Alkyl Hydroperoxides as Terminal Oxidant

Alkyl hydroperoxides are known to oxidize sulfides slowly in a noncatalyzed reaction [3, 15b, 60]. If silica gel is present there is a significant acceleration of the rate of reaction, showing that there is a catalytic effect by the silica [15bj. 

Most applications of sulfide oxidations by alkyl hydroperoxides have involved titanium catalysis together with chiral ligands for enantioselective transformations. The groups of Kagan in Orsay [61] and Modena in Padova [62] reported independently on the use of chiral titanium complexes for the asymmetric sulfoxidation by the use of BuOOH as the oxidant. A modification of the Sharpless reagent with the use of Ti(0 Pr)4 and (J ,J )-diethyl tartrate (J ,J )-DET) afforded chiral sulfoxides with up to 90% ee (Eq. (8.17)). 

The outcome of the reaction was later improved by replacing BuOOH by cumene hydroperoxide [63]. 

An improved catalytic reaction with the use of 10 mol% of titanium using a ratio Ti(0 Pr)4/(J ,J )-DET/ PrOH = 1 4 4 in the presence of molecular sieves gave an efficient sulfoxidation vfith ees up to 95% with various aryl methyl sulfoxides [64]. The asymmetric Ti-catalyzed sulfoxidations with alkyl hydroperoxides have been reviewed by Kagan [65[. 

The asymmetric titanium-catalyzed sulfoxidation with BuOOH also works with chiral diols as ligands [66-68]. Various l,2-diaryl-l,2-ethanediols were employed as ligands, and the use of 15 mol% of Ti(0 Pr)4 with l,2-diphenyl-l,2-ethanediol gave 

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The AE reaction catalyzed by titanium tartrate 1 and with alkyl hydroperoxide as terminal oxidant has been applied to a large variety of primary allylic alcohols containing all eight basic substitution patterns. A few examples are presented in Table 6.2. 

SCHEME 34.1. (A) Sharpless asymmetric epoxidation of allylic alcohols 1 mediated by Ti(IV)-diethyltartrate (DET) catalyst with alkyl hydroperoxide as terminal oxidant leading to enantioenriched epoxides 2. (B) Preferential attack of the oxygen atom as a function of the stereochemistry of the DET chiral ligand. (C) Schematic representation of the dimeric active catalytic species 3. 

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

Homogeneous Systems Using Molybdenum and Tungsten Catalysts and Alkyl Hydroperoxides or Hydrogen Peroxide as the Terminal Oxidant... 

This procedure, which is based entirely on commercially available reagents, is very easy to reproduce. Table 6.12 shows different aryl- and alkyl-substituted enones that can be epoxidised with high asymmetric induction with the in situ formed (/ )-BINOL-zinc-catalyst in diethyl ether, with cumene hydroperoxide as the terminal oxidant. 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

Soluble Co compounds are generally employed in the autoxidation of hydrocarbons, i.e., the oxidation with O2 as the oxidant. In neat hydrocarbons, low concentrations of Co compounds accelerate the autoxidation since the Co2+/Co3+ couple is excellent for decomposing alkyl hydroperoxides and thus initiates free radical chain reactions. However, at high conversions, the Co may be deactivated by formation of insoluble clusters with side products of the hydrocarbon autoxidation. Moreover, high concentrations of a Co compound may actually inhibit the reaction because Co also terminates radical chains by reaction with ROO radicals ... 

High-valent early transition metals like titanium(IV) and vanadium(V) have been shown to efficiently catalyze the epoxidation of alkenes. The preferred oxidants using these catalysts are various alkyl hydroperoxides, typically tert-butyl hydroperoxide (TBHP) or ethylbenzene hydroperoxide (EBH P). One of the routes for the industrial production of propylene oxide is based on a heterogeneous Ti ySi02 catalyst, which employs EBHP as the terminal oxidant [6]. 

Historically, the interest of using manganese complexes as catalysts for the epox-idation of alkenes comes from biologically relevant oxidative manganese porphyrins. The terminal oxidants compatible with manganese porphyrins were initially restricted to iodosylbenzene, sodium hypochlorite, alkyl peroxides and hydroperoxides, JV-oxides, KHSO5, and oxaziridines. Molecular oxygen can also be used in the... 

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