Mechanistic study of silver-mediated furan formation by oxidative coupling.

Density functional calculations and experiments have been carried out to unravel the mechanism of a silver-mediated furan formation by oxidative coupling. Various possible reaction paths were considered and the most favorable channel has been identified on the basis of the calculated solvent-corrected Gibbs free-energy profiles. The mechanism represented by this route consists of a radical and a subsequent ionic route. The silver cation has a double role in the mechanism: it is the oxidant in the radical steps and the catalyst for the ionic steps, which is in accordance with the experimental observations. The two most important aspects of the optimal route are the formation of a silver-acetylide, reacting subsequently with the enolate radical, and the aromatic furan-ring formation in a single step at the latter, ionic segment of the reaction path. Our findings could explain several experimental observations, including the "key-promoter role" of silver, the preference for ionic cyclization, and the reduced reactivity of internal acetylides.

[1]  Yang Gao,et al.  Silver-mediated C-H activation: oxidative coupling/cyclization of N-arylimines and alkynes for the synthesis of quinolines. , 2012, The Journal of organic chemistry.

[2]  J. Ke,et al.  Silver-mediated oxidative C-H/C-H functionalization: a strategy to construct polysubstituted furans. , 2012, Journal of the American Chemical Society.

[3]  Jun Wang,et al.  Palladium-catalyzed cross-dehydrogenative functionalization of C(sp(2))-H Bonds. , 2014, Chemistry, an Asian journal.

[4]  D. Truhlar,et al.  The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals , 2008 .

[5]  S. Stahl,et al.  Overcoming the "oxidant problem": strategies to use O2 as the oxidant in organometallic C-H oxidation reactions catalyzed by Pd (and Cu). , 2012, Accounts of chemical research.

[6]  Wei-Liang Duan,et al.  Silver-mediated oxidative C-H/P-H functionalization: an efficient route for the synthesis of benzo[b]phosphole oxides. , 2013, Journal of the American Chemical Society.

[7]  C. Cramer,et al.  Universal solvation model based on solute electron density and on a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions. , 2009, The journal of physical chemistry. B.

[8]  F. Glorius,et al.  Towards mild metal-catalyzed C-H bond activation. , 2011, Chemical Society reviews.

[9]  Fen Wang,et al.  Cross-dehydrogenative coupling between enamino esters and ketones: synthesis of tetrasubstituted pyrroles. , 2012, Organic letters.

[10]  A. Lei,et al.  Transition-metal catalyzed oxidative cross-coupling reactions to form C-C bonds involving organometallic reagents as nucleophiles. , 2011, Chemical Society reviews.

[11]  M. Beller,et al.  Katalytische oxidative Kupplungsreaktionen: selektive Bildung von C‐C‐ und C‐X‐Bindungen über radikalische Prozesse , 2013 .

[12]  B. Lipshutz Five-membered heteroaromatic rings as intermediates in organic synthesis , 1986 .

[13]  X. Chen,et al.  Palladium(II)‐katalysierte C‐H‐Aktivierung/C‐C‐Kreuzkupplung: Vielseitigkeit und Anwendbarkeit , 2009 .

[14]  B. Liu,et al.  Highly enantioselective insertion of carbenoids into N-H bonds catalyzed by copper complexes of chiral spiro bisoxazolines. , 2007, Journal of the American Chemical Society.

[15]  Donald G Truhlar,et al.  Density functionals with broad applicability in chemistry. , 2008, Accounts of chemical research.

[16]  M. M. Díaz‐Requejo,et al.  Coinage metal catalyzed C-H bond functionalization of hydrocarbons. , 2008, Chemical reviews.

[17]  J. Ke,et al.  Oxidative cross-coupling/cyclization to build polysubstituted pyrroles from terminal alkynes and β-enamino esters. , 2013, Chemical communications.

[18]  J. Iqbal,et al.  Transition Metal-Promoted Free-Radical Reactions in Organic Synthesis: The Formation of Carbon-Carbon Bonds , 1994 .

[19]  H. Davies,et al.  Catalytic C–H functionalization by metal carbenoid and nitrenoid insertion , 2008, Nature.

[20]  Chuan He,et al.  Heteroaromatic imidazo[1,2-a]pyridines synthesis from C-H/N-H oxidative cross-coupling/cyclization. , 2012, Chemical communications.

[21]  M. V. Sargent,et al.  3.10 – Furans and their Benzo Derivatives: (i) Structure , 1984 .

[22]  jin-quan yu,et al.  Palladium(II)-catalyzed C-H activation/C-C cross-coupling reactions: versatility and practicality. , 2009, Angewandte Chemie.

[23]  Chao‐Jun Li Cross-dehydrogenative coupling (CDC): exploring C-C bond formations beyond functional group transformations. , 2009, Accounts of chemical research.

[24]  Synthesis of pyrroles by click reaction: silver-catalyzed cycloaddition of terminal alkynes with isocyanides. , 2013, Angewandte Chemie.

[25]  Y. Lan,et al.  Visible-light-mediated decarboxylation/oxidative amidation of α-keto acids with amines under mild reaction conditions using O(2). , 2014, Angewandte Chemie.

[26]  Q. Zhang,et al.  Silver-catalyzed isocyanide-alkyne cycloaddition: a general and practical method to oligosubstituted pyrroles. , 2013, Angewandte Chemie.

[27]  B. Liu,et al.  Highly enantioselective insertion of carbenoids into O-H bonds of phenols: an efficient approach to chiral alpha-aryloxycarboxylic esters. , 2007, Journal of the American Chemical Society.

[28]  A. Lei,et al.  Bond formations between two nucleophiles: transition metal catalyzed oxidative cross-coupling reactions. , 2011, Chemical reviews.

[29]  T. Mei,et al.  Heterocycle Formation via Palladium-Catalyzed C-H Functionalization. , 2012, Synthesis.

[30]  M. Beller,et al.  Oxidative catalytic coupling reactions: selective formation of C-C and C-X bonds using radical processes. , 2013, Angewandte Chemie.

[31]  Elaine C. Lee,et al.  Copper-Catalyzed Asymmetric N−H Insertion Reactions: Couplings of Diazo Compounds with Carbamates to Generate α-Amino Acids , 2007 .