Oxidation with Ruthenium Complex Catalysts and Oxidants

Bridgehead carbons of adamantane [86], pinane [87], and fused norbornanes [85a, 88] undergo selective hydroxylation under similar reaction conditions. Alkyl-substituted cyclopropane is oxidized selectively at the a-position to cyclopropane ring (Eq. 3.54) [89]. The methyl group of toluene can be converted into the corresponding carboxylic acids (Eq. 3.55) [91]. 

Oxidation with Ruthenium Complex Catalysts and Oxidants 

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Oxidation with Ruthenium Complex Catalysts and Oxidants I 69... 

A variety of other oxidants that produce sulfoxides from sulfides are known, such as nitric acid, acyl nitrates, nitronium salts,N204, oxygen with ruthenium complex catalysts, - pressurized oxygen with CAN as a catalyst, ozone, - TTN, Ce salts, - and potas-... 

A range of variously substituted piperidines, piperazines and dialkylamines have been conveniently deuterated in a single step by isotopic exchange with deuterium oxide in the presence of an appropriate ruthenium complex catalyst.154 The isotopic exchange has been carried out efficiently in dimethylsulfoxide at positions both a and P to the NH group. 

The oxidation of selenoxides to selenones is slow requiring drastic conditions to be used. Diaryl, aryl methyl and dimethyl selenones are prepared by the oxidation of the ctxresponding selenoxides with prolonged exposure to KMn04 or ozone. The direct oxidation of selenides to selenones by FWO with ruthenium(II) complex catalyst, and C u(Mn04)2 has also been described. Aiyl trifluo-... 

Aqua(phosphine)ruthenium(II) complexes [121] are useful for activation of molecular oxygen, and catalytic oxidation of cyclohexene can be carried out with 1 atm of O2 [121a,bj. The ruthenium catalyst bearing perfluorinated 1,3-diketone ligands catalyzes the aerobic epoxidation of alkenes in a perfluorinated solvent in the presence of i-PrCHO [122]. Asymmetric epoxidations of styrene and stilbene proceed with 56-80% e.e. with ruthenium complexes 38-40 (Figure 3.2) and oxidants such as PhI(OAc)2, PhIO, 2,6-dichloropyridine N-oxide, and molecular oxygen [123-125]. 

Catalytic oxidation of electron rich alkenes such as styrene with ruthenium complexes gave mainly benzaldehyde rather than the expected epoxide. When styrene was oxidised with CHP as the oxidising agent benzaldehyde was the major detectable product. Polymerisation of styrene occurs in solvents like methanol. Table-4 shows the oxidation of styrene with CHP. Substitution of methyl groups in the pyridine ring increases the yield of benzaldehyde. When RuCl2(4-Mepy)4 was the catalyst, negligible amounts of styrene oxide was also detected. However with other catalysts benzaldehyde was the only detectable product. Other oxidation products like phenyl acetaldehyde and acetophenone were not detected [10,11]. 

The great advantage of this type of detection is to avoid the use of marks or indicators of hybridization, simplifying the experimental procedure. However, the detection based on the electroactivity of bases gives rise to a lack of sensitivity. Various proposals based on the use of oxidation products of adenine as catalysts of NADH oxidation [17], or those based in the use of mediators for the oxidation of bases, with ruthenium complex [18, 19] or osmium complex [20] have been proposed in order to get an amplification of the signal and thereby improve the sensitivity. 

High-valent ruthenium oxides (e. g., Ru04) are powerful oxidants and react readily with olefins, mostly resulting in cleavage of the double bond [132]. If reactions are performed with very short reaction times (0.5 min.) at 0 °C it is possible to control the reactivity better and thereby to obtain ds-diols. On the other hand, the use of less reactive, low-valent ruthenium complexes in combination with various terminal oxidants for the preparation of epoxides from simple olefins has been described [133]. In the more successful earlier cases, ruthenium porphyrins were used as catalysts, especially in combination with N-oxides as terminal oxidants [134, 135, 136]. Two examples are shown in Scheme 6.20, terminal olefins being oxidized in the presence of catalytic amounts of Ru-porphyrins 25 and 26 with the sterically hindered 2,6-dichloropyridine N-oxide (2,6-DCPNO) as oxidant. The use... 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... 

Cationic ruthenium complexes of the type [Cp Ru(MeCN)3]PF6 have been shown to provide unique selectivities for inter- and intramolecular reactions that are difficult to reconcile with previously proposed mechanistic routes.29-31 These observations led to a computational study and a new mechanistic proposal based on concerted oxidative addition and alkyne insertion to a stable ruthenacyclopropene intermediate.32 This proposal seems to best explain the unique selectivities. A similar mechanism in the context of C-H activation has recently been proposed from a computational study of a related ruthenium(ll) catalyst.33... 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

This chapter is essentially a review of those ruthenium complexes which have been used as oxidation catalysts for organic substrates, emphasis being placed on such species which have been chemically well-defined and are effective catalysts. Of all the ruthenium oxidants dealt with here those which have the greatest diversity of use are RuO, [RuO ] , [RuO ], the tetramesityl porphyrinato (TMP) complex fran.y-Ru(0)2(TMP), RuCl3(PPh3)3, and cw-RuCl3(dmso). Many other catalysts are covered, and the uses of two principal starting materials, RuO. nH O and RuClj. nH O as precursors for a number of catalysts, discussed. 

In 1989, a method for the peroxysilylation of alkenes nsing triethylsUane and oxygen was reported by Isayama and Mnkaiyama (eqnation 25). The reaction was catalyzed by several cobalt(II)-diketonato complexes. With the best catalyst Co(modp)2 [bis(l-morpholinocarbamoyl-4,4-dunethyl-l,3-pentanedionato)cobalt(n)] prodnct yields ranged between 75 and 99%. DiaUcyl peroxides can also be obtained starting from tertiary amines 87, amides 89 or lactams via selective oxidation in the a-position of the Af-fnnctional group with tert-butyl hydroperoxide in the presence of a ruthenium catalyst as presented by Murahashi and coworkers in 1988 ° (Scheme 38). With tertiary amines 87 as substrates the yields of the dialkyl peroxide products 88 ranged between 65 and 96%, while the amides 89 depicted in Scheme 38 are converted to the corresponding peroxides 90 in yields of 87% (R = Me) and 77% (R = Ph). 

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