Effect of Cp*Iridium(III) Complex and acid co-catalyst on conversion of furfural compounds to cyclopentanones or straight chain ketones

In this paper, Cp*Ir (III) Complex and acid co-catalyst system was developed. By using Cp*Ir and gamma-Al2O3 (Lewis acid), 5-hydroxymethylfurfural (5-HMF) can be converted efficiently to 3-hydroxymethyl cyclopentanone (HCPN). Meanwhile, Cp*Ir and Bronsted acid can promote conversion of 5-HMF to 1-Hydroxy-2,5-hexanedione (HHD). The effect of Lewis acid and Bronsted acid on the hydrogenation of furan derivatives was studied. Mechanism of conversion of 5-HMF to HCPN was discussed in detail and mechanism proposed by our pre-decessors was revised. Instead of being an intermediate for the formation of HCPN, it is believed that, HHD is a product of another reaction pathway. HHD condensed via Aldol reaction to produce 3-methylcyclopenten-2-ol-1-one (MCP) instead of HCPN. Under the promotion of Lewis acid, 5-HMF firstly convert to the precursor of HHD. After that, the reaction is through 4 pi-electrocyclic ring closure process and HCPN was formed ultimately. Furthermore, we found that our Cp*Ir and acid co-catalyst system is suitable for a variety of furfural compounds. By using Cp*Ir, Bronsted acid can promote conversion of furfural compounds to straight chain ketones and Lewis acid can promote the rearrangement of furfural compounds to cyclopentanone derivatives.