Elson Kathryn E, Jenkins Ian D, Loughlin Wendy A
School of Science, Griffith University, Nathan, Brisbane, QLD, 4111, Australia.
Org Biomol Chem. 2003 Aug 21;1(16):2958-65. doi: 10.1039/b305375j.
The alkoxytriphenylphosphonium ion intermediate of the Mitsunobu reaction can be generated using the Hendrickson reagent, triphenylphosphonium anhydride trifluoromethanesulfonate, 1. Strangely, while the reagent 1 can be used in place of the Mitsunobu reagents (triphenylphosphine and a dialkylazodicarboxylate) for the esterification of primary alcohols, secondary alcohols such as menthol undergo elimination. Evidence is presented to show that this unexpected result is due to the presence of trialkylammonium triflate salts. Such salts lead to a dramatic decrease in the rate of esterification relative to competing elimination. The Mitsunobu esterification of menthol with p-nitrobenzoic acid was re-examined and the occurrence of elimination reported for the first time. The presence of traces of tetrabutylammonium triflate led to a dramatic reduction in the yield of inverted ester and a corresponding increase in the yield of anti elimination product 2-menthene. The mechanism of the Mitsunobu reaction is discussed in the light of the dramatic salt effect on both the rate and outcome of the reaction and the possible involvement of ion pair clustering. In contrast, use of the reagent 1 resulted in syn elimination to give a 1:2 mixture of 2- and 3-menthenes. Finally, 1 and sodium azide can be used to convert a primary alcohol into an azide in high yield. There was no reaction under Mitsunobu conditions.
光延反应的烷氧基三苯基鏻离子中间体可以使用亨德里克森试剂三氟甲磺酸三苯基鏻酸酐（1）来生成。奇怪的是，虽然试剂1可以代替光延试剂（三苯基膦和二烷基偶氮二甲酸酯）用于伯醇的酯化反应，但诸如薄荷醇之类的仲醇会发生消除反应。有证据表明，这一意外结果是由于三氟甲磺酸三烷基铵盐的存在。相对于竞争性消除反应，此类盐会导致酯化反应速率显著下降。对薄荷醇与对硝基苯甲酸的光延酯化反应进行了重新研究，并首次报道了消除反应的发生。痕量三氟甲磺酸四丁基铵的存在导致构型翻转的酯的产率大幅降低，同时反式消除产物2-薄荷烯的产率相应增加。鉴于盐对反应速率和结果的显著影响以及离子对聚集的可能参与，对光延反应的机理进行了讨论。相比之下，使用试剂1会导致顺式消除，生成2-薄荷烯和3-薄荷烯的1:2混合物。最后，1与叠氮化钠可用于将伯醇高产率地转化为叠氮化物。在光延反应条件下则不发生反应。
