Catalytic Isomerization of Unprotected Mesoionic N‐Heterocyclic Olefins and Their Lewis Adducts

Abstract

Recently, novel C 5 -position-unprotected mesoionic N-heterocyclic olefins (mNHOs) demonstrated interesting reactivity with various Lewis acids during experiments. In this work, extensive DFT calculations show that the isomerization of such mNHOs to more Lewis-basic mesoionic carbenes (MICs) can be efficiently catalyzed by weakly protic acidic species including C 5 -unprotected mNHOs, their Lewis adducts and polar H 2 O molecules, thus enabling catalytic isomerization and H-isotope exchange of such mNHOs and their Lewis adducts.

Origin of the Ligand Ring‐Size Effect on the Catalytic Activity of Cationic Calcium Hydride Dimers in the Hydrogenation of Unactivated 1‐Alkenes

“…When very small dihydrogen is used as substrate for catalytic H 2 isotope exchange, the dimeric mechanism becomes more important due to evidently reduced steric hindrance and stronger dinuclear cooperative effects. As seen in Figure 3(A), according to our recent DFT calculations, a very facile H•••H•••Htype H 2 isotope exchange may occur via a cooperative CaÀ HÀ Ca bridge of THF-free cation dimer 1 2 + over a low barrier of 16.9 kcal mol À 1 (via TS5 2 + ) after THF elimination, [14] consistent with the very fast reaction observed at room temperature. [10] The three exchanging hydrogen atoms are placed evenly between two calcium ions and perpendicular to the other CaÀ HÀ Ca bridge, suggesting potentially strong dinuclear cooperative effects.…”

“…[14] In benzene solution, the dimer [(THF)(BDI)CaH] 2 was computed to be about 7.7 kcal mol À 1 higher in free energy than two (THF)(BDI)CaH monomers, [15] in sharp contrast to previous experimental [1c] and our recent dispersion-corrected DFT results (À 18.2 kcal mol À 1 lower dimer than two monomers). [14] It is thus crucial to include both dispersion corrections and suitable solvation in modeling such catalytic reactions in solution.…”

“…[10] The same monomeric mechanism was also found in our recent DFT calculations using the CH 2 =CHCye (Cye = cyclohexenyl) substrate. [14] When smaller ethylene (CH 2 =CH 2 ) is used as substrate instead, the overall catalytic hydrogenation becomes À 27.6 kcal mol À 1 over a 5.4 kcal mol À 1 lower barrier of 19.2 kcal mol À 1 , suggesting a much faster reaction even at room temperature. Due to reduced steric hindrance, direct CH 2 =CH 2 addition to a CaÀ HÀ Ca bridge of cation dimer 1 2 + becomes kinetically only 0.5 kcal mol À 1 less favorable (via eTS1 2 + , see Supporting Information).…”

“…[7,[10][11] Very recently, our extensive dispersion-corrected DFT calculations have shown that both monomeric and dimeric mechanisms can be involved depending on the nature of substrates (unactivated vinyl-cyclohexene, conjugation-activated styrene, and H 2 ) even with the same 1 2 + • THF catalyst. [14] In this DFT-mechanistic work, using 1 2 + • THF and 2 2 + as catalysts and CH 2 =CHBu, CH 2 =CH 2 and H 2 as substrates of different size, the origin of the experimentally observed ligand ring-size effect is explored. This effect is actually due to a subtle compromise of solvent, steric and dinuclear cooperative effects via competitive CaÀ HÀ Ca bridges and terminal CaÀ H bonds (Scheme 1B).…”

Reactions of Frustrated Lewis Pairs with Chloro‐Diazirines: Cleavage of N=N Double Bonds

“…[13] In addition, Melen and co-workers have reviewed borane mediated carbene transfer reactions using diazomethanes, [14] while more recently carbocations derived from protonation of diazomethane α-carbon atoms were shown to be key intermediate in such reactions. [15] In 2018, the seminal report by Braunschweig et al [16] demonstrated N 2 reduction by a B I species. To us, this further supported the notion that similar reactivity might also be achieved with a B III species and a two-electron donor (i.e., a FLP).…”