N-Oxide catalysts

Methyl trichlorosilyl ketene acetal reacts with aromatic and aliphatic ketones (the former enantioselectively), using chiral pyridine bis-N-oxide catalysts.134 Computations and an X-ray crystal structure of a catalyst-SiCU complex have helped to elucidate the mechanism. 

Two other types of catalysts have been investigated for the enantioselective Strecker-type reactions. Chiral N-oxide catalyst 24 has been utilized in the trimethylsilyl cyanide promoted addition to aldimines to afford the corresponding aminonitriles with enantioselectivities up to 73% ee [14]. Electron-deficient aldimines were the best substrates, but unfortunately an equimolar amount of catalyst 24 was used in these reactions. The asymmetric Strecker addition of trimethylsilyl cyanide to a ketimine with titanium-based BINOL catalyst 25 gave fast conversions to quarternary aminonitriles with enantiomeric excesses to 59%... 

Chai, Q., Song, C., Sun, Z., Ma, Y., Ma, C., Daia, Y., and Andrus, M.B. (2005) Asymmetric allylation of aldehydes with allyltrichlorosilane using aza-paracyclophane-oxazoline-N-oxide catalysts. Te.trahe.dron Lett., 47, 8511-8615. 

Figure 15.2 Further examples of chiral Lewis-basic N-oxide catalysts. The ee refers to the reaction of benzaldehyde with 2a.

Oxidation of poly(4-vinylpyridine) with hydrogen peroxide in acetic acid produced a pyridine N-oxide catalyst (60) that is useful for synthesis of mixed formic anhydrides as in equation (26). ... 

SiClj, quinoline type N-oxide catalyst DCM, i-PrjNEt, 2h... 

Nitropyridazines are reduced catalytically either over platinum, Raney nickel or palladium-charcoal catalyst. When an N-oxide function is present, palladium-charcoal in neutral solution is used in order to obtain the corresponding amino N-oxide. On the other hand, when hydrogenation is carried out in aqueous or alcoholic hydrochloric acid and palladium-charcoal or Raney nickel are used for the reduction of the nitro group, deoxygenation of the N- oxide takes place simultaneously. Halonitropyridazines and their N- oxides are reduced, dehalogenated and deoxygenated to aminopyridazines or to aminopyridazine N- oxides under analogous conditions. 

LEY - GRIFFITH Oxidation reagent Oxidation ol alcotwls to caitxnyl compounds with a pemjthenate catalyst and N-methylmorpholine - N-oxIde (NMO), In the presence of other functional gmups... 

Among the J ,J -DBFOX/Ph-transition(II) metal complex catalysts examined in nitrone cydoadditions, the anhydrous J ,J -DBFOX/Ph complex catalyst prepared from Ni(C104)2 or Fe(C104)2 provided equally excellent results. For example, in the presence of 10 mol% of the anhydrous nickel(II) complex catalyst R,R-DBFOX/Ph-Ni(C104)2, which was prepared in-situ from J ,J -DBFOX/Ph ligand, NiBr2, and 2 equimolar amounts of AgC104 in dichloromethane, the reaction of 3-crotonoyl-2-oxazolidinone with N-benzylidenemethylamine N-oxide at room temperature produced the 3,4-trans-isoxazolidine (63% yield) in near perfect endo selectivity (endo/exo=99 l) and enantioselectivity in favor for the 3S,4J ,5S enantiomer (>99% ee for the endo isomer. Scheme 7.21). The copper(II) perchlorate complex showed no catalytic activity, however, whereas the ytterbium(III) triflate complex led to the formation of racemic cycloadducts. 

Both amine oxides related to pyridines and aliphatic amine oxides (/25) are easily reduced, the former the more so. Pyridine N-oxide has been reduced over palladium, platinum, rhodium, and ruthenium. The most active was rhodium, but it was nonselective, reducing the ring as well. Palladium is usually the preferred catalyst for this type of reduction and is used by most workers 16,23,84 158) platinum is also effective 100,166,169). Katritzky and Monrol - ) examined carefully the selectivity of reduction over palladium of a... 

