Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S. A. R., P. R. China.
J Org Chem. 2010 Sep 3;75(17):5837-51. doi: 10.1021/jo100848b.
Time-resolved spectroscopic experiments were performed to investigate the kinetics and mechanisms of the photodeprotection reactions for p-methoxyphenacyl (pMP) compounds, p-methoxyphenacyl diethyl phosphate (MPEP) and diphenyl phosphate (MPPP). The experimental results reveal that compared to the previous reports for the counterpart p-hydroxyphenacyl (pHP) phosphates, the (3)npi*/pipi* mixed character triplet of pMP acts as a reactive precursor that leads to the subsequent solvent and leaving group dependent chemical reactions and further affects the formation of photoproducts. The MPPP triplet in H(2)O/CH(3)CN and in fluorinated alcohols shows a rapid heterolytic cleavage (tau approximately 5.4 ns) that results in deprotection and formation of a solvolytic rearrangement product, whereas the MPPP triplet in CH(3)CN and the MPEP triplet in CH(3)CN and H(2)O/CH(3)CN and fluorinated alcohols decay on a much longer time scale (tau approximately 100 ns) with little observation of the rearrangement product. The density functional theory (DFT) calculations reveal a substantial solvation effect that is connected with the methoxy versus hydroxyl substitution in accounting for the different deprotection reactivity of pMP and pHP compounds. The results reported here provide new insight in elucidating the solvent and leaving group dependent dual reactivity of pMP compounds on the formation of the rearrangement versus reductive photoproduct.
进行了时间分辨光谱实验，以研究对甲氧基苯乙酰基（pMP）化合物，对甲氧基苯乙酰基二乙酯（MPEP）和二苯基磷酸酯（MPPP）的光解保护反应的动力学和机制。实验结果表明，与以前报道的对羟基苯乙酰基（pHP）磷酸酯相比，pMP 的（3）npi*/pipi*混合特征三重态作为反应前体，导致随后的溶剂和离去基团依赖性化学反应，并进一步影响光产物的形成。在 H(2)O/CH(3)CN 和氟化醇中的 MPPP 三重态显示出快速的异裂（tau 约为 5.4 ns），导致去保护和形成溶剂解重排产物，而在 CH(3)CN 中的 MPPP 三重态和在 CH(3)CN 和 H(2)O/CH(3)CN 和氟化醇中的 MPEP 三重态在更长的时间尺度（tau 约为 100 ns）上衰减，几乎没有观察到重排产物。密度泛函理论（DFT）计算揭示了一种实质性的溶剂化效应，该效应与甲氧基与羟基取代有关，解释了 pMP 和 pHP 化合物在形成重排与还原光产物方面的不同去保护反应性。这里报道的结果为阐明 pMP 化合物在形成重排与还原光产物方面的溶剂和离去基团依赖性双重反应性提供了新的见解。
