Nickel-Mediated Cyclobutabenzene Syntheses. trans-7,8-Dibromocyclobutabenzenes:Their One Pot Preparation, X-ray Structure and Diels-Alder Reactions.
The use of nickel(0) complexes, [(1,2,3,4-h)-anthracene]bis(tributylphosphine)nickel, [(1,2,5,6-h)-1,5-cyclooctadiene]bis(tributylphosphine)nickel or metallic nickel to produce high yields of trans-7,8-dibromocyclobutabenzene (i.e., trans-7,8-dibromobicyclo[4.2.0]octa-1,3,5-triene) from a,a,a',a'-tetrabromo-o-xylene was reported. Trans-7,8-dibromobicyclo[4.2.0]octa-1,3,5-triene was characterized by x-ray crystallog., that confirmed the trans arrangement between the two bromine atoms. The cyclization reaction was also applied to substituted bis(bromomethyl)benzenes to yield the resp. trans-7,8-dibromocyclobutabenzenes. The reactions of trans-7,8-dibromocyclobutabenzenes with various dienophiles showed that the opening of the four membered ring of the cyclobutabenzenes is exclusively conrotatory (to produce the corresponding E,E-dibromo-o-xylylene), and that the addn. of the dienophile is exclusively endo. The reactions of trans-7,8-dibromobicyclo[4.2.0]octa-1,3,5-triene with unsym. substituted dienophiles yielded only one regioisomer.
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