Benzoquinones reoxidant

Isomerization of cyclopropanols. Ring opening of cyclopropanols gives enones on Pd-catalysis. Formation of a-methylene ketones is favored by the use of (dba)jPd2, molecular sieve 4A, and benzoquinone (reoxidant) in toluene. " ... 

A powerful piece of methodology has been developed involving nucleophilic attack on both an r and an T]p-complex in an adaptation of the Wacker reaction. Treatment of cyclohexadiene 9.268 with palladium acetate in acetic acid gives an -complex 9.281 (Scheme 9.76). If the reaction is done in the presence of sodium acetate, this salt will act as a nucleophile to give an t -complex 9.283 via an initial t -complex 9.282. Reductive elimination to form a bond between a ring carbon and the acetate ligand from palladium then gives the diacetate product 9.280. As the first acetate attacks trans to Pd, and the second acetate comes from Pd, the product is the trans isomer. The palladium is now in its zero oxidation state, but inclusion of benzoquinone reoxidizes it to palladium (II) and makes the entire process catalytic. There is an occasional... 

In order to make these oxidative reactions of 1,3-dienes catalytic, several reoxidants are used. In general, a stoichiometric amount of benzoquinone is used. Furthermore, Fe-phthalocyanine complex or Co-salen complex is used to reoxidize hydroquinone to benzoquinone. Also, it was found that the reaction is faster and stereoselectivity is higher when (phenylsulflnyl)benzoquinone (383) is used owing to coordination of the sulfinyl group to Pd, Thus the reaction can be carried out using catalytic amounts of PdfOAcji and (arylsulfinyl)benzoquinone in the presence of the Fe or Co complex under an oxygen atmosphere[320]. Oxidative dicyanation of butadiene takes place to give l,4-dicyano-2-butene(384) (40%) and l,2-dicyano-3-butene (385)[32l]. 

Larock has developed a new catalyst system for the Pd-catalyzed cyclization of olefinic tosylamides. Whereas typical conditions require either stoichiometric amounts of Pd(II) salts or catalytic amounts of Pd(II) in the presence of benzoquinone as a reoxidant, the new catalyst system utilizes catalytic Pd(OAc)2 under an atmosphere of O2 in DMSO with no additional reoxidant <96JOC3584>. Although o-vinylic tosylamides 76 can be cyclized to Af-tosylindoles 77 using this catalyst system, PdCla/benzoquinone is more effective for such cyclizations. Interestingly, in the case of o-allylic tosylanilides, the cyclization can be modulated to afford either dihydroindole or dihydroquinoline products. In a related approach involving a common 7i-aUyl Pd-intermediate, 2-iodoanilines were coupled with vinylic cyclopropanes or cyclobutanes in the presence of a Pd catalyst to afford dihydroindoles <96T2743>. 

The palladium(II)-mediated oxidative cyclization is also applied to the synthesis of carbazole-l,4-quinone alkaloids. The required arylamino-l,4-benzo-quinones are readily prepared by arylamine addition to the 1,4-benzoquinone and in situ reoxidation of the resulting hydroquinone [131]. 

The protection of a reactive intermediate complex by the DNA double helix versus a neutral oxidising agent in solution, has also been demonstrated by studying a photo-electron transfer process. In this example the intermediate complex is produced photochemically on the DNA, and is examined spectroscopically after a laser pulsed excitation [73]. Thus Ru(TAP)2(HAT) physically bound to nucleic acid is photo-reduced by hydroquinone during the laser pulse. The intermediate [Ru(TAP)2(HAT)] so-produced, detected by its absorption at 480 nm, is reoxidised by benzoquinone purposely added as oxidant to the solution. It is shown that this reoxidation of the mono-reduced complex is slower in the presence of polynucleotide than in its absence, indicating a protection of the transient mono-reduced complex in the DNA grooves. 

Akermark et al. applied (548) a catalytic version of Furukawa s palladium-mediated (stoichiometric) cyclization of 2-anilino-5-methyl-l,4-benzoquinone (842) to a total synthesis of murrayaquinone A (107) (see Scheme 5.101) (623). In this cyclization, only 5mol% of palladium(ll) acetate and an excess of TBHP as reoxidant were used (548). Subsequently, a catalytic cyclization of 842 to murrayaquinone A (107), using oxygen for the reoxidation of palladium, was reported (549) (Scheme 5.107). 

Under suitable conditions, further oxidation of the coupled products produces isolable dimer cation radicals. Thus treatment of 9-alkylcarbazoles with lead tetracetate in acetic acid-perchloric acid, or with 2,3-dichloro-5,6-dicyano-p-benzoquinone in acetic acid-perchloric acid, or with tris-(p-bromophenyl)ammoniumyl perchlorate in methylene chloride, or with nitrosonium borofluoride in acetonitrile all gave isolable cation radical perchlorates such as 17. These were reducible with aqueous sodium dithi-onite to the corresponding bicarbazoles the dimer cation radicals could be produced again by reoxidation of the dimer using 2,3-dichloro-5,6-dicyano-p-benzoquinone in acid solution. ... 

