Allyl bromide 1,4-Benzoquinone

A novel approach to the problem of effecting regiospecific quinone-isoprene coupling has been reported (Scheme 4). Addition of the allylic bromide (142) to the masked quinone (143) gave the epimeric masked quinol (144). Cope rearrangement of the latter, followed by oxidation gave 2-isopentenyl-p-benzoquinone (145). 

The boviquinones-3 and -4 have been synthesised in moderate yields from 2,5-dihydroxy-l,4-benzoquinone and the corresponding allylic bromide in the presence of a tertiary amine (Scheme 40) (389). It is interesting that the success of these reactions was crucially dependent on the direct use of farnesyl bromide and of geranylgeranyl bromide without special purification. The preparation and properties of the chromenols and chromanols of boviquinone-4 have been reported (71). 

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). 

Potassium chromate, K2Cr04, is used very rarely. It converts hydra-zomethane into azomethane [613]. In the presence of hexamethylphos-phoramide and crown ether, it transforms allylic and benzylic bromides into a,p-unsaturated aldehydes [614], and in the presence of dilute sulfuric acid, it oxidizes chlorohydroquinone to chloro-p-benzoquinone [615]. 

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