In all experiments, the initial reaction solution was 500 mM DMAD and 5 mM [Rh(dppb)(COD)]BF4 catalyst in acetone-d6. Deuterated acetone was used to extend the lifetime of hyperpolarized spin order. Figure shows a ZULF NMR spectrum acquired while bubbling para-enriched hydrogen gas into 2 ml of reaction solution in a titanium NMR tube. The spectrum is the average of 32 transients. Experimental details can be found in the Materials and Methods.