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

Abstract

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.

Chemoselective catalytic hydrodefluorination of trifluoromethylalkenes towards mono-/gem-di-fluoroalkenes under metal-free conditions

“…Building on our previous work 59 , we envisioned that diazaphospholenes of super hydricity 60 – 62 may provide a good chance to realize HDF of trifluoromethylalkenes via an S N 2’ path 63 . A preliminary attempt indicated that the reaction of α-trifluoromethyl-styrene 2a with diazaphospholene 1 primarily gave the hydrophosphination intermediate A .…”

“…Catalyst 1 has been synthesized and characterized in our previous work 59 , 73 , 74 . Trifluoromethyl alkenes 2 were synthesized according to references (see Supplementary Information for details).…”

Stay positive: catalysis with 1,3,2-diazaphospholenes

“…Recently, the potent nucleophilicity of the DAP-hydrides was quantified according to the Mayr nucleophilicity scale by Cheng and co-workers. 16 Their systematic study on the kinetics of hydride addition to several benchmark electrophiles established that 1 is, at the time of writing, the most nucleophilic hydride donor ever quantified, with a nucleophilicity parameter (N) of 25.54 (cf. for NaBH 4 , N = 14.74).…”

1,3,2‐Diazaphospholene‐Catalyzed Reductive Cyclizations of Organohalides**

“…[2] For, instance, diazaphospholene hydrides (DAP-Hs) are powerful nucleophiles and weakly basic organic hydride donors. [3] They efficiently reduce carbonyls [4] and, in conjugate fashion, α,β-unsaturated amides, [5] esters, [5b, 6] ketones, [5] and acids (Scheme 1). [7] A σ-bond metathesis between the alkoxysubstituted DAPs and pinacol borane (HBpin) regenerating DAP-H rendered these processes catalytic.…”