Benzoquinone, allylation

In the presence of oxidants, such as benzoquinone, allylic acetates can be obtained using only catalytic quantities of Pd complexes, as shown in Scheme 10. The reactions of cycloaUcenes may not involve n-allylpalladium intermediates, and they may merely be examples of the Wacker-type oxidation. 

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>. 

Photoaddhion of electron donor olefins such as vinyl ethers and stilbene to variously methyl and halogeno-substituted 1,4-benzoquinones resulted in the formation of dihydrobenzofurans via a dienone-phenol rearrangement of the primary product spirooxetanes <96H(43)619>. High-temperature water seems to be an alternative to use of acid catalysts or organic solvents by the cyclization of allyl phenyl ethers to dihydrobenzofurans <96JOC7355>. 

Benzoquinone, having a low triplet energy (Et= 50 kcal/mole),<104) reacts with dienes to yield spiro-pyrans, presumably through the following allylic radical intermediate 51 ... 

In 1971, Brown and Davidson reported that 1,3-cyclohexadiene undergoes a palladium-catalyzed 1,4-diacetoxylation of unspecified stereochemistry28. The oxidant employed was p-benzoquinone. They were uncertain about the mechanism at the time but later work has shown that the reaction proceeds via a (jr-allyl)palladium intermediate and subsequent nucleophilic attack by acetate6,7. 

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). 

Allylic acetoxylation.2 Pd(OAc)2 in HOAc can effect allylic acetoxylation of alkenes, probably via a TT-allylpalladium complex, and only a catalytic amount is required in the presence of a cooxidant such as benzoquinone-Mn02. The reaction is not useful in the case of simple alkenes because of lack of discrimination between the two allylic positions, but this acetoxylation can be regioselective in the case of alicyclic alkenes. 

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). 

Using a similar approach, l-acetoxy-4-diethylamino-2-butene and l-acetoxy-4-benzylamino-2-butene were prepared. Treatment of 1,3-butadiene with LiCl-LiOAc in the presence of Pd(OAc)2 and p-benzoquinone in acetic acid gave 91% l-acetoxy-4-chloro-2-butene (E/Z = 90/10). Subsequent allylic amination with diethylamine, catalyzed by Pd(PPh3)4 in THF, produced mainly ( )-l-acetoxy-4-diethylamino-2-butene13. 

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... 

The intramolecular addition of carbon nucleophiles to alkenes has received comparatively little attention relative to heterocyclization reactions. The first examples of Pd-catalyzed oxidative carbocyclization reactions were described by Backvall and coworkers [164-166]. Conjugaled dienes with appended al-lyl silane and stabilized carbanion nucleophiles undergo 1,4-carbochlorination (Eq. 36) and carboacetoxylation (Eq. 37), respectively. The former reaction employs BQ as the stoichiometric oxidant, whereas the latter uses O2. The authors do not describe efforts to use molecular oxygen in the reaction with allyl silanes however, BQ was cited as being imsuccessful in the reaction with stabihzed car-banions. Benzoquinone is known to activate Ti-allyl-Pd intermediates toward nucleophilic attack (see below. Sect. 4.4). In the absence of BQ, -hydride eUm-ination occurs to form diene 43 in competition with attack of acetate on the intermediate jr-allyl-Pd" species to form the 1,4-addition product 44. 

N 15-72%. The following isomer is known. N-Allyl-o-nitrooniline, red oil bp 174-5° at 12 mm. Was prepd by oxidation of N-allyl-o-aminoaniline with FeCls and p-benzoquinone... 

Fumigatin [484-89-9] (3-hydroxy-2-methoxy-5-methyl-/>-benzoquinone) is isolated from metabolism of Aspergillusjumigatus and is used as an antimicrobial. 5-Allyl-l,6-dimethoxy-2,3-methylenedioxybenzene (dillapiole) (62) is a synergist for pyrethrum. Derivatives have been prepared and evaluated (216). 

Allylic acetoxylation of cyclohexene can be selectively effected by palladium acetate in the presence of Mn02 and p-benzoquinone (bq) (equation 298).640... 

In 1956, Braude et al.92 showed that treatment of allylic, benzylic and propargylic alcohols with o-chloranil (tetrachloro-l,2-benzoquinone) (82) provided the corresponding aldehydes and ketones. Under the mild conditions employed, involving room temperature or refluxing ether, saturated alcohols remained unaffected. 

Simple, direct and regioselective allylation of 1,4-benzoquinones is an important objective in the synthesis of many isoprenoid quinones such as plastoquinone-1 and vitamin Ki, which play important roles in cellular metabolism and photosynthesis. In a new procedure where the use of a strongly coordinating solvent was found to play an important role, it was shown that reaction of 3 equivalents of (3-methyl-2-butenyl)trifluorosilane with one equivalent of 2,3-dimethyl-l,4-benzoquinone in formamide in the presence of 5 equivalents of FeCl3.6H20 for 23 hours at 40°C gave plastoquinone-1 1, in 90% yield. 

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 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]. 

A mixed oxide of ruthenium, copper, iron and alumnium has been developed as a catalyst for the synthesis of aldehydes and ketones from alcohols.258 Oxidation of chiral secondary 1,2-diols with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone under ultrasound wave promotion leads to the selective oxidation of benzylic or allylic hydroxyl group. The configuration of the adjacent chiral centre is retained.259 The kinetics of oxidation of ethylbenzene in the presence of acetic anhydride have been studied.260... 

A mild triple catalytic system consisting of Pd(OAc)2, hydroquinone, and a transition metal macrocycle (for example, iron phthalocyanine) was reported [243]. The catalytic effect is carried out by the interaction of Pd(II) with the substrate and the acquisition of two electrons, which are further transferred to the benzoquinone that is reduced to hydroquinone. The hydroquinone is then reorganized to benzoquinone by the 02/metal macrocycle system. The following types of transformations were carried out in mild conditions using the developed system 1,4-oxidation of conjugated dienes, oxidation of terminal olefins to methyl ketones, and allylic oxidation. 

Benzoquinone 6 was reported as one of a new class of amino alcohol-derived benzoquinones tested in the palladium-catalyzed 1,4-dialkylation of 1,3 dienes. These ligands were prepared by reaction of 1 with C2-symmetric l,4-diallyloxy-2,5-benzenedicarboxylic acid chloride followed by allyl deprotection.29... 

PMB ethers can be cleaved oxidatively with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)11 in dichloromethane/water tor with cerium ammonium nitrate (CAN) in acetonitrile/water.12 Many other protecting groups such as esters, isopropylidene acetals, benzyl ethers, allyl ethers and f-butyldiphenyl silyl (TBDMS) ethers are stable to these conditions (Scheme 2.4). The cleavage reaction, with DDQ is initiated with a single-... 

The regeneration of the benzoquinone can also be achieved with dioxygen in the absence of the cobalt cocatalyst. Thus, Hanyu et al. [86] showed that a combination of RuCl2(Ph3P)3, hydroquinone and dioxygen, in PhCF3 as solvent, oxidized primary aliphatic, allylic and benzylic alcohols to the corresponding aldehydes in quantitative yields (Eq. 20). 

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