Nickel Complexes of a Pincer NN_2 Ligand: Multiple Carbon-Chloride Activation of CH_2CI_2 and CHCI_3 Leads to Selective Carbon-Carbon Bond Formation

摘要

Significant advance has been made for transition-metal-catalyzed carbon-carbon coupling reactions employing aryl and alkenyl halides and pseudohalides as the electrophiles. However, catalytic coupling of unactivated alkyl halides remains a challenge. This is often attributed to the reluctance of alkyl halides to undergo oxidative addition and the tendency of metal alkyl intermediates to undergo β-H elimination, producing undesired products. These limitations are severe for unactivated alkyl chlorides. There are only a few reported examples of efficient C-C coupling catalysis employing these cheap and readily available starting materials.3 Coupling of a di- or polychloroalkane to an organometallic reagent to form new C-C bonds is even more scarce. The only example we are aware of is the Ag-catalyzed insertion of carbene into one C-Cl bond of CH_2Cl_2, CHCl_3, and CCl_4. Only one new C-C bond was formed, and all chlorines remained in the products. Herein we report a new class of isolated and well-characterized nickel(II) alkyl complexes that are able to cleave all the C-Cl bonds in CH_2Cl_2 and CHCl_3 under mild conditions. Most significantly, these reactions yield exclusively the C-C coupled products in which each chlorine atom has been replaced by an organic group originating from the nickel alkyl species. To the best of our knowledge, this is the first time that such transformation has been observed, which leads to the first efficient double alkyl-alkyl coupling of CH_2Cl_2.