Alkaline catalyst Peroxides

The scope of CAR-CLS in analytical determinations has been expanded with one other type of CL reaction (luminol-based CL reactions are restricted to direct determinations of metal ions and some indirect ones). The so-called energy transfer CL is one interesting alternative, with a high analytical potential. As stated above, PO-CL systems based on the reaction between an oxalate ester and hydrogen peroxide in the presence of a suitable fluorophore (whether native or derivatized) and an alkaline catalyst are prominent examples of energy transfer CL. This technique has proved a powerful tool for the sensitive (and occasionally selective) determination of fluorophores its implementation via the CAR technique is discussed in detail later. 

Hori et have recently reported aza crown ether chiral quaternary ammonium salts for the epoxidation of ( )-chalcone with alkaline hydrogen peroxide as the terminal oxidant. The oxidation proceeded in high yield and good enantio-selectivity the success of the reaction depended on the length of the carbon chain on the nitrogen atom. These PTC catalysts are shown in Figure 1.50. 

As discussed in Section 10.1, asymmetric epoxidation of C=C double bonds usually requires electrophilic oxygen donors such as dioxiranes or oxaziridinium ions. The oxidants typically used for enone epoxidation are, on the other hand, nucleophilic in nature. A prominent example is the well-known Weitz-Scheffer epoxidation using alkaline hydrogen peroxide or hydroperoxides in the presence of base. Asymmetric epoxidation of enones and enoates has been achieved both with metal-containing catalysts and with metal-free systems [52-55]. In the (metal-based) approaches of Enders [56, 57], Jackson [58, 59], and Shibasaki [60, 61] enantiomeric excesses > 90% have been achieved for a variety of substrate classes. In this field, however, the same is also true for metal-free catalysts. Chiral dioxiranes will be discussed in Section 10.2.1, peptide catalysts in Section 10.2.2, and phase-transfer catalysts in Section 10.2.3. 

An example of catalysts which are themselves heterogeneous are the poly-amino acids used for the asymmetric Julia-Colonna-type epoxidation of chalcones using alkaline hydrogen peroxide (Section 10.2) [8]. Because of the highly efficient synthesis of epoxides, this process also has attracted industrial interest (Section 14.3). Since recent work by the Berkessel group revealed that as few as five L-Leu residues are sufficient for epoxidation of chalcone, several solid-phase-bound short-chain peptides have been used, leading to enantioselectivity up to 98% ee [14], For example, (L-Leu)5 immobilized on TentaGel S NH2 , 8, was found to be a suitable solid-supported short-chain peptide catalyst for epoxidations. 

Oxygen is readily evolved at the ordinary temperature on adding water to a mixture of bleaching powder and an alkali or alkaline earth peroxide in the presence of a catalyst such ns ferrous or copper sulphate. If the solid mixture is pressed into small lumps or cubes, it may be used in a Kipp s apparatus and thus afford a convenient method of preparing the gas for lecture or laboratory purposes.4... 

C24H31N2O5I3, which corresponds to addition of one molecule of iodine to the methiodide. Hydrogenation of cimicidine by use of Adams catalyst yields a dihydro derivative, obtained as its dihydrate from methanol and characterized as its picrate and hydrochloride. The reaction of cimicidine with alkaline hydrogen peroxide is also different from that of haplophytine the product is an almost nonbasic, acidic substance of composition C23H28N2O6 (1, 3). 

Epoxldation of a, -uusaturated ketones. Some years ago, /-butyl hydroperoxide in the presence of a basic catalyst (Triton B) was reported to epoxidize unhindered a,/3-unsaturated ketones in yields comparable to those obtained with alkaline hydrogen peroxide (1, 88). This reaction results in Michael adducts in the case of acrylonitrile or methyl acrylate. 

Several oxidants were tested in an epoxidation reaction in the presence of iminium salt catalysts to determine which offers the best profile in the absence of water [42]. These reactions were carried out at 0 °C with 1-phenylcyclohexene as substrate and (17) and /or (24) as catalysts (5-20 mol%), in dichloromethane as solvent. Most of the systems examined showed either high levels of background epoxidation (alkaline hydrogen peroxide, peracids, persulphates) or very low rates of reaction, even in the presence of 20 mol% of the catalysts (perselenates, percarbonates, perborates and iodosobenzene diacetate). Tetra-N-butylammo-nium Oxone, reported by Trost [43], was also unsuccessful as oxidant. 

While luminol and isoluminol require an oxidant plus a catalyst for initiation of the chemiluminescent reaction, esters derived from A -methyl acridinium carboxylic acid require only alkaline hydrogen peroxide (W4). Acridinium esters were first introduced by McCapra s group (M23, M25, S32), based on earlier work on the bioluminescence of the lucigenin/luciferase system (G18), and reviewed in McCapra and Beheshti (M21). From Fig. 19, one can see the structural similarity between lucigenin and a typical acridinium ester. 

A two step synthesis of ( )-eucomol (10) has been published by Farkas et al. 23, 24). 5,7-Di-O-benzyleucomin (43) was transformed with alkaline hydrogen peroxide to the 3.9-epoxy compound (44). Hydrogenation with a special palladium catalyst under carefully controlled conditions led to (10) in moderate yield. Proof for structure (44) was adduced by boiling the compound with p-toluenesulfonic acid in methanol which gave the 9-methoxy derivative (45). 

Similar to luminol and lucigenin, both lophine and gallic acid also react with alkaline hydrogen peroxide to yield chemiluminescence. In all of these reactions, the emission intensity is proportional to transition metal ion catalyst concentration over finite ranges. Table 1 provides an abbreviated comparison of the selectivity and sensitivity available with these systems. 

Standard oxidation of alkyldibromoboranes with alkaline hydrogen peroxide affords alcohols. Conversion of terminal alkenes to carboxylic acids using alkyldibromoboranes works well, although hydrolysis prior to oxidation is needed." Chiral alkyldibromoboranes have been used as catalysts for the asymmetric Diels-Alder reaction. 

Hydroboration, the addition of B-H bonds to carbon-carbon multiple bonds, is an attractive route to organoboranes, which can be converted into a variety of functional groups such as alcohols upon alkaline hydrogen peroxide treatment to give the corresponding anti-Markovnikov products. Metal catalysts allow for these hydroborations to be carried out under milder reaction conditions, improved or even altered reaction selectivity and more importantly, enantioselectively. 

Ketones are oxidatively cleaved in 87—96% yield using potassium superoxide (KO2) and a phase-transfer catalyst. The oxidative cleavage of a-ketols, R COC(OH)R R, has been found to proceed smoothly with alkaline hydrogen peroxide in aqueous methanol, affording high yields of ketones R COR and carboxylic acids R C02H. Oxidation of enolizable ketones to a-nitrato-ketones can be achieved using thallium(iii) nitrate in acetonitrile. ... 

The asymmetric epoxidation of the chalcone type of substrate has also been accomplished using other types of chiral catalysts [15]. Wynberg was the first to use chiral ammonium salts, and obtained chalcone oxide with 55% ee using alkaline hydrogen peroxide as the stoichiometric oxidant and a quinine-derived quaternary ammonium salt as the chiral phase transfer catalyst [16]. More recently, Lygo... 

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