Mechanistically informed selection rules for competing β-hydride and β-heteroatom eliminations

Abstract

Alkylpalladium complexes are important intermediates in several industrially relevant catalytic reactions such as the Mizoroki-Heck, alkyl C-H activation and ethylene polymerisation. β-elimination -of either a hydride (β-Η) or a heteroatom (β-Χ) -is the most common decomposition pathway for these intermediates; this can either lead to the desired reaction as in the Mizoroki-Heck reaction, or it can hinder reaction progress as in ethylene/vinyl halide co-polymerisations. Despite the importance of these elimination processes, little mechanistic understanding exists with respect to the factors that control them. Here, we present a systematic investigation of the factors governing the competition between β-Η and β-Χ in catalytically relevant alkylpalladium complexes. These results enabled us to derive selection rules which dictate ligand choice to control selectivity for either elimination. This knowledge may allow chemists to manipulate β-eliminations in the design of chemoselective catalytic reactions for a wide range of applications.

Reciprocal Interactions of Abiotic and Biotic Dechlorination of Chloroethenes in Soil

“…Though there could be competition between β-H and β-Cl elimination in FeS-mediated dechlorination of CEs, the low pK a H (the aqueous pK a of the conjugate acid) of the chlorines was recently identified to favor the β-Cl elimination. 72 Therefore, β-Cl elimination was preferentially enabled in the abiotic dechlorination of TCE and DCEs. DCEs with two chlorines were converted to acetylene through dichlorine elimination.…”

“…Interestingly, this study found that OHRB could promote FeS-mediated PCE-to-acetylene dechlorination by providing less-chlorinated ethenes (e.g., TCE and DCEs). Though there could be competition between β-H and β-Cl elimination in FeS-mediated dechlorination of CEs, the low p K a H (the aqueous p K a of the conjugate acid) of the chlorines was recently identified to favor the β-Cl elimination . Therefore, β-Cl elimination was preferentially enabled in the abiotic dechlorination of TCE and DCEs.…”

Radical-Polar Crossover Catalysis with a d0 Metal Enabled by a Redox-Active Ligand

“…45 Additionally, an experiment (see pg S99) was run with an OMs leaving group which has been demonstrated by Morandi and co-workers to prefer β-hydride elimination over β-OMs elimination in an inner sphere mechanism but still generates a mixture of those two products. 45 Under our conditions only styrene�the product of β-OMs elimination�is observed. This further supports an outer sphere elimination mechanism from the anion.…”

“…We chose to explore the mechanism using derivatives of 3a due to the 19 F NMR spectroscopy tag and the availability of the starting materials. Three potential intermediates of the organic fragment that could form product are that of (1) a radical intermediate, 44 (2) an anionic intermediate formed from radical-polar crossover, and (3) an inner sphere β-halide elimination from an organometallic species generated from two-electron oxidative addition of 3 to 2 45,46 (Figure 4). The results of no competition between elimination of bromide versus fluoride in Figure 3e suggest the leaving group ability governs this selectivity.…”

“…Similarly, the preference of fluoride versus hydride elimination (Figure 3d) could provide support for ruling out an innersphere mechanism, although this is an understudied topic with early transition metals. 45 Additionally, an experiment (see pg S99) was run with an OMs leaving group which has been demonstrated by Morandi and co-workers to prefer β-hydride elimination over β-OMs elimination in an inner sphere mechanism but still generates a mixture of those two products. 45 Under our conditions only styrene�the product of β-OMs elimination�is observed.…”