Abstract:We have achieved the non-covalent immobilization of chiral primary amino acid-derived di-A C H T U N G T R E N N U N G amines on organic and inorganic sulfonated solid acids through acid-base interaction. With the commercial sulfonated fluoropolymer nafion NR50 as support an optimal balance was found between activity and stereoselectivity of the supported catalyst in direct asymmetric aldol reactions of linear ketones and aromatic aldehydes. Under optimized conditions aldol products were obtained in high yields and with excellent enantioselectivities for the syn-product (up to 98% ee). Furthermore, catalysis with the supported diamine was demonstrated to occur truly heterogeneously and the loaded nafion NR50 beads could be reused several times. Ultimately, the immobilized catalyst/nafion NR50 system was successfully implemented in a fixed-bed reactor set-up under continuous flow conditions.

Abstract:We have designed a novel class of chiral diamine organocatalysts based on natural primary amino acids that efficiently catalyze syn-selective aldol reactions of challenging linear ketones, such as 2-butanone, and aromatic aldehydes. In the presence of trifluoroacetic acid (TFA) as Brønsted acid and 2,4-dinitrophenol (DNP) as co-catalyst, synaldol products have been obtained with excellent enantioselectivities of up to > 99% ee.Keywords: aldol reaction; aliphatic ketones; chiral diamines; syn-diastereoselectivity; primary amino acids In the past decade intensive research has focused on the design of various chiral pyrrolidine derivatives for asymmetric enamine-catalyzed reactions.[1] Within this aminocatalytic gold rush, an overwhelming number of novel, highly efficient organocatalysts appeared in the literature and at present, asymmetric aminocatalysis is widely regarded as a well-established and powerful synthetic tool for the enantioselective functionalization of carbonyl compounds.[2] However, in spite of the tremendous success of secondary amines as enamine-based catalysts, primary amino acids have only rarely been considered for this kind of catalysis. In 2004/2005 the first reports on primary amines as enamine-based catalysts appeared in the literature. [3,4] Simple acyclic amino acids and peptides, as well as thiourea-primary amines have been successfully applied in asymmetric aldol reactions, Mannich reactions and/or Michael additions. These initial investigations demonstrate that enamine intermediates derived from primary amines can be generated effectively. Moreover, primary amine catalysts show advantages in asymmetric catalysis since the presence of a hydrogen on the nitrogen can play a role in both promoting the formation of an active intermediate and controlling the stereoselectivity. [4b,c] As a consequence of these findings, primary amines have been further established during the past few years as valuable enamine catalysts, complementing the traditional secondary amino catalysis. [4,5] Although important progress has been accomplished recently, the development of primary amine catalysts is still far behind in comparison with secondary amine catalysis. Hence, it is of great interest to further explore the potential of primary Figure 1. Overview of catalysts screened in the model reaction.

Abstract:The zeolite H-Beta catalyzes the retroDiels-Alder reaction of a range of cyclopentadiene cyclo-adducts at moderate temperatures and ambient pressure, in the presence of an active dienophile. The active catalyst was identified and optimum reaction conditions established after screening a range of zeolites in the retro-Diels-Alder reaction of the cyclopentadiene adduct of cyclopentenone. Our results suggest that retro-Diels-Alder reactions of tricyclodecadienones are catalyzed by Brønsted acids and the high catalytic performance of H-Beta catalysts can be ascribed to the optimal balance between the number of acid sites and their strength as well as to the accessibility of these sites. The methodology was then applied to a series of alkyl derivatives of cyclopentadienylcyclopentenones to provide a viable alternative synthetic route to 4-alkylcyclopentenones and the versatility of the approach was demonstrated by the successful cycloreversion of N-cyclohexyl-2-azanorborn-5-ene.

A straightforward synthetic route to chiral metal-organic frameworks is proposed that relies on an acid-base interaction between an acid linker and a chiral primary amino acid derived diamine organocatalyst. High ee values for the aldol condensation of linear ketones and aromatic aldehydes are reported with this heterogeneous catalyst. Three consecutive catalyst reuse experiments demonstrated that the majority of the activity was preserved, as was the enantioselectivity.Although organocatalysis with amino acids and their derivatives has proven very successful towards highly enantioselective condensation reactions, there is an urgent need to immobilize these elegant and bioinspired chiral catalysis for easy reuse. Many immobilization protocols that employ different organic and inorganic support materials have been discussed in the literature, and most of them follow a covalent anchoring strategy, sometimes with the prospect of using them in a flow setup. [1] Metal-organic frameworks (MOFs) [2] are among the most promising supporting materials: distinct from traditional inorganic materials, MOFs can be synthesized from a large variety of building blocks. In spite of the large degree of unpredictability in synthesis, [3] thousands of MOF structures have been reported to date. [4] Different strategies have been employed to prepare homochiral MOFs, including chiral-template and direct synthesis methods, either starting with enantiopure ligands or through self-resolution from achiral or racemic molecules. [5] The current methods for the synthesis of homochiral MOFs have shown limited flexibility: on the one hand, if the organocatalyst is used as a linker for MOF synthesis, catalysts with limited stability are formed owing to linker dimensions that are usually large. On the other hand, the templated approach is limited by the stability of the organocatalyst. [6] In this work, we present a straightforward postfunctionalization based on the noncovalent immobilization of a phenylalanine-derived chiral ligand on a sulfated achiral MOF derivative through acid-base pair interaction. The presence of the organocatalyst in the MOF structure was confirmed with N 2 sorption measurements, FTIR spectroscopy, and thermal gravimetrical analyses. The morphology and framework of the solid chiral catalyst was characterized by SEM and X-ray diffraction, respectively. Very high enantioselectivities were obtained in the direct asymmetric aldol reactions of various linear ketones with aromatic aldehydes. The presented postsynthetic modification strategy is very practical, straightforward, and flexible towards the synthesis of a variety of chiral MOF structures with very high stability.Postsynthetic modification [7] of a preassembled achiral MOF offers the advantage that a single, stable parental framework can be converted into an active chiral MOF by carefully selecting the suitable catalytic chiral units. In 2009, Banerjee et al. reported the synthesis of a chiral MOF by the attachment of l-proline-derived chiral ligands to the open metal...

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