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

Abstract

The metal-free cis selectiveh ydrogenation of alkynes catalyzed by ab oroxypyridine is reported. Av ariety of internala lkynesa re hydrogenated at 80 8Cu nder 5bar H 2 with good yields and stereoselectivity.F urthermore, the catalyst describedherein enablest he first metal-free semihydrogenation of terminal alkynes. Mechanistic investigations, substantiated by DFT computations, reveal that the mode of action by which the boroxypyridinea ctivates H 2 is reminiscent of the reactivity of an intramolecular frustratedL ewis pair.H owever,i ti st he change in the coordination mode of the boroxypyridine upon H 2 activationt hat allows the dissociationo ft he formedp yridone borane complex and subsequenthydroboration of an alkyne. This change in the coordinationm ode upon bond activation is described by the term boron-ligand cooperation.

Synthesis of 6-Adamantyl-2-pyridone and Reversible Hydrogen Activation by the Corresponding Bis(perfluorophenyl)borane Complex

“…The results for a prototypical substrate are compared to those obtained with 2a as the catalyst. 5,7 The adamantyl pyridone borane complex 2b indeed catalyzed the gem dimerization of phenylacetylene. However, the activity of 2b was inferior compared to that of 2a (Scheme 6).…”

“…4 Classic FLPs undergo irreversible C sp -H activations with terminal alkynes. 5 Likewise, pyridonate borane 1a reacts with terminal alkynes to give a pyridone alkynyl borane complex 5.…”

Element‐Ligand Cooperativity with p‐Block Elements

“…The individual elementary steps of the boron‐based catalytic cycle feature alkyne addition to 9 ‐open yielding the addition product 12 , followed by subsequent carboboration of a second equivalent of the alkyne and final product formation from 13 under regeneration of the free catalyst 9 (Figure b). It was recently found that the hydrogen activation capability of 9 ‐open can also be applied to the semihydrogenation of internal and terminal alkynes (Figure b) . The mechanism is believed to proceed comparably to the alkyne dimerization sequence (vide supra).…”

“…The alkene products were obtained in a cis ‐selective manner. However, prolonged reaction times brought the isomerizing properties of HB(C 6 F 5 ) 2 for the transformation of cis ‐ to trans ‐alkenes to effect and gave coupling products with trans configuration …”

Die Bildung nukleophiler Allylborane aus molekularem Wasserstoff und Allenen katalysiert durch ein Pyridonatboran, das die Reaktivität eines frustrierten Lewis‐Paares zeigt

“…24096 www.angewandte.de dukt von Acetonitril, in 83 %A usbeute.D ie Bildung von 11 zeigt, dass das Allylboran 8 tatsächlich nukleophil ist (Schema 5).Die11 B-NMR Verschiebung von 21.4 ppm und die C = N Streckschwingung bei 1864 cm À1 zeigen, dass 11 als monomeres Ketiminboran mit einer linearen Anordnung der Substituenten am Stickstoff vorliegt. Anschließend wurde das Pyridon 5 zum Ketiminboran 11 gegeben.…”