Palladium-Catalyzed Self-Cross Coupling ofo-Bromo-trans-stilbenes To Yield 9,10-Bis(arylmethylene)-9,10-dihydroanthracenes©

Abstract

Under Palladium catalysis [Pd(OAc)2, K2CO3, LiCl, Bu4NBr, 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.

The Heck Reaction as a Sharpening Stone of Palladium Catalysis

“…Going back to the arylation of aromatic molecules, it should be noted that while only electron-rich phenols or heterocycles were, so far, involved in the intermolecular version, intramolecular arylation is a well-known process, examples of which are numerous though random. Thus, the intramolecular reaction of cis - o -bromostilbenes under phase-transfer phosphine-free conditions has been found to lead to phenanthrenes in good yield …”

“…In contrast, the reaction of trans - o -bromostilbenes leads exclusively to a product of intermolecular double Heck reaction (Scheme ) 173 …”

Anomalous Stereoselectivity in the Wittig Reaction: The Role of Steric Interactions

“…Wittig reactions involving semistabilized ylides, such as 2 , typically show poor stereoselectivity, yielding mixtures of Z and E isomers. , Interestingly, for many ortho-substituted arylaldehydes ( 1 , X ≠ H), Wittig olefinations with arylmethylene ylides are highly Z -selective, and the effect is cooperatively enhanced by the presence of another ortho substitutent on the ylidic aryl group (X′). ,− This increase of stereoselectivity is preparatively useful when the X and X′ groups are halogens because the resulting ( Z )-stilbenes are valuable precursors to various heterocyclic systems, including fused azepines, , oxepines, borepines, − and silepines (Scheme ). Furthermore, direct couplings of mono- and di- o -halo-( Z )-stilbenes provide a regioselective alternative to photochemical benzannulations. − We have recently explored the utility of the latter strategy in our ongoing work on the synthesis of distorted π-aromatics , and have become interested in the mechanistic aspects of the Wittig reaction.…”

Cross‐Coupling of Organyl Halides with Alkenes – The Heck Reaction

“…Scheme 8.14 Dimerization of o-bromostilbenes to yield 9,10-bis(arylmethylene) dihydroanthracenes [310].…”