Crespi Stefano, Ravelli Davide, Protti Stefano, Albini Angelo, Fagnoni Maurizio
PhotoGreen Lab, Department of Chemistry, University of Pavia, Viale Taramelli 12, 27100 Pavia (Italy).
Chemistry. 2014 Dec 22;20(52):17572-8. doi: 10.1002/chem.201404787. Epub 2014 Oct 30.
The scope of the photochemical generation of α,n-didehydrotoluene diradicals from aryl sulfonates and phosphates and their chemistry are explored. The thermally inaccessible α,2- and α,4- intermediates are efficiently obtained by irradiation of ortho- and para-(trimethylsilylmethyl)phenyl triflates through heterolytic splitting of the ester anion from the substrate in the triplet state. Triplet phenyl cations are formed and the loss of trimethylsilyl cation from them affords the desired diradicals ((3) Me3 SiCH2 C6 H4 -OZ→(3) Me3 SiCH2 C6 H4 (+) →(⋅) CH2 C6 H4 (⋅)). Triplet sensitization is required, for which acetone is used throughout. Direct irradiation leads, on the contrary, to photo-Fries fragmentation ((1) Me3 SiCH2 C6 H4 O-Z→Me3 SiCH2 C6 H4 O(⋅) +Z(⋅)). With mesylates, where ester cleavage is less convenient, a further competition from the triplet is direct desilylation. Didehydrotoluenes are also obtained from the corresponding phosphates, although with poor efficiency.
探索了由芳基磺酸盐和磷酸盐光化学生成α,n - 二脱氢甲苯双自由基的范围及其化学性质。通过在三重态下从底物中杂裂酯阴离子，对邻位和对位 -（三甲基硅基甲基）苯基三氟甲磺酸酯进行辐照，可高效获得热不稳定的α,2 - 和α,4 - 中间体。形成三重态苯基阳离子，从中失去三甲基硅基阳离子得到所需的双自由基（(3) Me3SiCH2C6H4 - OZ→(3) Me3SiCH2C6H4(+) →(⋅)CH2C6H4(⋅)）。需要三重态敏化，全程使用丙酮。相反，直接辐照会导致光 - 弗里斯碎片化（(1) Me3SiCH2C6H4O - Z→Me3SiCH2C6H4O(⋅) + Z(⋅)）。对于甲磺酸酯，酯裂解不太方便，三重态的另一个竞争反应是直接脱硅。相应的磷酸盐也能得到二脱氢甲苯，不过效率较低。
