Palladacycles substitution mechanics

The palladium catalyzed reactions of substituted vinylallenes with unactivated 1,3-butadienes proceeded with high selectivity133. A multistep mechanism, involving several palladacycles, was proposed to explain the high selectivities observed. 

Internal alkynes will also readily undergo palladium-catalyzed annulation by functionally substituted aromatic or vinylic halides to afford a wide range of heterocycles and carbocycles. However, the mechanism here appears to be quite different from the mechanism for the annulation of terminal alkynes. In this case, it appears that the reaction usually involves (1) oxidative addition of the organic halide to Pd(0) to produce an organopalladium(II) intermediate, (2) subsequent insertion of the alkyne to produce a vinylic palladium intermediate, (3) cyclization to afford a palladacycle, and (4) reductive elimination to produce the cyclic product and regenerate the Pd(0) catalyst (Eq. 28). 

Interestingly, HR of < -bromobenzaldehyde (93) with acrylate gave the doubly substituted product 94 and the expected product 95 under Jeffery s ligandless conditions [60]. Eormation of 94 is explained by the following mechanism. Insertion of acrylate to 93, followed by oxidative addition of aldehyde generates 96. The palladacycle 97 is formed by decarbonylation, and its reductive elimination gives 98. The final product 94 is obtained by HR of 98 with acrylate. 

Larock and Reddy obtained the 2-alkylidenecyclopentanone 72 by the reaction of l-(l-alkynyl)cyclobutanol 71 with iodobenzene. The bicyclononanone 74 was obtained from 73. Selective formation of 74 demonstrates that the more substituted bond a in the eyelobutanol 73 undergoes exclusive cleavage (or migration) [8,13]. Larock proposed the mechanism of the reaction of 75 involving ring expansion of 76 to form palladacycle 77 and reductive elimination to give 78. 

Hydrometallation proceeds by proximal bond cleavage. Addition of HSnBus affords the homoallylstaimane 172 [56]. The addition may be understood formally by the formation of the palladacycle 171, but explanation by the formation of the cyclo-propylcarbinylpalladium 173 is more appropriate. The intermediate 173 undergoes i6-carbon elimination to generate the homoallylpalladium 174, which then gives rise to 172 by reductive elimination. Addition of HSnBus to a substituted MCP 175 to yield 176 can be understood by this mechanism. 

A proposed mechanism was described as oxidative addition of benzyl halide to Pd(0) takes place leading to a Pd(II) species, the plausible palladacycle 134 for the final reductive elimination formed via either C-H activation or electrophilic aromatic substitution (Scheme 2.26). 

Several carbon-hydrogen bond substitutions are involved in the palladium-catalysed reaction of arylsulfonic acids with arenes to yield aromatic sulfones. A plausible mechanism, shown in Scheme 9, involves the initial formation of the palladacycle (99) which, after eoupling with the arene, yields the biphenyl derivative (100). Further coordination and carbon-hydrogen activation gives the seven-membered palladacycle (101) that affords the sulfone, (102), after reductive elimination. ... 

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