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1.
Top Curr Chem (Cham) ; 378(1): 1, 2019 Nov 25.
Artigo em Inglês | MEDLINE | ID: mdl-31761979

RESUMO

Organocatalysis-the branch of catalysis featuring small organic molecules as the catalysts-has, in the last decade, become of central importance in the field of asymmetric catalysis, so much that it is now comparable to metal catalysis and biocatalysis. Organocatalysis is rationalized and classified by a number of so-called activation modes, based on the formation of a covalent or not-covalent intermediate between the organocatalyst and the organic substrate. Among all the organocatalytic activation modes, enamine and iminium catalysis are widely used for the practical preparation of valuable products and intermediates, both in academic and industrial contexts. In both cases, chiral amines are employed as catalysts. Enamine activation mode is generally employed in the reaction with electrophiles, while nucleophiles require the iminium activation mode. Commonly, in both modes, the reaction occurs through well-organized transitions states. A large variety of partners can react with enamines and iminium ions, due to their sufficient nucleophilicity and electrophilicity, respectively. However, despite the success, organocatalysis still suffers from narrow scopes and applications. Multicatalysis is a possible solution for these drawbacks because the two different catalysts can synergistically activate the substrates, with a simultaneous activation of the two different reaction partners. In particular, in this review we will summarize the reported processes featuring Lewis acid catalysis and organocatalytic activation modes synergically acting and not interfering with each other. We will focus our attention on the description of processes in which good results cannot be achieved independently by organocatalysis or Lewis acid catalysis. In these examples of cooperative dual catalysis, a number of new organic transformations have been developed. The review will focus on the possible strategies, the choice of the Lewis acid and the catalytic cycles involved in the effective reported combination. Additionally, some important key points regarding the rationale for the effective combinations will be also included. π-Activation of organic substrates by Lewis acids, via formation of electrophilic intermediates, and their reaction with enamines will be also discussed in this review.


Assuntos
Aminoácidos/química , Ácidos de Lewis/química , Catálise , Estrutura Molecular , Estereoisomerismo
2.
Molecules ; 24(9)2019 May 02.
Artigo em Inglês | MEDLINE | ID: mdl-31052604

RESUMO

Salen ligands are a class of Schiff bases simply obtained through condensation of two molecules of a hydroxyl-substituted aryl aldehyde with an achiral or chiral diamine. The prototype salen, or N,N'-bis(salicylidene)ethylenediamine has a long history, as it was first reported in 1889, and immediately, some of its metal complexes were also described. Now, the salen ligands are a class of N,N,O,O tetradentate Schiff bases capable of coordinating many metal ions. The geometry and the stereogenic group inserted in the diamine backbone or aryl aldehyde backbone have been utilized in the past to efficiently transmit chiral information in a variety of different reactions. In this review we will summarize the important and recent achievements obtained in stereocontrolled reactions in which Al(salen) metal complexes are employed. Several other reviews devoted to the general applications and synthesis of chromium and other metal salens have already been published.


Assuntos
Etilenodiaminas/química , Compostos Organometálicos/química , Catálise , Técnicas de Química Sintética , Reação de Cicloadição , Etilenodiaminas/síntese química , Estrutura Molecular , Compostos Organometálicos/síntese química , Polimerização , Estereoisomerismo , Uridina Difosfato N-Acetilglicosamina/análogos & derivados
3.
Chem Commun (Camb) ; 55(48): 6838-6841, 2019 Jun 11.
Artigo em Inglês | MEDLINE | ID: mdl-31093623

RESUMO

Here we report the application of dual nickel/photoredox catalysis to the allylation of aliphatic, aromatic and heteroaromatic aldehydes by using commercially available reagents. The process utilizes the combination of a Ni(ii) complex, [Ru(bpy)3]2+ as a photoredox catalyst, and allylacetate under blue LED irradiation, and allows the synthesis of a large variety of homoallylic alcohols.

4.
Angew Chem Int Ed Engl ; 57(42): 13853-13857, 2018 10 15.
Artigo em Inglês | MEDLINE | ID: mdl-30129685

RESUMO

A gas-phase nitrogen-nitrogen noncovalent interaction has been unveiled in the nitroethane-trimethylamine complex in an environment free from solvent and matrix effects using rotational spectroscopy in supersonic expansion. Different quantum chemical models (NOCV/CD and NBO) agree in indicating that this interaction largely prevails over the C-H⋅⋅⋅O and C-H⋅⋅⋅N hydrogen bonds. Furthermore, a SAPT analysis shows that electrostatic and dispersion interactions play a comparable role in stabilizing the complex. The conformational landscape exploration and stationary points characterization have been performed using state-of-the-art quantum-chemical computations providing significant insights on structure determination.

