Organocatalytic Cross-Coupling of Biofuranics to Multifunctional Difuranic C11 Building Blocks

To synthesize the still missing C11 furoins as potential trifunctional difuranic building blocks for biopolymers and biofuels, the route based on the cross-coupling of two biofuraldehydes, furfural (FF) and 5-hydroxymethylfurfural (HMF), has been investigated using organic N-heterocyclic carbene (NHC) catalysts. This NHC-catalyzed coupling reaction in solution gives a statistical mixture of products including homocoupled C10 and C12 furoins 1 and 4, as well as cross-coupled C11 furoins 2 and 3, regardless of the electronics or sterics of the NHC precatalyst used, but a slight preference for cross-coupled products (∼60% 2 + 3 combined) under neat conditions has been achieved. Cross-coupled products 2 and 3 have been isolated in 48.1% yield and further separated into their pure state in 22.2% and 19.9% yield for 2 and 3, respectively. The isolated 2 and 3 have been fully characterized by NMR, HRMS, and single crystal X-ray diffraction. A simple metal-free/in-air oxidation reaction converts these two isomeric furoins into the single α-diketone furanil structure 5, another new C11 bioderived building block.