Comprehensive Kinetic Screening of Palladium Catalysts for Heck Reactions

Abstract

Comprehensive kinetic screening of Pd catalysts for Heck reactions via a consecutive pulse reaction methodology allows a more informed choice of catalyst for a particular transformation, taking into account not only initial reactivity but also long-term catalyst stability. Competitive reactions also offer mechanistic information. The resting state and rate-limiting step within the catalytic cycle are found to be the same for phospha- and azapalladacycles and ligandless Pd(OAc)₂ but different for the zero valent complex Pd[P(t-Bu)₃]₂.

Key words

Heck reactions - Pd - kinetic screening - reaction calorimetry

References and Notes

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13

Although catalyst 3 is not as robust as the palladacycles 1 and 2 under this reaction protocol, we have shown that catalyst concentration is stable during the progress of the reactions using 3 at 140 °C. This suggests that the deactivation we observe occurs in between pulse reactions, when the catalyst is starved for aryl halide. Further, efficient reaction is possible with 3 at much lower temperatures where deactivation may not be as significant. Thus while the screening protocol may not apply directly to the conditions under which a catalyst may be employed, it can give general hints about relative stability. For instance, the relative stability in screening revealed here correlates with the relative solution stability of the three catalysts.

15

One rationalization for the difference in our results in Figure [3] compared to those of ref. 14 may lie in the methodology used to determine relative rates. We found that under conditions where the olefin is the limiting reagent, care must be taken to avoid a second arylation reaction, which consumes aryl halide, and which may alter the assumption that the monoarylated product ratio equals the ratio of the aryl bromide rate constants. In addition, product ratios changed during the reaction but reached constant values at higher conversion. Therefore, we only used data points of the part showing constant product ratios, at conversions prior to formation of double arylation products.