5.
Chem Commun (Camb) ; 54(72): 10044-10047, 2018 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-30039815

RESUMO

Here we report the application of readily prepared and available coumarin dyes for photoredox catalysis, which are able to mimic powerful reductant [Ir(iii)] complexes. Coumarin derivatives 9 and 10 were employed as photoreductants in pinacol coupling and in other reactions, in the presence of Et3N as a sacrificial reducing agent. As the electronic, photophysical, and steric properties of coumarins could be varied, a wide applicability to several classes of photoredox reactions is predicted.

6.
Langmuir ; 34(26): 7698-7707, 2018 07 03.
Artigo em Inglês | MEDLINE | ID: mdl-29889539

RESUMO

We combine ambient (air) and ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) investigations together with density functional theory (DFT) calculations to gain a subnanometer insight into the structure and dynamic of two-dimensional (2D) surface-supported molecular networks. The planar tetraferrocene-porphyrin molecules employed in this study undergo spontaneous self-assembly via the formation of hydrogen bonded networks at the gold substrate-solution interface. To mimic liquid phase ambient deposition conditions, film formation was accomplished in UHV by electro-spraying a solution of the molecule in chloroform onto an Au(111) substrate, thereby providing access to the full spectroscopic capabilities of STM that can be hardly attained under ambient conditions. We show that molecular assembly on Au (111) is identical in films prepared under the two different conditions, and in good agreement with the theoretical predictions. However, we observe the contrast found for a given STM bias condition to be different in ambient and UHV conditions despite the similarity of the structures, and we propose possible origins of the different imaging contrast. This approach could be valuable for the thorough characterization of surface systems that involve large molecules and are prepared mainly in ambient conditions.

7.
Chemistry ; 24(46): 11954-11960, 2018 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-29603481

RESUMO

Salophens and Salens are Schiff bases generated through the condensation of two equivalents of salicylaldehyde with either 1,2-phenylenediamines or aliphatic diamines, respectively. Both ligands have been extensively exploited as key building blocks in coordination chemistry and catalysis. In particular, their metal complexes have been widely used for various catalytical transformations with high yield and selectivity. Through the modification of the phenol unit it is possible to tune the steric hindrance and electronic properties of Salophen and Salen. The introduction of long aliphatic chains in salicylaldehydes can be used to promote their self-assembly into ordered supramolecular structures on solid surfaces. Herein, we report a novel method towards the facile synthesis of robust and air-stable [Al(Salophen)] derivatives capable of undergoing spontaneous self-assembly at the graphite/solution interface forming highly-ordered nanopatterns. The new synthetic approach relies on the use of [MeAlIII (Salophen)] as a building unit to introduce, via a simple acid/base reaction with functionalized acidic phenol derivatives, selected frameworks integrating multiple functions for efficient surface decoration. STM imaging at the solid/liquid interface made it possible to monitor the formation of ordered supramolecular structures. In addition, the redox properties of the Salophen derivatives functionalized with ferrocene units in solution and on surface were unraveled by cyclic voltammetry. The use of a five-coordinate aluminum alkyl Salophen precursor enables the tailoring of new Salophen molecules capable of undergoing controlled self-assembly on HOPG, and thereby it can be exploited to introduce multiple functionalities with subnanometer precision at surfaces, ultimately forming ordered functional patterns.

8.
Phys Chem Chem Phys ; 20(12): 8071-8076, 2018 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-29516066

RESUMO

A mechanism based on the sequential absorption of two photons by the components of a redox couple has been recently proposed for catalysis of the energetically demanding reduction of aryl halides. Here, we analyze the suggested photochemical mechanism of this reaction, which employs perylenediimide (PDI) as a photocatalyst, on the basis of spectroscopic, electrochemical and electron paramagnetic resonance data. Our results indicate that the photoexcited PDI radical anion (*PDI˙-) cannot play the role of a photosensitizer in the aforementioned process. Instead, the reduction of 4'-bromoacetophenone likely involves *PDI˙- decomposition products. The extremely short lifetime of the photoexcited transient species, as *PDI˙-, is a major general limitation for photocatalytic schemes based on sequential two-photon excitation. In order to better understand the potential of such schemes, we discuss them in the context of the Z-scheme in natural photosynthesis.

