Secondary green oxidation

The system (4- Bu-pyH)3[Ru(0)3Cl ]/NM0/PMS/CH2Cl3 catalysed the oxidation of primary alcohols to aldehydes and of secondary alcohols to ketones like TRAP (Tables 2.1 and 2.2), such oxidations did not attack double bonds. As stoich. trans-(PPh )2[Ru(0)2Cl ] -/CH3CN it is a two electron oxidant for alcohols [561]. For tran -[Ru(0)2Cy - in solution the effective oxidant or oxidant precursor is [Ru(0)2Cl3]", and this species is coordinatively unsaturated. That this is the case is suggested by the observation that addition of extra Cl" (as (PPh )Cl) to the green [Ru(0)2Cl3]" in solution (Eq. 1.4) generating the red franx-[Ru(0)3Cl ] ", a markedly less effective catalytic oxidant for alcohols than [Ru(0) Cl ]" [561]. 

Similarly, a water-soluble palladium complex of a sulfonated phenanthroline ligand catalyzed the highly selective aerobic oxidation of primary and secondary alcohols in an aqueous biphasic system in the absence of any organic solvent (Figure 1.8) [40]. The liquid product could be recovered by simple phase separation, and the aqueous phase, containing the catalyst, used with a fresh batch of alcohol substrate, affording a truly green method for the oxidation of alcohols. 

Chromic acid test. This test is able to distinguish primary and secondary alcohols from tertiary alcohols. Using acidified dichromate solution, primary alcohols are oxidized to carboxylic acids secondary alcohols are oxidized to ketones tertiary alcohols are not oxidized. (Note that in those alcohols which are oxidized, the carbon that has the hydroxyl group loses a hydrogen.) In the oxidation, the brown-red color of the chromic acid changes to a blue-green solution. Phenols are oxidized to nondescript brown tarry masses. (Aldehydes are also oxidized under these conditions to carboxylic acids, but ketones remain intact see Experiment 31 for further discussion.)... 

Other workers348 d 124 have reported that commencement of the cobalt-catalyzed autoxidation of pure hydrocarbons, i.e., nonpolar solvent, is accompanied by oxidation of Co(II) to Co(III). The transformation is easily observed by the change in color from pale violet or pink [Co(II)] to intense green [Co(III)]. Similarly, manganese-catalyzed autoxidations were observed to start when Mn(II) was converted to Mn(III). The concentration of Co(III) reached a maximum during the course of autoxidation and then decreased. This maximum coincided with the appearance of aldehydes in the reaction mixtures. The authors also showed by calculation124 that reduction of Co(III) by a secondary product accounted for the observed kinetics much better than reduction by hydroperoxide. Hence, the decrease in concentration of Co(III) after it had reached a maximum was attributed to reduction by aldehydes (see Section II.B.3.e), which was much more facile than reduction by alkyl hydroperoxide [reaction (96)]. 

The chromic acid test for primary and secondary alcohols exploits the resistance of tertiary alcohols to oxidation. When a primary or secondary alcohol is added to the chromic acid reagent, the orange color changes to green or blue. When a nonoxidizable substance (such as a tertiary alcohol, a ketone, or an alkane) is added to the reagent, no immediate color change occurs. 

Much effort has been devoted to finding synthetically useful methods for the palladium-catalyzed aerobic oxidation of alcohols. For a detailed overview the reader is referred to several excellent reviews [163]. The first synthetically useful system was reported in 1998, when Peterson and Larock showed that simple Pd(OAc)2 in combination with NaHC03 as a base in DMSO as solvent catalyzed the aerobic oxidation of primary and secondary allylic and benzylic alcohols to the corresponding aldehydes and ketones, respectively, in fairly good yields [164, 165]. Recently, it was shown that replacing the non-green DMSO by an ionic liquid (imidazole-type) resulted in a three times higher activity of the Pd-catalyst [166]. 

The palladium(II) complex of sulfonated bathophenanthroline was used in a highly effective aqueous biphasic aerobic oxidation of primary and secondary alcohols to the corresponding aldehydes or carboxylic acids and ketones respectively (Fig. 7.15) [52, 53]. No organic solvent was necessary, unless the substrate was a solid, and turnover frequencies of the order of 100 h-1 were observed. The catalyst could be recovered and recycled by simple phase separation (the aqueous phase is the bottom layer and can be left in the reactor for the next batch). The method constitutes an excellent example of a green catalytic oxidation with oxygen (air) as the oxidant, no organic solvent and a stable recyclable catalyst. 

Jones Oxidation. Dissolve 5 mg of the unknown in 0.5 mL of pure acetone in a test tube and add to this solution 1 small drop of Jones reagent (chromic acid in sulfuric acid). A positive test is formation of a green color within 5 s upon addition of the orange-yellow reagent to a primary or secondary alcohol. Aldehydes also give a positive test, but tertiary alcohols do not. 

The research that involves the end-of-process treatments to eliminate pollutants is termed green chemistry. As Ronald Breslow (Columbia University) pointed out, concern for the environment is as old as the biblical injunction, hurt not the earth, neither the sea, nor the trees. The following example indicates approaches to the environmentally benign chemistry. The process described is high yielding with water as the by-product. Sato et al. have developed an efficient, environmentally friendly method for oxidizing primary and secondary alcohols (Scheme 8). The Japanese... 

Of the compounds we have dealt with so far, alcohols also dissolve in sulfuric acid. Alcohols can be distinguished from alkenes, however, by the fact that alcohols give a negative test with bromine in carbon tetrachloride and a negative Baeyer test—so long as we are not misled by impurities. Primary and secondary alcohols are oxidized by chromic anhydride, CrOa, in aqueous sulfuric acid within two seconds, the clear orange solution turns blue-green and becomes opaque. 

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