CO₂, COS and CS₂ adducts of N-heterocyclic olefins and their application as organocatalysts for carbon dioxide fixation

Abstract

Various CO2, COS and CS2 adducts of N-Heterocyclic olefin (NHO) were synthesized and characterised by single crystal Xray crystallography. The length of C-O bond in NHO-COS adducts is slightly shorter than that in the corresponding NHO-CO2 adducts, while the C-S bond length in NHO-COS adducts is significantly longer than the corresponding C-S distance in the NHO-CS2 adducts, suggesting the negative charge of the NHO-COS adducts is preferentially delocalized on the sulfur atom. The length of the CCS2−CNHO bond is significantly shorter than that of the CCOS−CNHO or CCO2−CNHO bond in the corresponding NHO-COS or NHO-CO2 adducts, implying the relatively high thermal stability of NHO-CS2 adducts. The NHO-CO2, NHO-COS and NHO-CS2 adducts were found to be efficient in catalyzing the cycloaddition reaction of CO2 and epoxides to selectively afford the corresponding cyclic carbonates. Among them, NHO-CO2 adducts were found to be more active, while NHO-CS2 adducts exhibited the lowest activity for this reaction, probably due to their high stability for difficult release of the highly active NHO. NHOs with BBr 3 giving the neutral compound of NHO-BBr 3 , unexpectedly making the ring of THF open, has been shown by Robinson and colleagues 45 . Silyl-functionalized NHOs, obtained by the reaction of NHOs with HSiCl 3 , have been unveiled by the

N‐Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns

“…These reactions serve as the first examples of an N‐heterocyclic olefin (NHO) acting as an organic mediator for C−C bond‐coupling C ‐carboxylations of CH‐acidic positions by NHO‐activated CO 2 . A related C−O bond formation has been described by Wang and co‐workers who employed NHO–CO 2 adducts as catalysts for the O ‐carboxylation of propargylic alcohols and propylene oxides to obtain cyclic α‐alkylidene and propylene carbonates, respectively . The carboxylation of acetonitrile was also observed with a 1,3‐di‐ tert‐ butylimidazolium‐2‐carboxylate .…”

The Resurgence of the Highly Ylidic N‐Heterocyclic Olefins as a New Class of Organocatalysts

“…In summary, elegant studies from Lu and co‐workers and from Naumann, Thomas, and Dove have demonstrated the great synthetic potential of N‐heterocyclic olefins in promoting CO 2 fixation and polymerization processes. With their highly ylidic characters and inherently tunable structures, as observed with the parent NHC species, it is expected that NHOs will soon emerge as versatile organocatalysts for a much broader range of important Lewis/Brønsted base‐catalyzed reactions.…”

“…Furthermore, Lu and co‐workers have recently reported the synthesis of a series of NHO–COS and NHO–CS 2 adducts for comparison to their previously synthesized NHO–CO 2 catalysts. These new catalytic systems were used to promote coupling reactions of CO 2 with epoxides (Scheme c).…”

“…Although thiocarbonate esters are also compounds of significant synthetic interest, Lu and co‐workers did not report the similar catalyzed addition of COS or CS 2 themselves to propargylic alcohol or epoxide substrates. Most inspiringly, the authors proved that the free NHOs are probably the actual active species in the catalytic cycle, as the use of a NHO instead of NHO–CO 2 adduct as organocatalyst in the same reaction afforded comparable product yield …”

“…Due to the strong nucleophilicity of the α‐carbon, NHOs are often stronger Lewis bases than their parent NHCs, leading to several applications of NHOs as end‐on ligands in transition metal coordination and catalysis,– as well as stabilization of the metalloid boron and silicon in borenium ions, and silylenes . Very recently, NHOs have elicited great interest among synthetic chemists for their intriguing new roles as organocatalysts in important chemical transformations, such as CO 2 sequestration and polymerization reactions (Figure ) . With suitable structural design, NHOs can serve as a new class of Brønsted/Lewis base organocatalysts for a wide range of chemical reactions.…”