S Sankararaman

The synthesis of a variety of 1,3-diarylpropynones that contain not only substituted phenyl groups but also fluorophoric 1-pyrenyl, 3-carbazolyl, and 1-naphthyl groups was achieved in good to excellent yields by using a carbonylative alkynylation reaction in the presence of cis-(Tz)Pd(Cl)2(PPh3) (Tz = 1,2,3-triazol-5-ylidene) as a precatalyst under 1.0 atm of CO. Products resulting from competing Sonogashira coupling reactions, namely 1,2-diarylacetylenes, were not observed. The synthesized ynones exhibit fluorescence not only in solution but also in the solid state. The catalytically active palladium species was recovered and reused for up to three cycles by adsorbing the cis-(Tz)Pd(Cl)2(PPh3) precatalyst onto silica gel and carrying out the reaction under heterogeneous conditions.

Synthesis of normal and mesoionic N-heterocyclic carbene Pd(ii) complexes in near quantitative yields from the corresponding NHC precursor salts under base free and ambient conditions is reported. A plausible mechanism involving charge-assisted hydrogen bonded intermediates is proposed. © 2014 the Partner Organisations.

Chiral 1,2,3-triazol-5-ylidene-Pd complexes with the planar chiral [2.2]paracyclophane wing tip group have been synthesized and structurally characterized. The complex with a labile acetonitrile co-ligand is an excellent catalyst for chemoselective hydrogenation of alkynes and alkenes and enantioselective hydrogenation of prochiral alkenes at ambient conditions and 1.0 atmosphere of H2.

(-) Menthol was converted to 1-menthyl-3-methyl-4-phenyltriazolium iodide and 1-neomethyl-3-methyl-4-phenyltriazolium iodide in four steps. These triazolium salts that are enantio pure but diastereo isomers, served as precursors for the synthesis of the corresponding enantio pure bis(1,2,3-triazol-5-ylidene)PdI2 complexes with menthyl and neomenthyl wing tip groups. The palladium complexes were synthesized in high yields using a recently developed mild method under base free and silver free conditions at ambient temperature. These triazolium salts also served as precursors for the synthesis of the corresponding enantio pure PEPPSI type (1,2,3-triazol-5-ylidene) (pyridine)PdI2 complexes. All the complexes were thoroughly characterized by spectroscopic and single crystal XRD data. The relative catalytic activities of these complexes were compared for Suzuki-Miyaura C-C coupling.

A new class of cationic Pd(II) complexes of the type [Pd(Tz)(Cl)(bipy)]+Cl− and [Pd (Tz)(Cl)(phen)]+Cl− (Tz = 1,4-diaryl-3-methyl-1,2,3-triazol-5-ylidene, bipy = 2,2′-bipyridine and phen = 1,10-phenanthroline) with various wing tip groups were synthesized from the corresponding 1,2,3-triazolium iodide via the corresponding chloro bridged dinuclear complexes [(Tz)(Cl)Pd(μ-Cl)2Pd(Cl)(Tz)]. The synthesized cationic complexes were screened for their catalytic activity of hydration of alkynes and found to be excellent towards the selective conversion of internal alkynes to the corresponding 1,2-diketones in good yields. A plausible mechanism was proposed for this conversion.

Synthesis of mononuclear and PEPPSI type palladium diiodo complexes of 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene without the use of strong bases and silver salts and at ambient conditions using Pd(OAc)2 is reported. Using stoichiometric amounts of bidendate ligands such as pyrazine, 4,4′-bipyridine and DABCO bridged binuclear palladium diiodo complexes were obtained in excellent yields. By simple variation of reagents and their stoichiometry, one can control the reactions towards the selective formation of mononuclear [(Tz)2Pd(I)2] complexes, iodo bridged binuclear complexes [(Tz)Pd(I)(μ-I)2Pd(I)(Tz)], mononuclear PEPPSI type complexes [(Tz)Pd(I)2(Py)], bridged binuclear PEPPSI type complexes [(Tz)Pd(I)2-(bridge biPy)-(I)2Pd(Tz)]. The catalytic activities of these three structurally different types of complexes are compared for Suzuki-Miyaura coupling reaction.

A modular method for the synthesis of 3-aryl-4-(arylethynyl)-2H-chromen-2-ones from 2-iodoaryl 2-arylacetates and arylacetylenes has been developed. The carbonylative Sonogashira coupling–intramolecular aldol casade reaction was carried out in the presence of Pd(PPh3)2(Cl)2as the catalyst. The one-pot approach that involves the in situ formation of the 2-iodoaryl 2-arylacetates from the corresponding 2-iodophenols and 2-arylacetyl chlorides followed by the palladium-catalyzed carbonylative annulation in the presence of the arylacetylene has also been described for the formation of the 3-aryl-4-(arylethynyl)-2H-chromen-2-ones.

An abnormal or mesoionic N-heterocyclic carbene (NHC), namely 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene (Tz), formed from 1,4-diphenyl-3-methyl-1,2,3-triazolium iodide and silver oxide, gave the corresponding silver carbene complex. The transmetalation reaction of the silver carbene complex with PdCl2(CH3CN)2 gave a trans mononuclear complex, [(Tz)2PdCl2]. Reaction of the silver carbene complex with Pd(OAc)2 gave an acetate-bridged binuclear complex in which the palladium had undergone a C-H insertion at the ortho position of the phenyl group attached to the triazole nitrogen (1-position). The structures of these two complexes were established by single-crystal XRD data and spectroscopic data. The acetate complex was used as a catalyst for the stereoselective hydroarylation of alkynes. © 2011 American Chemical Society.

trans-Bis(1,4-dimesityl-3-methyl-1,2,3-triazol-5-ylidene)palladium(II) dichloride has been shown to be an excellent catalyst for the multiple Suzuki-Miyaura coupling reactions of polybromoarenes to the corresponding fully substituted polyarylarenes. The reactions proceeded in excellent yields and with high turnover numbers. With 1,4-dibromobenzene the catalyst was found to be active for up to 13 consecutive cycles with a turnover number of 1260. The polyarylarenes were obtained in pure form after crystallization once without recourse to chromatographic purification. The single-crystal X-ray structures of the chloro (1) as well as the corresponding acetato (2) complexes are also reported and compared with the corresponding complexes of 1,4-diphenyl-3-methyl- 1,2,3-triazol-5-ylidene as the ligand. © 2013 Shaik et al.

A palladium complex (3) of abnormal N-heterocyclic carbene (NHC) namely, 4-(hydroxymethyl)-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene was prepared via silver carbene transmetalation method and characterized by spectroscopic and single crystal XRD data. Single crystal analysis revealed the structure of the complex 3 to be a cis isomer. The synthesis of palladium complex 6 via silver carbene transmetalation method led to a mixture of cis-trans isomers. In this case trans isomer was formed as major product. We have reported earlier the synthesis and structural characterization of trans isomer (trans-6). However, the presence of cis isomer was not recognized until the crude reaction mixture was recrystallized from acetonitrile. Pure cis isomer (cis-6) was obtained by washing the cis-trans mixture with acetonitrile followed by slow evaporation of acetonitrile solution at 35 C. Structure of cis (cis-6) isomer was unambiguously established by single crystal XRD data.© 2013 Elsevier B.V. All rights reserved.