C3-Alkylation of furfural derivatives by continuous flow homogeneous catalysis

The C3-functionalization of furfural using homogeneous ruthenium catalysts requires the preinstallation of an ortho-directing imine group, as well as high temperatures, which did not allow scaling up, at least under batch conditions. In order to design a safer process, we set out to develop a continuous flow process specifically for the C3-alkylation of furfural (Murai reaction). The transposition of a batch process to a continuous flow process is often costly in terms of time and reagents. Therefore, we chose to proceed in two steps: the reaction conditions were first optimized using a laboratory-built pulsed-flow system to save reagents. The optimized conditions in this pulsed-flow mode were then successfully transferred to a continuous flow reactor. In addition, the versatility of this continuous flow device allowed both steps of the reaction to be carried out, namely the formation of the imine directing group and the C3-functionalization with some vinylsilanes and norbonene.

Modular synthesis of 2-furyl carbinols from 3-benzyldimethylsilylfurfural platforms relying on oxygen-assisted C–Si bond functionalization

3-Silylated furfurals, readily prepared in three steps from biomass-derived furfural and 5-methylfurfural, are converted into 3-silylated 2-furyl carbinols upon condensation with organomagnesium or organolithium reagents. The hydroxy unit of the carbinol adducts can be exploited to promote C3(sp²)–Si bond functionalization through intramolecular activation. Two approaches were contemplated for this purpose. Activation by alkoxides of the C3–SiEt₃ or C3–SiMe₂t-Bu bonds was ineffective. Conversely, treatment of the C3-benzyldimethylsilyl-appended derivatives with tetrabutylammonium fluoride led to cyclic siloxanes, which revealed to be competent donors for copper-catalyzed cross-coupling reactions, such as arylation reactions catalyzed by Pd₂(dba)₃/CuI, as well as allylation and methylation reactions catalyzed by CuI⋅PPh₃. C3-Benzyldimethylsilyl-appended furfurals are thus introduced as versatile platforms, providing a modular access to 3-substituted 2-furyl carbinols from renewable feedstock.

Redox-Neutral Ru(0)-Catalyzed Alkenylation of 2-Carboxaldimine-heterocyclopentadienes

A new Ru₃(CO)₁₂-catalyzed directed alkenylation of 2-carboxaldimine-heterocyclopentadienes has been accomplished. This process allows coupling of furan, pyrrole, indole, and thiophene 2-carboxaldimines with electron-poor alkenes such as acrylates, vinylsulfones, and styrenes. This regio- and chemoselective oxidative C-H coupling does not require the presence of an additional sacrificial oxidant. Density functional theory calculations allowed us to propose a mechanism and unveiled the nature of the H₂ acceptor.

Rhodium-Catalyzed Reductive trans-Alkylacylation of Internal Alkynes via a Formal Carborhodation/C-H Carbonylation Cascade

A rhodium-catalyzed reductive annulation cascade reaction that consists of a formal anti-carborhodation of a C≡C bond and an aromatic C-H carbonylation cascade for producing cyclopenta[de]quinoline-2,5(1H,3H)-diones is described. This method uses the Mn reductant to reductively regenerate the active rhodium species, hence obviating the need for prefunctionalization, and represents a new route to the carbonylation of aromatic C-H bonds with alkynes leading to aryl vinyl ketone frameworks.

Catalytic C-H Functionalization of Unreactive Furan Cores in Bio-Derived Platform Chemicals

C-H functionalization is one of the most convenient and powerful tools in the arsenal of modern chemistry, deservedly nominated as the "Holy Grail" of organic synthesis. A frequent disadvantage of this method is the need for harsh reaction conditions to carry out transformations of inert C-H bonds, which limits the possibility of its use for modifying less stable substrates. Biomass-derived furan platform chemicals, which have a relatively unstable aromatic furan core and highly reactive side chain substituents, are extremely promising and valuable organic molecules that are currently widely used in a variety of research and industrial fields. The high sensitivity of furan derivatives to acids, strong oxidants, and high temperatures significantly limits the use of classical methods of C-H functionalization for their modification. New methods of catalytic functionalization of non-reactive furan cores are urgently required to obtain a new generation of materials with controlled properties and potentially bioactive substances.

The Increasing Value of Biomass: Moving From C6 Carbohydrates to Multifunctionalized Building Blocks via 5-(hydroxymethyl)furfural

Recent decades have been marked by enormous progress in the field of synthesis and chemistry of 5-(hydroxymethyl)furfural (HMF), an important platform chemical widely recognized as the "sleeping giant" of sustainable chemistry. This multifunctional furanic compound is viewed as a strong link for the transition from the current fossil-based industry to a sustainable one. However, the low chemical stability of HMF significantly undermines its synthetic potential. A possible solution to this problem is synthetic diversification of HMF by modifying it into more stable multifunctional building blocks for further synthetic purposes.