When and How Do Diaminocarbenes Dimerize?

Abstract

No example of a simple uncatalyzed dimerization of a diaminocarbene has been clearly established, so it is timely to ask what factors control the thermodynamics of this reaction, and what mechanisms are responsible for the observed dimerizations? In agreement with qualitative experimental observations, the dimerizations of simple five- and six-membered-ring diaminocarbenes are calculated to be 100 kJ mol(-1) less favorable than those of acyclic counterparts. This large difference is semiquantitatively accounted for by bond and torsional angle changes around the carbene centers. Carbenes such as (Et(2)N)(2)C are kinetically stable in THF at 25 degrees C in agreement with calculated energy barriers, but they rapidly dimerize in the presence of the corresponding formamidinium ion. This proton-catalyzed process is probably the most common mechanism for dimer formation, and involves formation of C-protonated dimers, which can be observed in suitable cases. The possibility of alkali-metal-promoted dimerization is raised, and circumstantial evidence for this is presented.

Stable four-π-electron, four-membered heterocyclic cations and carbenes

“…Although the endocyclic B-N bonds are slightly shorter, the C-N bond lengths are quite similar to those of Da, because the phenyl ring at boron is coplanar with the four-membered ring and acts as a donor. However, this alteration enhanced the electrophilicity of the endocyclic carbon and boron atoms of Db as shown by the 13 C (194.1 ppm) and 11 B NMR signals (45.7 ppm), which are shifted downfield by 11.4 and 17.7 ppm, respectively, relative to those of Da. In fact, the heterocyclic salt Db appears to be very fragile; it quickly decomposes even in weakly coordinating solvents such as Et 2 O, THF, CH 2 Cl 2 , and CHCl 3 .…”

“…We have shown that six--electron NHCs B (6), isoelectronic with borazenes (7), are highly thermally stable. Even more strikingly, the two--electron cyclopropenylidene C is stable enough to be isolated at room temperature (8), despite the absence of -donor heteroatoms directly bonded to the carbene center, a necessary criterion for previously isolated carbenes (9)(10)(11)(12)(13)(14).…”

The Measure of All Rings—N‐Heterocyclic Carbenes

“…Besides the deprotonation of azolium salts, some additional, less commonly applied methods for the formation of free NHCs exist (Scheme 2), and they might have attractive features, such as a simpler purification or a slow in situ formation of the NHC: [17] -Desulfurization of thioureas with molten potassium in boiling THF (Scheme 2 b). An attractive feature of this method is the insolubility of the by-product (potassium sulfide) in THF.…”

Proton Transfer Reactions of Triazol-3-ylidenes: Kinetic Acidities and Carbon Acid pKa Values for Twenty Triazolium Salts in Aqueous Solution

“…[1][2][3][4][5][6][7][8][9][10][11][12] Structural classes embodied within the NHC family include the thiazol-2-ylidenes 1, imidazol-2-ylidenes 2 and imidazolin-2-ylidenes 3, trihydropyrimidin-2-ylidenes 4 and triazol-3-ylidenes 5. From the early seminal work of Breslow, 13 thiazol-2-ylidenes such as 1 have long been implicated as the catalytically active species in the benzoin condensation, however, until recently, applications beyond this C-C bond-forming reaction were relatively limited.…”