Reversible Hyperpolarization of Ketoisocaproate Using Sulfoxide‐containing Polarization Transfer Catalysts

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. Extending substrate scope: It is demonstrated that the scope of sulfoxide‐containing SABRE polarization transfer catalysts can be extended to hyperpolarize molecules with similar α‐keto acid motifs to pyruvate. This could lead to a larger range of potential applications of SABRE in monitoring chemical transformations.