Direct C-H bond arylation

06 May 2008 - Organic chemistry

CHbondArylationKawai2008  C-H bond activation is a hot topic in current chemistry potentially converting cheap and abundant alkanes into valuable fine chemicals. Breakthroughs in this field were made in 1982 by Bergman and Graham in 1982 with humble cyclohexane.

The functionalization of certain aromatic compounds is also considered C-H activation but the question is if this is rightly so. These reactions used to be called aromatic substitution but the label C-H bond activation makes them all the more trendy. Case at hand the arylation of naphthalene by palladium as investigated by a group of Japanese researchers (DOI) .

The reagents are phenyltintrichloride, a catalytic amount palladium(II) chloride and oxidant copper chloride and the optimised product distribution shows this reaction type has a long way to go.

The researchers propose that copper chloride (the sacrificial catalyst) oxidizes Pd(II) to Pd(IV) which after accepting a phenyl ligand in transmetallation acts as a strong electrophile in plain old fashioned electrophilic substitution with the arene.
They have plenty of evidence in support for this reaction mechanism: no reaction takes place without the copper chloride and a radical scavenger does not exactly spoil it.

But if there is nothing new here then where is it?

What makes this mechanism so interesting is that involvement of organopalladium (IV) complexes in organic reactions is of of those other hot topics with those arguing they exist and those who see a multitude of alternative mechanisms. To be continued!