Herein we report the use of polyether binders as regulation agents (RAs) to enhance the enantioselectivity of rhodium‐catalyzed transformations. For reactions of diverse substrates mediated by ...rhodium complexes of the α,ω‐bisphosphite‐polyether ligands 1–5,a–d, the enantiomeric excess (ee) of hydroformylations was increased by up to 82 % (substrate: vinyl benzoate, 96 % ee), and the ee value of hydrogenations was increased by up to 5 % (substrate: N‐(1‐(naphthalene‐1‐yl)vinyl)acetamide, 78 % ee). The ligand design enabled the regulation of enantioselectivity by generation of an array of catalysts that simultaneously preserve the advantages of a privileged structure in asymmetric catalysis and offer geometrically close catalytic sites. The highest enantioselectivities in the hydroformylation of vinyl acetate with ligand 4 b were achieved by using the RbB(3,5‐(CF3)2C6H3)4 (RbBArF) as the RA. The enantioselective hydrogenation of the substrates 10 required the rhodium catalysts derived from bisphosphites 3 a or 4 a, either alone or in combination with different RAs (sodium, cesium, or (R,R)‐bis(1‐phenylethyl)ammonium salts). This design approach was supported by results from computational studies.
Polyether binders were used as regulation agents (RAs) to enhance the enantioselectivity of rhodium‐catalyzed hydroformylations and hydrogenations of an array of diversely substituted alkenes (see figure).
Progress reaction profiles are affected by both catalyst activation and deactivation processes occurring alongside the main reaction. These processes complicate the kinetic analysis of reactions, ...often directing researchers toward incorrect conclusions. We report the application of two kinetic treatments, based on variable time normalization analysis, to reactions involving catalyst activation and deactivation processes. The first kinetic treatment allows the removal of induction periods or the effect of rate perturbations associated with catalyst deactivation from kinetic profiles when the quantity of active catalyst can be measured. The second treatment allows the estimation of the activation or deactivation profile of the catalyst when the order of the reactants for the main reaction is known. Both treatments facilitate kinetic analysis of reactions suffering catalyst activation or deactivation processes.
Keep calm and carry on (doing kinetics): Two different kinetic analysis methods, based on variable time normalization analysis (VTNA), are described for studying reactions with catalyst deactivation and activation processes. The cases studied are a hydroformylation reaction catalyzed by a supramolecular rhodium complex and an aminocatalytic Michael reaction.
Iridium(I) complexes with phosphine–phosphite ligands efficiently catalyze the enantioselective hydrogenation of diverse seven‐membered C=N‐containing heterocyclic compounds (eleven examples; up to ...97 % ee). The P‐OP ligand L3, which incorporates an ortho‐diphenyl substituted octahydrobinol phosphite fragment, provided the highest enantioselectivities in the hydrogenation of most of the heterocyclic compounds studied. The observed stereoselection was rationalized by means of DFT calculations.
Cl switch: Efficient enantioselective hydrogenation of seven‐membered N‐heterocycles mediated by Ir‐(P‐OP) complexes is described (11 examples, up to 97 % ee; see figure). The position of the Cl ligand in the catalytically relevant Ir species (amongst other factors) is key for rationalizing the stereochemical outcome.
Different methods for transforming N‐heteroarenes into more reactive derivatives for catalytic asymmetric hydrogenation are highlighted. The first strategy consists of facilitating hydrogenation by ...the formation of positively charged derivatives of the heteroarene. Catalyst deactivation processes arising upon binding of the substrate to the metal center can thus be prevented and, additionally, hydrogenation of positively charged heteroarenes may also be more favored than that of their neutral analogues. The second strategy is based on introducing a ligating group onto the substrate to assist its coordination to the metal center and facilitate hydrogenation by chelation assistance. The last strategy involves breaking the aromaticity of the heteroarene by inducing a double‐bond migration process. This microreview summarizes advances made in the above strategies, which have allowed the development of highly enantioselective catalytic hydrogenation of N‐heteroarenes for the production of fully or partially saturated chiral heterocycles.
This microreview highlights progress in synthetically manipulating heteroaromatic compounds in order to increase their reactivity towards asymmetric hydrogenation mediated by enantiomerically pure transition‐metal complexes.
Conspectus Over hundreds of new organic semiconductor molecules have been synthesized as hole transport materials (HTMs) for perovskite solar cells. However, to date, the well-known N 2,N 2,N 2′,N ...2′,N 7,N 7,N 7′, octakis-(4-methoxyphenyl)-9,9-spirobi-9,9′-spirobi9H-fluorene-2,2′,7,7′-tetramine (spiro-OMeTAD) is still the best choice for the best perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term stable devices, and its market price makes its use in large-scale production costly. Novel synthetic routes for new HTMs have to be sought that can be carried out in fewer synthetic steps and can be easily scaled up for commercial purposes. On the one hand, synthetic chemists have taken, as a first approach, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the spiro-OMeTAD molecule as a reference to synthesize molecules with similar energy levels, although these HOMO and LUMO energy levels often have been measured indirectly in solution using cyclic voltammetry. On the other hand, the “spiro” chemical core has also been studied as a structural motif for novel HTMs. However, only a few molecules incorporated as HTMs in complete functional perovskite solar cells have been capable of matching the performance of the best-performing perovskite solar cells made using spiro-OMeTAD. In this Account, we describe the advances in the synthesis of HTMs that have been tested in perovskite solar cells. The comparison of solar cell efficiencies is of course very challenging because the solar cell preparation conditions may differ from laboratory to laboratory. To extract valuable information about the HTM molecular structure–device function relationship, we describe those examples that always have used spiro-OMeTAD as a control device and have always used identical experimental conditions (e.g., the use of the same chemical dopant for the HTM or the lack of it). The pioneering work was focused on well-understood organic semiconductor moieties such as arylamine, carbazole, and thiophene. Those chemical structures have been largely employed and studied as HTMs, for instance, in organic light-emitting devices. Interestingly, most research groups have reported the hole mobility values for their novel HTMs. However, only a few examples have been found that have measured the HOMO and LUMO energy levels using advanced spectroscopic techniques to determine these reference energy values directly. Moreover, it has been shown that those molecules, upon interacting with the perovskite layer, often have different HOMO and LUMO energies than the values estimated indirectly using solution-based electrochemical methods. Last but not least, porphyrins and phthalocyanines have also been synthesized as potential HTMs for perovskite solar cells. Their optical and physical properties, such as high absorption and good energy transfer capabilities, open new possibilities for HTMs in perovskite solar cells.
