Pablo Espinet

Transition metal liquid crystals: Advanced materials within the reach of the coordination chemist

This review serve to open the field to many coordination chemistry who can use specific expertise of new transition metal liquid crystals. Bibliography

Phosphine-pyridyl and related ligands in synthesis and catalysis

Abstract This review summarizes the advances produced in the last 5 years in the use of mixed ligands containing at least one phosphino and one pyridyl or related group (bipyridyl, quinolyl, etc.) bonded to the same or different metals. In this period the publications have doubled since the last review on this topic. The present paper covers the structural applications of these ligands, and also the catalytic applications of the complexes produced with them.

Unlikeliness of Pd-free gold(I)-catalyzed Sonogashira coupling reactions.

The Sonogashira coupling reaction is not catalyzed by AuI/dppe in the absence of Pd complexes. However, addition of 0.1 mol % of Pd(0) led to efficient cross-coupling reactions. The most plausible catalytic cycles for the Au-catalyzed cross-coupling reactions have been examined and are unlikely in the absence of Pd contamination.

Die Mechanismen der Stille‐Reaktion

Vor achtzehn Jahren hat John K. Stille in der Angewandten Chemie uber eine neuartige Kupplung von Organostannanen mit organischen Elektrophilen berichtet, die schlieslich unter seinem Namen bekannt wurde. Seither hat sich diese Methode zu einem vielseitigen und facettenreichen Gebiet voller versteckter Moglichkeiten zum Forschen, Entdecken und Geniesen entwickelt. Die neuesten Modifizierungen lassen Syntheseziele in greifbare Nahe rucken, von denen man vor einigen Jahren nur zu traumen wagte. Auch beim Verstandnis der mechanistischen Details der Umsetzungen wurden grose Fortschritte erzielt. Daher sollte einer gezielten Anwendung dieser wichtigen Reaktion und ihrer neuen Varianten, die sich auf mehr als nur empirische Befunde stutzt, nichts mehr im Wege stehen. Dieser Aufsatz legt uber diese Fortschritte kritisch Rechenschaft ab.

Bimetallic Catalysis using Transition and Group 11 Metals: An Emerging Tool for CC Coupling and Other Reactions

Bimetallic catalysis refers to homogeneous processes in which either two transition metals (TM), or one TM and one Group 11 (G11) element (occasionally Hg also), cooperate in a synthetic process (often a C-C coupling) and their actions are connected by a transmetalation step. This is an emerging research area that differs from the isolated or tandem applications of the now classic processes (Stille, Negishi, Suzuki, Hiyama, Heck). Most of the reactions used so far combine Pd with a second metal, often Cu or Au, but syntheses involving very different TM couples (e.g., Cr/Ni in the catalyzed vinylation of aldehydes) have also been developed. Further development of the topic will soon demand a good knowledge of the mechanisms involved in bimetallic catalysis, but this knowledge is very limited for catalytic processes. However, there is much information available, dispersed in the literature, coming from basic research on exchange reactions occurring out of any catalytic cycle, in polynuclear complexes. These are essentially the same processes expected to operate in the heart of the catalytic process. This Review gathers together these two usually isolated topics in order to stimulate synergy between the bimetallic research coming from more basic organometallic studies and the more synthetic organic approaches to this chemistry.

Gold(I) Complexes with Hydrogen-Bond Supported Heterocyclic Carbenes as Active Catalysts in Reactions of 1,6-Enynes

Complexes [AuCl{C(NHR)(NHPy-2)}] (Py-2 ) 2-pyridyl; R ) Me, tBu, nBu, iPr, nheptyl) have been prepared in amodular way from [AuCl(CNPy-2)]. The carbene moiety has a hydrogen-bond supported heterocyclic structure similar to the nitrogen heterocyclic carbenes in the solid state, and in CH2Cl2 or acetone solution, which is open in the presence of MeOH. The compounds are good catalysts for the skeletal rearrangement of enynes, and for the methoxycyclization of enynes. In contrast, the complexes [AuCl{C(NHR)(NHPy-4)}] are scarcely active due to the blocking effect of the coordination position required for the catalysis by the nitrogen of the NHPy-4 group.

