Heteropolyacids reoxidants

Since S03/H2S04 is clearly not the most desirable system for industrial applications, a formidable challenge is to find an oxidant that oxidizes Pt(II) much faster than S03 does, operates in an environmentally friendly solvent, and can be (like SVI/SIV) reoxidized by oxygen from air. Ideally, the reduced oxidant would get reoxidized in a continuous process, such that the oxidant acts as a redox mediator. In addition, the redox behavior has to be tuned such that the platinum(II) alkyl intermediate would be oxidized but the platinum(II) catalyst would not be completely oxidized. Such a system that efficiently transfers oxidation equivalents from oxygen to Pt(II) would be highly desirable. A redox mediator system based on heteropolyacids has been reported for the Pt-catalyzed oxidation of C-H bonds by 02, using Na8HPMo6V6O40... 

The Wacker PdCl2-CuCl2 catalyst is a highly corrosive one, and its use requires special vessels and apparatus, such as titanium or tantalum alloys. Heteropolyacids have been used as alternative noncorrosive reoxidants of palladium for a variety of organic transformations,410 for example in reaction (155).411... 

In the group of Backvall a method was developed involving palladium and benzoquinone as cocatalyst (Fig. 4.42) [103]. The difficulty of the catalytic reaction lies in the problematic reoxidation of Pd(0) which cannot be achieved by dioxygen directly (see also Wacker process). To overcome this a number of electron mediators have been developed, such as benzoquinone in combination with metal macrocycles, heteropolyacids or other metal salts (see Fig. 4.42). Alternatively a bimetallic palladium(II) air oxidation system, involving bridging phosphines, can be used which does not require additional mediators [115]. This approach would also allow the development of asymmetric Pd-catalyzed allylic oxidation. 

Alternatively TEMPO can be reoxidized by metal salts or enzyme. In one approach a heteropolyacid, which is a known redox catalyst, was able to generate oxoammonium ions in situ with 2 atm of molecular oxygen at 100 °C [223]. In the other approach, a combination of manganese and cobalt (5 mol%) was able to generate oxoammonium ions under acidic conditions at 40 °C [224]. Results for both methods are compared in Table 4.9. Although these conditions are still open to improvement both processes use molecular oxygen as the ultimate oxidant, are chlorine free and therefore valuable examples of progress in this area. Alternative Ru and Cu/TEMPO systems, where the mechanism is me-... 

The success of this triple catalytic system relies on a highly selective kinetic control. From a thermodynamic point of view, there are 10 possible redox reactions that could occur in this system. However, the energy barrier for six of these (O2 + diene, O2 + Pd(0), etc.) are too high, and only the kinetically favored redox reactions shown in Scheme 11.14 occur. A likely explanation for this kinetic control is that the barrier is significantly lowered by coordination. Thus, the diene coordinates to Pd(II), BQ coordinates to Pd(0), HQ coordinates to (ML,), and Oj coordinates to ML ,. In a related system for aerobic oxidation, a heteropolyacid was employed in place of the metal macrocyclic complex (ML ,) as oxygen activator and electron transfer mediator [72]. Recent immobilization of the macrocyclic complex in ZeoHte-Y, led to eflBcient reoxidation of the HQ in the palladium-catalyzed 1,4-diacetoxylation [73]. 

All 1,4-oxidations discussed so far have relied on the use of 1,4-benzoquinone or Mn02 (in combination with 1,4-benzoquinone) as the stoichiometric oxidant. For obvious reasons molecular oxygen would constitute a much more attractive oxidant. Recent developments have made it possible to use molecular oxygen as the terminal reoxidant, either via activation by metal macrocycles or heteropolyacids. In both cases the oxidation of palladium is done by benzoquinone, which in turn is reoxidized by the metal macrocycle or heteropolyacid (Scheme 13). 

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