María Teresa Herrero

Palladium NCN and CNC pincer complexes as exceptionally active catalysts for aerobic oxidation in sustainable media

The oxidation of secondary benzyl alcohols is catalyzed by two palladacycles at atmospheric pressure in PEG-400, a sustainable reaction media. Recycling of the active catalytic species is performed up to the 5th run, and catalyst loadings decreased down to 10−8 mol%, thus achieving unprecedented TON and TOF values. In addition, the same conditions proved to be effective for the aerobic oxidation of benzyl methylene compounds, a scarcely explored process by palladium catalysts.

Toward Safer Processes for C—C Biaryl Bond Construction: Catalytic Direct C-H Arylation and Tin-Free Radical Coupling in the Synthesis of Pyrazolophenanthridines

A series of pyrazolo[1,5-f]phenanthridine derivatives has been efficiently synthesized by a short, straightforward sequence. A tandem amine-exchange/heterocyclization of enaminones was successfully applied to the regioselective preparation of 1,5-diarylpyrazole intermediates with structure resemblance to relevant nonsteroidal anti-inflammatory drugs such as celecoxib or tepoxalin. The final key step, cyclization by intramolecular biaryl bond formation, was accomplished by two alternative methodologies: radical coupling and catalytic direct arylation via C-H activation. The scope and limitations of the two methodologies have been explored and their complementariness has been established. In addition, polymer-supported heterogeneous catalysts have been compared with homogeneous analogues. In the radical process, toxic tin derivatives have been avoided in order to employ environmentally safer protocols.

Synthesis, Structure, and Catalytic Applications for ortho- and meta-Carboranyl Based NBN Pincer-Pd Complexes

o- and m-Carborane-based NBN pincer palladium complexes (oCB-L1)Pd, (oCB-L2)Pd, and (mCB-L1)Pd are synthesized in two steps from commercially available starting materials. The pincer complexes were prepared by the reaction of bis-[R(hydroxy)methyl]-1,2-dicarba-closo-dodecaborane (R = 2-pyridyl oCB-L1, 6-methyl-2-pyridyl oCB-L2) or bis-[2-pyridyl (hydroxy)methyl]-1,2-dicarba-meta-dodecaborane (mCB-L1) with [PdCl2(MeCN)2] under mild conditions. The X-ray structure determination of all carboranyl pincer complexes shows unambiguously B-H activation of the carborane cages. The results agree with the Pd-B bonds in all complexes exhibiting strong σ-electron donation. Theoretical calculations reveal the importance of considering the solid state intermolecular hydrogen bonding when investigating the trans influence in organometallic chemistry. A localized orbitals approach has also been applied to analyze the metal oxidation state in the carboranyl pincer complexes. Catalytic applications of (oCB-L1)Pd and (mCB-L1)Pd have shown the complexes are good catalyst precursors in Suzuki coupling in water and with very low amounts of catalyst loadings.

STEREOSELECTIVE SYNTHESIS OF POLYENIC ALARM PHEROMONES OF CEPHALASPIDEAN MOLLUSCS

Abstract The stereospecific thallium-accelerated palladium-catalyzed cross-coupling of 1-alkenyl boronic acids and 1-alkenyl iodides (the Suzuki reaction) is the key step in an efficient approach to several polyenic pheromones isolated from cephalaspidean opisthobranch molluscs.

Enaminoketone derivatives as key intermediates for the synthesis of 4-quinolones

N-substituted 4-quinolones can be obtained in good yields via intramolecular amine exchage reaction from N-substituted (E)-1-(2-aminophenyl)-3-(dimethylamino)prop-2-en-1-ones or, alternatively, from (E)-1-(2-aminophenyl)-3-(dimethylamino)prop-2-en-1-one through intramolecular amine exchange/N-benzylation. Both strategies can lead to N-(2-halobenzyl)-4-quinolones, functionalized substrates which could undergo further transformations to provide interesting polyheterocycles.

A more efficient entry to phenanthridinone

The presence of the phenanthridinone core in natural products and biologically active compounds has encouraged research on more efficient approaches to such valuable tricyclic framework.[1] Due to the lack of transmetallating agents and the atom-economy implied, palladium-catalyzed direct arylation is an appealing alternative for the ring closure step. However, the relative high amount of the catalyst employed may become a serious drawback from a practical view.[2] In this context, we wish to present the application of a palladacyclic complex to this reaction and the significant reduction of the catalytic amount achieved (0.05 mol%) in the successful preparation of a series of phenanthridinone derivatives. [1] Bhakuni, B. S.; Kumar, A.; Balkrishna, S. J.; Sheikh, J. A.; Konar, S.; Kumar, S. Org. Lett. 2012, 14, 2838-2841. [2] Rousseaux, S.; Gorelsky, S. I.; Chung, B. K. W.; Fagnou, K. J. Am. Chem. Soc. 2010, 132, 10692-10705.

New catalytic systems for oxygen-mediated oxidative processes

The oxidation of alcohols to the corresponding carbonyl compounds is a current transformation in laboratory and industrial chemistry. Traditionally this reaction involves oxidants used in stoichiometric or overstoichiometric amounts so that relatively large quantities of waste are generated. Molecular oxygen is the ideal oxidant (readily available, safe, environmentally friendly, water as waste, etc.), but the need of metal catalysts to control the reaction outcome in relatively high amounts can become a serious drawback.[1],[2] We wish to present two pallacyclic systems with remarkable catalytic properties in the aerobic oxidation of a number of alcohols. [1] Sheldon, R.A.; Arends, I.W.C.E.; ten Brink, G.J.; Dijksman, A. Acc. Chem. Res. 2002, 35, 774-778. [2] Verho, O., Dilenstam, M.D.V.; Karkas, M.D., Johnston, E.V.; Akermark, T.; Backvall, J.E.; Akermark, B. Chem. Eur. J. 2012, 18, 16947-16954.