Metal‐Free σ‐Bond Metathesis in 1,3,2‐Diazaphospholene‐Catalyzed Hydroboration of Carbonyl Compounds

Abstract

The first metal‐free catalytic hydroboration of carbonyl derivatives has been developed in which a catalytic amount of 1,3,2‐diazaphospholene effectively promotes a hydroboration reaction of aliphatic and aromatic aldehydes and ketones. The reaction mechanism involves the cleavage of both the PO bond of the alkoxyphosphine intermediate and the BH bond of pinacolborane as well as the formation of PH and BO bonds. Thus, the reaction proceeds through a non‐metal σ‐bond metathesis. Kinetic and computational studies suggest that the σ‐bond metathesis occurred in a stepwise but nearly concerted manner.

Aluminum Hydride Catalyzed Hydroboration of Alkynes

“…[1,2] In particular, the reductive functionalization of unsaturated polar bonds (e.g.,C =O, C=NR) with silicon and boron reagents is well-established, with both sand p-block based catalysts having been reported. [3][4][5][6][7][8] Although aluminum compounds have found use as Lewis acid catalysts and stoichiometric hydride reagents, [9] there is adearth of applications in wider catalysis.…”

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.…”

“…[7] A σ-bond metathesis between the alkoxysubstituted DAPs and pinacol borane (HBpin) regenerating DAP-H rendered these processes catalytic. [4] Reports by Kinjo and Speed exploited σ-bond metathesis between the PÀ N and BÀ H bonds of ammonia borane and HBpin, respectively to enable catalytic reductions of azobenzenes [8] and imines. [5b, 9] Cheng demonstrated the cleavage of the PÀ F bond in DAP fluorides by employing phenylsilane.…”

Chiral 1,3,2‐Diazaphospholenes as Catalytic Molecular Hydrides for Enantioselective Conjugate Reductions

“…[10b] Thed iscovery of facile s-bond metathesis between alkoxy DAPs and pinBH by Kinjo and coworkers was an important advancement as it allows the regeneration of the DAP, thus enabling catalytic ketone reductions. [11] Moreover,t his report led other groups to consider the use of more stable alkoxy DAPs as viable precatalysts that are in situ activated by pinBH. Independently,the groups of Speed and Kinjo reported 1,4-reductions of a,b-unsaturated carbonyl derivatives with achiral DAP catalysts in 2017, [12a,b] followed by the hydroboration of pyridines.…”