Dimerisation of nitrile oxides: a quantum-chemical study

Area of Science:

• Computational Chemistry
• Organic Reaction Mechanisms
• Theoretical Chemistry

Background:

• Nitrile oxides are versatile synthetic intermediates known to undergo cyclodimerisation reactions.
• Understanding the mechanisms and energetics of these reactions is crucial for controlling synthetic outcomes and predicting stability.
• Experimental observations suggest furoxan formation as a common pathway, but detailed theoretical investigations are needed.

Purpose of the Study:

• To investigate the [3+2] and [3+3] cyclodimerisation pathways of small nitrile oxides (XCNO) using high-level ab initio theory.
• To determine the preferred reaction mechanism, identify key intermediates, and elucidate the factors controlling reaction rates.
• To explore alternative reaction pathways, including those leading to different heterocyclic products and potential polymerization initiation.

Main Methods:

• Ab initio coupled cluster theory, including CCSD, CCSD(T), and MR-AQCC levels of theory.
• Utilisation of density functional theory (UB3LYP/cc-pVTZ) for geometry optimizations.
• Analysis of kinetic energy barriers and thermodynamic parameters (DeltaG(298)) for various reaction steps.

Main Results:

• The favoured dimerisation pathway involves a multi-step reaction to furoxans via dinitrosoalkene intermediates with diradical character.
• The rate-determining step is typically the initial C-C bond formation, with kinetic barriers varying based on the substituent (X).
• Barriers increase with decreasing electronegativity and increasing pi-donor ability of X: F < Cl < Br < CH3 < CN.
• Alternative one-step concerted pathways to other heterocycles exhibit significantly higher energy barriers.
• New low-energy pathways for dinitrosoethylene intermediates were identified, with implications for their detection and reactivity.
• Polymerization initiation involving the nitrile substituent of NCCNO was also explored.

Conclusions:

• The theoretical findings support experimental observations of furoxan formation as the dominant pathway for nitrile oxide dimerisation.
• The substituent effects on kinetic barriers provide a basis for predicting and controlling the reactivity of different nitrile oxides.
• The identified alternative pathways offer new insights into the complex reaction landscape of nitrile oxides and their potential applications.