The confined cavity of a coordination cage suppresses the photocleavage of α-diketones to give cyclization products through kinetically unfavorable pathways
The confined cavity of a coordination cage suppresses the photocleavage of α-diketones to give cyclization products through kinetically unfavorable pathways
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DOI：
10.1002/anie.200701250
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Fujita, Makoto
中科院分区：
文献类型：
--
作者：
Furusawa, Takahito;Kawano, Masaki;Fujita, Makoto
与酮丰富的光化学相反，[1] α-二酮的光化学反应在很大程度上尚未探索，主要是因为α-二酮的主要反应途径是通过均裂成酰基自由基引发的，酰基自由基随后产生许多降解产物的混合物。[2，3]通过与酮类比，α-二酮应该潜在地具有导致合成有用物质的光化学途径。然而，这种化合物的形成通常由均裂裂解主导。如果α-二酮被限制在笼状的受限空腔中，则由均裂裂解形成的酰基立即重新结合，得到起始的α-二酮。[4]因此，裂解途径是可忽略的，否则不利的反应途径可能是主要的反应过程。考虑到α-二酮会形成动力学上不利的光化学产物，我们研究了笼状化合物对α-二酮的包合反应，并研究了笼状化合物中随后的光化学反应。我们采用了自组装的M6 L4笼（1），它具有良好的中性有机分子的结合能力。[5]我们发现，α-二酮（2）的前所未有的分子内环化可以导致环化产物，而不会裂解α-二酮骨架（方案1）。二苯乙二酮（2）被用作客体分子。当过量（约5当量）的2在100 ℃下悬浮于1的水溶液（8 mm）中1小时时，水不溶性2部分溶解，无色溶液变成浅黄色。在过滤过量的2之后，1H NMR光谱表明客体信号显著地向高场移动（Δδ%<2. 1- 103 5），这是包合复合物形成的诊断（图1）。根据积分比，我们估计每个笼中大约包封两个客体分子，得到络合物1 '（2）2。适合于X射线晶体结构分析的单晶通过在一周内缓慢蒸发水从水中获得。晶体学分析证实了1 ′（2）2配合物的结构。单晶X射线衍射分析显示，两个客体分子正交地堆积在1的空腔中，其中它们中的每一个都采用两个羰基之间的二面角为828的扭曲构象（图2）。
In contrast with the rich photochemistry of ketones,[1] the photochemical reactions of α-diketones have been largely unexplored, mainly because the major reaction pathways from α-diketones are initiated by homolytic cleavage into acyl radicals that subsequently give a mixture of many degradation products.[2, 3] By analogy with ketones, α-diketones should potentially have photochemical pathways leading to synthetically useful species. However, the formation of such compounds is normally dominated by homolytic cleavage. If αdiketones are confined in a restricted cavity of cages, the acyl radicals formed by the homolytic cleavage are immediately recombined to give the starting α-diketones.[4] As a result, the cleavage pathway is negligible and otherwise unfavorable reaction pathways can be major reaction courses. Expecting the formation of kinetically unfavorable photochemical products from α-diketones, we examined the enclathration of an α-diketone by a cage compound and studied subsequent photochemical reactions in the cage. We employed the selfassembled M6L4 cage (1), which has a well-established binding ability for neutral organic molecules.[5] We found that the unprecedented intramolecular photocyclization of the α-diketone (2) can lead to cyclized products without cleavage of the α-diketone framework (Scheme 1). Diphenylethanedione (2) was employed as a guest molecule. When an excess amount (approximately 5 equivalents) of 2 was suspended in an aqueous solution of 1 (8 mm) at 1008C for 1 h, water-insoluble 2 was partially dissolved and the colorless solution turned pale yellow. After the filtration of excess 2, 1H NMR spectroscopy indicated that the guest signals were significantly shifted upfield (Δδ% À2. 1–À3. 5), which was diagnostic of the formation of an inclusion complex (Figure1). From the integral ratio, we estimated that approximately two guest molecules were encapsulated per cage, giving the complex 1'(2) 2. Single crystals suitable for X-ray crystal structure analysis were obtained from water by the slow evaporation of water over a week. The crystallographic analysis confirmed the structure of the 1'(2) 2 complex. The single-crystal X-ray diffraction analysis revealed that the two guest molecules were orthogonally packed in the cavity of 1, wherein each of them adopted a twisted conformation with a dihedral angle of 828 between the two carbonyl groups (Figure 2).