ω‐Hydroxy‐ortho‐quinodimethane: trapping with trifluoroacetone and photocyclization

Abstract

ω‐Hydroxy‐ortho‐quinodimethane (2), generated by photoenolization of 2‐methylbenzaldehyde (1) in benzene, is quantitatively trapped with trifluoroacetone to give the bicyclic hemiacetal 6 and the hydroxy aldehyde 7. A less effective trapping reaction is observed with benzaldehyde with the formation of the bicyclic hemiacetal 5. The formation of benzocyclobutenol 4 is probable due to a two‐photon process involving electronically excited 2.

o-Quinodimethanes: Efficient Intermediates in Organic Synthesis

“…The explanation given in the literature is the assumption of a twophoton process for the photochemical reaction, i.e., the reaction of triplet excited 2-methylbenzaldehyde (61a*) with the photochemically generated oQDM 66. 91 Griesbeck and Stadtmu ¨ller have recently reported the first example of a photochemically generated hydroxy-oQDM trapped by a ketone 90 and propose that a rather low bimolecular reaction rate constant for dimerization and not the previously proposed photochemical formation must be responsible for the exclusive formation of 67 during thermolysis of cyclobutanol 19b.…”

“…In the absence of trapping reagents, the photolysis of 61a leads to a dimerization product (hemiacetal, 67) besides a photoreduction product (1,2-ditolylglycol) (Scheme 23). 90 The dimerization product 67 was not observed when (E)-ω-hydroxy-oQDM (66) was generated by thermolysis of the benzocyclobutanol 19b. The explanation given in the literature is the assumption of a twophoton process for the photochemical reaction, i.e., the reaction of triplet excited 2-methylbenzaldehyde (61a*) with the photochemically generated oQDM 66.…”

“…( E )- 66 could be trapped with a variety of dienophiles such as dimethyl butynedioate, maleic anhydride, tetracyanoethylene, and even with triplet oxygen. In the absence of trapping reagents, the photolysis of 61a leads to a dimerization product (hemiacetal, 67 ) besides a photoreduction product (1,2-ditolylglycol) (Scheme ) …”

Application ofo-Quinodimethanes in Organic Synthesis

“…有些产生邻苯碳醌的方法在合成上非常有用, 这些 过程有的只能通过光化学法来实现, 或者采用光化学法 是最佳合成途径. 与前面的热反应合成法相比, 光化学 法的反应条件和反应浓度有着明显的不同, 这是因为前 者涉及的是基态反应, 而后者则是激发态反应 [37] . 在各 种产生邻苯碳醌的光化学法中, 特别需要注意三种类型 的反应: i)酮 44 光解脱去 CO; ii)邻烷基苯甲基醛(酮) (45) 的烯醇化; iii)邻烷基苯乙烯(46)的 1,5-氢迁移(Scheme 18) [1,31] .…”

“…其中 E-50 可以 被多种亲二烯体所捕获, 如丁炔二酸二甲酯、顺丁烯二 酸酐、四氰基乙烯以及三线态氧等. 在没有以上捕获剂 的情况下, 光解 45 除了能得到光还原产物 1,2-二甲苯乙 二醇之外, 还能得到半缩醛二聚化合物 51 (Scheme 20) [37] . [4,39] .…”