Dimeric palladium hydroxide

The catalytic ability of dimeric palladium hydroxide in carbonylative Sonogashira coupling was demonstrated by Alper and his team in 1994 [29]. In this report, terminal alkynes and alkynols were coupled with aryl iodides in the presence of carbon monoxide in moderate to good yields (Scheme 5.6). In 1995 Cacchi and colleagues presented a general methodology for 5-(2-acylethynyl)-... 

Scheme 5.6 Dimeric palladium hydroxide-catalyzed carbonylative Sonogashira coupling...

Reaction efficiency was improved using ethoxy acrylate (42, R = Et Scheme 11) to avoid methoxide generation. Methoxide is capable of forming bridged dimeric palladium complexes that hinder palladium catalysis, analogous to the process observed with hydroxide. The use of ethoxy acrylate reduced palladium catalyst loading from 5-10 mol% to 0.5-2.0 mol%. 

It has been shown that hydroxide bridged dimeric palladium complexes can form and impede catalysis, see Hartwig, J.F. Palladium-catalyzed amination of aryl halides mechanism and rational catalyst design. Synlett 1997, 4, 329-340. 

The negatively charged base reacts with the arylpalladium(II) halide to give the arylpalladium hydroxide or alkoxide complex, which is able to form the dimeric palladium-boron complex XXIII what is crucial for the transmetallation process [2-6]. It is apparent that the metal cation (from the base) accelerates the formation of the latter, as clearly showed by Zhang and coworkers [15]. They have developed the SM coupling procedure for sterically bulky arylboronic acids when the clear influence of the anion basicity and the cation effect were discovered. The cationic radius is presumably an important parameter which influences the formation of dimeric... 

The reaction rate is half-order in palladium and dimeric hydroxides of the type shown are very common for palladium. The reaction is first order in alcohol and a kinetic isotope effect was found for CH2 versus CD2 containing alcohols at 100 °C (1.4-2.1) showing that probably the (3-hydride elimination is rate-determining. Thus, fast pre-equilibria are involved with the dimer as the resting state. When terminal alkenes are present, Wacker oxidation of the alkene is the fastest reaction. Aldehydes are prone to autoxidation and it was found that radical scavengers such as TEMPO suppressed the side reactions and led to an increase of the selectivity [18],... 

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