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‐[(13)C]. This is achieved by forming [Ir(H)(2)(κ (2)‐ketoisocaproate)(N‐heterocyclic carbene)(sulfoxide)] which transfers latent magnetism from p‐H(2) via the signal amplification by reversible exchange (SABRE) process. The effect of polarization transfer field on the formation of enhanced (13)C magnetization is evaluated. Consequently, performing SABRE in a 0.5 μT field enabled most efficient magnetisation transfer. (13)C NMR signals for α‐ketoisocaproate‐1‐[(13)C] in methanol‐d (4) are up to 985‐fold more intense than their traditional Boltzmann derived signal intensity (0.8 % (13)C 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.