Reversible Hyperpolarization of Ketoisocaproate Using Sulfoxide‐containing Polarization Transfer Catalysts

Abstract

The substrate scope of sulfoxide‐containing magnetisation transfer catalysts is extended to hyperpolarize α‐ketoisocaproate and α‐ketoisocaproate‐1‐[13C]. This is achieved by forming [Ir(H)2(κ2‐ketoisocaproate)(N‐heterocyclic carbene)(sulfoxide)] which transfers latent magnetism from p‐H2 via the signal amplification by reversible exchange (SABRE) process. The effect of polarization transfer field on the formation of enhanced 13C magnetization is evaluated. Consequently, performing SABRE in a 0.5 μT field enabled most efficient magnetisation transfer. 13C NMR signals for α‐ketoisocaproate‐1‐[13C] in methanol‐d4 are up to 985‐fold more intense than their traditional Boltzmann derived signal intensity (0.8 % 13C polarisation). Single crystal X‐ray diffraction reveals the formation of the novel catalyst decomposition products [Ir(μ‐H)(H)2(IMes)(SO(Ph)(Me)2)]2 and [(Ir(H)2(IMes)(SO(Me)2))2(μ‐S)] when the sulfoxides methylphenylsulfoxide and dimethylsulfoxide are used respectively.

Advancing homogeneous catalysis for parahydrogen-derived hyperpolarisation and its NMR applications

“…Ir–NHC catalysts are typically highly soluble in organic solvents and therefore polarisation transfer reactions are typically performed in methanol- d 4 (ref. 29 , 38 , 146 , 157 and 158 ) or even dichloromethane- d 2 , 37,38 chloroform- d 159 or ethanol- d 6 . 146 The efficiency of SABRE catalysis in other solvents can be improved by alteration of the auxiliary NHC ligand.…”

“…For example, 13 C NMR enhancements of 108-fold for 13 C-acetate were achieved using [Ir(H) 2 (IMes)(acetate)(pyridine) 2 ]. 126 More notably, sulfoxides have emerged as a versatile class of SABRE coligand 38,125,146,147,157 and as a result, novel SABRE catalysts of the form […”

Hyperpolarised NMR to aid molecular profiling of electronic cigarette aerosols

“…d ¼ À28 to À30 ppm range expected for iridium(III) hydrides located trans to oxygen. [42][43][44][45][46] Interestingly, when the shaking process is repeated several times following addition of fresh pH 2 , these enhanced acrolein and hydride signals decrease in intensity until they are no longer visible (Fig. 3d), while signals of nicotine remain enhanced with similar signal intensity.…”

Ultrafast Laplace NMR to study metal–ligand interactions in reversible polarisation transfer from parahydrogen

“…[59][60][61] Laplace NMR can be useful for monitoring processes in which a substrate undergoes a change in T 2 or D, particularly in cases where there may be no discernible change in chemical shift. This approach may be particularly useful to monitor rapid catalyst activations (for example, those of sulfoxide-based SABRE catalysts 33,34,47,62 ) or even catalyst dimerisation that may be accompanied by a large change in D. 34,61,63 This approach may be of particular relevance for reactions studied at low, ultra-low, or even zero field. 64,65 It is also possible to use sequences with selective chirp pulses to excite only the pyridine resonance(s).…”

“…24 This is necessary to unlock the latent magnetism of pH 2 , which can then be transferred to 1 H or heteronuclear sites of ligated molecules within the active catalyst. 24 This transfer occurs through a temporary J-coupled network [25][26][27][28] formed between pH 2 -derived hydride ligands and target sites at low magnetic field (mT or µT for transfer to 1 H 24,29,30 or X-nuclei [31][32][33][34] respectively). Reversible dissociation yields hyperpolarised ligand free in solution.…”