The Pt II complexes [( x bpy)Pt(Ph)(THF)] + ( x bpy = 4,4′-X 2 -2,2′bipyridyl; x = OMe (1a), t Bu (1b), H (1c), Br (1d), CO 2 Et (1e) and NO 2 (1f)] catalyze the formation of n-propylbenzene and cumene from benzene and propene. The catalysts are selective for branched products, and the cumene/n-propylbenzene ratio decreases with increasing donor ability of the x bpy ligand. DFT(D) calculations predict more favorable activation barriers for 1,2-insertion into the Pt−Ph bond to give branched products. The calculations indicate that 1,2-insertion of propene should be faster than 2,1-insertion for all Pt(II) catalysts studied, but they also indicate that cumene/n-propylbenzene selectivity is under Curtin−Hammett control.
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...
A DFT study of methane C−H activation barriers for neutral NHC−Pt II −methoxy complexes yielded 22.8 and 26.1 kcal/mol for oxidative addition (OA) and oxidative hydrogen migration (OHM), respectively. Interestingly, this is unlike the case for cationic NHC−Pt II −methoxy complexes, whereby OHM entails a calculated barrier of 26.9 kcal/mol but the OA barrier is only 14.4 kcal/mol. Comparing transition state (TS) and ground state (GS) geometries implies an ∼10 kcal/mol "penalty" to the barriers arising from positioning the NHC and OMe ligands into a relative orientation that is preferred in the GS to the orientation that is favored in the TS. The results thus imply an intrinsic barrier arising from C−H scission of ∼15 ± 2 kcal/mol for NHC−Pt II −methoxy complexes. Calculations show the importance of designing C−H activation catalysts where the GS active species is already structurally "prepared" and which either does not need to undergo any geometric perturbations to access the methane C−H activation TS or is not energetically prohibited from such perturbations.
DFT studies are reported of a monomeric iron dialkyl for which oxygen atom insertion into metal-methyl bonds occurs with O2: FeMe2 + O2 → Fe(OMe)2. Computation of the reaction coordinate implicates the intermediacy of Fe(III)-peroxo, Fe(VI)-dioxo, and Fe(IV)-oxo intermediates, connected by O2 oxidative addition and two methyl migration steps. Analysis of the reaction of O2 with d(6)-Fe(Me)2 indicates that oxy-insertion for this iron complex occurs with lower free energy barriers than competing homolytic/radical pathways, exploiting "spin-flip" processes via minimum energy crossing points (MECPs).
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