Mechanism and Selectivity of N-Triflylphosphoramide Catalyzed (3+ + 2) Cycloaddition between Hydrazones and Alkenes
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- 作者：Xin Hong ; Hatice Ba艧p谋nar K眉莽眉k ; Modhu Sudan Maji ; Yun-Fang Yang ; Magnus Rueping ; K. N. Houk
- 刊名：Journal of the American Chemical Society
- 出版年：2014
- 出版时间：October 1, 2014
- 年：2014
- 卷：136
- 期：39
- 页码：13769-13780
- 全文大小：1090K
- ISSN：1520-5126
文摘
Br酶nsted acid catalyzed (3+ + 2) cycloadditions between hydrazones and alkenes provide a general approach to pyrazolidines. The acidity of the Br酶nsted acid is crucial for the catalytic efficiency: the less acidic phosphoric acids are ineffective, while highly acidic chiral N-triflylphosphoramides are very efficient and can promote highly enantioselective cycloadditions. The mechanism and origins of catalytic efficiencies and selectivities of these reactions have been explored with density functional theory (M06-2X) calculations. Protonation of hydrazones by N-triflylphosphoramide produces hydrazonium鈥損hosphoramide anion complexes. These ion-pair complexes are very reactive in (3+ + 2) cycloadditions with alkenes, producing pyrazolidine products. Alternative 1,3-dipolar (3 + 2) cycloadditions with the analogous azomethine imines are much less favorable due to the endergonic isomerization of hydrazone to azomethine imine. With N-triflylphosphoramide catalyst, only a small distortion of the ion-pair complex is required to achieve its geometry in the (3+ + 2) cycloaddition transition state. In contrast, the weak phosphoric acid does not protonate the hydrazone, and only a hydrogen-bonded complex is formed. Larger distortion energy is required for the hydrogen-bonded complex to achieve the 鈥渋on-pair鈥?geometry in the cycloaddition transition state, and a significant barrier is found. On the basis of this mechanism, we have explained the origins of enantioselectivities when a chiral N-triflylphosphoramide catalyst is employed. We also report the experimental studies that extend the substrate scope of alkenes to ethyl vinyl ethers and thioethers.