A phosphine-catalyzed
oligomerization of arynes using selenocyanates
was developed. The use of JohnPhos as a bulky phosphine is the key
to accessing α,ω-bisfunctionalized oligo(ortho-arylenes) with RSe as the substituent at one terminus and CN as
the substituent at the other. The in situ formation
of R3PSeR′ cations, serving as sterically encumbered
electrophiles, hinders the immediate reaction that affords the 1,2-bisfunctionalization
product and instead opens a competitive pathway leading to oligomerization.
Various optimized conditions for the predominant formation of dimers,
but also for higher oligomers such as trimers and tetramers, were
developed. Depending on the electronic properties of the electrophilic
reaction partner, even compounds up to octamers were isolated. Optimization
experiments revealed that a properly tuned phosphine as catalyst is
of crucial importance. Mechanistic studies demonstrated that the cascade
starts with the attack of cyanide; aryne insertion into n-mers leading to (n+1)-mers was ruled out.
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