This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. ...For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.
A manganese/copper co-catalyzed electrochemical Wacker–Tsuji-type oxidation of aryl-substituted alkenes has been developed. The process involves the use of 5 mol % MnBr2 and 7.5 mol % CuCl2, in 4:1 ...acetonitrile/water in an undivided cell at 60 °C, with 2.8 V constant applied potential. α-Aryl ketones are formed in moderate to excellent yields, with the advantages of avoidance of palladium as a catalyst and any external chemical oxidant in an easily operated, cost-effective procedure.
A polystyrene‐supported 1,1’‐bi‐2‐naphthol derived phosphoric acid has been synthesized and applied in the enantioselective Friedel–Crafts reaction of indoles and sulfonylimines. The immobilized ...catalyst was highly active and selective, and gave rise to a broad range of 3‐indolylmethanamines (19 examples) in high yields and excellent enantioselectivities (up to 98 % enantiomeric excess) after short reaction times under very convenient reaction conditions (RT in dichloromethane). Moreover, repeated recycling (14 cycles) was possible with no substantial loss in catalytic performance and the system could be adapted to a continuous‐flow operation (6 h). Finally, the applicability of the system was further confirmed by rapid access to a library of compounds with three points of diversity in a single continuous‐flow experiment that involved sequential pumping of different substrate combinations.
A solid‐supported phosphoric acid has been prepared that shows excellent catalytic activity in the Friedel–Crafts reaction between indoles and sulfonylimines (see figure). Diverse libraries of indoles can be sequentially prepared by a continuous‐flow process with high productivities.
Abstract
The importance of discovering the true catalytically active species involved in photocatalytic systems allows for a better and more general understanding of photocatalytic processes, which ...eventually may help to improve their efficiency. Bi
2
O
3
has been used as a heterogeneous photocatalyst and is able to catalyze several synthetically important visible-light-driven organic transformations. However, insight into the operative catalyst involved in the photocatalytic process is hitherto missing. Herein, we show through a combination of theoretical and experimental studies that the perceived heterogeneous photocatalysis with Bi
2
O
3
in the presence of alkyl bromides involves a homogeneous Bi
n
Br
m
species, which is the true photocatalyst operative in the reaction. Hence, Bi
2
O
3
can be regarded as a precatalyst which is slowly converted in an active homogeneous photocatalyst. This work can also be of importance to mechanistic studies involving other semiconductor-based photocatalytic processes.
A chemically modified, first generation MacMillan imidazolidin-4-one has been anchored onto 1% DVB Merrifield resin and Fe3O4 (5.3 ± 1.4 nm) magnetic nanoparticles through copper-catalyzed alkyne ...azide cycloaddition (CuAAC) reactions. The resulting immobilized catalysts have been successfully used in the asymmetric Friedel–Crafts alkylation of N-substituted pyrroles with α,β-unsaturated aldehydes. The PS-supported catalyst (B) showed higher catalytic activity and enantioselectivity, while the MNP-supported one (A) showed higher recyclability and could be used in a sequential process with intermediate magnetic decantation.
The development of enantioselective catalytic processes that make use of sunlight as the energy source and nontoxic, affordable materials as catalysts represents one of the new and rapidly evolving ...areas in chemical research. The direct asymmetric α‐alkylation of aldehydes with α‐bromocarbonyl compounds can be successfully achieved by combining bismuth‐based materials as low‐band‐gap photocatalysts with the second‐generation MacMillan imidazolidinone as the chiral catalyst and simulated sunlight as a low‐cost and clean energy source. This reaction also proceeded with high efficiency when the reaction vial was exposed to the morning sunlight on a clear September day in Tarragona, Spain.
Now it's bismuth time! The asymmetric intermolecular α‐alkylation of aldehydes with α‐bromocarbonyl compounds can be achieved under visible‐light irradiation by combining the second‐generation MacMillan catalyst and an inexpensive, nontoxic, and commercially available Bi2O3 powder. This reaction also proceeded with high efficiency when the reaction vial was exposed to the morning sunlight in Tarragona, Spain.
A set of six solid-supported diarylprolinol catalysts (varying on the anchoring strategy and the type of polymeric support) has been prepared and applied to the enantioselective cyclopropanation ...reaction. The selected candidate allows implementation of a long flow experiment (48 h) and generates a library of 12 cyclopropanes by sequential flow experiments. The mildness and utility of the method have enabled a telescoped process in which the outstream is directly used in a Wittig flow reaction.
An efficient, highly regio- and stereoselective 4 + 2 cycloaddition reaction to generate tetrahydropyranopyrazole frameworks has been developed. To this end, a dienamine-based catalytic strategy that ...relies on the H-bond-directing effect of the hydroxy group of a dinaphthylprolinol-type aminocatalyst has been used. This enables the synthesis of multifunctionalized heterocyclic derivatives with three contiguous stereocenters in good yields and excellent enantioselectivities.
A new tris(1-benzyl-1H-1,2,3-triazol-4-yl)methanol ligand 3 has been prepared by a triple Cu(I)-catalyzed alkyne−azide 1,3-dipolar cycloaddition (CuAAC). Ligand 3 forms a stable complex with CuCl, ...which catalyzes the Huisgen 1,3-dipolar cycloaddition on water or under neat conditions. Low catalyst loadings, short reaction times at room temperature, and compatibility with free amino groups make 3·CuCl an outstanding catalyst for CuAAC.
Cucurbiturils are a family of supramolecular hosts obtained by condensation of glycoluril and formaldehyde. Cucurbit7uril, CB7, is the most prominent member of the family for its biomolecular ...interest, arising from its mild solubility in water and for its strong binding with a large variety of guests containing nonpolar fragments such as adamantanes and ferrocene. For instance, CB7 encapsulates diamantane diammonium iodide with an attomolar dissociation constant, a value unmatched even in natural encapsulation processes. Computational chemistry has been extensively employed to describe the enthalpic–entropic compensation principle of the molecular recognition process of cucurbituril hosts, but the synergistic contribution of experimental data is required for accurate results to be obtained. This paper proposes the first fully theoretical model able to reconcile the calculated thermodynamics of the complexation process with the experimental data obtained by calorimetry (ITC) for cucurbit7uril. The model allows the isolation and estimation of all of the enthalpic and entropic contributions coming from solute and solvent alike to the whole host–guest binding event and enables the straightforward calculation of the contribution of the solvation entropy to the binding.
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