Alkylperoxo complexes

A mononuclear diastereopure high-spin Fe alkylperoxo complex with a pen-tadentate N,N,N,0,0-ligand 33 (Scheme 17) was reported by Klein Gebbink and coworkers [109, 110]. The complex is characterized by unusual seven-coordinate geometry. However, in the oxidation of ethylbenzene the iron complex with 33 and TBHP yielded with large excess of substrate only low TON s (4) and low ee (6.5%) of 1-phenylethanol. 

The alkyl complexes [TpBut]MgR (R = Me, Et, Pr , Bul) react with excess 02 at room temperature to give alkylperoxo complexes [TpBut]-MgOOR (Scheme 5), which have been characterized by 170 NMR spectroscopy. Each complex exhibits two 170 NMR resonances in the ranges 8 102-183 and 8 323-427 for the two distinct oxygen atoms of the alkylperoxo (Mg-O-O-R) moiety (Table IV). Organometallic derivatives generally react with oxygen to produce complex mixtures (69,... 

Cobalt(III)-alkylperoxo complexes find use in the oxidation of hydrocarbons.1342,1343 Since they release ROO and RO radicals upon mild heating in solution, they are effective oxidants under mild conditions, and produce catalytic systems in the presence of excess ROOH. Aliphatic C—11 bond oxidation by ConOOR (R = Con, alkyl, H) complexes including a hydrotris(pyrazolyl) borate ligand have also been reported, with homolysis of the peroxo O—O bond believed to be important in oxygenation of the C—H bond.1344... 

Ti alkylperoxo complexes based on a NMR analysis has been also reported [118]. 

B3LYP calculations as described in Section 2. b Structures optimized with a 6-31 G(d) basis for the main group elements. c Also alkylperoxo complexes. d Attack of Oa of the hydroperoxo (alkylperoxo) group.e Attack of OP of the hydroperoxo group. 

The 0-0 stretch for the hydroperoxo complexes (60,86,88,90,94,95,99- ) falls in the range 780 900 cm-1. In the case of a nonheme iron complex it was found that the low-spin hydroperoxo form absorbs at a lower frequency than the high-spin peroxo complex (89,102). This trend also holds for some iron alkylperoxo complexes, and DFT calculations have shown the high-spin alkylperoxide form to have a larger activation energy for the 0-0 bond cleavage than the corresponding low-spin alkylperoxo form (104). 

In complexes (3), (7) and (8), dioxygen can be inserted between two metal atoms (bimetallic //-peroxo complexes), between one metal and one carbon atom (alkylperoxo complexes) or between one metal and one hydrogen atom (hydroperoxo complexes). 

These complexes can exist in a triangular peroxo form (7a) for early d° transition metals, or in a bridged (7b) or linear (7c) form for Group VIII metals. They can be obtained from the reaction of alkyl hydroperoxides with transition metal complexes (equations 9 and 10),42-46 from the insertion of 02 into a cobalt-carbon bond (equation ll),43 from the alkylation of a platinum-peroxo complex (equation 12),44 or from the reaction of a cobalt-superoxo complex with a substituted phenol (equation 13).45 Some well-characterized alkylperoxo complexes are shown (22-24). 

Very few of these compounds have been characterized by X-ray crystallographic methods. As with alkylperoxo complexes, they can have a linear or a bridged structure, but no triangular form... 

The reaction of Pt02(PPh3)2 with alkyl halides results in the formation of alkylperoxo complexes, presumably via an SN2 nucleophilic attack of the terminal oxygen on the carbon atom, and occurs with inversion of configuration.44 Addition of excess alkyl halide to the alkylperoxo complex results in the formation of the dialkyl peroxide (equation 4S).44... 

Although no direct oxygen transfer reactions from well-characterized rhodium-hydroperoxo or -alkylperoxo complexes to alkenes have yet been reported, these species are probably involved in the rhodium-copper catalyzed ketonization of terminal alkenes by 02, as previously shown in Section 61.3.2.1.3. Rhodium trichloride has been used to catalyze the ketonization of terminal alkenes by H202 or TBHP in alcoholic solvents, but these reactions occur less efficiently than with the Rh/Cu/02 system.207... 

Et2Cd and O2 has also been studied and it has been claimed that an unusual adduct of composition Et2Cd 2O2 is formed reversibly prior to formation of the alkylperoxo complex. Mixed alkylperoxide derivatives of the type MeCd(00-t-Bu) may be obtained by the reaction of Me2Cd with r-BuOOH (equation 16). Thermal decomposition of MeCd(OO-t-Bu) gives an ill-defined product of composition [MeCd0C(0)H CdO]. 

Irradiation of 118 in deoxygenated benzene solution gave an insoluble cobalt(II) complex which was not further characterized. Irradiation in the presence of O2 gave a clean photoreaction to yield an organometallic complex tentatively formulated as an alkylperoxo complex. Irradiation of dimethylcobalt complexes such as [(CH3)2Co(bipy)2]" (bipy = bipyridyl) 130) and (CH3)2Co(bdml,3pn) [bdml,3pn = bis-l,3-(diacetylmono-... 

However, it is noteworthy that oxygen atom transfer to the magnesium alkyl derivative does not occur rapidly at room temperature. This is presumably one of the factors that is responsible for the observation that the reactions of [Tp ]MgR with O2 are selective in the formation of alkylperoxo complexes, rather than the alkoxo derivatives. It is likely that this is a consequence of the sterically encumbered nature of the magnesium centers in these complexes. 

The alkylperoxo complexes [Tp jMgOOR also effect oxygen atom transfer to PPhj at room temperature, giving [Tp " ]MgOR and PhjPO [Eq. (13)]. 

The mechanism of oxygen transfer from the peroxide (29) to the substrate still remains a matter of controversy. Current opinions favor the formation of a high-valent FeO (or Fe =0 or Fe —0-) (30) active species, which acts as a homolytic hydrogen abstractor from the substrate. An alternative mechanism considers the Fe -peroxide complex as the actual hydroxylating reagent, by analogy with the reactivity of vanadium(V)-peroxo and -alkylperoxo complexes and that of chromium(Vl)-peroxo complexes (see below). ... 

In spite of intensive efforts, the knowledge accumulated to date about polar oxygen-transfer reactions on peroxo, hydroperoxo, and alkylperoxo complexes is not yet sufficient to decide whether to agree with the direct attack of the olefin at the positively polarized oxygen center [9] ( butterfly mechanism without an organometallic intermediate, step A in Scheme 1) discussed by Chong and Sharpless, or with the mechanism postulated by Mimoun based on an olefin coordination. 

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