Pyridine Hydroboration with a Diazaphospholene Precatalyst

Abstract

We demonstrate pyridine hydroboration catalyzed by a diazaphospholene hydride precatalyst. Pyridines bearing electron-withdrawing groups in the 3-position are hydroborated efficiently. This system features low catalyst loadings, fast reaction times at ambient temperature, and tolerance of other reducible functionality. Off-cycle products of pyridine hydrophosphination were also obtained in one case from a stoichiometric reaction and structurally characterized.

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

“…These advantages make synthetic applications of the PÀHh ydrides ab urgeoning field. [9] Moreover, their chiral analogues (such as 1g)w ere also exploited in catalysis of asymmetric hydride-initiated transformations with high enantioselectivity. [9] Moreover, their chiral analogues (such as 1g)w ere also exploited in catalysis of asymmetric hydride-initiated transformations with high enantioselectivity.…”

“…Consequently,an umber of N-heterocyclic phosphorus hydrides ( Figure 1) and their analogues have been developed in recent years,rendering many originally infeasible reactions to occur under metal-free conditions,s uch as reductions of polar olefins, [5] imines, [5a] ketones, [6] and azo compounds, [7] valorization of CO 2 , [8] and hydroboration of pyridines. [9] Moreover, their chiral analogues (such as 1g)w ere also exploited in catalysis of asymmetric hydride-initiated transformations with high enantioselectivity. [10] In these processes,t he propensity of PÀHs pecies to cleavage their PÀHb onds dominates the feasibility of the reaction.…”

Alkoxide‐Promoted Selective Hydroboration of N‐Heteroarenes: Pivotal Roles of in situ Generated BH3 in the Dearomatization Process

“…At the outset, we hypothesized a working mode based on the literature: pyridines will be activated by forming an adduct with HBpin, which will undergo a hydride transfer from a presupposed active species, alkoxide‐HBpin (MOR⋅HBpin) via a concerted outer‐sphere pathway. The sterically congested C2‐position was predicted to be less reactive than C4 in the hydride transfer process, thus eventually affording an N‐ borylated 1,4‐dihydropyridyl product (Scheme ).…”

A 1,3,2‐Diazaphospholene‐Catalyzed Reductive Claisen Rearrangement

“…According to Cheng and co‐workers, certain DAPs are among the most nucleophilic hydride donors quantified according to the Mayr nucleophilicity scale . They have been shown to reduce aldehydes and ketones, imines, pyridines, and azobenzenes, as well as α,β‐unsaturated carbonyl derivatives . Kinjo et al.…”