Studies on the Mechanism of Allylic Coupling Reactions: A Hammett Analysis of the Coupling of Aryl Silicate Derivatives

Abstract

Hammett analysis of the palladium-catalyzed allyl-aryl coupling reaction has demonstrated that the rate of the coupling reaction is enhanced by electron-withdrawing groups on the aryl siloxane. The positive slope of the Hammett plot indicated involvement of a charged transition state in which negative charge on the aryl ring is stabilized inductively. This result is consistent with either transmetalation or reductive elimination being the rate-determining step in the coupling process. Furthermore, the influence of ligand on the metal site has been assessed from competition studies as a function of ligand type, cone angle, and electronic effects. From the relative ratios of coupling products produced in the Hammett study, it is possible to gather insight into the role of the electronic as well as the steric effects of ligands on the mechanism of the coupling reaction.

Estratégias para estudo das correlações de energia livre em acoplamento aril-aril de Suzuki: elucidando ciclos catalíticos através da equação de Hammett

“…Portanto, grupos eletrorretiradores como R' aumentarão a velocidade da transmetalação quando o acoplamento ocorrer com bromoarenos e isso corrobora a observação feita por DeShong e Shukla. 16 Contudo, tais suposições carecem de maiores investigações para total comprovação.…”

Substrate‐Selective Catalysis

“…DeSong and co-workers performed a Hammett analysis of the Pd-catalyzed coupling between ethyl 2-cyclohexenylcarbonate (396) and para-substituted phenyl siloxanes. [116] A linear Hammett correlation (log(k rel ) versus s) was obtained from a series of pair-wise competitive experiments in which a mixture of triethoxy(phenyl)silane (397) and 398 (R = OMe, Me, Cl, CO 2 Et, 1.42 mmol of each) was reacted with 396 (0.712 mmol) as the coupling partner and [Pd(dba) 3 ] (0.0712 mmol) as the pre-catalyst in the presence of TBAF (2.85 mmol) (Scheme 80). The competitive experiments demonstrated that phenyl siloxanes carrying an electron-withdrawing para-substituent were the most reactive.…”

In Situ Monitoring of Transmetallation in Electric Potential-Promoted Oxidative Coupling in a Single-Molecule Junction

“…For example, the electronic influence of organic reagents on transmetallation is not well understood and contradictory conclusions have been reported. [19][20][21][22] Organoboron compounds are one of Page 2 of 18 CCS Chemistry 3 the most diverse classes of easily obtained reagents, 23 and provide access to a raft of valuable indispensable transformations such as Suzuki-Miyaura coupling, Chan-Evans-Lam coupling, 24 oxidative coupling 25 and so on. 26 They undergo facile transmetallation with various transition metals in the presence of base.…”