Alkyl hydroperoxides reaction with transition metals

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). 

The dual role of transition metals as either prodegradant or stabilizer is rationalized by the observation that the free-radical chemistry which dominates depends on the concentration of the metal ion. At low concentrations Cu I/Cu II may be a pro-oxidant, whereas at high concentrations it may be a stabilizer. The explanation lies in the complexation of hydroperoxides with transition metals (Black, 1978). Thus, taking the example of Co II/Co III, the reactions in Scheme 1.71 are recognized (Black, 1978), which together give the usual reaction for the metal-catalysed bimolecular decomposition of hydroperoxides to alkyl peroxy and alkoxy radicals. 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

The phenomenon that early transition metals in combination with alkyl hydroperoxides could participate in olefin epoxidation was discovered in the early 1970s [30, 31]. While m-CPBA was known to oxidize more reactive isolated olefins, it was discovered that allylic alcohols were oxidized to the corresponding epoxides at the same rate or even faster than a simple double bond when Vv or MoVI catalysts were employed in the reaction [Eq. (2)] [30]. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

The most common pathway for catalysis of autoxidations by transition metal complexes involves the decomposition of alkyl hydroperoxides. Another route that may be possible for chain initiation involves direct oxygen activation, whereby the complexation of molecular oxygen by a transition metal would lower the energy of activation for direct reaction with the substrate [reaction (9)]. For example, oxygen coordinated to a metal might be expected to possess properties similar to alkylperoxy radicals and undergo hydrogen transfer with a hydrocarbon ... 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

Early transition metal ions in their highest oxidation states, such as Ti(IV), V(V), W (VI), and Mo(VI), tend to be stable toward changes in their oxidation states. Consequently, in epoxidation reactions with hydrogen peroxide or alkyl hydroperoxides they form adducts (M-OOH and M-OOR) that are the key intermediates in the... 

The most well-known example is the catalytic epoxidation of olefins with alkyl hydroperoxides that is used for the commercial production of propylene oxide (see earlier). The reaction is catalyzed by high-valent compounds of early transition metals, e.g. Movl, WVI, Vv and TiIV, and involves a peroxometal type mechanism [6,7] as shown (reaction 12). Mo compounds are particularly effective homo-... 

Table 3.5. Alkene Epoxidation by Transition Metal Catalyzed Reaction with Alkyl Hydroperoxides SS...

Sulfides react faster with hydrogen peroxide and alkyl hydroperoxides than do alkenes. For this reason, transition metal catalysts are rarely necessary, but these reactions are acid catalyzed and first order in both sulfide and peroxide. The acid (HX) can be as weak as alcohol or water but the "effectiveness (of the oxidation) is determined by the pXa of the acid. Sulfides also react faster with peroxides than do ketones (see the Baeyer-Villiger reaction, sec. 3.6). Formation of the sulfone in these reactions is straightforward, but requires more vigorous reaction conditions. It is usually easy to isolate the sulfoxide from oxidation of a sulfide. Direct conversion of a sulfide to a sulfone requires excess peroxide and vigorous reaction conditions (heating, long reaction times, more concentrated peroxide). 

The oxidation of hydrocarbons by molecular oxygen in the absence of metal complexes has been discussed in Chapter II. The oxidation of hydrocarbons with molecular oxygen, as well as with donors of an oxygen atom (hydrogen peroxide, alkyl hydroperoxides and some other compounds), is a very important field since many industrial processes are based on these reactions [1], In many cases, chain radical non-catalyzed autoxidation of samrated hydrocarbons is not very selective and the yields of valuable products are often low. The use of salts and complexes of transition metals creates great possibilities for solving problems of selective oxidation, as has been demonstrated for a number of important processes. 

The electron transfer producing free radicals as shown above is preceded by the formation of unstable coordination complexes of the metal ions with alkyl hydroperoxides. The relative importance of Reaction 1.76 and Reaction 1.77 depends upon the relative strength of the metal ion as an oxidizing or reducing agent. When the metal ion has two valence states of comparable stability, both Reaction 1.76 and Reaction 1.77 will occur, and a trace amount of the metal can convert a large amount of peroxide to free radicals according to the sum of the two reactions (Reaction 1.78). This is true of compounds of metals such as Fe, Co, Mn, Cu, Ce, and V, commonly called transition metals. 

Fujita et al. used a catalytic amount of a binuclear titanium(IV) complex in an attempt to find an efficient system to oxidize sulfides with high enantioselectivity [102]. Prior to this study, they investigated other systems with several transition metals. A similar asymmetric sulfoxidation was discovered [105] using a catalytic amount of nonracemic Schiff base oxovanadium complex (Table 1.4) under atmospheric conditions at room temperature in dichloromethane. With 0.1 mol% of catalyst and cumene hydroperoxide as oxidant, oxidation produces sulfoxides in excellent yields. However, the reaction is limited to alkyl aryl sulfide substrates, and the best enantioselectivity obtained was 40% ee, for (S)-methyl p-methoxy phenyl sulfoxide. 

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