Dioxazolones are a convenient class of acyl nitrene transfer reagents. Their application in homogeneous transition-metal catalysis has led to many new amidation reactions. Dioxazolones are typically activated by transition metals at relatively low reaction temperatures. The metal nitrenoids formed by decarboxylative activation of dioxazolones are generally electron deficient and commonly react in a concerted fashion. “Intermolecular” nitrene insertion reactions involving preactivated C–H bonds (“inner-sphere” mechanism) easily compete with the Curtius-type rearrangement, but for intramolecular “direct” nitrene transfer/insertion reactions involving nonpreactivated substrates (i.e., without preceding formation of metal–carbon or metal–hydride bonds) extensive ligand optimization is important to prevent such unwanted side reactions. The ease of dioxazolone synthesis, formation of CO2 gas as the sole byproduct from reactions with dioxazolones, and the importance of nitrene transfer reactions in general has led to the development of several interesting reactions producing N-aryl amides, oxazoles, and lactams. Since the activation of dioxazolones proceeds under mild reaction conditions, stereo- and enantioselective reactions are also possible, which is useful for the synthesis of bioactive nitrogen-containing compounds. This review provides an overview of these reactions reported in recent years.
Direct synthetic routes to amidines are desired, as they are widely present in many biologically active compounds and organometallic complexes. N-Acyl amidines in particular can be used as a starting material for the synthesis of heterocycles and have several other applications. Here, we describe a fast and practical copper-catalyzed three-component reaction of aryl acetylenes, amines, and easily accessible 1,4,2-dioxazol-5-ones to N-acyl amidines, generating CO2 as the only byproduct. Transformation of the dioxazolones on the Cu catalyst generates acyl nitrenes that rapidly insert into the copper acetylide Cu–C bond rather than undergoing an undesired Curtius rearrangement. For nonaromatic dioxazolones, [Cu(OAc)(Xantphos)] is a superior catalyst for this transformation, leading to full substrate conversion within 10 min. For the direct synthesis of N-benzoyl amidine derivatives from aromatic dioxazolones, [Cu(OAc)(Xantphos)] proved to be inactive, but moderate to good yields were obtained when using simple copper(I) iodide (CuI) as the catalyst. Mechanistic studies revealed the aerobic instability of one of the intermediates at low catalyst loadings, but the reaction could still be performed in air for most substrates when using catalyst loadings of 5 mol %. The herein reported procedure not only provides a new, practical, and direct route to N-acyl amidines but also represents a new type of C–N bond formation.
Size‐selective hydroformylation of terminal alkenes was attained upon embedding a rhodium bisphosphine complex in a supramolecular metal–organic cage that was formed by subcomponent self‐assembly. The catalyst was bound in the cage by a ligand‐template approach, in which pyridyl–zinc(II) porphyrin interactions led to high association constants (>10 5 m −1 ) for the binding of the ligands and the corresponding rhodium complex. DFT calculations confirm that the second coordination sphere forces the encapsulated active species to adopt the ee coordination geometry (i.e., both phosphine ligands in equatorial positions), in line with in situ high‐pressure IR studies of the host–guest complex. The window aperture of the cage decreases slightly upon binding the catalyst. As a result, the diffusion of larger substrates into the cage is slower compared to that of smaller substrates. Consequently, the encapsulated rhodium catalyst displays substrate selectivity, converting smaller substrates faster to the corresponding aldehydes. This selectivity bears a resemblance to an effect observed in nature, where enzymes are able to discriminate between substrates based on shape and size by embedding the active site deep inside the hydrophobic pocket of a bulky protein structure.
An efficient synthetic strategy towards beta-lactams, amides, and esters involving "in situ" generation of ketenes and subsequent trapping with nucleophiles is presented. Carbonylation of carbene radical intermediates using the cheap and highly active cobalt(II) tetramethyltetraaza [14]annulene catalyst [Co(MeTAA)] provides a convenient one-pot synthetic protocol towards substituted ketenes. N-tosylhydrazones are used as carbene precursors, thereby bridging the gap between alde- [a] Homogeneous, Supramolecular and Bio-Inspired Catalysis (HomKat), van '2251 Scheme 2. Proposed mechanism for catalytic ketene synthesis via cobalt(III)carbene radical carbonylation.
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