Alkenes metal catalyzed oxidations with

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

Metal-catalyzed oxidation of alcohols to aldehydes and ketones is a subject that has received significant recent attention [21,56,57]. One such method that utilizes NHC ligands is an Oppenauer-type oxidation with an Ir or Ru catalyst [58-62]. These alcohol oxidation reactions consist of an equilibrium process involving hydrogen transfer from the alcohol substrate to a ketone, such as acetone (Eq. 5), or an alkene. Because these reactions avoid the use of a strong oxidant, the potential oxidative instability of NHC ligands is less problematic. Consequently, these reactions represent an important target for future research into the utility of NHCs. 

Punniyamurthy T, Velusamy S, Iqbal J. Recent advances in transition metal catalyzed oxidation of organic substrates with molecular oxygen. Chem. Rev. 2005 105 2329-2363. McGarrigle EM, GUheany DG. Chromium- and manganese-salen promoted epoxidation of alkenes. Chem. Rev. 2005 105 1563-1602. 

One method of bypassing the stoichiometry problem involves the deliberate addition of a sacrificial reagents, which accept one of the 0-atom equivalents. Examples are metal-catalyzed oxidations of alkenes with O2 in the presence of alcohols, aldehydes etc. as co-reductants. While this approach is acceptable on a small-scale [32], it is impracticable for the production of bulk chemicals. More practicable co-reductants are molecular hydrogen and carbon monoxide, provided that Reactions 13 and 14 are appropriately mediated by a catalyst. 

In this chapter we discussed the transition metal catalyzed oxidative carbonylation of alkenes, alkynes and organometallic reagents. In these types of reactions, an additional oxidant is needed to reoxidize the catalyst back to an active state after a reductive elimination step. The oxidants applied are normally Cu(OAc)2 or BQ, air or O2, as more green oxidants should be investigated and applied in oxidative carbonylation reactions. In contrast, carbonylative reduction reactions using CO as a reductant are also interesting. In the next chapter, the reduction of C-NO2 with CO will be discussed. 

The first, and so far only, metal-catalyzed asymmetric 1,3-dipolar cycloaddition reaction of nitrile oxides with alkenes was reported by Ukaji et al. [76, 77]. Upon treatment of allyl alcohol 45 with diethylzinc and (l ,J )-diisopropyltartrate, followed by the addition of diethylzinc and substituted hydroximoyl chlorides 46, the isoxazolidines 47 are formed with impressive enantioselectivities of up to 96% ee (Scheme 6.33) [76]. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

Transition Metal-Catalyzed Epoxidation of Alkenes. Other transition metal oxidants can convert alkenes to epoxides. The most useful procedures involve f-butyl hydroperoxide as the stoichiometric oxidant in combination with vanadium or... 

The oxidative cleavage of C=C bond is a common type of reaction encountered in organic synthesis and has played a historical role in the structural elucidation of organic compounds. There are two main conventional methods to oxidatively cleave a C=C bond (1) via ozonol-ysis and (2) via oxidation with high-valent transition-metal oxidizing reagents. A more recent method developed is via the osmium oxide catalyzed periodate oxidative cleavage of alkenes. All these methods can occur under aqueous conditions. 

Wilkinson s catalyst has also been utilized for the hydroboration of other alkenes. Sulfone derivatives of allyl alcohol can be hydroborated with HBcat and subsequently oxidized to give the secondary rather than primary alcohol. This reactivity proves to be independent of substituents on the sulfur atom.36 Similarly, thioalkenes undergo anti-Markovnikoff addition to afford a-thioboronate esters.37 The benefits of metal-catalyzed reactions come to the fore in the hydroboration of bromoalkenes (higher yields, shorter reaction times), although the benefits were less clear for the corresponding chloroalkenes (Table 3).38,39 Dienes can be hydroborated using both rhodium and palladium catalysts [Pd(PPh3)4] reacts readily with 1,3-dienes, but cyclic dienes are more active towards [Rh4(CO)i2].40... 

A number of additional metal-catalyzed epoxidations have been reported in the past year. Platinum is a rarely used catalyst in oxidation reactions. The use of chiral Pt-catalyst 2 in the epoxidation of terminal alkenes provides the epoxide products in moderate yield and enantiomeric excess <06JA14006>. The chiral hydroxamide 3 is used with a Mo catalyst to provide the epoxide product in excellent yields and moderate enantioselectivity <06AG(I)5849>. A bis-titanium catalyst, 4, has also been used to epoxidize the usual set of alkenes with H202 as the oxidant <06AG(I)3478>. 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

Asymmetric epoxidation of olefins is an effective approach for the synthesis of enan-tiomerically enriched epoxides. A variety of efficient methods have been developed [1, 2], including Sharpless epoxidation of allylic alcohols [3, 4], metal-catalyzed epoxidation of unfunctionalized olefins [5-10], and nucleophilic epoxidation of electron-deficient olefins [11-14], Dioxiranes and oxazirdinium salts have been proven to be effective oxidation reagents [15-21], Chiral dioxiranes [22-28] and oxaziridinium salts [19] generated in situ with Oxone from ketones and iminium salts, respectively, have been extensively investigated in numerous laboratories and have been shown to be useful toward the asymmetric epoxidation of alkenes. In these epoxidation reactions, only a catalytic amount of ketone or iminium salt is required since they are regenerated upon epoxidation of alkenes (Scheme 1). 

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. 

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

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