Reducing agents as components in catalytic systems of the Heck reaction

Abstract

Using of reducing agents for arylation of alkenes has been found to increase the activity of the catalytic system and allow to avoid the use of traditional tertiary amines as bases in the Heck reaction.

Effect of macrokinetic factors on the ligand-free Heck reaction with non-activated bromoarenes

“…Unlike the reaction with iodobenzene where the major deactivation process is Pd oxidation (Scheme 1, F and B) [18][19][20]30], in the reaction with bromobenzene the influence of agglomeration (Scheme 1, C and D) and Pd dissolution (Scheme 1, E) on catalytic activity was expected to be more significant (due to low reactivity of PhBr [11]). The agglomeration of metal atoms non-stabilized in the solution occurs at a great rate because of considerable energetic winning; this rate may potentially exceed the diffusion rate of both single atoms and growing aggregates in the solution.…”

“…However, that problem might be solved by the addition of reducing agent small amounts (for example, sodium formate) [15,20]. The second undesirable process is an agglomeration of catalytically active Pd(0) complexes (Scheme 1, C and D), an acceleration of which is favored by a lowering of ArX reactivity, i.e.…”

Interplays between Reactions within and without the Catalytic Cycle of the Heck Reaction as a Clue to the Optimization of the Synthetic Protocol

“…79 As a model system a super-active catalytic system containing additives of sodium formate [Pd(acac) 2 + HCOONa + NaOAc] was studied. 91 As noted, the Pd(acac) 2 reduction is an autocatalytic reaction (reaction autoacceleration is distinctly fixed). It turned out that kinetics of the reaction at high conversion of starting Pd(acac) 2 is impossible to describe without considering a process which leads to a decrease in soluble Pd(0) concentration.…”

“…127 The catalytic activity of this system was one order higher than the most effective catalytic system using tertiary amines as bases.…”

In situ NMR studies of the mechanism of homogeneously and heterogeneously catalysed Heck reactions of aryl chlorides and bromides

“…2a,11 But some authors consider the colloidal Pd as plausible reservoir for the Pd(0) regeneration in the solution. 11a,b,d The above scheme of the catalyst deactivation-regeneration makes it possible to suggest a number of methods for enhancement of the catalytic system efficiency: the application of catalytic quantities of reducing agent for the Pd recovery from nonactive oxidated state; 9 an increase of aryl halogenide concentration for the more successful competition of the oxidative addition with the aggregation and for Pd recovery into catalytic cycle from a possible colloidal state; 2a,11b,d a decrease of catalyst concentration 11,12a to build up an added barrier to the catalyst agglomeration, which is of the higher kinetic order relative to Pd(0) than the oxidative addition; 12b the use of oxidizing agent for the oxidation of "dead" Pd(0) agglomerates to a state from which Pd could be involved in catalysis again. 3,10,13 All discussed processes are connected with the Pd oxidation-reduction transformation.…”