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.