Metal carbenes appended with two electron-donating groups, known as "donor/donor" carbenes, undergo diastereo- and enantioselective rhodium-catalyzed C-H insertion reactions with ether substrates to form benzodihydrofurans. Unlike the reactions of metal carbenes with electron-withdrawing groups attached, the attenuated electrophilicity enables these reactions to be conducted in Lewis basic solvents (e.g., acetonitrile) and in the presence of water. The diazo precursors for these species are prepared in situ from hydrazone using a mild and chemoselective oxidant (MnO ). Although this sequence often can be performed in one-pot, control experiments have elucidated why a "two-pot" process is often more efficient. A thorough screening of achiral catalysts demonstrated that sterically encumbered catalysts are optimal for diastereoselective reactions. Although efficient insertion into allylic and propargylic C-H bonds is observed, competing dipolar cycloaddition processes are noted for some substrates. The full substrate scope of this useful method of benzodihydrofuran synthesis, mechanisms of side reactions, and computational support for the origins of stereoselectivity are described.
Background Biomass valorization has been suggested as a sustainable alternative to petroleum-based energy and commodities. In this context, the copper radical oxidases (CROs) from Auxiliary Activity Family 5/Subfamily 2 (AA5_2) are attractive biocatalysts for the selective oxidation of primary alcohols to aldehydes. Originally defined by the archetypal galactose 6-oxidase from Fusarium graminearum, fungal AA5_2 members have recently been shown to comprise a wide range of specificities for aromatic, aliphatic and furan-based alcohols. This suggests a broader substrate scope of native CROs for applications. However, only 10% of the annotated AA5_2 members have been characterized to date. Results Here, we define two homologues from the filamentous fungi Fusarium graminearum and F. oxysporum as predominant aryl alcohol oxidases (AAOs) through recombinant production in Pichia pastoris, detailed kinetic characterization, and enzyme product analysis. Despite possessing generally similar active-site architectures to the archetypal FgrGalOx, FgrAAO and FoxAAO have weak activity on carbohydrates, but instead efficiently oxidize specific aryl alcohols. Notably, both FgrAAO and FoxAAO oxidize hydroxymethyl furfural (HMF) directly to 5-formyl-2-furoic acid (FFCA), and desymmetrize the bioproduct glycerol to the uncommon L-isomer of glyceraldehyde. Conclusions This work expands understanding of the catalytic diversity of CRO from AA5_2 to include unique representatives from Fusarium species that depart from the well-known galactose 6-oxidase activity of this family. Detailed enzymological analysis highlights the potential biotechnological applications of these orthologs in the production of renewable plastic polymer precursors and other chemicals.
A comprehensive mechanistic analysis of the copper-catalyzed azide–alkyne cycloaddition to form 5-protio-1,2,3-triazoles (from terminal alkynes) or 5-iodo-1,2,3-triazoles (from 1-iodoalkynes) is presented. Through various mechanistic probes, we elucidate several salient features of this well-known reaction that have yet to be fully articulated in the literature: Kinetic evidence is provided that supports (i) the copper-catalyzed cycloadditions to form 5-protiotriazoles and 5-iodotriazoles are mechanistically distinct, (ii) the catalyst counterion has a linchpin role in facilitating the chemoselective generation of 5-iodotriazoles from 1-iodoalkynes in the presence of terminal alkynes, (iii) “activation” of the requisite catalyst for protiotriazole synthesis is highly influenced by the nature of the catalyst counterion, and last (iv) a more nuanced interpretation of the role of copper acetylides in triazole synthesis is required. An expanded reaction manifold is offered to provide the most comprehensive image to date of the different copper-catalyzed processes active during triazole synthesis, which are obscured behind what appears to be a simple catalytic system. Ultimately, mechanistic and kinetic insight is provided that can be utilized in the development of chemoselective methods where 1-iodoalkynes and terminal alkynes are simultaneously present.
Detailed kinetic profiles of the copper-catalyzed exchange between the acetylenic proton and iodide of terminal and 1-iodophenylacetylenes are reported. The electronic nature of the alkynes does not influence the equilibrium of the exchange (K eq = 1), only the rate of equilibration. Notably, the profiles are the same for electron-rich, methyl-substituted phenylacetylenes but are divergent for electron-deficient, trifluoromethyl-substituted variants. The heretofore unreported exchange process yields practical considerations regarding reactions involving iodo and terminal alkynes.
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