Palladium-benzoquinone-based 1,4-oxidation

In the catalytic cycle of the palladium-benzoquinone-based 1,4-oxidation of 1,3-dienes, benzoquinone is reduced to hydroquinone. The diacetoxylation reaction is conveniently performed with p-benzoquinone in catalytic amounts, employing Mn02 as the stoichiometric oxidant. In this process, the hydroquinone formed in each cycle (cf. Scheme 8-6) is reoxidized to p-benzoquinone by MnO,. For example, the catalytic reaction of cyclohexa-1,3-diene using catalytic amounts of both Pd(OAc)2 and p-benzoquinone with stoichiometric amounts of Mn02 in acetic acid in the presence of lithium acetate afforded a 93% yield of rranj-l,4-diacetoxycyclohex-2-ene (>91% tram) [51b]. The corresponding reaction in the presence of lithium chloride gave ci5-l,4-diacetoxycyclohex-2-ene in 79% yield (>96% cis). 

In the palladium-catalyzed 1,4-oxidations of conjugated dienes described so far, only heteroatom nucleophiles have been employed. There is an intrinsic problem in using free carbanions in an oxidation reaction since the oxidant can readily remove an electron and oxidize the carbanion to a radical. Furthermore, in the procedure associated with the best selectivity, i.e., the benzoquinone-based process, acid is required to regenerate the Pd(0)-benzoquinone complex to Pd(II) and hydroquinone. 

The choice of the most effective catalyst system is highly dependent on the type of olefin under consideration. Polymerization of CO and aliphatic a-olefines is most suitably carried out employing a catalytic system modified with a symmetrical, Cs-bridged aryldiphosphine ligand (2,71). However, these systems are not suitable for copolymerization of CO and styrene (41). For this reaction palladium(II)-based catalysts modified with a conjugated diimine (39,41), a bisoxazoline (43,44), a phosphine-phosphite (43), or a phosphine-imine ligand (43) have been employed, in, under chain-transfer conditions, combination with an oxidant promotor, such as 1,4-benzoquinone or 1,4-naphthoquinone (39-47,72), or a polar, acidic type of solvent (73,74). 

Retrosynthetic analysis of carbazoquinocin C (274) and (+ )-carquinostatin A ( + )-278 based on our highly convergent palladium(II)-mediated intramolecular oxidative coupling of arylamino-l,2-benzoquinones provides aniline (839) and 4-heptyl-3-methyl-l,2-benzoquinone (946) as precursors for 274, and 4-prenylaniline (945) and 4-(2-hydroxypropyl)-3-methyl-l,2-benzoquinone (947) as precursors for ( + )-278 (646) (Scheme 5.126). 

To overcome the problems encountered in the homogeneous Wacker oxidation of higher alkenes several attempts have been undertaken to develop a gas-phase version of the process. The first heterogeneous catalysts were prepared by the deposition of palladium chloride and copper chloride on support materials, such as zeolite Y [2,3] or active carbon [4]. However, these catalysts all suffered from rapid deactivation. Other authors applied other redox components such as vanadium pentoxide [5,6] or p-benzoquinone [7]. The best results have been achieved with catalysts based on palladium salts deposited on a monolayer of vanadium oxide spread out over a high surface area support material, such as y-alumina [8]. Van der Heide showed that with catalysts consisting of H2PdCU deposited on a monolayer vanadium oxide supported on y-alumina, ethene as well as 1-butene and styrene... 

Three new syntheses of 7-methoxymitosene (3) were published in the last five years. They were based on entirely different approaches. The one by Luly and Rapoport was a forerunner of the decarbamoylaziridinomitosene synthesis described above, and the palladium-catalyzed ring closure was developed in it [33]. In this synthesis (Scheme 11), 5,6-dibromo-2-methoxy-3-methyl-l,4-benzoquinone (86) was treated with proline ethyl ester to give 87. Photochemical oxidation-reduction then afforded 88 in 71% overall yield. Palladium-catalyzed oxidation of 88 furnished 89, which was converted to 90 by palladium-acetate-catalyzed ring closure in 93% overall yield. The carbethoxy group was then transformed into a carbamoyloxymethyl substituent by standard procedures, affording 3 in 67% yield. 

Palladium(ii) Complexes.—The oxidation of quinol by palladiumfii) occurs in two stages. In the first, reaction of the conjugate base of the Pd aquo ion results in the formation of a Pd -benzoquinone intermediate, which has been isolated from the reaction mixture. The observed rate constant is of the form (in excess reductant)... 

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