Ley et al. reported oxidation of alcohols catalyzed by an ammonium perruthenate catalyst dissolved in [NEtJBr and [EMIM][PFg] [60]. Oxygen or N-methylmorpholine N-oxide is used as the oxidant and the authors describe easy product recovery by solvent extraction and mention the possibility of reusing the ionic catalyst solution. 

Catalytic oxidation of n-butane at 490° over a cerium chloride, Co-Mo oxide catalyst produces maleic anyhydride ... 

Manganese naphthenate may he used as an oxidation catalyst. Rouchaud and Lutete have made an in-depth study of the liquid phase oxidation of n-hexane using manganese naphthenate. A yield of 83% of C1-C5 acids relative to n-hexane was reported. The highest yield of these acids was for acetic acid followed hy formic acid. The lowest yield was observed for pentanoic acid. 

The use of molybdenum catalysts in combination with hydrogen peroxide is not so common. Nevertheless, there are a number of systems in which molybdates have been employed for the activation of hydrogen peroxide. A catalytic amount of sodium molybdate in combination with monodentate ligands (e.g., hexaalkyl phosphorus triamides or pyridine-N-oxides), and sulfuric acid allowed the epoxidation of simple linear or cyclic olefins [46]. The selectivity obtained by this method was quite low, and significant amounts of diol were formed, even though highly concentrated hydrogen peroxide (>70%) was employed. 

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... 

Nakajima reported the use of a chiral bipyridine N,N -dioxide 18 in the desym-metrization of acyclic meso epoxides (Figure 7.3). Although the enantioselectivity was not as high as in the method developed by Fu for meso-stilbene oxide (90% ee vs. 94% ee), it was higher for the same aliphatic epoxide (74% ee vs. 50% ee) [57]. Nakajima showed that mono-N-oxide derivatives 19 and 20 were much less effective than 18 in tenns of both yield and enantioselectivity, and accordingly proposed a unique mechanism for 18 involving a hexacoordinate silicon intermediate coordinated to both N-oxides of the catalyst. 

The reaction scheme is rather complex also in the case of the oxidation of o-xylene (41a, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Ni-kieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-Al2Oa catalyst (87f) or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of n-and sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87h). 

Polyhydric Alcohols. (Polyols). An alcohol with three or more hydroxyl groups, each attached to a different carbon atom. They are w-sol and of sweetish taste, which tends to intensify with increasing hydroxyl content. Examples of polyols of ordn interest are listed below. Polyvinyl alcohol is considered in a separate entry as a polymer although it is defined as a polyhydric alcohol. Polyols, when nitrated, make excellent expls, proplnt binders, plasticizers, etc. Prepn can follow the procedure of Lenth DuPuis (Ref 3) which uses a methanol suspension of either sucrose or dextrose and a special Cu-Al oxide catalyst to yield 60-65% distillable polyols at 240° and 1500psi Refs 1) Beil — refs found under individual compds 2) CA, under Alcohols, Polyhydric for compds of current ordn interest 3) C.W. Lenth R.N. DuPuis, "Polyhydric Alcohol Production by Hydrogenolysis of Sugars in the Presence of Copper-Aluminum Oxide , IEC 37, 152-57 (1945) CA 39, 1391 (1945)... 

Tertiary amine N-oxides may also be used to convert sulphoxides to sulphones16. The reaction proceeds by initial attack by the N-oxide oxygen atom on the sulphoxide moiety, followed by subsequent elimination of the amine. In order to obtain good yields, the reaction must be carried out at 190°Cfor 20 hours with a 20-fold excess of N-oxide in the presence of acid catalysts. The sulphone must then be separated by chromatography, thus making the method less attractive than other procedures and so it has not been employed synthetically. 

N.J. Bjerrum The group of Professor Bjerrum was the first to demonstrate NEMCA with a molten salt catalyst (Chapter 10) and also with a commercial V205-based S02 oxidation catalyst (Chapter 12). Both discoveries are of significant practical importance. 

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