Synthesis of chromane derivatives by palladium-catalyzed intramolecular allylation of aldehydes with allylic acetates or chlorides using indium and indium(III) chloride

Since chromanes are found in many natural products, efficient construction of this ring structure has attracted much attention. We have investigated indium-mediated intramolecular allylation and allenylation to carbonyl groups as well as imines and proved that these methods are efficient for constructing chromane ring structures. Recently, Araki and co-workers reported the transmetallation of π-allylpalladium(II) by an indium(I) salt and showed that allylation to carbonyl groups could be achieved by the resulting organoindium species. This process appears to involve a πallylpalladium(II) species which was, then, transmetallated with an indium salt. As a result, an allylindium reagent was generated, which is believed to be the active organometallic species to undergo allylation. Also, a very similar allylation using In, InCl3, and catalytic Pd(0) to carbonyl groups was also reported by Kim and co-workers. The actual active species involved for transmetallation in this process was also believed to be InCl, which was formed by the reaction of indium and indium(III) chloride. Interestingly, other metal chloride salts with higher reduction potential than In(I) can be utilized in the allylation reactions. Thus, SnCl4, FeCl3, or CuCl paired with indium metal in the presence of Pd(0) was successfully employed in the allylation with various allylic compounds. More recently, Araki and co-workers reported preparation of allylic indium from allylic alcohols via πallylnickel with InI. Allylindiums generated by abovementioned reductive transmetallation of π-allylpalladium(II) complexes were also used in allyl cross-coupling reactions. Because these methods seem to be efficient in allylation in intermolecular fashion, it is natural to consider possibility of applying these to intramolecular allylations. In connection with our continuing efforts to develop more efficient methods to synthesize chromane and chromane-related ring structures using organoindium species, we decided to investigate the allylation involving In, InCl3, and Pd(0) species to prepare chromane derivatives. Here we wish to disclose the results of our investigation on acquiring chromane and the related structures via an organoindium species generated by the transmetallation of π-allylpalladium(II) according to the allylation method reported by Kim and co-workers. First, we have focused our investigation on the construction of chromanes which possess 6-membered rings by the intramolecular allylation under the conditions involving In and InCl3 in the presence of Pd(PPh3)4 (Eq. 1).