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M. Schnürch, R. Pollice:
"Mechanistic and Kinetic Studies of the Direct Alkylation of Benzylic Amines - Surprisingly an sp2 C-H Activation";
Poster: OMCOS18 Organometallic Chemistry Directed Towards Organic Synthesis, Sitges - Barcelona; 28.06.2015 - 02.07.2015; in: "OMCOS18", (2015), S. 18.

Upon carrying out mechanistic investigations of a Rh(I)-catalyzed direct C−H alkylation of benzylic amines with alkenes it was observed that this reaction, even though formally an C(sp3)−H activation, 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.
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_241518.pdf