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Second Comes First: Switching Elementary Steps in Palladium-Catalyzed Cross-Coupling Reactions.

ABSTRACT: The electron-poor palladium(0) complex L3 Pd (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) reacts with Grignard reagents RMgX and organolithium compounds RLi via transmetalation to furnish the anionic organopalladates [L2 PdR]- , as shown by negative-ion mode electrospray-ionization mass spectrometry. These palladates undergo oxidative additions of organyl halides R'X (or related SN 2-type reactions) followed by further transmetalation. Gas-phase fragmentation of the resulting heteroleptic palladate(II) complexes results in the reductive elimination of the cross-coupling products RR'. This reaction sequence corresponds to a catalytic cycle, in which the order of the elementary steps of transmetalation and oxidative addition is switched relative to that of palladium-catalyzed cross-coupling reactions proceeding via neutral intermediates. An attractive feature of the palladate-based catalytic system is its ability to mediate challenging alkyl-alkyl coupling reactions. However, the poor stability of the phosphine ligand L against decomposition reactions has so far prevented its successful use in practical applications.

SUBMITTER: Kolter M

PROVIDER: S-EPMC7540703 | biostudies-literature | 2020 Sep

REPOSITORIES: biostudies-literature

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Second Comes First: Switching Elementary Steps in Palladium-Catalyzed Cross-Coupling Reactions.

Kolter Marlene M Koszinowski Konrad K

Chemistry (Weinheim an der Bergstrasse, Germany) 20200907 53

The electron-poor palladium(0) complex L3 Pd (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) reacts with Grignard reagents RMgX and organolithium compounds RLi via transmetalation to furnish the anionic organopalladates [L2 PdR]- , as shown by negative-ion mode electrospray-ionization mass spectrometry. These palladates undergo oxidative additions of organyl halides R'X (or related SN 2-type reactions) followed by further transmetalation. Gas-phase f ...[more]

PMID: 32428266

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Project description:Detailed mechanistic studies on the coupling of aryl halides with thiols catalyzed by palladium complexes of the alkylbisphosphine ligand CyPF-(t)Bu (1-dicyclohexylphosphino-2-di-tert-butylphosphinoethylferrocene) are reported. The elementary steps that constitute the catalytic cycle, i.e. oxidative addition, transmetalation and reductive elimination, have been studied, and their relative rates are reported. Each of the steps of the catalytic process occurs at temperatures that are much lower than those required for the reactions catalyzed by a combination of palladium precursors and CyPF-(t)Bu. To explain these differences in rates between the catalytic and stoichiometric reactions, studies were conducted to identify the resting state of the catalyst of the reactions catalyzed by a combination of Pd(OAc)(2) and CyPF-(t)Bu, a combination of Pd(dba)(2) and CyPF-(t)Bu, or the likely intermediate Pd(CyPF-(t)Bu)(Ar)(Br). These data show that the major palladium complex in each case lies off of the catalytic cycle. The resting state of the reactions catalyzed by Pd(OAc)(2) and CyPF-(t)Bu was the palladium bis-thiolate complex [Pd(CyPF-(t)Bu)(SR)(2)] (R = alkyl or aryl). The resting state in reactions catalyzed by Pd(2)(dba)(3) and CyPF-(t)Bu was the binuclear complex [Pd(CyPF-(t)Bu)](2)(mu(2),eta(2)-dba) (9). The resting states of reactions of both aromatic and aliphatic thiols catalyzed by [Pd(CyPF-(t)Bu)(p-tolyl)(Br)] (3a) were the hydridopalladium thiolate complexes [Pd(CyPF-(t)Bu)(H)(SR)] (R= alkyl and aryl). All these palladium species have been prepared independently, and the mechanisms by which they enter the catalytic cycle have been examined in detail. These features of the reaction catalyzed by palladium and CyPF-(t)Bu have been compared with those of reactions catalyzed by the alkylbisphosphine DiPPF and Pd(OAc)(2) or Pd(dba)(2). Our data indicate that the resting states of these reactions are similar to each other and that our mechanistic conclusions about reactions catalyzed by palladium and CyPF-(t)Bu can be extrapolated to reactions catalyzed by complexes of other electron-rich bisphosphines.

Dataset Information

Second Comes First: Switching Elementary Steps in Palladium-Catalyzed Cross-Coupling Reactions.

Publications

Second Comes First: Switching Elementary Steps in Palladium-Catalyzed Cross-Coupling Reactions.

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