Abstract Palladium-catalyzed oxidation reactions are among the most diverse methods available for the selective oxidation of organic molecules, and benzoquinone is one of the most widely used terminal oxidants for these reactions. Over the past decade, however, numerous reactions have been reported that utilize molecular oxygen as the sole oxidant. This chapter outlines the fundamental reactivity of benzoquinone and molecular oxygen with palladium(O) and their catalyst reoxidation mechanisms. The chemical similarities... 

The original work shows that benzoquinone can be added to reoxidize the Pd reagent however, the reaction proceeds well without it. 

Several Pd-catalyzed oxidations with different reoxidants have been developed. In these reactions PdCl2 or Pd(OAc)2 in acetic acid is usually employed, with tert-BuOOH and Te02,699 or p-benzoquinone and MnO2 70°... 

The vinyl substitution reaction often may be achieved with catalytic amounts of palladium. Catalytic reactions are carried out in different ways depending on how the organopalladium compound is generated. Usually copper(II) chloride or p-benzoquinone is employed to reoxidize palladium(0) to palla-dium(II) in catalytic reactions when methods (i) or (ii) are used for making the organopalladium derivative. The procedures developed for making these reactions catalytic are not completely satisfactory, however. The best catalytic reactions are achieved when the organopalladium intermediates are obtained by the oxidative addition procedures (method iii), where the halide is both the reoxidant and a reactant. Reviews of some aspects of these reactions have been published.u-le... 

A phenolic oxygen participates in facile oxypalladation. 2-Allylphenol (55) undergoes clean cyclization to 2H-l-benzopyran (56) in DMSO under air with a catalytic amount of Pd(OAc)2 without a reoxidant. 2-Methylbenzofuran (57) is obtained when PdCl2 is used [50]. But different chemoselectivity with the Pd(II) salts was also reported [51]. Catalytic asymmetric cyclization of the tetrasubstituted 2-allylic phenol 58 using the binaphthyl-type chiral ligand 60, called (.S, .S )-ip-borax, afforded the furan 59 with 96% ee. Use of Pd(CF3 002)2 as a catalyst is essential in the presence of benzoquinone [52]. Formation of the benzofuran 62 from 61 has been utilized in the synthesis of aklavinione [53]. The intramolecular reaction of 2-hydroxychalcone (63) produces the flavone 64 [54]. 

Pd(OAc)2 to give the cyclohexenone 256. In clavulone synthesis, only the silyl enol ether in 257 reacts with Pd(OAc)2 to give the enone 258 [153]. The dehydrogenation can be carried out with a catalytic amount of Pd(OAc)2 using benzoquinone as the reoxidant. Cyclopentenone (260) is prepared from cyclopentanone (259) by using a supported Pd catalyst under 02 atmosphere [154], The enone 261 is converted to the dienone 263 via the dienol silyl ether 262 [155],... 

Additions to l,3-dienes6 (12, 367-368 14, 249-250 15, 245). This reaction can be used to effect intramolecular cyclization of cyclic 1,3-dienes substituted by a suitable nitrogen nucleophile. Thus reaction of the amido diene 1 with lithium acetate catalyzed by Pd(OAc)2 (with benzoquinone as reoxidant) provides the ds-fused heterocycle cis-2, in which the acetoxy group is cis to the ring fusion, formed by an overall trans-1,4-oxyamidation of the diene system. Addition of a trace of LiCl improves the yield and results in an overall cis- 1,4-oxyamidation (equation I). Acetamides and carbamates can also be used in place of amides. 1,4-Chloroamidation can also be effected by use of 2 equiv. of LiCl. 

In a recent review it was argued that such additives of copper, benzoquinone, and HPMOV are not really needed all that is needed is the presence of oxidation-resistant ligands that prevent palladium metal formation [15]. Indeed, activation of the C-H bond is not as slow as, for example, the Wacker reaction of ethene in which reoxidation of palladium must be performed by copper oxidation, although in this catalytic system the additives may still play a role in stabilizing the intermediate low-valent palladium species and thus prevent catalyst decomposition. This thesis was corroborated by the work of de Vos and Jacobs, who showed that addition of benzoic acid to the oxidative arylation reaction in the presence of oxygen led to superior results in the coupling of a variety of substituted arenes with acrylates, cinnamates, and ,/f-unsaturated ketones. Very good yields and TON up to 762 were obtained at 90 °C. A mixture of the o, m, and p isomers is obtained if substituted arenes are used [16]. 

In 2008, the same group developed an asymmetric version of this reaction (Scheme 10.10).24 Run under similar conditions, but with more silver oxide (1 equiv) and thus less reoxidant (benzoquinone 0.5 equiv) and in the presence of catalytic amounts of chiral ligand (20 mol%), the best enantiomeric excesses and yields were obtained with menthyl-L-leucine ester as the chiral ligand. 

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