Aldehyde Reduction by a Pyridone Borane Complex through Boron‐Ligand‐Cooperation: Concerted or Not?

Abstract

The reduction of benzaldehyde by a pyridone borane complex was investigated by NMR, X‐ray analysis as well as DFT and DLPNO‐CCSD(T) computations. The reaction leads to the formation of a pyridone boronic ester complex stabilized by an NH···O hydrogen bond. The computations show that the hydrogenation takes place in a concerted fashion, yielding a transient benzyl alcohol boroxypyridine complex that undergoes a barrierless O–H addition. The concerted hydrogen transfer is associated with a transformation of a pyridone borane to a boroxypyridine, and thus with the interconversion of a neutral donor ligand to a covalently bound substituent.

Semihydrogenation of Alkynes Catalyzed by a Pyridone Borane Complex: Frustrated Lewis Pair Reactivity and Boron–Ligand Cooperation in Concert

“…The bispyridone complex 8 that was previously described and characterized in detail was observed by 1 H NMR as the resting state of the catalytic reaction (Figure ) . Furthermore, 1 H and 11 B NMR proved formation of boroxypyridine 3 with progressing reaction and hydrogen consumption.…”

“…The bispyridone complex 8 that was previously described and characterizedi nd etail was observed by 1 HNMR as the resting state of the catalytic reaction( Figure 1). [8] Furthermore, 1 Ha nd 11 BNMR proved formation of boroxypyridine 3 with progressing reaction and hydrogen consumption.T his finding strongly supports the assumption that 3 is part of the catalytic cycle. [9] To elucidate whether the envisioned protonolysiso ft he alkenylborane can be assumed to be part of the catalytic reaction, 5 was added to the borane 9,d erived from the reaction of Piers borane 6 and 3-hexyne.…”

Element‐Ligand Cooperativity with p‐Block Elements

“…Pan and others provided additional quantum chemical insights into this process and in silico ‐designed modified versions of 9 for hydrogenation of CO 2 . Gellrich's H 2 ‐activation product 10 exhibited reducing properties toward benzaldehyde resulting in the formation of the pyridone borinate ester adduct 11 (Figure b) . The mechanism of this transformation was studied, disclosing concerted hydrogenation, followed by barrierless trapping of the benzyl alcohol to give 11 .…”

Boron–Ligand Cooperation: The Concept and Applications

“…After hydrogen activation by the pyridonate borane 18',t he resulting pyridone borane complex 19 dissociates into the pyridone 21 and Piers borane 22.This endergonic reactioni sr endered thermodynamically more favorable by the complexation of the pyridone 21 with the pyridonate borane 18',y ielding the bispyridone complex 23. [13,23] After hydroboration of the alkyne by Piers borane 22,t he resulting alkenylborane 24 has to re-coordinate to the pyridone 21 to undergo protonolysis. Therefore, the bispyridonec omplex 23 has to dissociate again into the pyridonate borane 18' and the pyridone 21.T he latter coordinates to alkenylborane 24,y ieldingt he alkenylborane pyridone complex 25.P rotodeborylationl iberates the (Z)-alkene and regenerates the catalyst 18'.…”

“…After hydrogen activation by the pyridonate borane 18′ , the resulting pyridone borane complex 19 dissociates into the pyridone 21 and Piers borane 22 . This endergonic reaction is rendered thermodynamically more favorable by the complexation of the pyridone 21 with the pyridonate borane 18′ , yielding the bispyridone complex 23 [13, 23] . After hydroboration of the alkyne by Piers borane 22 , the resulting alkenylborane 24 has to re‐coordinate to the pyridone 21 to undergo protonolysis.…”