic and fan-shaped pendants with multiple (S)-3, 7-dimethyloctyloxy chains were prepared by Meijer et al. 6 and Percec et al., 7 respectively, and the steric effects on their helical conformations were extensively investigated. These helical polyacetylenes formed liquid crystalline (LC) macrostructures in the thermotropic system. 6, 7 Recently, we reported an unprecedented helix-sense-controlled polymerization of enantiomerically pure phenyl isocyanides bearing an L-or D-alanine pendant with a long n-decyl chain. 8 In sharp contrast with amino acid-or peptide-bound helical poly (isocyanopeptide) s, 1e, 3a, b, d, f the polymerization with an achiral nickel catalyst (NiCl2) diastereoselectively proceeded, resulting in either a right-or a left-handed helical poly (phenyl isocyanide) whose helix-sense could be controlled by the polymerization solvent and temperature. 8 An intermolecular hydrogen bond between the pendant amide residues of the growing chain end and the monomer during the propagation reaction appeared to play a crucial role in the poly (phenyl isocyanide) either kinetically or thermodynamically forming a stable right-or left-handed helical structure. This speculation was supported by the fact that a poly (phenyl isocyanide) having L-lactic acid residues with the same n-decyl chain as that of the pendants exhibited a very weak Cotton effect independent of the polymerization conditions. 9 In this study, we synthesized a series of novel optically active phenyl isocyanides bearing mono-, di-, or tri-(S)-alkoxy chains with stereogenic centers at a different position than that of the pendants (1a-3a and 1b-3b), and they were polymerized with an achiral NiCl2 or the µ-ethynediyl palladium-platinum (Pd-Pt) complex, 10 which is known to be an effective catalyst for the living polymerization of phenyl isocyanides. The resulting poly (phenyl isocyanide) s (poly-1a-poly-3a, poly-1b-poly-3b, poly-3a′, and poly-3b′)(Chart 1) possess optically active alkoxy chains, and a preferred-handed helix formation cannot be expected through attractive interactions such as hydrogen bonds. We then investigated the steric effects and the position of the stereogenic center (odd-even effect) of the pendant groups of the polymers on their chiroptical and LC properties and helical structures by using circular dichroism (CD) and absorption …