Flavin-Catalyzed Insertion of Oxygen into Rhenium–Methyl Bonds

Abstract

Flavins and related molecules catalyze organic Baeyer−Villiger reactions. Combined experimental and DFT studies indicate that these molecules also catalyze the insertion of oxygen into metal−carbon bonds through a Baeyer−Villiger-like transition state.S elective oxy functionalization reactions are among the most important classes of chemical transformations for both biological and nonbiological processes. In contrast to the broad progress on oxygen transfer reactions for olefins, 1 significant barriers remain for the development of catalysts for the selective partial oxidation of saturated hydrocarbons. 2 Transition-metal-mediated partial oxidation of alkanes involves two key steps: C−H bond cleavage and C−O bond formation. In the Pt-based Shilov reaction and related systems, 3 C−H activation occurs at Pt II , and C−O bond formation likely involves a nucleophilic addition to an electrophilic hydrocarbyl coordinated to Pt IV . The required formal two-electron redox sequence between Pt II and Pt IV has limited further development of this process, since scalable reactions with practical oxidants have not been developed. 3,4 A potential alternative to the Shilov pathway for partial oxidation of alkanes is shown in Scheme 1. 5 In this pathway, a metal alkoxide complex activates a C−H bond for 1,2-addition across the M−OR bond to form a metal hydrocarbyl complex and a free alcohol. Insertion of an oxygen atom into the M−R bond reforms the initial alkoxide complex.The 1,2-addition of C−H bonds across metal−heteroatom bonds is known. In 2004, we reported intramolecular C−H activation by a parent Ru II amido complex 6 followed by intermolecular C−H activation of benzene by Ru II hydroxide and anilido complexes. 7 Similar reactions of Ir III , 8 Rh I , 9 and other Ru II complexes 10 have also been observed and extensively modeled. 11 Although limited in number, examples of insertion of an oxygen atom into M−C bonds are also known. Insertion of an oxo ligand into a Re−Ph bond of a cationic Re VII complex has been observed. 12 A similar mechanism has been proposed for several Pd complexes. 13 Hillhouse and co-workers have studied the transfer of oxygen from N 2 O into the Ni−alkyl and Ni−aryl bonds of Ni II metallacycles. 14 Methylrhenium trioxide (MTO) reacts with oxidants to produce methanol. 15 Goddard, Periana, and co-workers 16 proposed a reaction pathway similar to the organic Baeyer− Villiger (BV) reaction that proceeds by coordination of the oxidant (YO) and subsequent methyl migration and loss of Y (Scheme 2 for YO = HOO − ). Related reactions of arylrhenium trioxides have been studied. 17 However, theoretical studies have indicated that the activation barrier for this pathway can be high, especially for late-transition-metal complexes. 18 Thus, the implementation of a combined C−H activation/oxygen insertion strategy for catalytic hydrocarbon oxidation depends on uncovering methods for lowering the activation barrier for oxygen insertion into metal−hydrocarbyl bonds. This work led us to consider...

Methyl Complexes of the Transition Metals

“…172b-d Remarkably, the insertion of an oxygen atom into a Re-CH3 bond has been shown to be catalyzed by flavins, with rate enhancements up to 600-fold. 173 Although group 12 elements are not included in this review, it is worth mentioning here the controlled insertion of O2 into a Cd-CH3 bond, which allowed for isolation of the first structurally authenticated cadmium organoperoxide. 174 Cationic [ R Cp2ZrCH3] + zirconocenes have been frequently employed as catalysts in olefin polymerization.…”

“…In a general approach, the insertion of oxygen into Group 9 and 10 M −CH 3 bonds has been studied by computational methods, pointing out the benefits of using hemilabile ligands, that is, chelating ligands with one substitutionally labile group that can be displaced from the metal center while remaining available for recoordination –. Remarkably, the insertion of an oxygen atom into a Re−CH 3 bond has been shown to be catalyzed by flavins, with rate enhancements up to 600‐fold . Although Group 12 elements are not included in this review, it is worth mentioning here the controlled insertion of O 2 into a Cd−CH 3 bond, which allowed for isolation of the first structurally authenticated cadmium organoperoxide …”

PtII and RhIII Hydrocarbyl Complexes Bearing Coordinated Oxygen Atom Delivery Reagents

“…[37,38] The reaction has also been extended to an analogous Re-aryl system, O 3 ReAr (Ar = mesityl, 2,6-xylyl), to produce the corresponding phenol, and a flavincatalyzed variant with MTO has been reported. [39,40] Finally, we have recently reported W VI complexes, in which oxygen-atom insertion into a W-CH 2 SiMe 3 bond can be controlled by both acids and bases. [41] Net oxygen-atom insertion into Ni II -R and Pd II -R bonds demonstrates that M-R + YO Ǟ M-OR + Y is possible with late-transition-metal systems.…”

Molecular Editing of Flavins for Catalysis

“…51 Re-oxidation of reduced N5substituted flavinium catalysts with molecular oxygen has since been broadly applied including iodine-mediated reactions, 52 Template for SYNTHESIS © Thieme Stuttgart • New York 2021-03-18 page 6 of 10 catalytic water oxidation, 53 and catalytic oxygenation of methylrhenium trioxide (MTO). 54 Scheme 21 Metallic zinc serves as competent reductant in Baeyer-Villiger oxidation of cyclobutanone substrates with molecular oxygen as terminal oxidant.…”