Palladium‐Catalyzed Self‐Cross Coupling of o ‐Bromo‐ trans ‐stilbenes To Yield 9,10‐Bis(arylmethylene)‐9,10‐dihydroanthracenes©

Under Palladium catalysis [Pd(OAc) 2 , K 2 CO 3 , LiCl, Bu 4 NBr, DMF] o ‐bromo‐ trans ‐stilbene ( trans ‐ 7a ) reacts to give 9,10‐dibenzylidene‐9,10‐dihydroanthracene ( 4a ) with formation of a new six‐membered ring. The ( Z ) diastereomer crystallizes preferentially to give pure ( Z )‐ 4a , as proved by X‐ray crystal structure analysis. A variety of substituted o ‐bromostilbenes and heterocyclic analogs 7 were prepared by Wittig olefination of o ‐bromobenzaldehyde with substituted benzyltriphenylphosphonium ylides, Wittig‐Horner‐Emmons olefination of arenecarbaldehydes with diethyl o ‐bromobenzylphosphonate or Wittig olefination of substituted benzaldehydes with substituted ( o ‐bromobenzyl)‐diphenylphosphonium ylides, respectively. The cis ‐ o ‐bromostilbenes were photoisomerized to the trans diastereoisomers trans ‐ 7 by irradiation in the presence of diphenyl disulfide. All of these o ‐bromo‐ trans ‐stilbenes trans ‐ 7a – g and trans ‐ 7j , k under palladium catalysis reacted to the corresponding 9,10‐bis(arylmethylene)‐9,10‐dihydroanthracenes 4 , mostly as mixtures of ( E ) and ( Z ) diastereomers (50‐97 % yield). The ( Z ) diastereomer of the parent 4a and the alkyl‐substituted compounds 4c and 4e could be purified by simple crystallization, and in some runs, only ( Z )‐ 4a , c , e were obtained. Among the heterocyclic analogs trans ‐ 7h , i only the furyl derivative trans ‐ 7h reacted (76 % yield) cleanly, whereas the pyridine analog trans ‐ 7i gave a mixture of products from which the rather sensitive product 4i could not be isolated in pure form. The cis‐o ‐bromostilbenes cis ‐ 7a , c cyclized to phenanthrenes under the same conditions (70‐71 % yield). The UV spectra of compounds 4a , c – k are similar to that of anthracene, and so are the oxidation and reduction potentials of ( Z )‐ 4a .