Supramolecular catalysis is a rapidly expanding discipline which has benefited from the development of both homogeneous catalysis and supramolecular chemistry. The properties of classical metal and ...organic catalysts can now be carefully tailored by means of several suitable approaches and the choice of reversible interactions such as hydrogen bond, metal-ligand, electrostatic and hydrophobic interactions. The first part of these two subsequent reviews will be dedicated to catalytic systems for which non-covalent interactions between the partners of the reaction have been designed although mimicking enzyme properties has not been intended. Ligand, metal, organocatalyst, substrate, additive, and metal counterion are reaction partners that can be held together by non-covalent interactions. The resulting catalysts possess unique properties compared to analogues lacking the assembling properties. Depending on the nature of the reaction partners involved in the interactions, distinct applications have been accomplished, mainly (i) the building of bidentate ligand libraries (
intra
ligand-ligand), (ii) the building of di- or oligonuclear complexes (
inter
ligand-ligand), (iii) the alteration of the coordination spheres of a metal catalyst (ligand-ligand additive), and (iv) the control of the substrate reactivity (catalyst-substrate). More complex systems that involve the cooperative action of three reaction partners have also been disclosed. In this review, special attention will be given to supramolecular catalysts for which the observed catalytic activity and/or selectivity have been imputed to non-covalent interaction between the reaction partners. Additional features of these catalysts are the easy modulation of the catalytic performance by modifying one of their building blocks and the development of new catalytic pathways/reactions not achievable with classical covalent catalysts.
Non-covalent interactions constitute a suitable tool for the building and modification of catalytic systems (L = ligand, M = metal, LA = ligand additive, S = substrate, cat. = either organic or metal catalyst). This review discusses the various strategies used for the design of supramolecular catalysts.
A detailed study is disclosed on the Rh‐mediated hydrogenative kinetic resolution of α,β‐unsaturated sulfoxides with alkyl and aryl substituents at the α‐, E‐ and Z‐positions of the double bond. This ...stereoselective catalytic methodology has enabled the preparation of highly enantioenriched (or even enantiopure) alkyl and aryl‐substituted (un)saturated sulfoxides via a simple and efficient synthetic operation. Moreover, the application of the hydrogenative KR to the preparation of valuable optically active sulfoxide‐containing building blocks or biologically active intermediates is described.
The ability of a rhodium catalyst derived from phosphine‐phosphite ligands to hydrogenatively resolve a set of structurally diverse α,β‐unsaturated vinyl sulfoxides is reported. The practicality of the methodology was applied to the preparation of precursors of biologically active compounds.
Set the N free! The reactivity of the amino group of P‐stereogenic aminophosphines allows the further elaboration of the aminophosphine unit whilst preserving the original chirality of the phosphorus ...atom (see picture; Rh green). P‐stereogenic aminodiphosphine ligands can easily be prepared in optically pure forms, feature distinct structural and electronic characteristics, and can be used in asymmetric hydrogenation reactions.
A second generation of phosphine–phosphite (P–OP) ligands, incorporating a more sterically bulky phosphite group than previous P–OP ligand designs, gave very efficient catalysts for the Rh‐catalysed ...asymmetric hydrogenation of a diverse array of substrates (11 examples, 93–99 % ee) containing structurally diverse substituents and chelating groups at the C=C double bond. The presence of the sterically bulky (Sa)‐3,3′‐diphenyl‐5,5′,6,6′,7,7′,8,8′‐octahydro‐1,1′‐binaphthalene‐2,2′‐diol‐derived phosphite fragment caused significant increases in enantioselectivity (up to Δee = 58 %), and provided improved results compared to those obtained with the first generation of P–OP‐derived rhodium catalysts {i.e., rhodium complexes incorporating phosphine–phosphite ligands with (Ra)‐ and (Sa)‐BINOL‐derived phosphite groups; BINOL = 1,1′‐binaphthalene‐2,2′‐diol}. Overall, the optimal ligand L8 provided very high enantioselectivities for a range of structurally diverse olefins (up to 99 % ee).
A second generation of phosphine–phosphite (P–OP) ligands, incorporating a larger phosphite group than previous P–OP ligand designs, led to very efficient catalysts for the Rh‐catalyzed enantioselective hydrogenation of a diverse array of substrates (11 examples, 93–99 % ee, mean value of 98 % ee).
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