Stable Singlet Carbenes as Organic Superbases

Abstract

A simple experimental procedure for scaling carbene Brønsted basicity is described. The results highlight the strong basicity of pyrazol-4-ylidenes, a type of mesoionic carbene, also named cyclic-bentallenes (CBA). They are more basic (pK aH > 42.7 in acetonitrile) than the popular proazaphosphatrane Verkade bases, and even the Schwesinger phosphazene superbase P 4 ( t Bu). The basicity of these compounds can readily be tuned, and they are accessible in multigram quantities. These results open new avenues for carbon centered superbases.

Genuine carbene versus carbene‐like reactivity

“…This reactivity aligns with the predicted high ambiphilicity of this family of carbenes [7] . In an attempt to isolate 1 , we envisaged the deprotonation of 1 Cl with a more basic carbene as a sacrificial base [15] . In this case, deprotonation of 1 Cl with CAAC A in d 8 ‐toluene from −78 °C to room temperature resulted in the clean formation of benzocyclobutene 3 , along with the protonated CAAC A Cl .…”

A Combination of Computational and Experimental Studies to Correlate Electronic Structure and Reactivity of Donor–Acceptor Singlet Carbenes

“…Carbene chemistry has laid to the foundation of numerous attractive areas of research with respect to organic transformation. 1 2 3 4 5 6 7 8 9 10 11 12 13 Metal carbenoids generated from donor–acceptor (D–A) diazo compounds (like aryl diazoacetates) have been investigated extensively. 14–23 Aryl ring adjacent to carbene contributes as a donor moiety and the ester part imposes an electron-withdrawing effect to the carbene carbon thus constructing the acceptor moiety.…”

Compact Rotaxane Superbases

“…While superbase definitions vary, a common description is that an organic superbase is a neutral organic compound with a basicity greater than that of proton-sponge (p K a H + = 18.6 in MeCN) . The most frequently used families of organic superbases include amidines (p K a H + = 21–25), guanidines (p K a H + = 23–26), and phosphazenes (p K a H + = 27–43). , While the conjugate acids of these organic superbases are often stabilized by resonance, other driving forces can be important in their design, such as aromatization in Lambert’s cyclopropenimine superbases or electron-density donation to vacant orbitals in the Verkade superbases and the singlet carbene superbases reported by Bertrand . In recent years, a number of new organic superbases have been introduced that have proved efficacious in various aspects of synthetic methodology , and other fields of chemistry. − …”

“…3,6 While the conjugate acids of these organic superbases are often stabilized by resonance, 7 other driving forces can be important in their design, such as aromatization in Lambert's cyclopropenimine superbases 8 or electron-density donation to vacant orbitals in the Verkade superbases 9 and the singlet carbene superbases reported by Bertrand. 10 In recent years, a number of new organic superbases have been introduced that have proved efficacious in various aspects of synthetic methodology 1,11 and other fields of chemistry. 12−14 Hydrogen bonding between ammonium groups and crown ethers is a common way to template (pseudo)rotaxane assembly (Figure 1A).…”