Mechanistic Significance of the Si–O–Pd Bond in the Palladium-Catalyzed Cross-Coupling Reactions of Alkenylsilanolates

Abstract

Through the combination of reaction kinetics (both catalytic and stoichiometric) and solid-state characterization of arylpalladium(II) alkenylsilanolate complexes, the intermediacy of covalent adducts containing Si–O–Pd linkages in the cross-coupling reactions of organosilanolates has been unambiguously established. Two mechanistically distinct pathways have been demonstrated: (1) transmetalation via a neutral 8-Si-4 intermediate that dominates in the cross-coupling of potassium alkenylsilanolates, and (2) transmetalation via an anionic 10-Si-5 intermediate that dominates in the cross-coupling of cesium alkenylsilanolates. Arylpalladium(II) alkenylsilanolate complexes bearing various phosphine ligands (both bidentate and monodentate) have been isolated, fully characterized, and evaluated for their kinetic competence under thermal (stoichiometric) and anionic (catalytic) conditions. Comparison of the rates for thermal and anionic activation demonstrates that intermediates containing the Si–O–Pd linkage are involved in the cross-coupling process.

Cooperative redox activation for carbon dioxide conversion

“…A second transmetalation step from Si to Pd finally generates the precursor 41 for the reductive elimination step to diarylketone formation. This step proceeds possibly via the intervention of an anionic species 40a involving the activation of 40 with an additional silanolate in accord with the recent results from the Denmark group3132. If transmetalation of 40 or 40a in Cycle B is slow as is the case with electron rich aryl iodides run at lower reaction temperatures than 110 °C, then reductive elimination leads to the silylated carboxylate 42 (ref.…”

Mechanistic Significance of the Si–O–Pd Bond in the Palladium-Catalyzed Cross-Coupling Reactions of Arylsilanolates

“…44 Turnover-limiting reductive elimination has already been discounted. 1 , 43a However, this rate equation cannot differentiate between direct transmetalation via 18 and activated transmetalation via M + 20 – , as the ratio of silanolate/Pd is high.…”

“…In the preceding paper in this issue, the mechanism of palladium-catalyzed cross-coupling of alkenylsilanolates with aryl halides was investigated. 1 From a combination of kinetic analysis of catalytic reactions, X-ray structure determination of stable complexes bearing phosphine ligands, and stoichiometric reactions of those complexes, two distinct catalytic cycles were revealed (Figure 1 ). Under “ligandless” conditions (dpppO 2 ), the potassium alkenylsilanolate K + 2 – forms an 8-Si-4 2 intermediate ii containing a discrete Si–O–Pd linkage, which undergoes direct transmetalation to diorganopalladium species iii , which suffers reductive elimination to form product 3 .…”

“…In the preceding paper in this issue, the mechanism of palladium-catalyzed cross-coupling of alkenyl­silanol­ates with aryl halides was investigated . From a combination of kinetic analysis of catalytic reactions, X-ray structure determination of stable complexes bearing phosphine ligands, and stoichiometric reactions of those complexes, two distinct catalytic cycles were revealed (Figure ).…”

Pd‐Catalyzed Vinylation of Aryl Halides with Inexpensive Organosilicon Reagents Under Mild Conditions

“…The observation is expected because, according to recent studies, Pd(OAc) 2 /X‐phos is capable of activating aryl chlorides under mild conditions. The coordination of the aromatic ring in X‐phos could increase the electron density of the metal center (Pd) and promote the oxidative‐addition step, which is the rate‐determining step in this vinylation reaction . Reducing the amount of catalyst (1 mol %) generated a lower yield (68 %, entry 8).…”