In one study a reaction mechanism for a specific aza-BH reaction is proposed.[3] Given a set of reaction conditions the reaction is found to be first-order in the triphenylphosphine nucleophile, MVK and the tosylimine concentration in the rate determining step in the presence of a Brønsted acid such as phenol or benzoic acid. The presence of an acid facilitates the elimination reaction in the zwitterion by proton transfer which becomes much faster and no longer rate determining. A 6 membered cyclic transition state is proposed for this reaction step. Because this step is also reversible the presence of acid causes a racemisation process simply by mixing chiral aza-BH adduct, phosphine and acid.
^Bifunctional Activation and Racemization in the Catalytic Asymmetric Aza-Baylis–Hillman Reaction Pascal Buskens, Jürgen Klankermayer, and Walter Leitner J. Am. Chem. Soc.; 2005; 127(48) pp 16762 - 16763; (Communication) doi:10.1021/ja0550024Abstract[permanent dead link]
^Gausepohl, Rolf; Buskens, Pascal; Kleinen, Jochen; Bruckmann, Angelika; Lehmann, Christian W.; Klankermayer, Jürgen; Leitner, Walter (2006). "Highly Enantioselective Aza-Baylis–Hillman Reaction in a Chiral Reaction Medium". Angewandte Chemie International Edition. 45 (22): 3689–3692. doi:10.1002/anie.200600327. PMID16708413.