Anionic diarylrhodium complexes, generated by reacting [RhCl(cod)] 2 with 2 equiv of aryl Grignard reagents, were found to be effective active catalysts in cross-coupling reactions of vinyl ethers with aryl Grignard reagents, giving rise to the production of vinyl arenes. In this catalytic system, vinyl-O bonds were preferably cleaved over Ar−O or Ar−Br bonds. A lithium rhodate complex was isolated, and its crystal structure was determined by X-ray crystallography.A rylrhodium(I) complexes (B) are readily formed by the transmetalation of the corresponding rhodium salts (A) with various organometallic reagents 1 and play important roles as key catalytic species in C−C bond forming reactions. 2 Some useful examples include the addition of aromatic moieties to carbonyl groups and enones (1,2-and 1,4-addition), 3,4 alkenes (also Heck-type reactions), 5 and alkynes. 6 Asymmetric addition to carbonyl compounds provides a powerful tool for creating chiral carbon centers. 4 Another fundamental reaction course of arylrhodium(I) complexes is oxidative addition with organo (pseudo)halides to give Rh(III) complexes, which is involved as a key process in cross-coupling reactions. 7 In our previous studies, we reported that anionic transitionmetal species such as nickelate, 8a−c palladate, 8d and cobaltate 9 complexes efficiently catalyze cross-coupling reactions of unactivated alkyl (pseudo)halides with organometallic reagents. Despite the importance and usefulness of Rh catalysts in organometallic chemistry and in organic synthesis, anionic diarylrhodium complexes C and their potent catalytic activities remained undeveloped in sharp contrast with neutral and cationic rhodium species. 1 Garcia et al. synthesized anionic diarylrhodium complexes and revealed a crystal structure of a bimetallic complex having Ag within the sum of covalent radii of Rh and Ag but their catalytic activities were not examined. 10 Here we report on the first examples in which anionic rhodium(I) species C, generated by the addition of arylmetal reagents to Rh salts A via arylrhodium(I) B (Scheme 1, A to B to C), play important roles as an active catalytic intermediate in crosscoupling reactions of vinyl ethers with aryl Grignard reagents. 11−13 In our previous work on anionic transition-metal-catalyzed C− C bond formation, unsaturated hydrocarbon additives were employed as the ligand or ligand precursor, 8,9,14 to stabilize the complexes by withdrawing electrons through π-back-donation from the anionic metal center. Therefore, our initial efforts involved reacting [RhCl(cod)] 2 with PhMgBr to generate a rhodate species, followed by its application to cross-coupling reactions. When phenyl vinyl ether (1a) was reacted with PhMgBr in the presence of 1 mol % of [RhCl(cod)] 2 (2 mol % based on metal), a coupling reaction via C−O bond cleavage proceeded giving styrene (2a) in 64% yield, accompanied by a 3% yield of stilbene (3a) formed probably by the Heck-type arylation of the coupling product 2a with PhMgBr (Table 1, entry 1). 5 The fact that a bip...
Iron-catalyzed cross-coupling reaction of vinylic ethers with aryl Grignard reagents is described. The reaction proceeded at room temperature with catalytic amounts of an iron salt without the aid of costly ligands and additives. In this catalytic system, vinylic C-O bonds were preferentially cleaved over aromatic C-O bonds of aryl ethers or aryl sulfonates.
The Schmidt rearrangement of substituted 3-phenyl-2-butanone with trimethylsilyl azide in 90% (v/v) aqueous TFA gave two types of product, fragmentation and rearrangement, the ratio of which depends on the substituent: more fragmentation for a more electron-donating substituent. Rate measurements by azotometry indicated the presence of an induction period, and the pseudo-first-order rate constants showed saturation kinetics with respect to the azide concentration. It was indicated that the reaction proceeds through pre-equilibrium in the formation of iminodiazonium (ID) ion and that the N(2) liberation from the ID ion is rate-determining. Under high azide concentration conditions, where the effective reactant is the ID ion, the reaction gave a linear Hammett plot with a ρ value of -0.50. The observed substituent effects on the rate and the product selectivity imply that path bifurcation on the way from the rate-determining TS to the product states occurs, as suggested by previous molecular dynamics simulations, in a similar manner to the analogous Beckmann rearrangement/fragmentation reactions.
Diarylrhodates as Promising Active Catalysts for the Arylation of Vinyl Ethers with Grignard Reagents. -(IWASAKI, T.; MIYATA, Y.; AKIMOTO, R.; FUJII, Y.; KUNIYASU, H.; KAMBE*, N.; J. Am. Chem. Soc. 136 (2014) 26, 9260-9263, http://dx.
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