Reactive intermediates are key to halting and promoting chemical transformations; however, due to their elusive nature, they are not straightforwardly harnessed for reaction design. Herein, we describe studies aimed at stabilizing reactive intermediates in the N-heterocyclic carbene (NHC) catalytic cycle, which enabled the full shutdown of the known benzoin coupling pathway, while rerouting its intermediates toward deuteration. The reversible nature of NHC catalysis and the selective stabilization of reaction intermediates facilitated clean hydrogen–deuterium exchange reactions of aromatic aldehydes by D2O, even for challenging electron-withdrawing substrates. In several cases, the addition of catalytic amounts of phenyl boronic acid was used to further stabilize highly reactive intermediates and mitigate the formation of benzoin coupling byproducts. The mechanistic understanding at the foundation of this work resulted in unprecedented mild conditions with base and catalyst loadings as low as 0.1 mol %, and a scalable deuteration reaction applicable to a broad substrate scope with outstanding functional group tolerance. More importantly, adopting this approach enabled the construction of a guideline for identifying the appropriate catalyst and conditions for different substrates. Experimental studies combined with machine learning and computational methods shed light on the nontrivial mechanistic underpinnings of this reaction.

Using the tunneling-controlled reactivity of cyclopropylmethylcarbene, we demonstrate the viability of isotope-controlled selectivity (ICS), a novel control element of chemical reactivity where a molecular system with two conceivable products of tunneling exclusively produces one or the other, depending only on isotopic composition. Our multidimensional small-curvature tunneling (SCT) computations indicate that, under cryogenic conditions, 1-methoxycyclopropylmethylcarbene shows rapid H-migration to 1-methoxy-1-vinylcyclopropane, whereas deuterium-substituted 1-methoxycyclopropyl-d-methylcarbene undergoes ring expansion to 1-d-methylcyclobutene. This predicted change in reactivity constitutes the first example of a kinetic isotope effect that discriminates between the formation of two products.

Scheme1. Cyclo[18]carbon (C 18 )int he D 9h (polyynic) and D 18h (cumulenic) structures, and their geometricalv ariables (r 1 = r 2 = 1.28 for the D 18h , r 1 = 1.34 > r 2 = 1.22 for the D 9h ,a nd q 1 = q 2 for both).Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

Dative Bond (IUPAC): "The coordinationb ond formed upon interaction between molecular species, one of which servesa sadonora nd the other as an acceptor of the electron pair to be shared in the complex formed… The distinctive feature of dative bonds is that their minimum-energy rupture in the gas phase or in inert solvent follows the heterolytic bond cleavage path." This definition encompasses an immense number of molecules such as Lewis adducts, transition-metal complexes, supposedly hypervalento rh ypovalent systems, and many molecules with multifacetedL ewiss tructures. Still, there is al arge reticence to include dative bonds in the regular depiction of molecules, and even al arger unawareness of the dative bond arrow in many chemical circles. Herein we will discuss in simplec hemical terms the past, presenta nd future of such bonds. In addition, we will try to provide cleaner options to represent intricatem olecules without sacrificing physical accuracy.Scheme1.Standard and dative bond style of compact Lewis notation (i.e., only with lines) for (A) ozone, (B) sulfuric acid, (C) nitric acid, and (D) carbon monoxide.Scheme2.Hexamminecobalt(III) and ammonia borane, two prototypical systems with strongc harge transfer interaction depicted as adative bond.[a] A. Concepting methods. [17][18][19][20][21][22][23][24][25][26][27][28][29] This article deals with many historical cases, but it is hardly aR eview (andt herefore we might miss many important references). It deals with chemical bonds, but it is not aq uantum mechanical dissertation. It is just ab iased overview of at opic that has not been on the front line up to recent times.Scheme3.Correlation diagram between orbitalsofC ,C 2 and ethyne. 764 Concept Scheme13. (A) Dinitrogen tetroxide, nitric acid, and nitrous oxide. (B) Alternant aromatic cyclic S 2 N 2 .(C) At riazolium nitrenium. Scheme14. (A) The four resonant structureso ft rifluoramineoxide. (B) Methylenea mine and methylene phosphine.

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