9.
Angew Chem Int Ed Engl ; 56(42): 12820-12821, 2017 10 09.
Artigo em Inglês | MEDLINE | ID: mdl-28857385

RESUMO

The photocatalytic mechanism reported in a recent Communication to produce the radical anion of pyrenes postulates a highly endergonic electron transfer process. An analysis of the thermodynamics is reported together with the proposal of an alternative thermodynamically feasible mechanism.

10.
Chem Sci ; 8(2): 1613-1620, 2017 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-28451291

RESUMO

A readily accessible iridium(iii) phenyl-tetrazole complex ([Ir(ptrz)2(tBu-bpy)]+, 2; Hptrz = 2-methyl-5-phenyl-tetrazole; tBu-bpy = 4,4'-di-tert-butyl-2,2'-bipyridine) is shown to be a versatile catalyst for a new photocatalytic Michael reaction. Under light irradiation in the presence of 2, a dithiane 2-carboxylic acid, obtained by simple hydrolysis of a commercially available ethyl ester, generates a 1,3-dithiane radical capable of performing addition to a variety of Michael acceptors (e.g., unsaturated ketones, esters, amides and malonates). This broad scope reaction with high yields is a formal photo-redox addition of the elusive methyl radical and the adducts obtained can be starting materials for a variety of functionalized products. The excited-state oxidation potential of catalyst 2 allows selective formation of radicals only from α-heterosubstituted carboxylates. Chemical modification of this metal complex can tune the electrochemical properties, opening a route to new highly selective catalytic photo-oxidation reactions.

11.
Chimia (Aarau) ; 71(9): 562-567, 2017 Sep 27.
Artigo em Inglês | MEDLINE | ID: mdl-30188285

RESUMO

Chiral Schiff bases are privileged chiral ligands, capable of transmitting chiral information in many stereoselective processes. Their modular nature, the possibility to modify their electronic character, and their ability to coordinate many metals, gives these chiral ligands broad utility and these have been investigated by many research groups around the world. For more than 20 years our research group has devoted attention to the application of chiral Schiff bases in the development of new stereoselective processes and in this brief survey we present our principal results and achievements in this field.

12.
Chemistry ; 21(52): 18949-52, 2015 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-26549317

RESUMO

The first catalytic asymmetric addition of TosMIC to unactivated ketones is presented. A combination of Me2Zn and aminoalcohol catalyst promoted the aldol addition/cyclization reaction to render oxazolines possessing a fully substituted stereocenter with excellent yields (up to 92%), high enantioselectivities (up to 96%), and complete diastereoselectivity. The chiral oxazolines were then used to give, after a straightforward acid hydrolysis, enantioenriched building blocks bearing tertiary alcohol motifs such as hydroxylaldehydes, hydroxylacids, and hydroxylesters without racemization.

13.
Chemistry ; 21(39): 13689-95, 2015 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-26239866

RESUMO

The combination of a practical and highly enantioselective organocatalytic reaction, which allows the stereoselective introduction of a benzodithiol group, with a fluorination step, gives a new and effective strategy for the stereoselective synthesis of difluorinated building blocks. The benzodithiol group is a versatile and chameleonic group that can be further functionalized before fluorination, giving customized and tailored useful synthetic strategies. As an example of the application of this facile strategy, the effective enantioselective synthesis of difluoroarundic acid is described.

14.
J Org Chem ; 80(9): 4791-6, 2015 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-25867788

RESUMO

A representative number of bench-stable nonsymmetric diarylcarbenium tetrafluoroborates have been isolated via the direct coupling of aryl (or heteroaryl) aldehydes and N-heteroarenes and fully characterized. They have proven to be highly stable in the presence of both EDG and EWG substituents. An (E)-iminium vinylogous substructure has been shown as the common cation scaffold by X-ray analysis and by NOE determination.

15.
ChemSusChem ; 7(12): 3534-40, 2014 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-25336345

RESUMO

The use of immobilized metal-free catalysts offers the unique possibility to develop sustainable processes in flow mode. The challenging intermolecular organocatalyzed enantioselective alkylation of aldehydes was performed for the first time under continuous flow conditions. By using a packed-bed reactor filled with readily available supported enantiopure imidazolidinone, different aldehydes were treated with three distinct cationic electrophiles. In the organocatalyzed α-alkylation of aldehydes with 1,3-benzodithiolylium tetrafluoroborate, excellent enantioselectivities, in some cases even better than those obtained in the flask process (up to 95% ee at 25 °C), and high productivity (more than 3800 h(-1) ) were obtained, which thus shows that a catalytic reactor may continuously produce enantiomerically enriched compounds. Treatment of the alkylated products with Raney-nickel furnished enantiomerically enriched α-methyl derivatives, key intermediates for active pharmaceutical ingredients and natural products.


