Suzuki without metal

22 June 2014 - Orgo

aggerwal 2014 boron cross coupling  The Aggerwal group has recently described an alternative to the well-known Suzuki carbon - carbon coupling reaction that still utilises a boronic acid but does away with the platinum catalyst (Bonet et al. DOI). The novel method is applicable to aromatics such as furan, thiophene and electron-rich arenes. The metal is replaced by NBS acting as an electrophile. This novel research was triggered by a reinvestigation of some very old chemistry dating from the 1970's (DOI, DOI, DOI) with the key difference being that the Aggerwal boron shuttles are now chiral.

Chiral boron compound A (described in a 2007 publication) was synthesised from a carbamate of phenylpropanol that via its alkyllithium was stereoselectively methylated with pinacol methylboronate using sparteine. Compound A was then reacted with lithiumfuran to first give a boronate complex which was then brominated with NBS triggering a 1,2-shift (akin a Matteson homologization) followed by an elimination reaction with rearomatization to the chiral alkylated furan as final product.

Interestingly in the Aggerwal publication the final elimination is triggered by a nucleophile at least according to the published reaction scheme. Details on what the nucleophile might be however are lacking. In their Nature Chemistry editorial on the same topic Ho-Yan Sun and Dennis G. Hall must have struggled with the same problem and in their infographic the nucleophile was simply scrapped.

Update 23-06: the image did contain some errors. Corrected.