Wacker-type oxidations

PdCOTfj CIPr) generated in situ from [Pd(p,-Cl)(Cl)(IPr)]j and AgOTf was reported to catalyse the copper-free Wacker-type oxidation of styrene derivatives using ferf-butyl hydroperoxide (TBHP) as the oxidant (Table 10.7) [41]. Reaction conditions minimised oxidative cleavage of styrene, which is a common side-reaction in Wacker-type oxidations. However, when franx-stilbene was used as a substrate, a significant amount of oxidative cleavage occurred. 

The system consisting of [PMo12 BVBO40](3+, ) and Pd2+ salts can catalyze the Wacker-type oxidation. What is the role of [PMo12 V O40]( 3+, ) ... 

The electrochemical Wacker-type oxidation of terminal olefins (111) by using palladium chloride or palladium acetate in the presence of a suitable oxidant leading to 2-alkanones (112) has been intensively studied. As recyclable double-mediatory systems (Scheme 43), quinone, ferric chloride, copper acetate, and triphenylamine have been used as co-oxidizing agents for regeneration of the Pd(II) catalyst [151]. The palladium-catalyzed anodic oxidation of... 

In 1960, Moiseev and coworkers reported that benzoquinone (BQ) serves as an effective stoichiometric oxidant in the Pd-catalyzed acetoxylation of ethylene (Eq. 2) [19,20]. This result coincided with the independent development of the Wacker process (Eq. 1, Scheme 1) [Ij. Subsequently, BQ was found to be effective in a wide range of Pd-catalyzed oxidation reactions. Eor example, BQ was used to achieve Wacker-type oxidation of terminal alkenes to methyl ketones in aqueous DMF (Eq. 3 [21]), dehydrogenation of cyclohexanone (Eq. 4 [22]), and alcohol oxidation (Eq. 5 [23]). In the final example, 1,4-naphthoquinone (NQ) was used as the stoichiometric oxidant. 

Palladium-catalyzed, Wacker-type oxidative cycHzation of alkenes represents an attractive strategy for the synthesis of heterocycles [139]. Early examples of these reactions typically employed stoichiometric Pd and, later, cocat-alytic palladium/copper [140-142]. In the late 1970s, Hegedus and coworkers demonstrated that Pd-catalyzed methods could be used to prepare nitrogen heterocyles from unprotected 2-allylanilines and tosyl-protected amino olefins with BQ as the terminal oxidant (Eqs. 23-24) [143,144]. Concurrently, Hosokawa and Murahashi reported that the cyclization of allylphenol substrates can be accomplished by using a palladium catalyst with dioxygen as the sole stoichiometric reoxidant (Eq. 25) [145]. 

PdS04/H9PV6Mo604o/CuS04 in the presence of chemically modified P-cyclodextrins were also used as CPTC systems in the Wacker-type oxidation of higher a-olefins (Cg-Cje) to the corresponding 2-ketones (Equation 18) with high yields (90-98%) in an aqueous/organic two phase system.545,571... 

Stereoselective aldol condensation. 2-Butenyllilhium (1) reacts with aldehydes to form the threo- and erythro-fl-methyl alcohol in equal amounts. However, if a trialkylboranc is present, r/ireo-products predominate. Presumably an allylboronate complex (a) is involved.1 An example is formulated in equation (1). The products are converted into 0-hydroxy ketones (4) by a Wacker-type oxidation.2... 

The Pd(II) hinge in cage 2 can also participate in a chemical transformation. The catalytic Wacker-type oxidation of olefins took place when 8-nonen-l -ol (32) was heated for 5 h at 80 °C in the presence of cage 2 (5 mol%), giving 9-hydroxynonan-2-one (33)... 

The field of homogeneous palladium catalysis traces its origin to the development of the Wacker process in the late 1950s (Eq. 7) [83]. Since this discovery, palladium-catalyzed reactions have evolved into some of the most versatile reactions for the synthesis of organic molecules [84,85]. Palladium-catalyzed Wacker-type oxidation of alkenes continues to be an active field of research [86-88], and several recent applications of NHC-coordinated Pd catalysts have been reported for such reactions. 

