Palladium–Borane Cooperation: Evidence for an Anionic Pathway and Its Application to Catalytic Hydro‐/Deutero‐dechlorination

Abstract

Metal–Lewis acid cooperation provides new opportunities in catalysis. In this work, we report a new type of palladium–borane cooperation involving anionic Pd0 species. The air‐stable DPB palladium complex 1 (DPB=diphosphine‐borane) was prepared and reacted with KH to give the Pd0 borohydride 2, the first monomeric anionic Pd0 species to be structurally characterized. The boron moiety acts as an acceptor towards Pd in 1 via Pd→B interaction, but as a donor in 2 thanks to B‐H‐Pd bridging. This enables the activation of C−Cl bonds and the system is amenable to catalysis, as demonstrated by the hydro‐/deutero‐dehalogenation of a variety of (hetero)aryl chlorides (20 examples, average yield 85 %).

Evaluation of Pd→B Interactions in Diphosphinoborane Complexes and Impact on Inner‐Sphere Reductive Elimination

“…The 11 B NMR resonances are shifted linearly towards higher field with an increasing Pd,B distance for Pd 0 complexes, regardless of the valence electron count at the Pd center (Figure ). Complex [( Ph DPB Ph )Pd 0 (PPh 3 )] ( 2 ) reported by Kameo and Bourissou also fits perfectly into this correlation (d(Pd,B)=2.294(2) Å, δ ( 11 B) 27 ppm). In contrast, the 11 B NMR resonance shifts linearly towards lower field with an increasing Pd,B distance in case of Pd II complexes.…”

“…Thus, facile access to complexes featuring transition metals with formally negative oxidations states is realized (Figure a) . This stabilization of low oxidation states appears to inhibit oxidative addition reactions . However, we demonstrated that this obstacle can be overcome for complex 1 by addition of catalytic amounts of acetate, which competes with Pd 0 for the free coordination site at the borane, thus reversibly breaking the Pd 0 →B interaction (Figure b) .…”

“…However, we demonstrated that this obstacle can be overcome for complex 1 by addition of catalytic amounts of acetate, which competes with Pd 0 for the free coordination site at the borane, thus reversibly breaking the Pd 0 →B interaction (Figure b) . This concept allowed for the application of 1 in catalytic allylic amination, and most recently of 2 in the catalytic hydro‐/deutero‐dechlorination of aryl chlorides . Alternatively, bifunctional substrate activation across the M→Z interaction has been described .…”

Catalysis using transition metal complexes featuring main group metal and metalloid compounds as supporting ligands

“…37 Recently, Kameo and Bourissou reported another type of Pd-B cooperation involving anionic Pd species in a hydrodechlorination reaction of aryl chlorides. 38 The reaction of a Pd complex 17 having a Z-type PBP-ligand with KH and [2.2.2]cryptand afforded an anionic Pd complex 18, which possessed a bridging hydride ligand between Pd and B (Scheme 5). The anionic Pd complex 18 smoothly reacted with chlorobenzene to give benzene and the neutral Pd-B complex 17 whereas the same oxidative addition of PhCl to 17 did not proceed at all.…”

The Pincer Platform Beyond Classical Coordination Patterns

“…[ 27 ] Subsequently, Z‐type ligands based on the element boron were incorporated in pincer‐type complexes, [ 28–42 ] and used as catalysts in reactions like (de)hydrogenations. [ 34,38,43–45 ]…”