The Pd-Catalyzed Coupling of Allyl Halides and Tin Aryls: Why the Catalytic Reaction Works and the Stoichiometric Reaction Does Not

Arylallylpalladium complexes [Pd(5-C6F5-eta3-cyclohexenyl)(C6Cl2F3)(NCMe)] (10) and [Pd2(mu-C6Cl2F3)2(5-C6F5-1,3-eta3-cyclohexenyl)2] (13) have been synthesized. Complex 13 is an example of a rare class of metal complexes with aryl bridges and its X-ray crystal diffraction structure has been determined. These arylallylpalladium complexes are involved in the coupling of Bu3SnRf (1, Rf = dichlorotrifluorophenyl) and [Pd2(mu-Br)2(5-C6F5-1,3-eta3-cyclohexenyl)2] (2); complex 10 has been detected in the course of the stoichiometric coupling reaction in acetonitrile. Decomposition experiments of 10 and 13 in different conditions, and comparison with the reactions of 1 and 2, allow us to determine that reductive elimination does not occur in the absence of additives. p-Benzoquinone coordinates to Pd to give complex 15 and promotes reductive elimination to give the coupling products selectively. The outcome of the coupling reaction is controlled by the reductive elimination step, but the overall rate is controlled by the faster preequilibrium, which determines the concentration of 10 or 13. Palladium-catalyzed coupling of allyl halides and tin aryls works better than the stoichiometric allyl-aryl reductive coupling on isolated allylarylpalladium complexes, because they benefit from the presence in the solution of substrate allylic halides acting as electron-withdrawing olefins and promoting reductive elimination. More efficient allyl-aryl couplings, whether stoichiometric or catalytic, can be achieved upon addition of p-benzoquinone to the reaction mixture in a noncoordinating solvent.

Monomeric and dimeric ortho-metallated palladium(II)-imine systems as liquid crystals

Abstract Several types of ortho -palladated complexes derived from the imines HL n = p -C n H 2 n +1 O-C 6 H 4 -CH=N-C 6 H 4 -OC n H 2 n +1 - p ( n =2, 4, 6, 8 and 10) have been prepared and their mesogenic properties studied. The types include the dinuclear complexes Pd( μ -X)L n ] 2 with X=Cl, OAc, ( R )-2-propionato or ( S )-2-propionato, and the mononuclear complexes [Pd(acac)L n ] and [Pd(NHacac)L n ] (NHacac=4-amino-3-penten-2-onate). Most of the complexes except the acetato derivatives and some others with very short alkoxy chains) show liquid-crystal behavior. Nematic, smectic A and smectic C phases are found. Most of the derivatives with chiral carboxylates display double melting behavior.

Palladium round trip in the Negishi coupling of trans-[PdMeCl(PMePh2)2] with ZnMeCl: an experimental and DFT study of the transmetalation step.