The Pd-catalyzed conversion of terminal alkenes to methyl ketones is a reaction that has found widespread use in organic chemistry [87,88]. These reactions, as well as the industrial Wacker process, typically employ CuCh as a co-catalyst or a stoichiometric oxidant. Recently Cu-free reaction conditions were identified for the Wacker-type oxidation of styrenes using fBuOOH as the oxidant. An NHC-coordinated Pd complex, in-situ-generated (I Pr)Pd(OTf)2, served as the catalyst (Table 5) [101]. These conditions min-... 

Table 5 Wacker-type oxidation of alkenes employing (I Pr)Pd(OTf)2...

On the other hand, Wacker-type oxidative cyclization is a versatile approach for the construction of oxygenated stereocenters [213,214]. The synthesis of a number of dihydrobenzofurans catalyzed by an in situ-formed carbene-palladium complex has been reported by Muniz [215]. When Pd(TFA)2 in combination with IMes were employed, high yields in pure cyclized products were obtained after simple work-up (Table 9). However, palladium salts containing chlorine or acetate groups led to the formation of mixtures containing the desired product and its six-membered ring isomer. 

L. Hintermann, Wacker Type Oxidations, in. Transition Metals for Organic Synthesis, Second Ed. (Eds. M. Beller and C. Bolm, Wiley-VCH, Weinheim, 2004, Vol. 2, Chapter 2.8). 

A solvent combination of. vcC02- C4Ciim PF6 was found to afford superior selectivity in the Wacker-type oxidation of 1-hexene with PdCl2-CuCl as catalyst to afford 2-hexanone as the main product as shown in Scheme 5.16.[73] In the absence of either scC02 or ionic liquid, considerably lower selectivity for 2-hexanone was observed. Catalyst solutions were recycled five times with a low, but steady, decrease in activity. 

Cu " Pd " -, Cu " -, and Pd -TSMs are completely different from each other in catalytic activity. Cu - and Pd " -TSMs catalyze no reaction and the total oxidation of propylene, respectively, whereas Cu Pd " -TSM catalyzes the oxidation to form acetone selectively, suggesting that the Wacker type oxidation takes place over the catalyst (41). The results are shown in Fig. 6. The higher initial activity is observed for Cu Pd -TSM with the lower Cu Pd ratio, namely the higher Pd " loading. This might be explainable by the second order dependency of the reaction rate on Pd " concentration, observed for the homogeneous system by Vargaftik et al. in the... 

It is surprising that the Wacker-type oxidation of 1-octene to 2-octanone is faster with the Co-salophen/zeolite catalyst than with the free complex. However, it is known that the Pd(II)-catalyzed oxidation of terminal olefins to ketones is accelerated by the presence of a catalytic amount of strong acid [1,2]. An explanation of the fester rate of the zeolite-encapsulated Co-salophen in this case is therefore that the acidic sites in the zeolite accelerate the reaction. 

The first synthesis of the hexacyclic himandrine skeleton was achieved by L.N. Mander and co-workers. The last six-membered heterocycle was formed via an intramolecular Wacker-type oxidation in which the terminal alkene side-chain reacted with the secondary amine functionality. The oxidation was conducted in anhydrous acetonitrile to insure that the Pd-alkene complex was substituted exclusively by the internal nucleophile. The resulting six-membered enamine was then hydrogenated and the MOM protecting groups removed to give the desired final product. 

Monflier, E., Mortreux, A. Wacker-type oxidations. Aqueous-Phase Organometallic Catalysis 998, 513-518. 

Lambert, A., Derouane, E. G., Kozhevnikov, I. V. Kinetics of One-Stage Wacker-Type Oxidation of C2-C4 Olefins Catalysed by an Aqueous PdCl2-Heteropoly-Anion System. J. Catai. 2002, 211, 445-450. 

Pellissier, H., Michellys, P.-Y., Santelli, M. Regiochemistry of Wacker-type oxidation of vinyl group in the presence of neighboring oxygen functions. Part 2. Tetrahedron 1997, 53,10733-10742. 

Wacker-type oxidative reactions of olefins with nucleophiles, reactions of zr-allyl-palladium complexes with nucleophiles, reactions based on chelation, and trans-metallation of organomercury compounds. 

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