Aryl halides tolans

The reaction of aryl halides 1 with alkynylcopper(I) 2 under reflux in pyridine to prepare internal arylacetylenes is known as the Stephens-Castro reaction [Eq. (1)] [2]. The reaction has proved to be particularly important in the synthesis of a wide range of tolan and heteroaromatic alkynes [3]. Vinyl and allenic halides can also be used and several reviews... 

The Stille coupling between aryl halides and bis(tributylstannyl)ethyne furnishes symmetrical tolanes. Dienyl triflones are available in three steps from l-alkynes, with the reaction sequence terminated by a Stille coupling. 

Arene-1-alkynes can be coupled to aryl halides (iodides and bromides) in the presence of palladium phosphine complexes to yield tolanes directly [6,93]. Reaction conditions usually are mild and many functional groups are tolerated. 

The most simple a-aryl substituted vinyl halide studied, a-bromostyrene, flash photolytically yields the enol of acetophenone in aqueous solution, which has been used to measure rates of ketonization302,303. With a,/2-diarylvinyl halides, loss of the /2 (vinylic) proton from the intermediate vinyl cation is competitive with nucleophilic trapping Irradiation of compounds such as 96 in methanol gives about equal amounts of tolan and... 

The research groups of Taniguchi, Zupan, and Lodder have extensively studied the photobehavior of aryl-substituted vinyl halides. Numerous examples of competing radical and ionic photobehavior have been observed detailed review is beyond the scope of this report. Two of the earliest studies involved the 2,2-diarylvinyl halides 207, which underwent predominant rearrangement-elimination to the tolans 208, along with competing formation of several radical products. 

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