Liao Xuebin, Weng Zhiqiang, Hartwig John F
Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, USA.
J Am Chem Soc. 2008 Jan 9;130(1):195-200. doi: 10.1021/ja074453g. Epub 2007 Dec 13.
The asymmetric alpha-arylation of ketones with aryl triflates is described, and the use of this electrophile with nickel and palladium catalysts containing a segphos derivative increases substantially the scope of highly enantioselective arylations of ketone enolates. The combination of aryl triflates as reactant, difluorphos as ligand, palladium catalysts for reactions of electron-neutral or electron-rich aryl triflates, and nickel catalysts for reactions of electron-poor aryl triflates led to a series of alpha-arylations of tetralone, indanone, cyclopentanone, and cyclohexanone derivatives. Enantioselectivities ranged from 70% to 98% with 10 examples over 90%. Systematic studies on these alpha-arylations have revealed a number of factors that affect enantioselectivity. Ligands containing biaryl backbones with smaller dihedral angles generate catalysts that react with higher enantioselectivity than related ligands with larger dihedral angles. In addition, faster rates for reactions of aryl triflates versus those for reactions of aryl bromides allow the alpha-arylations of aryl triflates to be conducted at lower temperatures, and this lower temperature improves enantioselectivity. Finally, studies that compare the enantioselectivities of catalytic reactions to those of stoichiometric reactions of isolated [(segphos)Pd(Ar)(Br)], [(segphos)Pd(Ar)(I)], and [(segphos)Ni(C6H4-4-CN)Br] suggest that catalyst decomposition affects enantioselectivity.
描述了酮与芳基三氟甲磺酸酯的不对称α-芳基化反应，并且这种亲电试剂与含有segphos衍生物的镍和钯催化剂的使用大大扩展了酮烯醇盐的高对映选择性芳基化反应的范围。芳基三氟甲磺酸酯作为反应物、二氟膦作为配体、用于电子中性或富电子芳基三氟甲磺酸酯反应的钯催化剂以及用于贫电子芳基三氟甲磺酸酯反应的镍催化剂的组合，导致了一系列四氢萘酮、茚满酮、环戊酮和环己酮衍生物的α-芳基化反应。对映选择性范围为70%至98%，其中10个例子超过90%。对这些α-芳基化反应的系统研究揭示了许多影响对映选择性的因素。具有较小二面角的联芳基骨架的配体生成的催化剂，与具有较大二面角的相关配体相比，反应时对映选择性更高。此外，芳基三氟甲磺酸酯的反应速率比对溴苯的反应速率快，这使得芳基三氟甲磺酸酯的α-芳基化反应能够在较低温度下进行，而这种较低温度提高了对映选择性。最后，将催化反应的对映选择性与分离的[(segphos)Pd(Ar)(Br)]、[(segphos)Pd(Ar)(I)]和[(segphos)Ni(C6H4-4-CN)Br]的化学计量反应的对映选择性进行比较的研究表明，催化剂分解影响对映选择性。
