A Nucleophilicity Scale for the Reactivity of Diazaphospholenium Hydrides: Structural Insights and Synthetic Applications.

Nucleophilicity parameters (N, sN) of a group of representative diazaphospholenium hydrides were derived by kinetic investigations of their hydride transfer to a series of reference electrophiles with known electrophilicity (E) values, using the Mayr equation log k2=sN(N+E). The N scale covers over ten N units, ranging from the most reactive hydride donor (N=25.5) to the least of the scale (N=13.5). This discloses the highest N value ever quantified in terms of Mayr's nucleophilicity scales reported for neutral transition‐metal‐free hydride donors and implies an exceptional reactivity of this reagent. Even the least reactive hydride donor of this series is still a better hydride donor than those of many other nucleophiles such as the C−H, B−H, Si−H and transition‐metal M−H hydride donors. Structure–reactivity analysis reveals that the outstanding hydricity of 2‐H‐1,3,2‐diazaphospholene benefits from the unsaturated skeleton. P−H superhydrides: The unexpected philicity of P−H bonds in diazaphospholenes has made their applications to catalytic reductions a burgeoning field. A three‐parameter kinetic equation was used to evaluate their nucleophilicity parameters (N), which cover over ten N units. Kinetic studies imply their much superior hydricity over commonly used hydrides, with 2‐H‐1,3,2‐diazaphospholene being the strongest nucleophilic donor ever quantified by the Mayr equation. [ABSTRACT FROM AUTHOR]