Introducing annulative chloroaryl dimerization

31 January 2018 - Orgo

Koga 2018 science.PNGIn the journal Science this week Yoshito Koga et al. report on a new organic reaction type they call annulative dimerization (DOI). Imagine 2-chloro-p-terphenyl (carbon framework one up from biphenyl with a chlorine substituent in the middle section) get fused together at three edges. Apparently this only takes 0.05 eq. palladium chloride, 0.1 eq. di(1-adamantyl)-n-butylphosphine and 3 eq. of caesium carbonate in cyclopentylmethylether at 140°C for 18 hours with a 82% yield. Typically where one phospine compound is successful with bulky adamantyl groups , nine others fail miserably.

Koga sure does not want to be pinned down on a reaction mechanism, the article describes a total of three reaction mechanisms with a total of 11 reaction intermediates. Two of the pathways are equally plausible. For comparison: the same reactant but different reagents (nickel bromide, triphenylphoshine and zinc) gives the ordinary biaryl coupling product.

More interesting stuff happens: the same fused reaction product can be used in a Scholl reaction with iron chloride to form the start of a nanoribbon. This strategy was extended to the formation of a ribbon containing 60 carbon atoms.