β-X vs. β-H Elimination. Selection Rules for Chemoselectivity Enabled by Mechanistic Studies

Abstract

Alkylpalladium complexes are important intermediates in several industrially relevant catalytic reactions such as the Mizoroki–Heck, alkyl C–H activation and ethylene polymerisation. Beta-elimination - of either a hydride (β-Η) or a heteroatom (β-Χ) - is the most common decomposition pathway for these intermediates; this can either promote 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 15 processes, little mechanistic understanding exists with respect to the factors that control them. 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 beta-eliminations in the design of chemoselective catalytic reactions for a wide range of applications.

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

“…Similarly, the preference of fluoride versus hydride elimination confirms this. 38 Additionally, no competitive cross-coupling is observed between the organic fragments in the inner sphere mechanism of this coordinatively saturated complex further ruling out mechanism 3. While mechanism 1 is also unlikely, because catalyst 1 is known to induce radical mechanisms, further studies were needed to rule it out.…”

“…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 37 , 2) an anionic intermediate formed from radical-polar crossover, and 3) an inner sphere b-halide elimination from an organometallic species 38,39 (Figure 4). The results of no competition between elimination of bromide versus fluoride in Figure 3e suggest the leaving group ability governs this selectivity.…”