Abstract
The [3+2] cycloaddition (32CA) reactions of strongly nucleophilic norbornadiene (NBD), with simplest diazoalkane (DAA) and three DAAs of increased electrophilicity, have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G (d,p) computational level. These pmr-type 32CA reactions follow an asynchronous one-step mechanism with activation enthalpies ranging from 17.7 to 27.9 kcal·mol−1 in acetonitrile. The high exergonic character of these reactions makes them irreversible. The presence of electron-withdrawing (EW) substituents in the DAA increases the activation enthalpies, in complete agreement with the experimental slowing-down of the reactions, but contrary to the Conceptual DFT prediction. Despite the nucleophilic and electrophilic character of the reagents, the global electron density transfer at the TSs indicates rather non-polar 32CA reactions. The present MEDT study establishes the depopulation of the N–N–C core in this series of DAAs with the increase of the EW character of the substituents present at the carbon center is responsible for the experimentally found deceleration.
Highlights
Hept-2-ene) and related compounds serve as key intermediates in natural product synthesis [1,2,3] and polymer chemistry [1,4] owing to their angularly strained unusual geometry and high reactivity
The behavior of NBD derivatives in the Wagner–Meerwein rearrangement [6,7], photochemical di-π methane rearrangement [8], Diels Alder reactions [9], catalytic reactions with alkynes [10] and several other instances [1] have established their uniqueness as organic reagents and fostered their wide synthetic applications in [3+2] cycloaddition (32CA) reactions to form isoxazolidines, triazoles, carbocycles and other heterocycles of biological and environmental relevance [11,12,13,14,15]
Solvent effects were studied by modeling the solvent acetonitrile using the polarizable continuum model [54,55] (PCM) with the self-consistent reaction field [56,57,58] method, and the stationary points were optimized at MPWB1K (PCM)/6
Summary
Hept-2-ene) and related compounds serve as key intermediates in natural product synthesis [1,2,3] and polymer chemistry [1,4] owing to their angularly strained unusual geometry and high reactivity. NBD has found applications in molecular photoswitches to absorb solar radiation [5], and by 2018, more than 30 thousand publications and patents had reported their production and applications in various fields [1]. In 2001, Tam and coworkers [16,17] reported the highly regio- and stereoselective intramolecular 32CA reactions of NBD-tethered nitrones and nitrile oxides (Scheme 1). Chemistry 2021, 3, FOR PEER REVIEW CChheemmiissttrryy22002211,,33, FOR PEER REVIEW. Intramolecular 32CA reaction of norbornadiene (NBD)‐tethered nitrile oxide 1. Intramolecular 32CA reaction of norbornadiene (NBD)-‐tethereedd nniittrriillee ooxxiiddee 11
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