Optimizing the Synthesis of Deuterated Isotopomers and Isotopologues of Cyclohexene using Molecular Rotational Resonance Spectroscopy.

Despite advances in reactions such as hydrogen isotope exchange (HIE) and reductive deuteration, achieving controlled and selective deuteration remains challenging. Moreover, the difficulty of developing successful deuteration platforms is compounded by a lack of means to assess the stereoisotopic purity of deuterated products. We previously reported a highly regio- and stereoselective approach for generating semideuterated cyclohexenes via tandem protonation (H+/D+) and reduction (H-/D-) sequences of a dihapto-coordinate tungsten-benzene complex. While NMR and HRMS analyses suggested successful deuterium incorporation, molecular rotational resonance (MRR) spectroscopy identified numerous over-, under-, and mis-deuteration impurities. At the time of publication, these impurities were attributed to H/D scrambling that could occur during thermolysis of the tungsten-bound cyclohexene ligand prior to MRR analysis. In this work, we describe the analysis of semideuterated cyclohexenes using MRR spectroscopy with an improved thermolysis apparatus that eliminates deuterium scrambling during analysis. Quantitative analysis of both racemic and enantiopure samples enables the optimization of deuteration conditions by providing multiple mechanistic insights into the formation of impurities.