Compared with the detailed mechanistic knowledge of the Stille reaction, little is known about the Negishi reaction. Recently, we experimentally uncovered the complicated behavior of the transmetalation of transACHTUNGTRENNUNG[PdRfClACHTUNGTRENNUNG(PPh3)2] (Rf= 3,5-dichloro-2,4,6-trifluorophenyl) with ZnMe2 or ZnMeCl, showing that each methylating reagent afforded stereoselectively a different isomer (trans or cis, respectively) of the coupling intermediate [PdRfMe ACHTUNGTRENNUNG(PPh3)2].[4] Moreover, the study revealed the occurrence of undesired transmetalations, such as those shown in Scheme 1, which could eventually produce homocoupling products; the corresponding undesired intermediates were detected and identified by NMR spectroscopy techniques. The formation of undesired intermediates in related reactions with aryl zinc derivatives was later observed by Lei et al. Herein, we report an experimental mechanistic study of the reaction of trans-[PdClMe ACHTUNGTRENNUNG(PMePh2)2] (1) with ZnMeCl, which affords the first experimental determination of thermodynamic parameters of a Negishi transmetalation. This is complemented with a theoretical DFT study, which provides a detailed view of the reaction pathway, consistent with the experimental parameters. The reactions of 1 with ZnMeCl were carried out (with one exception) in 1:20 ratio, simulating catalytic conditions with 5 % Pd, in THF at different temperatures. At room temperature, the only product observed was cis[PdMe2ACHTUNGTRENNUNG(PMePh2)2] (2), in equilibrium with the starting material 1. In these conditions, complex 2 undergoes slow decomposition (reductive elimination) to give ethane. When the reaction was monitored by P NMR spectroscopy at 223 K (Figure 1 a), the coupling rate to give ethane became negligible and the formation of trans-[PdMe2ACHTUNGTRENNUNG(PMePh2)2] (3), as well as cis-[PdMe2ACHTUNGTRENNUNG(PMePh2)2] (2), was observed. The trans isomer 3 seemed to be formed first and then disappeared. The same reaction, carried out at 203 K in 1:1 ratio to get a slower rate of transformation, confirmed that 3 is formed noticeably faster than 2 (Figure 1 b). Thus, the observation of the cis isomer at room temperature is deceptive for the stereoselectivity of the transmetalation. Snapshots of two moments of the transmetalation reaction at 203 K, as seen by P NMR spectroscopy, are shown in Figure 1 c. The behavior of 3 is typical of a kinetic product of noticeably lower stability than the thermodynamic product (2): eventually it disappears from observation as the reaction proceeds and gets closer to the equilibrium concentrations, where the concentration of 3 is very small. In effect, during the progress of the reaction at 223 K (Figure 1 a), the concentration of 2 increases continuously; in contrast, a small accumulation of 3 is produced initially and then its concentration decreases, so that in 300 min 3 has practically disappeared. After about 10 h at 223 K, the system has reached equilibrium between the starting complex 1 and the final thermodynamic product 2 ([1]=5.8 10 3 mol l , [2]=4.4 10 3 mol l , and Keq =2.0 10 ); the concentration of 3 is below the limit of NMR observation. [a] B. Fuentes, Dr. J. A. Casares, Prof. P. Espinet IU CINQUIMA/Qu mica Inorg nica Facultad de CienciasUniversidad de Valladolid 47071 Valladolid (Spain) Fax: (+34) 983423231/ ACHTUNGTRENNUNG(+34) 983186336 E-mail : casares@qi.uva.es espinet@qi.uva.es [b] M. Garc a-Melchor, Prof. A. Lled s, Prof. F. Maseras, Dr. G. Ujaque Qu mica F sica, Edifici C.n, Universitat Aut noma de Barcelona 08193 Bellaterra, Catalonia (Spain) Fax: (+34) 935812920 E-mail : gregori@klingon.uab.es [c] Prof. F. Maseras Institute of Chemical Research of Catalonia (ICIQ) Av. Pa sos Catalans, 16, 43007 Tarragona, Catalonia (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001332. Scheme 1.

Cationic Intermediates in the Pd-Catalyzed Negishi Coupling. Kinetic and Density Functional Theory Study of Alternative Transmetalation Pathways in the Me–Me Coupling of ZnMe2 and trans-[PdMeCl(PMePh2)2]

The complexity of the transmetalation step in a Pd-catalyzed Negishi reaction has been investigated by combining experiment and theoretical calculations. The reaction between trans-[PdMeCl(PMePh(2))(2)] and ZnMe(2) in THF as solvent was analyzed. The results reveal some unexpected and relevant mechanistic aspects not observed for ZnMeCl as nucleophile. The operative reaction mechanism is not the same when the reaction is carried out in the presence or in the absence of an excess of phosphine in the medium. In the absence of added phosphine an ionic intermediate with THF as ligand ([PdMe(PMePh(2))(2)(THF)](+)) opens ionic transmetalation pathways. In contrast, an excess of phosphine retards the reaction because of the formation of a very stable cationic complex with three phosphines ([PdMe(PMePh(2))(3)](+)) that sequesters the catalyst. These ionic intermediates had never been observed or proposed in palladium Negishi systems and warn on the possible detrimental effect of an excess of good ligand (as PMePh(2)) for the process. In contrast, the ionic pathways via cationic complexes with one solvent (or a weak ligand) can be noticeably faster and provide a more rapid reaction than the concerted pathways via neutral intermediates. Theoretical calculations on the real molecules reproduce well the experimental rate trends observed for the different mechanistic pathways.