Tunneling Assists the 1,2‐Hydrogen Shift in N‐Heterocyclic Carbenes

Abstract

At room temperature, 1,2-hydrogen-transfer reactions of N-heterocyclic carbenes, like the imidazol-2-ylidene to give imidazole is shown to occurr almost entirely (>90 %) by quantum mechanical tunneling (QMT). At 60 K in an Ar matrix, for the 2, 3-dihydrothiazol-2-ylidene→thiazole transformation, QMT is shown to increase the rate about 10(5) times. Calculations including small-curvature tunneling show that the barrier for intermolecular 1,2-hydrogen-transfer reaction is small, and QMT leads to a reduced rate of the forward reaction because of nonclassical reflections even at room temperature. A small barrier also leads to smaller kinetic isotope effects because of efficient QMT by both H and D. QMT does not always lead to faster reactions or larger KIE values, particularly when the barrier is small.

Complexes Bearing Protic N-Heterocyclic Carbene Ligands

“…The name first appeared in the literature in 2008 to emphasize the potential Brønsted-acidic character of the wingtip. The NH group in close proximity to the nucleophilic carbene carbon atom actually renders the free pNHC less stable than the corresponding azole isomer, − thereby differing from the nonprotic (N-alkylated) classical NHCs (Scheme ). Coordination to a metal center, however, may shift the relative stability of the tautomers.…”

Synthesis of PdII Complexes Bearing pNHC Ligands by Oxidative Addition of 8-Halogenotheophyllines and 8-Bromoadenine to Pd0

“…Cyclic diaminocarbenes (NHCs) that feature at least one NH function within the carbene heterocycle are named protic NHCs or p NHCs . Such NHCs are normally not stable in the free form due to rapid tautomerization to the corresponding azole. − They can, however, be stabilized as ligands in transition-metal complexes, and the synthesis of such p NHC complexes has recently attracted considerable interest , due to multiple applications . One possible pathway for the preparation of complexes with protic p NHC ligands is the oxidative addition of unsubstituted azoles , or N -alkylhalogenoazoles to low-valent transition metals such as Ni 0 , Pd 0 , and Pt 0 .…”

Beyond static structures: Putting forth REMD as a tool to solve problems in computational organic chemistry

“…Often, a picture of this type successfully captures the key aspects of a system allowing, for example, identification of the primary mechanistic pathway [3][4][5][6][7][8][9] or rationalization of the presence of a specific intermediate. [9][10][11][12][13] Such descriptions, however, are occasionally insufficient to chemistry occurring in an experimental setting, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] in the most extreme cases leading to disastrous failures. [34] Inside the computer, a host of factors that govern "real world" chemical reactions must necessarily be approximated or ignored altogether in a static picture.…”