A Coherent Mechanistic Rationale for Additive Effects and Autoinductive Behaviour in Proline‐Mediated Reactions

Abstract

Abstract. Differences in the kinetic behavior of aldol reactions compared to aminoxylation and amination reactions are rationalized by consideration of the ratedetermining step in each case. Both autoinductive behavior and the rate-enhancing effect of additives are attributed to an effect on the enamine formation step.Keywords: organocatalysis · kinetics · autoinduction · amino acid catalysis · reaction mechanism Proline catalysis, elegantly revived by List, Lerner and Barbas in 2000 [1], has demonstrated efficient and enantioselective carbon-carbon and carbonheteroatom bond-forming reactions, including those shown in eq. 1, and has inspired the development of many other catalysts for a myriad of selective transformations [2]. Computational studies [3] have suggested a mechanistic analogy to reactions of aldolase enzymes [4]. Our recent work provided the first kinetic support for the enamine mechanism in aldol reactions and clarified the role of water on and off the catalytic cycle [5,6].(1a) (1b) (1c)A number of intriguing experimental observations have yet to be rationalized for these reactions. One unusual feature is a striking temporal increase in reaction rate that is observed in the α-aminoxylation [7,8] (eq 1a) and α-amination of aldehydes [9,10] (eq 1b), but is not observed in aldol reactions (eq 1c). In addition, additives including acids and bases have been observed to influence reactions employing proline as well as other pyrrolidine-based catalysts [11,12]. Here we present kinetic results that offer a coherent mechanistic rationalization for both the role of additives and the autoinductive behavior in proline-mediated reactions. These findings suggest an important role for detailed kinetic analysis in understanding and improving catalyst efficiency.The autoinductive behavior observed in the reactions of eqs 1a and 1b suggests that the product plays a role in accelerating rate, as is confirmed in Figure 1 for addition of product 3b to the α-amination reaction of eq 1b. The figure also shows that addition of aldol product 7 to the aldol reaction of eq 1c has no influence on the rate of this reaction. 2The two reactions exhibit distinctly different kinetic profiles, with the aldol reaction characterized by well-behaved positive order kinetics while the reactions of eqs 1a-1b show the sigmoidal conversion profile characteristic of an autoinductive process. Product ee is unaffected by added product in either reaction. The reactions of eqs. 1a and 1b do not proceed in the in the presence of product and absence of proline, confirming that the reactions are productaccelerated, or autoinductive, but not truly autocatalytic [14]. Careful studies, including those using fully soluble proline derivatives, showed that the effect could not be attributed to increasing solubilization of proline as the reaction proceeds [13].Interestingly, however, our further studies show that the source of the rate enhancement in the reactions of eqs 1a and 1b is not limited to the product specific to that reaction: Figure 2 show...

Organocatalyzed Aldol Reactions

“…All these discrepancies could mislead the interpretation for a simple nonlinear effect [141]. Finally, kinetic studies for this process demonstrated that it was characterized by well-behaved positive order kinetics, contrary to that observed in other related organocatalytic processes [142].…”

Quantum Mechanical Investigations of Organocatalysis: Mechanisms, Reactivities, and Selectivities

“…56,29f It was concluded that the autoinduction occurs in these reactions but not in the aldol reaction because of a difference in the rate-determing steps. 57 …”

Stabilization of Proline Enamine Carboxylates by Amine Bases

“…However, to promote specific reactions the application of dual or cooperative catalyst combinations [6] or the use of (co-catalytic) additives is slowly emerging because this often can prove beneficial for tuning both the reactivity and selectivity of existing catalytic systems. With respect to proline, additives [7][8][9] such as water, acids, or bases have successfully been shown to influence the outcome of the reactions and are already understood in part. [10] Even so, detailed experimental studies in solution, for example, with regards to the effect of additives on the reactive enamine intermediates, are rather scarce.…”