This review focuses on the effects that confinement of molecular and heterogeneous catalysts with well-defined structure has on the selectivity and activity of these systems. A general introduction about catalysis and how the working principles of enzymes can be used as a source of inspiration for the preparation of catalysts with enhanced performance is provided. Subsequently, relevant studies demonstrate the importance of second coordination sphere effects in a broad sense (in homogeneous and heterogeneous catalysis). Firstly, we discuss examples involving zeolites, MOFs and COFs as heterogeneous catalysts with well-defined structures where confinement influences catalytic performance. Then, specific cases of homogeneous catalysts where non-covalent interactions determine the selectivity and activity are treated in detail. This includes examples based on cyclodextrins, calix[n]arenes, cucurbit[n]urils, and self-assembled container molecules. Throughout the review, the impact of confined spaces is emphasized and put into context, in order to get a better understanding of the effects of confinement on catalyst performance. In addition, this analysis intends to showcase the similarities between homogeneous and heterogeneous catalysts, which may aid the development of novel strategies.
Confinement of a catalyst can have a significant impact on catalytic performance and can lead to otherwise difficult to achieve catalyst properties. Herein, we report the design and synthesis of a novel caged catalyst system Co−G@Fe8(Zn−L ⋅ 1)6, which is soluble in both polar and apolar solvents without the necessity of any post‐functionalization. This is a rare example of a metal‐coordination cage able to bind catalytically active porphyrins that is soluble in solvents spanning a wide variety of polarity. This system was used to investigate the combined effects of the solvent and the cage on the catalytic performance in the cobalt catalyzed cyclopropanation of styrene, which involves radical intermediates. Kinetic studies show that DMF has a protective influence on the catalyst, slowing down deactivation of both [Co(TPP)] and Co−G@Fe8(Zn−L ⋅ 1)6, leading to higher TONs in this solvent. Moreover, DFT studies on the [Co(TPP)] catalyst show that the rate determining energy barrier of this radical‐type transformation is not influenced by the coordination of DMF. As such, the increased TONs obtained experimentally stem from the stabilizing effect of DMF and are not due to an intrinsic higher activity caused by axial ligand binding to the cobalt center ([Co(TPP)(L)]). Remarkably, encapsulation of Co−G led to a three times more active catalyst than [Co(TPP)] (TOFini) and a substantially increased TON compared to both [Co(TPP)] and free Co−G. The increased local concentration of the substrates in the hydrophobic cage compared to the bulk explains the observed higher catalytic activities.
An efficient Ugi‐type three‐component reaction (U‐3CR) for the synthesis of pipecolic amides is reported. The U‐3CR between electronically diverse isocyanides, carboxylic acids and 4‐substituted Δ1‐piperideines proceeds in a highly diastereoselective fashion. The Δ1‐piperideines are obtained by NCS‐mediated oxidation of the corresponding 4‐substituted piperidines, which in turn are generated by an efficient two‐step procedure involving the alkylation of 4‐picoline and subsequent catalytic hydrogenation of the pyridine ring. We demonstrate the utility of this U‐3CR, in combination with the convertible isocyanide 2‐bromo‐6‐isocyanopyridine, in the synthesis of the anticoagulant argatroban.
In this study, three novel cubic cages were synthesized and utilized to encapsulate a catalytically active cobalt(II) meso‐tetra(4‐pyridyl)porphyrin guest. The newly developed caged catalysts (Co‐G@Fe8(Zn‐L ⋅ 1)6, Co‐G@Fe8(Zn‐L ⋅ 2)6 and Co‐G@Fe8(Zn‐L ⋅ 3)6) can be easily synthesized and differ in exo‐functionalization, which are either none, polar or apolar groups. This leads to a different polarity of the peripheral environment surrounding the cage, which affects the (relative) local concentration of the substrates surrounding the cage and hence indirectly influences the substrate availability of the catalysis embedded in the active site of the caged catalyst systems. The resulting increased local substrate concentrations give rise to higher catalytic activities of the respective caged catalyst in metalloradical catalyzed cyclopropanation reactions. Interestingly, the catalytic activity is the highest when the apolar cage catalyst (Co‐G@Fe8(Zn‐L ⋅ 1)6) is used, and lowest with the polar analog (Co‐G@Fe8(Zn‐L ⋅ 3)6). In addition, the catalytic activity of the cage without exo‐functionalities (Co‐G@Fe8(Zn‐L ⋅ 2)6) is nearly two times lower than that of Co‐G@Fe8(Zn‐L ⋅ 1)6 and three times higher than that of Co‐G@Fe8(Zn‐L ⋅ 3)6, which further demonstrates the effect of the peripheral functionalities on the cyclopropanation reaction.
In an attempt to synthesize a mononuclear rhodium nitridyl complex with a reduced tendency to undergo nitridyl radical N-N coupling, we synthesized a bulky analog of Milstein's bipyridine-based PNNH ligand, bearing a tert-butyl group at the 6′ position of the bipyridine moiety. A three-step synthetic route toward this new bulky tBu 3 PNNH ligand was developed, involving a selective nucleophilic substitution step, followed by a Stille coupling and a final hydrophosphination step to afford the desired 6-(tert-butyl)-6′-[(di-tert-butylphosphino)methyl]-2,2′-bipyridine (tBu 3 PNNH) ligand. This newly developed tBu 3 PNNH ligand was incorporated in the synthesis of the sterically protected azide complex [Rh(N 3 )(tBu 3 PNNH)]. We explored N 2 elimination form this species using photolysis and thermolysis, hoping to synthesize a mononuclear rhodium com-
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