Benzoquinone ligands

Transition metal complexes of o-benzoquinone, o-semiquinone and catecholate ligands. C. G. Pier-pontand R. M. Buchanan, Coord. Chem. Rev., 1981, 38, 45-87 (72). 

Yamamoto and co-workers reported abase- and ligand-free palladium (II) catalysed method, in methanol at room temperature and under air [49], While the conversion of arylboronic acids bearing an electro-donating group was very efficient, the presence of an electro-withdrawing substituent led to lower conversions. To solve this problem and also prevent the fast formation of palladium black, Yamamoto and co-workers described a new approach where the reaction was catalysed by NHC-bearing complexes 21 and 22 in the presence of an oxidant (Fig. 7.4) [50]. The best results were obtained when complex 21b was used in methanol at room temperature, in the presence of a small excess of p-benzoquinone. 

The development of catalysts for the efficient oxidation of catechol and its derivatives in water is topic of ongoing work in this laboratory. Towards this end, polyethylene glycol side-chains were incorporated in a pentadentate salen ligand to enhance the water solubility of the complexes derived thereof. A dinuclear copper(II) complex is found to catalyze the oxidation of 3,5-di-tert.-butylcatechol into 3,5-di-tert-butyl-o-benzoquinone more than twice as fast in aqueous organic solution as in purely organic solvents (ly,at/knon= 140,000). Preliminary data are discussed. 

Coordination studies of acyclic thioether ligands to silver(I) centers has also been studied. Poly(alkylthio)aromatic systems have been used to form supramolecular silver(I) compounds. With the ligand 2,3,5,6-tetrakis(isopropylthio)benzoquinone the compound has a linear chain structure in which silver(I) has a tetrahedral coordination.1156 A similar structure has the compound with the hexakis(methylthio)benzene,1157 but with the hexakis(tolylthio)benzene the silver... 

Asymmetric induction has also been achieved in the cyclization of aliphatic alcohol substrates where the catalyst derived from a spirocyclic ligand differentiates enantiotopic alcohols and alkenes (Equation (114)).416 The catalyst system derived from Pd(TFA)2 and (—)-sparteine has recently been reported for a similar cyclization process (Equation (115)).417 In contrast to the previous cases, molecular oxygen was used as the stoichiometric oxidant, thereby eliminating the reliance on other co-oxidants such as GuCl or/>-benzoquinone. Additional aerobic Wacker-type cyclizations have also been reported employing a Pd(n) system supported by A-heterocyclic carbene (NHC) ligands.401,418... 

When 1,3-dienes containing a tethered alcohol are subjected to Wacker-type reactions, the initial intramolecular oxypalladation event creates a 7r-allylpalladium species, which can then undergo an additional bond-forming process to effect an overall 1,4-difunctionalization of the diene with either cis- or // -stereochemistry (Scheme 18).399 An array of substrate types has been shown to participate in this reaction to generate both five- and six-membered fused or ro-oxacycles.435-437 Employing chiral benzoquinone ligands, progress toward the development of an asymmetric variant of this reaction has also been recorded, albeit with only modest levels of enantioselectivity (up to 55% ee).438... 

Transition metal complexes with o-dioxolene ligands constitute one of the most intriguing classes of complexes as far as their electrochemical behaviour is concerned, in that, as already mentioned in Chapter 5, Section 1, such ligands are able to shuttle through the oxidation states o-benzoquinone)o-benzosemiquinone/catecholate illustrated in Scheme 250 (a process carried out in nature by the dicopper (I I)-based enzyme catechol oxidase through a single two-electron step see Chapter 9, Section 1.2). 

Let us now illustrate some significant examples of the redox propensity of related (but less widely studied61) quinone diimines. As happens for 1,2-dioxolenes, such ligands also exhibit the reversible electron transfer sequence o-benzoquinone diiminejo-benzosemiquinone diimine monoanion/ o-phenylendiamine dianion illustrated in Scheme 4. 

For example, reductive elimination of benzonitrile from (Et3P)2Pd(CN)Ph is promoted by the addition of triethyl phosphite [4], addition of benzoquinone promotes reductive eliminations in palladium chemistry (Chapter 12), etc., but in all cases different roles can be envisaged, such as simple ligand exchange. 

Group VI Donors. Oxygen and sulphur donor ligands. Oxidative addition of tetrachloro-l,2-benzoquinone to rrans-[Ir(CO)Cl(PR3)2] (R3 — Ph3 or Ph2Me) has been shown to give the Ir ° products [Ir(02CgCl4)(C0)Cl(PR3)2], whose structures (45) were confirmed from far-i.r. and H n.m.r. data. Comparison... 

Ru(H30)(bpy)(amp)]+ (H3amp=A-(2-hydroxyphenyl)salicyldimine) is made from RuClj, the ligand and (bpy) electronic spectra were measured [789]. The reagent [Ru(H20)(bpy)(amp)]VTBHP/(BTBAC)/C Hg oxidised benzene to phenol and benzoquinone, while phenol was oxidised to benzoquinone. The intermediate species is likely to be [Ru (0)(bpy)(amp)]+ [789]. 

The synthesis of acrylic acid or its ester (228) from ethylene has been investigated in AcOH from the standpoint of its practical production 12]. The carbonylation of styrene is a promising commercial process for cinnamate (229) production[207,213,214]. Asymmetric carbonylation of styrene with Pd(acac)2 and benzoquinone in the presence of TsOH using 2,2 -dimethoxy-6, 6 -bis(diphenylphosphino)biphenyl (231) as a chiral ligand gave dimethyl phenylsuccinate 230 in 93% ee, although the yield was not satisfactory, showing that phosphine coordination influences the stereochemical course of the oxidative carbonylation with Pd(II) salt[215]. 

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