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R. Pollice, M. Schnürch:
"Mechanistic Investigations and Kinetic Modelling of the Direct Alkylation of Benzylic Amines reveal a surprising role of K2CO3";
Poster: 19th European Symposium on Organic Chemistry, Lissabon; 12.07.2015 - 16.07.2015; in: "19th European Symposium on Organic Chemistry - Programme", (2015), S. 69.

Within this work a Rh(I)-catalyzed direct C-H alkylation of benzylic amines with alkenes co-catalyzed by K2CO3 was studied as a benchmark reaction to gain insight into the main kinetic influence factors associated with heterogeneous bases in metal-catalyzed reactions and to elucidate one of the associated underlying reaction mechanisms. Even though formally an C(sp3)−H activation, this reaction actually proceeds via imine intermediates and, hence, via C(sp2)−H activation. To proof this hypothesis, series of synthetic experiments were carried out which allowed us to show that the formation of 4 from 1 actually proceeds via intermediates 2 and 3, which are interestingly detected only in trace amounts in the reaction mixture.[1]
Additionally, to get further insight into the reaction mechanism, kinetic experiments were carried out using the method of initial rates. There, an interesting influence of the base was observed showing that K2CO3 is actually only needed at the beginning of the reaction to form a catalytically active species. However, the reaction rate is dependent on the amount and on the source of base, respectively its specific surface area, to some extent, which will be elaborated in this contribution. Furthermore, the reaction shows a primary kinetic isotope effect of 4.3 at the benzylic C−H position together with a reversible H−D exchange at the same position, which indicates that there are at least two distinct steps in which the corresponding C−H bonds are broken. The presented transformation shows an interesting side product profile as well, indicating that the catalyst is not only capable of activating C-H but also C-C bonds. Based on our results we were able to propose a kinetic model of this direct alkylation which is in agreement with all our experimental findings.

C-H activation, alkylation, kinetic studies

http://publik.tuwien.ac.at/files/PubDat_241520.pdf