After surveying various alkyl phenylphosphonites, we selected as a model reaction the dealkylation of dibenzyl phenylphosphonite 2a with HCl (Fig. ). Through a systematic evaluation of HBDs featuring a tert-leucine-bis(trifluoromethyl)aniline scaffold (30), we found that a thiourea (1c) promoted the reaction with higher enantioselectivities than its squaramide (1a) or urea (1b) counterparts (Fig. ). Furthermore, the size of the (poly)aromatic substituent as well as α-quarternary substitution at the pyrrolidine correlated positively with enantioselectivity. Ultimately, we found that thiourea 1g bearing a 2-phenanthryl substituent promoted dealkylation in 90% ee and quantitative yield. Although catalyst optimization studies were performed using two equivalents of HCl in methyl tert-butyl ether (MTBE), measurable enhancements in enantioselectivity were achieved by changing the reaction solvent to toluene (Fig. , entry 1), employing a single equivalent of HCl (entry 2), and diluting the reaction mixture to 50 mM (entry 3) to furnish 3a in 95% ee.