含噻吩鄰醌二烯及Non-Kekulé四自由基前驅物之合成及其熱化學、光化學反應

Abstract

本研究的目的在合成 non-Kekulé 四自由基前驅物，並研究自由基的基態電子自旋狀態。我們分別成功的合成出 meta-disultine 32與para-disultine 36，探討其在 Diels-Alder 反應上的應用，並進行光化學和電子順磁共振光譜 (EPR) 等研究，藉由實驗結果探討 disultine 化合物是否為一類良好的四自由基前驅物。 將化合物32及36與 dienophile 進行高溫封管反應，除了可獲得重排的異構物 disulfolene 57及58，也可在脫去 SO2 後與烯類加成獲得 fused Type A 及 Type B 產物，與四自由基分子16a 之反應極為相似。而化合物 2,5-dimethylthienosultine 42a 與 2,5-dicholrothienosultine 42b 與 C60 反應時，均可成功的得到和 C60 行單一加成產物82a 及82b。經由變溫 1H-NMR 實驗研究我們發現，82a 與82b 六員環構形交換步驟的活化能 (ΔGc≠) 分別是14.3及13.5 kcal/mol，噻吩上的2,5-取代基對 boat-to-boat 構形翻轉活化能只有些微影響，其值與文獻上記載的碳環衍生物相近。 比較同一系列 C60 單一加成產物82a-d，我們發現化合物82a 是唯一在空氣中不安定的產物。此雙甲基噻吩取代的 C60 分子為極佳單態氧的光敏化劑，與 Komatsu 等人 (J. Org. Chem. 2001, 66, 8187.) 所發表的結果有異曲同工之妙，皆可看到 1O2 參與反應。 將化合物36與 N-phenylmaleimide (NPM) 進行光化學反應，以300 nm 波長照光，可得與熱化學反應相同的產物，且300 nm 波長對 disultine 36混合異構物之光解具有選擇性。在低溫介質的電子順磁共振光譜的研究中，化合物32、36、42a 及42b 皆可以偵測到三態雙自由基中間體的生成，此結果與 Berson 的單態雙自由基中間體5不同，其可能的原因是碳氧鍵斷裂生成三態雙自由基。而 disultine 的低溫光解量子產率 (Φ) 非常低導致我們在低溫 EPR 光譜沒有觀測到 quintet 四自由基。

The objectives of our research are to synthesize stable precursors for non-Kekulé tetraradicals, and to study the electronic states of their ground states. After successfully synthesized meta-disultine 32, and para-disultine 36, we have explored their potential application in Diels-Alder reaction and studied their photochemistry and EPR spectroscopy. From these studies, sultines were found to be good precursors for non-Kekulé tetraradicals. When compounds 32 and 36 were reacted with dienophiles in a sealed tube at high temperature, one obtained not only the rearranged disulfolenes 57 and 58 but also fused adducts of Type A and Type B. The reaction patterns of these disultines are very similar to those of corresponding tetraradical molecule 16a. When sultines 42a and 42b were reacted with C60, the mono-adducts of the 3,4-bismethylene thiophenes with C60 were obtained in good yields. Variable temperature 1H-NMR experiments show that the coalescence temperature (Tc) and activation free energy (ΔGc≠) for the boat-to-boat inversion of C60 mono-adducts 82a and 82b are 14.3 and 13.5 kcal/mol, respectively. These ΔGc≠ are similar to those reported for carbocyclic derivatives in the literatures. Compare the mono adducts of C60, 82a-d, we found that compound 82a is the only product that is unstable in air (with room light). This dimethylthienoadduct of C60 is a very good singlet oxygen sensitizer which is very similar to those by Komatsu (J. Org. Chem., 2001, 66, 8187.) The products from photochemical reactions of these disultines 36 (irradiated with 300 nm UV-light) are similar to those from thermochemical reactions. Furthermore, it was interesting to find that one of these disultine isomers is selectively photolyzed by 300 nm light. ESR on these sultine molecules give spectra consistent with triplet biradical intermediates, which is different from the result of singlet biradical intermediates 5 reported by Berson in diazene photolysis. A possible reason for this observation is the cleavage of carbon-oxygen bond of a triplet excited sultine. The extremely low quantum yield (Φ) for the photolysis of sultine explains why we did not observe a quintet in low temperature EPR spectroscopy.