Chemical exchange in novel spirobicyclic zwitterionic Janovsky complexes using dynamic ¹H NMR spectroscopy

Abstract

Highly coloured Janovsky complexes have been known for over 120 years, being used in many colourimetric analytical procedures. In this present study, two novel and stable nitrocyclohexadienyl spirobicyclic, zwitterionic Janovsky anionic hydantoin σ -complexes, rac-1,3-diisopropyl-6-nitro-2,4-dioxo-1,3-diazaspiro[4.5]deca-6,9-dien-8-ylideneazinate, ammonium internal salt (1) and 1,3-diisopropyl-2,4-dioxo-1,3-diazaspiro[4.5]deca-6,9-dien-8-ylideneazinate, ammonium internal salt (2) have been prepared and characterised by NMR, electrospray ionization mass spectrometry (ESI-MS) and UV/visible methods. For the p-mononitro-substituted complex (2), we discovered chemical exchange behaviour using 1D saturation transfer and 2D exchange spectroscopy (EXSY) 1 H NMR techniques. The coalescence temperature was determined to be 62• Cind 3 -acetonitrile. Analysis of these data provided a Gibbs free energy of activation, G ‡, of +67 kJ mole −1 , a rate constant, k, coalescence of 220 Hz and an equilibrium constant, K eqm , of 0.98 as estimates of the exchange process in this solvent. Of the two mechanisms proposed for this fluxional behaviour, ring opening to a substituted benzene or proton exchange, a further theoretical modelling study of 1D 1 H NMR spectra was able to confirm that simple proton exchange between the two nitrogen sites of the hydantoin ring provided an accurate simulation of the observed experimental evidence. Interestingly, the o,p-dinitro-substituted complex (1) did not show any chemical exchange behaviour up to 150• Cind 3 -acetonitrile (to 75• C) and d 6 -dimethyl sulfoxide (DMSO). Molecular modelling at the MM2 level suggests that steric collisions of an N-acyl isopropyl substituent of the hydantoin ring with the ortho-nitro group of the spirofused cyclohexadienyl ring prevents the proposed proton exchange mechanism occurring in this case.

Applications of spin-spin couplings

“…3 J HH couplings have been determined by Culf et al 227 to characterize two novel and stable nitrocyclohexadienyl spirobicyclic zwitterionic Janovsky anionic hydantoin s-complexes, rac-1,3-diisopropyl-6-nitro-2,4-dioxo-1,3diazaspiro [ 4,5]deca-6,9-dien-8-ylideneazinate, ammonium internal salt and 1,3-diisopropyl-2,4-dioxo-1,3-diazaspiro [4,5]deca-6,9-dien-8-ylideneazinate, ammonium internal salt. 3 J HH couplings in the polymethine chain have been used by Mustroph et al 228 to characterize the bond localisation in the ground state.…”

Polymeric and polymer-ligated spirobicyclic zwitterionic Janovsky complexes

“…8 The need for chromatographic purification would be circumvented as by-products and reagents can be efficiently separated by filtration and solvent washing. Following from our previous synthesis of Janovsky complexes in solution phase, 9 we proposed to move to organic (polystyrene, PS) and inorganic (silica, Si) supported carbodiimide resins to effect synthesis of the targeted Janovsky complexes by solid-phase methods.…”

“…4C). 9 The broad nature of these signals (Fig. 4B) may be due to a lack of molecular mobility or the possibility of a composite of two or more molecular environments for the poly-Janovsky complex in this polymeric system (Scheme 1).…”

“…9,16 IR spectroscopy is an extremely useful spectroscopic tool in identifying functional group type, particularly those with polarized chemical bonds. Bands corresponding to N-O stretching vibrations 17a have been reported for nitro-substituted Janovsky 10f,17b…”