Insights into the Mechanism of the Negishi Reaction: ZnRX versus ZnR 2 Reagents

Self Cite

217

131

Bimetallic catalysis refers to homogeneous processes in which either two transition metals (TM), or one TM and one Group 11 (G11) element (occasionally Hg also), cooperate in a synthetic process (often a C-C coupling) and their actions are connected by a transmetalation step. This is an emerging research area that differs from the isolated or tandem applications of the now classic processes (Stille, Negishi, Suzuki, Hiyama, Heck). Most of the reactions used so far combine Pd with a second metal, often Cu or Au, but syntheses involving very different TM couples (e.g., Cr/Ni in the catalyzed vinylation of aldehydes) have also been developed. Further development of the topic will soon demand a good knowledge of the mechanisms involved in bimetallic catalysis, but this knowledge is very limited for catalytic processes. However, there is much information available, dispersed in the literature, coming from basic research on exchange reactions occurring out of any catalytic cycle, in polynuclear complexes. These are essentially the same processes expected to operate in the heart of the catalytic process. This Review gathers together these two usually isolated topics in order to stimulate synergy between the bimetallic research coming from more basic organometallic studies and the more synthetic organic approaches to this chemistry.

Section: Introductionmentioning

confidence: 99%

Bimetallic Catalysis using Transition and Group 11 Metals: An Emerging Tool for CC Coupling and Other Reactions

Pérez‐Temprano

Casares

Espinet

2012

Self Cite

217

131

“…Keywords: cross-coupling · density functional calculations · N-heterocyclic carbenes · palladium · zinc steps: 1) oxidative addition (OA): insertion of Pd 0 into the carbon-halide bond with Pd 0 being oxidized to Pd II ; 2) transmetalation (TM): transfer of the second carbon fragment to Pd II from an organometallic compound concomitant with inorganic salt formation, which dissociates from the complex; 3) reductive elimination (RE): expulsion of the product hydrocarbon with regeneration of the Pd 0 catalyst. The importance of Pd-mediated cross-coupling has led to numerous computational studies that have revealed important insights into the structure and behavior of NHC-Pd [7][8][9] and phosphane-Pd [10,11] catalysts and actual or putative catalytic cycle intermediates as well as the nature of oxidative addition, [11][12][13] transmetalation, [14] and reductive elimination [15] steps. Computational analysis of whole catalytic cycles [9,16] have been performed much less frequently.…”

mentioning

confidence: 99%

Density Functional Theory Investigation of the Alkyl–Alkyl Negishi Cross‐Coupling Reaction Catalyzed by N‐Heterocyclic Carbene (NHC)–Pd Complexes

Chass

O’Brien

Hadei

et al. 2009

A novel mechanism is proposed for the Pd-1,3-(2,6-diisopropylphenyl)imidazolyl-2-ylidene (1) catalyzed Negishi reaction. DFT computations supported by atoms-in-molecules (AIM) analyses of non-truncated models show that a "steric wall" created by the N-substituent on the ligand guides reactants to and from the Pd center. Notably, transmetalation and not oxidative addition is found to be rate-limiting. Additionally, a key Pd-Zn interaction (approximately = 2.4 A, rho(b) approximately = 0.0600 au) is identified in the mechanism. This interaction persists beyond reductive elimination and, in combination with the ligand, facilitates reductive elimination by creating a highly sterically crowded environment in the coordination sphere of the Pd center.

“…Notably, both aryl groups are transferred from their organozinc reagents to the organic halide. [16] The highest rates of conversion and yields were observed when the reactions were performed in NMP/ THF ( % 4:1) mixtures (NMP = N-methylpyrrolidone). [17] When diphenylzinc was allowed to react, for example, with various, non-ortho-substituted aryl bromides and aryl chlorides with either electron-withdrawing or electron-donating groups, such as 1-(4-chlorophenyl)ethanone, 1-(3-chlorophenyl)ethanone, (4-bromophenyl)A C H T U N G T R E N N U N G (phenyl)methanone, ethyl 4-bromobenzoate, and 5-bromo-2-benzofuran-1A C H T U N G T R E N N U N G (3 H)-one, as well as with 1-chloro-4-methylbenzene, 1-bromo-4-ethenylbenzene, or imines such as N-[(1E)-(4-bromophenyl)methyliden]aniline, or with 1-bromo-3-(diethoxymethyl)benzene, 1-bromo-4-methoxybenzene, or 1-bromo-3,5-dimethoxybenzene in the presence of the catalyst (0.1 mol %), the corresponding biaryls were cleanly formed in yields of > 80 % in only 30 min when aryl bromides are used and within 1 h when aryl chlorides were applied (Scheme 2).…”

Section: Resultsmentioning

confidence: 99%

“…[16] Reductive elimination of the cross-coupling product (aryl-aryl') closes the catalytic cycle and regenerates the catalyst (cycle A).…”

Section: A C H T U N G T R E N N U N G Iden]aniline)mentioning

confidence: 99%

Negishi Cross‐Coupling Reactions Catalyzed by an Aminophosphine‐Based Nickel System: A Reliable and General Applicable Reaction Protocol for the High‐Yielding Synthesis of Biaryls

Gerber

Frech

2011

Treatment of NMP solutions of NiCl(2) with 1,1',1''-(phosphanetriyl)tripiperidine (≈2.05 equiv), dissolved in THF, in air at 25 °C forms a highly active catalytic system for the cross-coupling of a large variety of electronically activated, non-activated, deactivated, and ortho-substituted, heterocyclic, and functionalized aryl bromides and aryl chlorides with diarylzinc reagents. Very high levels of conversion and yields were obtained within 2 h at 60 °C in the presence of only 0.1 mol% of catalyst (based on nickel) and thus at catalyst loadings far lower than typically reported for nickel-catalyzed versions of the Negishi reaction. Various aryl halides-which may contain trifluoromethyl groups, fluorides, or other functional groups such as acetals, ketones, ethers, esters, lactones, amides, imines, anilines, alkenes, pyridines, quinolines, and pyrimidines-were successfully converted into the corresponding biaryls. Electronic and steric variations are tolerated in both reaction partners. Experimental observations indicate that a molecular (Ni(I)/Ni(III)) mechanism is operative.