Facile Functionalization of a Metal Carbon Bond by O-Atom Transfer

Abstract

A key challenge to developing selective, low-temperature hydrocarbon oxidation catalysts based on the CH activation reaction is integration with a compatible functionalization reaction. 1,2 We recently reported a CH activation reaction (Figure 1) with an alkoxo complex, M-OR, that simultaneously generates a functionalized product, ROH, and a metal alkyl, M-R, where M is Ir(III). 3a,b As shown in Figure 1, a catalytic cycle for the conversion of RH to ROH could be possible by regeneration of M-OR from M-R with O-atom donors, YO. Pt(IV) or Hg(II) alkyls are M-C σ+ polarized and readily undergo reductive functionalization with O-nucleophiles. 1b,4 However, M-Rs of more electropositive metals, such as Ir or Re, in the lower oxidation states useful for C-H activation do not undergo facile reductive functionalizations and are likely M-C σ-polarized. Consequently, functionalization of these M-Rs may be more facile in nonredox, insertion reactions with electrophilic YOs (Figure 1) if free-radical pathways or formal oxidation of the metal centers could be minimized. 5 Conversion of Y to YO with O 2 could complete the overall catalytic cycle for the overall oxidation of RH with O 2 .The conversion of M-R to M-OR is not well-known, and the few reported examples proceed with O 2 by free-radical pathways 6 or by slow redox reactions involving alkyl to metal oxo migration. 7 Consequently, identification of a facile pathway for this reaction, especially with non-peroxo 8 YOs that could potentially be recycled with O 2 , could be useful. We report here combined experimental and theoretical evidence for a facile Re-R to Re-OR bond conversion with non-peroxo YOs that proceeds via a low-energy, Baeyer-Villiger (BV)-type, electrophilic O-atom insertion.BV and alkyl borane oxidation reactions to generate oxyesters and alkoxy boranes, respectively, are well-known organic reactions involving electrophilic O-insertions with YOs. Significantly, both peroxo and non-peroxo YOs can be utilized, and the reactions proceed without free radicals or formal redox changes. 9 Methyltrioxorhenium, MTO, with peroxo YOs is well-known to catalyze olefin epoxidation and other oxidation reactions likely via Re η 2 -peroxo intermediates. 10 A reported observation that attracted our attention was that an undesirable side reaction is the decomposition of a MTO catalyst to methanol at room temperature. 11 We were intrigued because, despite the high Re(VII) oxidation state, unlike Pt(IV) alkyls, 1b,4 treatment of MTO in basic or acidic water does not generate Re(V) and methanol. Consistent with the observations of the initial investigators, 11 we find that the formation of methanol from MTO in water requires added H 2 O 2 as the oxidant. The reaction is facile, selective, quantitative, and significantly proceeds without a change in oxidation state of the Re to generate the ReO 4 -anion.In the initial studies on decomposition of MTO to methanol, only H 2 O 2 was investigated and two non-BV-type mechanisms proposed: reaction via a η 2 -peroxo intermediate or...

Catalytic Oxy‐Functionalization of Methane and Other Hydrocarbons: Fundamental Advancements and New Strategies

“…At ambient temperatures the dioxophenyl complex undergoes phenyl-to-oxo migration. [133] With [136] Mechanistic studies show that the reaction pathway for the MTO system does not likely occur by an oxo-insertion mechanism. A mechanism similar to the classic Baeyer-Villiger [137] organic reaction has been proposed (Scheme 23).…”

Activation of Carbon−Hydrogen Bonds via 1,2-Addition across M−X (X = OH or NH2) Bonds of d6Transition Metals as a Potential Key Step in Hydrocarbon Functionalization: A Computational Study

“…20 More recently, Periana et al have reported the reaction of methylrheniumtrioxo with external oxidants (e.g., hydrogen peroxide, pyridine-N-oxide, periodate, and iodosyl benzene) to form Re(OMe)O 3 . 21 Importantly, these transformations occur relatively rapidly at room temperature, and preliminary mechanistic studies indicate that the oxygen atom in the final methoxo ligand does not originate from a Re-oxo moiety. A mechanism similar to the classic Baeyer-Villiger organic reaction 54 has been proposed.…”

“…20,21 For the present models we have kept the d-electron count fixed at six, which has necessitated variation of the overall charge. While the precise relationship between the kinetics and thermodynamics of these transformations Vis-à-Vis d-electron counts and metal formal oxidation states is not entirely understood, it is reasonable to expect that a marriage of these two transformations may require a catalyst that satisfies both requirements (such as Co III , Pt IV , etc.).…”

The para-substituent effect and pH-dependence of the organometallic Baeyer–Villiger oxidation of rhenium–carbon bonds

“…Facile oxy-functionalization reactions of nucleophilic M δ+ -R δ− complexes are rare, [1][2][3][4][5] although functionalizations are well-known for M δ− -R δ+ intermediates of more electronegative metals. [6][7][8] Some of us reported recently both theory and experiments on the reaction of methylrhenium trioxide (CH 3 -ReO 3 , MTO) with oxidants (H 2 O 2 , PhIO, and IO 4 − ) in the aqueous phase.…”

“…[6][7][8] Some of us reported recently both theory and experiments on the reaction of methylrhenium trioxide (CH 3 -ReO 3 , MTO) with oxidants (H 2 O 2 , PhIO, and IO 4 − ) in the aqueous phase. 2,9 We concluded that the reaction proceeds through a Baeyer-Villiger (BV) type transition state 9 leading to formation of methanol in high yields (>80%). More recently, we studied the oxidation of 2,4,6-trimethylphenylrhenium trioxide (Mes-ReO 3 , MesTO) in similar conditions.…”