||Yohei Matsumoto, Jun Sawamura, Yumi Murata, Takashi Nishikata, Ryo Yazaki, Takashi Ohshima, Amino Acid Schiff Base Bearing Benzophenone Imine as a Platform for Highly Congested Unnatural α-Amino Acid Synthesis, Journal of the American Chemical Society, 10.1021/jacs.0c02707, 2020.05.
||Tsukushi Tanaka, Ryo Yazaki, Takashi Ohshima, Chemoselective Catalytic α-Oxidation of Carboxylic Acids
Iron/Alkali Metal Cooperative Redox Active Catalysis, Journal of the American Chemical Society, 10.1021/jacs.0c00727, 2020.01, We developed a chemoselective catalytic activation of carboxylic acid for a 1e<sup>-</sup> radical process. α-Oxidation of a variety of carboxylic acids, which preferentially undergo undesired decarboxylation under radical conditions, proceeded efficiently under the optimized conditions. Chemoselective enolization of carboxylic acid was also achieved even in the presence of more acidic carbonyls. Extensive mechanistic studies revealed that the cooperative actions of iron species and alkali metal ions derived from 4 Å molecular sieves substantially facilitated the enolization. For the first time, catalytic enolization of unprotected carboxylic acid was achieved without external addition of stoichiometric amounts of Brønsted base. The formed redox-active heterobimetallic enediolate efficiently coupled with free radical TEMPO, providing synthetically useful α-hydroxy and keto acid derivatives..
||Ryo Yazaki, Takashi Ohshima, Recent strategic advances for the activation of benzylic C–H bonds for the formation of C–C bonds, Tetrahedron Letters, 10.1016/j.tetlet.2019.151225, 60, 45, 2019.11, Alkylarenes, obtained from abundant hydrocarbon feedstock sources, are an attractive starting material for the formation of complex molecular architectures. Conventional activation strategies of the relatively inert sp3-hybridized benzylic C–H bonds usually require relatively harsh conditions and are difficult to apply to the synthesis of fine chemicals. The present review describes recent strategic advances for the activation of benzylic C–H bonds for the catalytic formation of C–C bonds. In particular, two activation methods, i.e., strategies that generate benzylic radicals or benzyl anions, are discussed..
||Takafumi Tanaka, Tsukushi Tanaka, Taro Tsuji, Ryo Yazaki, Takashi Ohshima, Strategy for Catalytic Chemoselective Cross-Enolate Coupling Reaction via a Transient Homocoupling Dimer, Organic Letters, 10.1021/acs.orglett.8b01313, 20, 12, 3541-3544, 2018.06, A new strategy, a transient homocoupling dimer strategy, for direct catalytic oxidative cross-enolate coupling reactions is developed. Cross-enolate coupling products bearing a (contiguous) tetrasubstituted carbon center were obtained chemoselectively without the need for stoichiometric amounts of strong bases/metal oxidants, and thus, the present catalysis provides a general method for the synthesis of unnatural α,α-disubstituted amino acid motifs. The distinct transformation of azlactone and 2-acylimidazole units highlighted the synthetic utility of the present catalysis..
||Tsukushi Tanaka, Kayoko Hashiguchi, Takafumi Tanaka, Ryo Yazaki, Takashi Ohshima, Chemoselective Catalytic Dehydrogenative Cross-Coupling of 2-Acylimidazoles
Mechanistic Investigations and Synthetic Scope, ACS Catalysis, 10.1021/acscatal.8b02361, 8, 9, 8430-8440, 2018.09, Chemoselective iron-catalyzed dehydrogenative cross-coupling using 2-acylimidazoles is described. The addition of a phosphine oxide ligand substantially facilitated the generation of tert-butoxy radicals from di-tert-butyl peroxide, allowing for efficient benzylic C-H bond cleavage under mild conditions. Extensive mechanistic studies revealed that the enolization of 2-acylimidazole proceeded through dual iron catalyst activation, followed by subsequent chemoselective cross-coupling with a benzyl radical over an undesired benzyl radical-derived homocoupling dimer that inevitably formed in earlier reported conditions. A variety of alkylarenes, aliphatic alkane, and functionalized 2-acylimidazoles were applicable, demonstrating the synthetic utility of the present catalysis. Contiguous all-carbon quaternary carbons were constructed through dehydrogenative cross-coupling. The catalytic chemoselective activation of 2-acylimidazole over bidentate coordinative and much more acidic malonate diester was particular noteworthy. Catalytic oxidative cross-enolate coupling of two distinct carboxylic acid equivalents was also achieved using acetonitrile as a coupling partner..