Competitive Chemisorption between Pairs of Cinchona Alkaloids and Related Compounds from Solution onto Platinum Surfaces

Abstract

Quinoline-derived compounds exhibit the following relative chemisorption strengths from CCl4 solutions onto platinum surfaces, as determined by in-situ infrared spectroscopy: quinine, quinidine > cinchonidine > cinchonine > 6-methoxyquinoline > lepidine > quinoline. This sequence explains nonlinear enantioselectivity effects with cinchona chiral modifiers in hydrogenation catalysis.

Novel Evidence on the Role of the Nucleophilic Intermediate Complex in the Orito-Reaction: Unexpected Inversion in the Enantioselective Hydrogenation of 2,2,2-Trifluoroacetophenone on Pt-Cinchona Chiral Catalyst Using Continuous-Flow Fixed-Bed Reactor

“…A review of the reports published in the past few years on the enantioselective hydrogenation of activated ketones over Pt catalyst modified by cinchona alkaloids (the Orito reaction) reveals that all (or most) that can be found out about the reaction on Pt-CD using the currently available experimental methods is already known (see summarized since 2006 in [8][9][10][11][12][13]). The results achieved are hallmarked by the extension of the scope of the reaction and by the deeper understanding of the reaction mechanism in general and that of chiral induction in particular: (1) ee values over 90% are already attainable in the hydrogenation of certain substrates [14][15][16][17][18][19][20][21]; (2) the success of studies on the reaction mechanism has been made possible by the application of a great variety of instrumental techniques (see summarized in references [22][23][24][25][26][27][28][29][30][31]; however, studies similar to those performed on Pt-CD should also be repeated on Pt-CN, Pt-QN and Pt-QD catalysts).…”

New Data of Nonlinear Phenomenon in the Heterogeneous Enantioselective Hydrogenation of Activated Ketones

“…Independently of all these differences, it should be emphasized that an ee exceeding 90% is achievable with all four parent alkaloids and this observation also calls for further research. The recent in situ RAIRS results [20] based on the earlier studies by Zaera and coworkers [31] revealed a different adsorption order of the cinchona alkaloids, thus, the interpretation of the phenomenon (K ads solvent dependence, adsorption mode of the alkaloids) calls the attention on the importance of further studies.…”

“…The results of studies carried out to date (using modifier mixtures, transient method, continuous flow measurements) can be summarized as follows. Studies on NLP have mainly been performed in enantioselective hydrogenations on Pt [10][11][12][13][14][15][16][17][18][19][20][21] and Pd [22][23][24][25] catalysts. A study using Rh catalyst has also been published [26].…”

“…One component of the chiral modifiers used was a parent cinchona alkaloid (CD, CN, QN, QD), whereas the other component was the opposite enantiomer of CD, CN, QN, QD or their C9-O-R derivatives [12,13,16,18,19,24,[26][27][28]. In addition to cinchona alkaloids and their derivatives, other chiral modifiers including synthetic chiral compounds and achiral modifiers were also investigated [11,15,20,22].…”

Adsorption of 1‐(1‐Naphthyl)ethylamine from Solution onto Platinum Surfaces: Implications for the Chiral Modification of Heterogeneous Catalysts

“…This effect was certainly proven important in determining the relative uptakes of different cinchona alkaloids. [4,19,[21][22][23] However, in the present case, all compounds studied are liquid and fully soluble in the solvent (CCl 4 ).…”