Journal
CHEMICAL SCIENCE
Volume 3, Issue 6, Pages 1987-1995Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c2sc20103h
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Funding
- National Science Foundation [CHE-1059084]
- Natural Science and Engineering Research Council of Canada (NSERC)
- NSERC
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The mechanism of Pd(0)-catalyzed carbohalogenation of alkenes has been investigated with density functional theory. The catalytic cycle involves oxidative addition of the aryl halide, alkene insertion, and a novel C(sp(3))-I reductive elimination. The C(sp(3))-I reductive elimination leads to a weakly bonded Pd-product complex, which undergoes ligand exchange via a dissociative mechanism to regenerate the catalyst. In the rate-determining reductive elimination step, bromides and chlorides have higher barriers than iodides, because the stronger Pd-Br and Pd-Cl bonds are being cleaved in these transition states. Bulky ligands, such as P(t-Bu)(3) and Q-Phos, facilitate the C(sp(3))-I reductive elimination by preventing the formation of tetracoordinated intermediates. The mechanism of the competing beta-hydrogen elimination pathway was also investigated. For reactions involving a syn-beta-hydrogen atom in the alkyl Pd(II) iodide intermediate, beta-hydrogen elimination is much more favorable, leading to Heck-type side products. Blocking beta-elimination by the choice of substrates is the main reason why this example of carboiodination works.
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