Photolyses of 1,2;4,5- and 1,2;3,4-benzenetetracarboxylic dianhydrides (3a and 3b), which would be precursors of 1,4- and 1,3-benzdiyne (2a and 2b), were studied by a matrix isolation technique and a selective irradiation technique using three kinds of excimer lasers in order to directly observe intermediates produced stepwise. The photolyzed products in the matrix were characterized by FT-IR and UV–vis spectroscopies. As a result, sequential decarboxylation and decarbonylation from one anhydride moiety of 3a and 3b produced corresponding benzocyclopropenone and benzyne intermediates in the initial stage. In both the photolyses, further decomposition proceeded to form 1,3,5-hexatriyne (4) as a final product. Although neither 2a nor 2b was observed directly, it seems that the benzdiynes including another isomer, 1,2,3,5-tetradehydrobenzene (2c), participated as precursors to acyclic C4H–Câ–·C–H biradical 13 and/or carbene 14, which were formed in the reaction from benzdiyne to 4. Additionally, as a result of CCSD(T)/6-31G**//CASSCF(4,4)/6-31G** level calculations for 2a, 2b and 2c, it is clear that the energy of 2c was comparable to those of 2a and 2b, which supports the formation of 13 from 2a/2b and of 14 from 2c after isomerization of 2a→2c and 2b→2c.
对1,2;4,5-和1,2;3,4-
苯四
羧酸二酐(3a和3b)的光解进行了研究,这些化合物是1,4-和1,3-
苯二炔(2a和2b)的前体。采用基质隔离技术和选择性照射技术,使用三种类型的准分子激光器,以便直接观察逐步产生的
中间体。在基质中光解产物通过FT-IR和紫外-可见光光谱进行表征。结果表明,从3a和3b的一个酐部分进行的顺序
脱羧和
脱羰反应在初始阶段产生了相应的
苯环丙烯酮和
苯二炔
中间体。在这两种光解反应中,进一步分解最终形成了1,3,5-六炔(4)作为最终产物。尽管没有直接观察到2a或2b,但似乎包括另一异构体1,2,3,5-四
脱氢苯(2c)在内的
苯二炔作为前体,参与了反应形成非环状
C4H–Câ–·C–H双自由基13和/或
碳烯14。此外,对2a、2b和2c进行的CC
SD(T)/6-31G**//CASSCF(4,4)/6-31G**等级计算表明,2c的能量与2a和2b相当,这支持了在2a→2c和2b→2c异构化后,13是由2a/2b形成,14是由2c形成的。