Photochemical generation and methanol trapping of localized 1,3 and 1,4 singlet diradicals derived from a spiroepoxy-substituted cyclopentane-1,3-diyl
Photochemical generation and methanol trapping of localized 1,3 and 1,4 singlet diradicals derived from a spiroepoxy-substituted cyclopentane-1,3-diyl
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DOI：
10.1021/ja9822761
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发表时间：
1998-11-11
影响因子：
15
通讯作者：
Nau, WM
中科院分区：
文献类型：
--
作者：
Abe, M;Adam, W;Nau, WM
偶氮烷烃2，3-二氮杂双环-[2.2.1]庚-2-烯-7，3 '-螺-2'，2 '-二苯基环氧乙烷1在环境温度下的直接和二苯甲酮敏化的光脱氮（约100 ℃）。15 ℃）在苯中的溶液中，仅得到6-氧杂双环[3.2.0]庚-1-烯3。不稳定的氧杂环丁烷3建议形成的C-O键的环氧环中的最初光生的，螺环氧取代的，本地化的环戊烷-1，3-二基二自由基1，3-DR的1，4-DR二自由基和随后的环化后者的选择性裂解。即使在-100摄氏度，高度应变（应变能估计为约。94 kcal/mol）螺环氧housane 2不能通过H-1 NMR（600 MHz）光谱观察到。在MeOH中，代替氧杂环丁烷3，以高产率获得两种区域异构的MeOH加合物6（双自由基1，3-DR和1，4-DR的捕获产物）和6'（3的甲醇解产物）。第一次甲醇捕获的环戊烷-1，3-二基单线态双自由基实现了所需的偶极字符大概是诱导在这个对称物种的极性甲醇介质。计算工作（UB 3LYP/6- 31 G *）揭示了S-1，3-DR单重双自由基具有单重基态（Δ E-ST ca. 1 kcal/mol），环闭合（E-A ca. 2.6 kcal/mol）转化为螺环环氧housane 2需要比环氧化物开环（E-A Ca. 0.7 kcal/mol）; T-1，3-DR三重态物质自发开环（E-A Ca.立体电子效应决定了C-O键与1，3-DR中的自由基p轨道的适当构象排列，以选择性地裂解环氧环中的C-O键而不是C-C键。
The direct and benzophenone-sensitized photodenitrogenation of the azoalkane 2,3-diazabicyclo-[2.2.1]hept-2-ene-7,3'-spiro-2',2'-diphenyloxirane 1 at ambient temperature (ca. 15 OC) in benzene afforded exclusively the 6-oxabicyclo[3.2.0]hept-1-ene 3. The labile oxetane 3 is proposed to be formed by the selective cleavage of the C-O bond in the epoxy ring of the initially photogenerated, spiroepoxy-substituted, localized cyclopentane-1,3-diyl diradical 1,3-DR to the 1,4-DR diradical and subsequent cyclization of the latter. Even at -100 degrees C, the highly strained (strain energy estimated to be ca. 94 kcal/mol) spiroepoxy housane 2 could not be observed by H-1 NMR (600 MHz) spectroscopy. In MeOH, instead of the oxetane 3, the two regioisomeric MeOH adducts 6 (trapping product of the diradicals 1,3-DR and 1,4-DR) and 6' (methanolysis product of 3) were obtained in high yields. For the first time methanol trapping of a cyclopentane-1,3-diyl singlet diradical was achieved; the required dipolar character is presumably induced in this symmetric species by the polar methanol medium. Computational work (UB3LYP/6-31G*) reveals that the S-1,3-DR singlet diradical possesses a singlet ground state (Delta E-ST ca. 1 kcal/mol), for which ring closure (E-A ca. 2.6 kcal/mol) to the spiroepoxy housane 2 requires more activation than epoxide-ring opening (E-A Ca. 0.7 kcal/mol); the T-1,3-DR triplet species ring-opens spontaneously (E-A Ca. 0 kcal/mol) to the corresponding T-1,4-DR. Stereoelectronic effects dictate the appropriate conformational alignment of the C-O bond with the radical p orbital in the 1,3-DR for the selective cleavage of the C-O rather than C-C bond in the epoxy ring.