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 (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 ¹³ C magnetization is evaluated. Consequently, performing SABRE in a 0.5 microT field enabled most efficient magnetisation transfer. ¹³ C NMR signals for α-ketoisocaproate-1-[¹³ C] in methanol-d₄ 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(micro-H)(H)₂ (IMes)(SO(Ph)(Me)₂ )]₂ and [(Ir(H)₂ (IMes)(SO(Me)₂ ))₂ (micro-S)] when the sulfoxides methylphenylsulfoxide and dimethylsulfoxide are used respectively.