Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rhodium(I)-Catalyzed Decarbonylative Aerobic Oxidation of Cyclic α‑Diketones: A Regioselective Single Carbon Extrusion Strategy Gangadhararao Golime, Hun Young Kim, and Kyungsoo Oh* Center for Metareceptome Research, College of Pharmacy, Chung-Ang University, 84 Heukseok-ro, Dongjak, Seoul 06974, Republic of Korea S Supporting Information *
ABSTRACT: A rhodium-catalyzed decarbonylative aerobic oxidation of cyclic α-diketones has been developed for the first time, where the regioselective formations of α-pyrones and isocoumarins have been achieved. The current decarbonylative aerobic oxidation pathway proceeds via the C−C bond cleavage followed by a C−O bond formation, representing a biomimetic oxidation approach to unsaturated six-membered cyclic lactones. The unique ability of rhodium catalysts to induce the decarbonylative aerobic oxidation opens up a new synthetic toolbox that utilizes the “regioselective single carbon” extrusion strategy.
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decarbonylative cycloaddition of cyclobutendiones with a pendant alkene.11 Recently, the Dong group also explored the Rh(I)-catalyzed decarbonylative coupling of alkynes12 and the decarbonylative coupling of isatins with alkynes.13 While the ketone decarbonylation strategy holds great synthetic value in new aryl−aryl bond-forming reactions and provides ample opportunity for redesigning the molecular structures, these methods typically require strict anaerobic reaction conditions due to the sensitive nature of organometallic species. To advance the selective activation strategy of C−C bonds, we envisioned a biomimetic oxidation approach inspired by mechanistic studies of nonenzymatic C−C bond cleavage by nonheme metal dioxygenase models.14 To the best of our knowledge there has been no research effort toward effecting C−C bond cleavage/C− O bond formation: the decarbonylative aerobic oxidation strategy (Scheme 1C). In the course of development of a new synthetic strategy to biologically relevant isocoumarin derivatives,15 we envisaged the decarbonylative aerobic oxidation of ortho-naphthoquinones to isocoumarins (Table 1). Thus, we subjected ortho-naphthoquinone 1a,16 prepared in two steps using Friedel−Crafts acylation followed by Cu(I)-catalyzed aerobic oxidation, under refluxing chlorobenzene conditions in the presence of 5 mol % dirhodium(I) catalyst in open air (entry 1). While the reaction temperature was as high as 130 °C, 1a did not undergo any reaction, demonstrating the stability of 1a under the aerobic conditions with the dirhodium catalyst. Next, we screened a variety of phosphine ligands, otherwise under the same reaction conditions (entries 2−5). Gratifyingly, we observed the formation of two new products, isocoumarin 2a and coumarin 3a, in a 6:1 ratio. Among the screened phosphine ligands, the use
elective activation of carbon−carbon bonds remains a formidable challenge in organic synthesis,1 as they typically rely on the oxidative addition of transition metal complexes to highly strained molecules such as cyclopropanes,2 cyclobutanes,3 cyclobutanones,4 and directing-group-containing substrates5 (Scheme 1A). The recent development of Rh-catalyzed6 and Scheme 1. Activation of C−C Bonds via Rh(I) Catalysis
Ni-mediated7 decarbonylative C−C bond forming reactions significantly expanded the scope of the C−C bond cleavage/C− C bond formation in catalytic reactions beyond the traditionalmetal-mediated decabonylation reaction of aldehydes.8 Thus, after the pioneering work of Tehanishi in 1974 on the decabonylative C−C bond formations of α- and β-diketones where RhCl(PPh3)3 was transformed to RhCl(CO)(PPh3)3,9 the Murakami group in 1994 reported the Rh(I)-mediated decarbonylation of cyclic ketones (Scheme 1B).10 The Yamamoto group in 2006 reported the Rh(I)-catalyzed © XXXX American Chemical Society
Received: December 8, 2017
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DOI: 10.1021/acs.orglett.7b03837 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Scheme 2. Electronic Effect of ortho-Naphthoquinones
Table 1. Optimization of the Rh(I)-Catalyzed Decarbonylative Aerobic Oxidationa
entry
Rh (mol %)
ligand (%)
solvent
yield (%)b
1 2 3 4 5 6 7 8 9c 10 11 12d 13e 14e 15e,f 16e,g
[Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (5) Rh(PPh)3Cl (10) Rh(PPh3)3(CO)H (10) RhCl3 (10) [Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (2.5) [Rh(COD)Cl]2 (5) [Rh(COD)Cl]2 (5)
− dppe (10) dppb (10) dppbz (10) Xant (10) dppb (10) dppb (10) dppb (10) dppb (10) dppb (10) dppb (10) dppb (10) dppb (10) dppb (5) dppb (10) dppb (10)
PhCl PhCl PhCl PhCl PhCl PhCl PhCl PhCl DMI o-Xyl PhEt PhMe PhCl PhCl PhCl PhCl
NR 62/8 70/12 62/17 58/11 15/6 12/6 NR 15/2 66/11 61/10 28/9 82/14(68/9) 44/8 73/13
