Palladium -benzoquinone complexes

The acid sensitive 2-methyl-3-buten-2-ol (10) was oxidized to the corresponding methyl ketone (11) with a palladium-nitro complex (90% conversion, 90% selectivity) or PdCl2-benzoquinone (98% conversion, 90% selectivity) (equation 10). ... 

The catalytic cycle of the palladium-catalyzed diacyloxylation follows the cycle depicted in Scheme 8-6 (X = RCOO, Y = R COO ). The coordination of a quinone in the Ji-allyl)palladium intermediate was demonstrated by NMR studies including T, measurements [58]. Attack by the second nucleophile results in the formation of the 1,4-addition product and a palladium(0)-benzoquinone complex. In an independent mechanistic study, it was shown that such Pd(0)-benzoquinone complexes, which are stable at neutral conditions (pH 7), react with weak acids to give hydroquinone and the palladium(II) salt of the acid [Eq.(36)] [59],... 

Interestingly, this 1,4-carbochlorination occurs syn, which constrasts with that via the vinylpalladation in Eq.(49), which occurs anti. An explanation for this difference is that the allylsilane attacks the palladium-diene complex anti, leading to a rrans-carbopalladation of the double bond. This is the first example of nucleophilic attack by an allylsilane on an olefin coordinated to a metal. Direct evidence for a frans-carbopalladation was provided by the isolation of the proposed 7r-allyl intermediate of Eq.(51) as its chlorodimer 98a from reaction of 97 with Li2PdCl4 in the absence of benzoquinone [Eq.(52)] [119b]. The trans relationship between palladium and the carbon that has attacked the diene was established by the reporter ligand technique used for 41 in Section 8.3.1.1 under Intramolecular 1,4-diacyloxylation . 

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. 

Following the work by Li, Shi and co-workers reported both an inter- and intramolecular oxidative allylic alkylation reaction utilizing palladium(II) complex 49 as a catalyst and using benzoquinone (BQ) with oxygen gas as the oxidant (Scheme 28) [51]. 

An interesting Pd-catalyzed diene carboamination reaction that involves urea-directed C—H activation was recently reported [56]. For example, treatment of N-aryl urea 63 with an activated diene in the presence of 10 mol% Pd(0 Ac) 2,50 mol% TsO H, AC2O, and benzoquinone provided 64 in 90% yield (Eq. (1.29)). The transformation is initiated by directed palladation of the arene by a palladium tosylate complex (formed... 

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)... 

Recently, Kapitan and Gracza and co-workers have disclosed further oxycarbonylative annulations of unsaturated polyols, either diastereoselective or enantioselective, using chiral palladium(II) complexes. In their initial report, the method was investigated for the kinetic resolution of pent-4-ene-l,3-diols 97 through their asymmetric oxycarbonylative bicyclization and a conversion controlled by the amount of benzoquinone. Besides the chiral ligand, the efficiency depends on the anionic part of the catalyst and the solvent. Under optimum conditions, the lactone 98a was isolated in 29% yield and 62% ee. Similarly, the iyK-diols ( )-97b and ( )-97c provided the corresponding natural... 

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 mechanism of this new reaction is shown in Scheme 14. Coordination of the diene to palladium(II) makes the diene double bond electrophilic enough to be attacked by the allylsilane. The attack by the allylsilane takes place on the face of the diene opposite to that of the palladium (anti). This is the first example of an anti attack by an allylsilane on a 7T-(olefin)metal complex. Benzoquinone (BQ)-induced anti attack by chloride ion produces the product 58. 

In a stoichiometric reaction the 6jr-allyl)palladium complex 66 was isolated and characterized5815. In a subsequent reaction the jr-allyl complex was reacted with benzoquinone in acetic acid to give an allylic acetate, which was hydrolyzed and oxidized to theaspirone. Interestingly, a quite high diastereoselectivity for the turns methyl isomer was obtained in the palladium-mediated spirocyclization (equation 28). 

In 1997, Backvall and Jonasson published a procedure for the 1,2-oxidation of terminal allenes 7 [5]. In this case the reaction conditions were chosen so that the (vinyl)palladium complex equilibrates back to the allene complex. Using bromide instead of chloride as a nucleophile, the 2-bromo-jt-allyl complex 9 is the major intermediate present in the reaction mixture. A catalytic reaction was developed with the use of 5 mol% palladium acetate and p-benzoquinone (BQ) as terminal oxidant (Scheme 17.5). 

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use is made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-x-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast... 

Dioximes form bis complexes (35) where the ligand normally chelates through the nitrogen atoms.251-253 Detailed studies have been made of the partially oxidized compounds obtained from reaction of bis(diphenylglyoximato)- and bis(benzoquinone dioximato)-palladium with iodine254,255 and bromine.256 These complexes contain polyhalide anions and possess the high anisotropic electrical conductivity associated with many complexes which crystallize as stacked molecular units.257... 

The elegant asymmetrization methodology of a meso compound, achieved in high enantioexcess under chiral environment, was the highlight of the total synthesis of (+)-pancratistatin (94) reported by Trost and Pulley (31]. The synthesis commenced with ( )-conduritol-A (130), obtained from p-benzoquinone, (Scheme 18) which was converted into the acetonide 131 and thence, via the dialkoxide to the cis-bis carbonate 132 (Scheme 19). The chiral n-ailyl palladium complex A formed on treatment erf 132 with the catalyst generated from chiral bis-amide 133 and n-allyl palladium chloride underwent azide substitution from the less hindered face of the molecule to provide the monocarbonate 134 in excellent yield and with high optical induction. 

A series of palladium(O) mono-imidazolylidene complexes have also been prepared and used for the Heck reaction of aryl chlorides in [NBu4]Br.29 The authors report that two of their catalysts, (1,3-dimesi-tylimidazolylidene)(naphthoquinone)palladium(0) and (1,3-dimesitylimida-zolylidene)(benzoquinone)palladium(0) (Fig. 5) remain stable throughout the reaction and will couple even non-activated aryl chlorides in good yields. Clearly, the authors believe that the imidazolylidene complex is responsible for this activity. 

Cyclic ethers and amines can be formed if the nucleophile is an intramolecular alcohol or amine. Stoichiometric palladium can be avoided by using benzoquinone as the stoichiometric oxidant with a catalytic amount of palladium. In this example intramolecular oxypalladation of a diene is followed by attack of an external nucleophile on a TC-allyl complex. 

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