Assuntos
Aldeídos/química , Alquilação , Catálise , Estereoisomerismo
16.
Chirality ; 26(10): 607-13, 2014 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-24639288

RESUMO

Recently, the direct substitution of allylic, benzylic, and tertiary alcohols has been achieved via SN 1-type reactions with catalytic amounts of Brønsted or Lewis acids. When a new stereogenic center is formed most of these transformations produce the desired product as a racemate, as these reactions proceed through carbenium ions. The arsenal of activation modes available in organocatalysis can be used to set up suitable reaction conditions in which chiral nucleophiles (enamine catalysis) or chiral electrophiles (iminium catalysis, chiral counterion catalysis) can easily be generated. Recently, we have used stabilized carbenium ions, directly available or obtained from the corresponding alcohols, in new organocatalytic stereoselective SN 1-type reactions. The commercially available carbenium ion benzodithiolylium tetrafluoroborate can be used for the straightforward organocatalytic stereoselective alkylation of aldehydes. In this account we will illustrate the application of this methodology in the total synthesis of natural products and the preparation of valuable starting materials.

17.
Chem Asian J ; 9(4): 984-95, 2014 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-24677815

RESUMO

In recent years there has been an accelerated rate of development in the field of organocatalysis, with asymmetric organocatalysis now reaching full maturity. The invention of new organocatalytic reactions and the exploration of new concepts now appear in tandem with the application of organocatalytic techniques in the synthesis of natural products and active pharmaceutical ingredients (APIs). After a "golden rush" in organocatalysis, researchers are now starting to combine different methods, thereby taking advantage of the significant benefits of synergy. Metals are used in combination with organocatalytic processes, thus reaching complexity that is found in nature, where enzymes take advantage of the presence of certain metals to increase the arsenal of organic transformations available. In this Focus review, we illustrate the possibility of a "happy marriage" between Lewis acids and organocatalytic stereoselective processes. Questions have been raised about the combination of Lewis acids and organocatalysis owing to the presence of water and/or strong bases in these processes. Some Lewis acids have been shown to be compatible with organocatalysis and concepts relating to their use will be illustrated herein. To summarize the fruitful use of Lewis acids in stereoselective organocatalytic processes, we will draw attention to the advantages and selectivity achieved using this method.


Assuntos
Ácidos de Lewis/química , Aminas/química , Catálise , Complexos de Coordenação/química , Reação de Cicloadição , Ouro/química , Cetonas/química , Estereoisomerismo
18.
Chembiochem ; 14(10): 1185-7, 2013 Jul 08.
Artigo em Inglês | MEDLINE | ID: mdl-23733510

RESUMO

A ringing belt: An artificial molecular machine was invented to build up a programmable peptidic sequence. By molecular movements, the rotaxane-based machine progressively introduces amino acids to form a short peptide chain. The journey towards an artificial ribosome is still quite long, but a conceivable future can be envisaged.


Assuntos
Peptídeos/química , Rotaxanos/química , Sequência de Aminoácidos , Estrutura Molecular , Peptídeos/genética , Peptídeos/metabolismo , Ribossomos/química , Ribossomos/genética , Ribossomos/metabolismo , Rotaxanos/metabolismo
19.
Chemistry ; 19(24): 7696-700, 2013 Jun 10.
Artigo em Inglês | MEDLINE | ID: mdl-23649888

RESUMO

Ironing out the details: Proline and pyrrolidine derivatives (Hayashi- Jørgensen catalysts) are considered "work horses" in organocatalysis. This report describes a new effective ferrocenyl pyrrolidine catalyst that is able to perform well in benchmark organocatalytic reactions (see figure). The ferrocene moiety controls the conformational space and a simple alkyl group effectively covers a face of the derived enamine. This new framework can find applications in organocatalysis, and in general, in new ligand design.


Assuntos
Aminas/química , Compostos Ferrosos/síntese química , Prolina/análogos & derivados , Pirrolidinas/síntese química , Catálise , Compostos Ferrosos/química , Estrutura Molecular , Prolina/síntese química , Prolina/química , Pirrolidinas/química , Estereoisomerismo
20.
Chem Commun (Camb) ; 48(30): 3614-6, 2012 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-22396955

RESUMO

We herein present a general enamine-mediated α-alkylation of α-substituted aldehydes with carbenium ions for the stereoselective construction of quaternary stereogenic centers.

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