Mechanistic Duality in Palladium-Catalyzed Cross-Coupling Reactions of Aryldimethylsilanolates. Intermediacy of an 8-Si-4 Arylpalladium(II) Silanolate

Abstract

The mechanism of palladium-catalyzed cross-coupling reactions of potassium aryl(dimethyl) silanolate K + 1 − has been investigated. Under catalysis by (t-Bu 3 P) 2 Pd the coupling with 2-bromofluorobenzene (2) displays the following rate equation: rate = k obs [R 3 SiOK] 0 [ArylBr] 0 with k obs = k[(t-Bu 3 P) 2 Pd] 0.98 . An independent study of the individual steps of the catalytic cycle has revealed a dual mechanistic pathway. The transmetalation can occur by a thermal process via an 8-Si-4 intermediate without the need for anionic activation. Additionally, arylsilanolates can serve as activators for transmetalation via a hypervalent 10-Si-5 siliconate intermediate.Palladium-(and nickel-)catalyzed cross-coupling reactions have emerged as the premier methods for construction of carbon-carbon (and carbon-heteroatom) bonds between unsaturated organic moieties. The historical development, diversification and applications of the myriad variants of this process are amply chronicled in recent monographs and authoritative volumes. 1 The generally accepted mechanism for these transition-metal-catalyzed, crosscoupling reactions involve: (1) oxidative addition to an electrophilic halide or pseudo halide (2) transmetalation from an organometallic donor and (3) reductive elimination to generate a new C-X bond. Although the oxidative addition of an organic halide to a Pd(0) complex is common among most coupling reactions, the transmetalation step is distinctive to the organometallic donor employed. The transmetalation of organostannanes has been extensively studied but is still a matter of some debate.2 These transfers have been suggested to follow an inter-molecular S E 2 mechanism by means of stereochemical models. Similarly, organosilane transmetalations can proceed via either a four-centered, closed transition structure (retentive) or an acyclic, open process (invertive). 3 Cross-coupling reactions of organoboranes are highly base dependent and a pre-coordination of the transferring agent with the transition metal followed by delivery has been proposed for the transmetalation step.4, Although kinetic studies have provided independent evidence for these pathways, the ability to isolate and characterize pre-transmetalation precursors is needed to unambiguously establish the course of palladiumcatalyzed, cross-coupling reactions.5 , 6Recent kinetic investigations from these laboratories on the mechanism of palladium-catalyzed cross-coupling with alkenyl(dimethyl)silanols 7 revealed that the transmetalation step is dependent on the reaction conditions. Cross-coupling of (E)-dimethyl(1-heptenyl)silanol under "classical activation" 8 by TBAF proceeds via a pentacoordinate siliconate complex that engages in a bimolecular transmetalation with the organoPd(II)-X acceptor.9a However, in a second mechanistic study, the observation of saturation kinetics suggested that upon silanol deprotonation, the resulting silanolate (iia) forms an adduct (iiia) with the organoPd(II)-X intermediate (i) from which (turnover limiting)...

Probing the electronic demands of transmetalation in the palladium-catalyzed cross-coupling of arylsilanolates

“…Previous studies on the mechanism of activated cross-coupling of arylsilanolates, 7 revealed an interesting insight that potentially complicates the interpretation of the studies herein. Specifically, the rate of cross-coupling of stoichiometrically formed complexes 4 shows a first order dependence on the concentration of added silanolate, whereas under catalytic conditions, a zeroth-order dependence on the silanolate is seen.…”

“…6 > Moreover, transmetalation via both neutral 8-Si-4 and anionic 10-Si-5 intermediates have been demonstrated. 78 Most of the evidence available to support these pathways comes from kinetic analysis of the catalytic and, in some cases, stoichiometric reactions.…”

“…14 Recent studies from these laboratories on the mechanism of the cross-coupling reaction of arylsilanolates with aryl halides in the presence of ( t -Bu 3 P) 2 Pd documented the intermediacy of a three-coordinate arylpalladium(II) arylsilanolate complex as the pretransmetalation species. 7 These studies included the isolation and spectroscopic, crystallographic and kinetic analysis of this intermediate. 15 The ability to prepare the pretransmetalation intermediate enables a direct interrogation of the electronic characteristics of the critical step in the catalytic cycle.…”

On the Stereochemical Course of Palladium-Catalyzed Cross-Coupling of Allylic Silanolate Salts with Aromatic Bromides

“…Previous studies in these laboratories on the cross-coupling of alkenyl-44 and arylsilanolates,45 have led to the formulation of a mechanism which involves the formation of an arylpalladium silanolate by attack of the starting silanolate salt on the arylpalladium(II)halide. The intermediacy of a discrete species containing an Si–O–Pd linkage has since been demonstrated in a number of different cross-coupling reactions 46.…”

Palladium-Catalyzed Cross-Coupling Reactions of Organosilanols and Their Salts: Practical Alternatives to Boron- and Tin-Based Methods

“…Finally, complex 12 was independently synthesized and fully characterized including single crystal X-ray analysis (Figure 4). 19…”