Alcohol oxidation bathophenanthroline

Figure 5.17 Selectivity in oxidation of unsaturated alcohol oxidation using the Pd-bathophenanthroline catalyst [82] versus the Pd-neocuproin catalyst [84]. For conditions see Eq. (5.12) and Figure 5.16.

Ten Brink et al. (2000) have shown how biphasic systems, sometimes with the sparingly soluble alcohols as one phase and an aqueous phase as the other phase, benefit from the strategy for air oxidation to aldehydes/ketones by using water soluble Pd complex of bathophenanthroline disulphonate. This is a nice example of green technology. 

Recently, great advancement has been made in the use of air and oxygen as the oxidant for the oxidation of alcohols in aqueous media. Both transition-metal catalysts and organocatalysts have been developed. Complexes of various transition-metals such as cobalt,31 copper [Cu(I) and Cu(II)],32 Fe(III),33 Co/Mn/Br-system,34 Ru(III and IV),35 and V0P04 2H20,36 have been used to catalyze aerobic oxidations of alcohols. Cu(I) complex-based catalytic aerobic oxidations provide a model of copper(I)-containing oxidase in nature.37 Palladium complexes such as water-soluble Pd-bathophenanthroline are selective catalysts for aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic... 

A prime advantage of such biphasic systems is that the catalyst resides in one phase and the starting materials and products are in the second phase, thus providing for easy recovery and recycling of the catalyst by simple phase separation. A pertinent example is the aerobic oxidation of alcohols catalyzed by a water-soluble Pd-bathophenanthroline complex (Figure 9.5). The only solvent used is water, the oxidant is air, and the catalyst is recycled by phase separation. 

Figure 9.5 Aerobic oxidation of alcohols catalyzed by Pd(II)/bathophenanthroline in...

Pd(II) catalysts have been widely used for aerobic oxidation of alcohols. The catalytic systems Pd(OAc)2-(CH3)2SO [14] and Pd(OAc)2-pyridine [15] oxidize allylic and benzylic alcohols to the corresponding aldehydes and ketones. Secondary aliphatic alcohols, with relatively high water solubility, have been oxidized to the corresponding ketones by air at high pressure, at 100 °C in water, by using a water-soluble bathophenanthroline disulfonate palladium complex [PhenS Pd(OAc)2] [5d]. The Pd catalyst has also been successfully used for aerobic oxidative kinetic resolution of secondary alcohols, using (-)-sparteine [16]. 

A much more active catalyst is constituted by a water-soluble palladium(II) complex of sulfonated bathophenanthroline as a stable, recyclable catalyst for the aerobic oxidation of alcohols in a two-phase aqueous-organic medium, e.g. in Fig. 4.64 [16, 172, 173]. Reactions were generally complete in 5 h at 100°C/ 30 bar air with as little as 0.25 mol% catalyst. No organic solvent is required (unless the substrate is a solid) and the product ketone is easily recovered by phase separation. The catalyst is stable and remains in the aqueous phase which can be recycled to the next batch. 

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. 

Nonactivated secondary alcohols were oxidized to the corresponding ketones with initial TOFs up to 100 mol mol h . Even less water-soluble and less reactive alcohols like 2-octanol could be oxidized with rates up to 20 mol moF h . Primary alcohols were oxidized to the corresponding acids. By adding TEMPO (2,2,6,6-tetramethylpiperdinyl-l-oxyl) the intermediate aldehyde could be trapped. As catalyst, the Pd complex of bathophenanthroline disulfonate (Structure 1) was used (bathophenanthroline is commercially available at approx. US 300/5 g). 

Compared to most existing systems for the aerobic oxidation of alcohols the Pd-bathophenanthroline system is an order of magnitude more reactive, requires no organic solvent, involves simple product isolation and catalyst recycling, and has broad scope in organic synthesis. 

Compared to existing systems for the aerobic oxidation of alcohols, the Pd-bathophenanthroline system is among the fastest catalytic systems reported to date. It requires no solvent, and product/catalyst isolation involves simple phase separation. The system has broad scope but is not successful with all alcohols. Some examples of unreactive alcohols are shown in Figure 5.14. Low reactivity was generally observed with alcohols containing functional groups which could strongly coordinate to the palladium. 

Figure S.IS Mechanism of Pd-bathophenanthroline catalyzed oxidation of alcohols.

Recently, we described the use of a water-soluble palladium(II) complex of sulfonated bathophenanthroline as a stable, recyclable catalyst for the aerobic oxidation of alcohols in a two-phase aqueous-organic medium, e.g. in Reaction 16 ... 

Oxidative Heck reaction. Homoallylic alcohols react with various boronic acids in the presence of Pd(TFA)2, bathophenanthroline and benzoquinone as reoxidant to conventional Heck adducts (eq 11). The E stereoisomer is highly favored. ... 

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