p-Tolylimido rhenium(v) complexes with phenolate-based ligands: synthesis, X-ray studies and catalytic activity in oxidation with tert-butylhydroperoxide

Abstract

The reactions of mer-[Re(p-NTol)X3(PPh3)2] (X = Cl, Br) with chelating phenolate-based ligands (2-(2-hydroxy-5-methylphenyl)benzotriazole (HL(1)), 2-(2-hydroxyphenyl)benzothiazole (HL(2)) or 2-(2-hydroxyphenyl)benzoxazole (HL(3))) afforded a series of p-tolylimido rhenium(v) complexes cis- or trans-(X,X)-[Re(p-NTol)X2(L)(PPh3)]·yMeCN (where X = Cl, Br; L = L(1), L(2), L(3) and y = 0-2) and [Re(p-NTol)X(L)(PPh3)2]Z·pPPh3 (where X = Cl, Br; Z = ReO4, PF6; L = L(1), L(2), L(3) and p = 0 or 1). The reported compounds were characterized by elemental analysis, FT-IR, NMR ((1)H, (13)C and (31)P) and X-ray crystallography. Interestingly, the halide ions of [Re(p-NTol)Cl2(L(1))(PPh3)]·MeCN (1) and [Re(p-NTol)Cl2(L(2))(PPh3)]·2MeCN (3) are in cis relative dispositions, whereas the complexes [Re(p-NTol)Br2(L)(PPh3)] (L(1) for 2, L(2) for 4 and L(3) for 6) and [Re(p-NTol)Cl2(L(3))(PPh3)] (5) were found to be trans-(X,X) isomers. The compounds [Re(p-NTol)X(L)(PPh3)2](PF6) (X = Cl, Br; L = L(1) and L(2)) and [Re(p-NTol)X(L(3))(PPh3)2](PF6)·PPh3 (X = Cl, Br) have been tested in oxidative catalysis. A few compounds exhibited very good catalytic properties in oxidation of alcohols with tert-BuOOH (TBHP) in acetonitrile solution at moderate temperatures. Complex [Re(p-NTol)Cl(L(2))(PPh3)2]PF6 (13) is the catalyst of choice for oxidation of 1-phenylethanol to acetophenone (in 80% yield; turnover number attained 290 after 30 h) and cyclooctanol to cyclooctanone (in 88% yield). Notably lower activity has been found in the oxidation of alkanes with TBHP. Product distribution in the oxidation of methylcyclohexane indicates some steric hindrance around the reaction center.

High Catalytic Activity of Heterometallic (Fe6Na7 and Fe6Na6) Cage Silsesquioxanes in Oxidations with Peroxides

“…It can be seen that hydrogen atoms in position 4 posess lower activity, aparently due to some sterical hindrance [50,57,60]. The bond-selectivity parameter (1 • :2 • :3 • ; the relative normalized reactivities of hydrogen atoms at the primary, secondary, and tertiary carbons) in the oxidation of methylcyclohexane (1.0:6.7:17.5) is close to the corresponding values found for the systems oxidizing alkanes with hydroxyl radicals (see, for example, References [69][70][71][72][73][74][75]). The oxidation of cis-1,2-dimethylcyclohexane proceeds non-stereoselectively, because the trans/cis ratio [the ratio of isomers of tert-alcohols with mutual transand cis-orientation of two methyl groups] of isomeric alcohols (after reduction with PPh 3 ) was 0.8.…”

New Trends in Oxidative Functionalization of Carbon–Hydrogen Bonds: A Review

“…During the past several decades, many metals have been studied in catalytic activation and functionalization of carbon-hydrogen bonds. The reader is directed to selected recent examples with gold [123], cobalt [124][125][126] chromium [127], copper [128][129][130][131] iron [132][133][134][135], iridium [136], manganese [137][138][139], molybdenum [140], nickel [141], osmium [142][143][144][145][146][147][148], palladium [149][150][151], rhenium [152], rhodium [153,154], ruthenium [155,156], and vanadium [157,158].…”

Metal Complexes Containing Redox-Active Ligands in Oxidation of Hydrocarbons and Alcohols: A Review

“…Machura et al [79] described the oxidation of inert alkanes to alkyl hydroperoxides by H2O2 catalyzed with monomeric oxovanadium(V) complexes of 8-hydroxyquinoline derivatives as noninnocent ligands (Scheme 4). The analogous ligands in complexes with copper or rhenium were used in oxidations by peroxides [80][81][82][83] Gushchin, Kuznetsov, et al recently [84,85] observed high activity among the vanadium complexes shown in Scheme 5 in the oxidation of hydrocarbons with peroxides. The mechanism for the generation of OH radicals is presented in Scheme 6.…”

“…Machura et al [79] described the oxidation of inert alkanes to alkyl hydroperoxides by H2O2 catalyzed with monomeric oxovanadium(V) complexes of 8-hydroxyquinoline derivatives as noninnocent ligands (Scheme 4). The analogous ligands in complexes with copper or rhenium were used in oxidations by peroxides [80][81][82][83] Scheme 4. Cleavage of the HO-OH bond and intramolecular electron transfer in a vanadium complex in the absence of PCA.…”