NMR and Computational Studies of the Configurational Properties of Spirodioxyselenuranes. Are Dynamic Exchange Processes or Temperature-Dependent Chemical Shifts Involved?

• Published in: Journal of organic chemistry 2012-10, Vol.77 (20), p.9268-9276
• Main Authors: Press, David J, McNeil, Nicole M. R, Rauk, Arvi, Back, Thomas G
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts: preparations and properties; Chemistry; Exact sciences and technology; General and physical chemistry; Magnetic Resonance Spectroscopy; Molecular Structure; Noncondensed benzenic compounds; Organic chemistry; Organoselenium Compounds - chemistry; Preparations and properties; Quantum Theory; Spiro Compounds - chemistry; Temperature; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo301846a

Spirodioxyselenurane 4a and several substituted analogs revealed unexpected 1H NMR behavior. The diastereotopic methylene hydrogens of 4a appeared as an AB quartet at low temperature that coalesced to a singlet upon warming to 267 K, suggesting a dynamic exchange process with a relatively low activation energy. However, DFT computational investigations indicated high activation energies for exchange via inversion through the selenium center and for various pseudorotation processes. Moreover, the NMR behavior was unaffected by the presence of water or acid catalysts, thereby ruling out reversible Se–O or benzylic C–O cleavage as possible stereomutation pathways. Remarkably, when 4a was heated beyond 342 K, the singlet was transformed into a new AB quartet. Further computations indicated that a temperature dependence of the chemical shifts of the diastereotopic protons results in convergence upon heating, followed by crossover and divergence at still higher temperatures. The NMR behavior is therefore not due to dynamic exchange processes, but rather to temperature dependence of the chemical shifts of the diastereotopic hydrogens, which are coincidentally equivalent at intermediate temperatures. These results suggest the general need for caution in ascribing the coalescence of variable-temperature NMR signals of diastereotopic protons to dynamic exchange processes that could instead be due to temperature-dependent chemical shifts and highlight the importance of corroborating postulated exchange processes through additional computations or experiments wherever possible.