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.

At wo-step( nucleophilic substitution/palladation by oxidative addition) sequence provides ah igh-yielding access to an on-symmetrical palladium NNC pincerc omplex. An umber of terminal and internal alkynoic acids with different substitution patternsa tt he a-a nd b-positions are regioand diastereoselectively cycloisomerizedt ot he corresponding exocyclic enol lactones in the presence of exceedingly low amounts of the latter palladium complex, so that unprecedented turnovern umbers and frequencies ranging from 1,000,000 to 700,000 and from 41,667 to 9722h À1 ,r espectively, are achieved. Theo ptimized protocol, based on the use of ac atalytic amount of triethylamine as base,a llows an easy real-time monitoring of the reactionb y NMR spectroscopy.S everal pieces of evidence in favor of the direct participation of the above pincer complex as the catalyst of the reaction have been gathered from kinetic and poisoning experiments

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Thed irect arylation of N-substituted obromobenzanilides and benzenesulfonamides via C À Hb ondf unctionalization hasb eend eveloped using very low catalyst loadings.T his novel cost-effective and more sustainable method relies on aP CN-type palladium pincerc omplexa sahighly active palladium source,p roviding ag enerala nd efficient access to phenanthridinones,b iaryl sultams and related heterocyclic systems. Theb eneficial effect of water as cosolvent has been observed in this process,w hich is not seriously influencedb y electronice ffects at the arene moieties or sterically demanding substituents at the amide or sulfonamide nitrogen. In addition, an umber of experiments (kinetic plot, poisoning assays,T EM,E SI) have been performed in order to understand the role of the employed palladium complex in this reaction.Palladium-catalyzed direct functionalization of C À H bonds has emerged over the last years as am odern and sustainablet ool in organic synthesis. [1] Thei ncreasing demand for greenc hemistry in industry as well as in academia has lead to the development of innovative,e nvironmentally friendly and highly efficient synthetic strategies. [2] Thus,r egarding both atom and stepe conomy,t he formationo fc arbon-carbon bonds throughC À Hf unctionalization hasa ppeared as ac onvenient methodology with broad applicationi n total synthesis and medicinal chemistry. [3] Phenanthridinones are important structural scaffolds found in manyn atural productst hat exhibit remarkable biological andp harmaceutical properties. [4] In the last years,n ovel alternative synthetic approachesh aveb eend evelopedt ot he synthesis of the phenanthridinone core and related lactams,m ost of them based on palladium-catalyzed direct functionalization of arenes. [5] Although recent efforts have been focused on the discovery of complementary routes in order to avoid the substantial costs associated to the relativeb ig amounts of catalysts required( 2-10 mol %), [6] still, the palladium-catalyzed intramolecular direct C À Ha rylation reactionh as provedt ob e one of the most versatile,r eliablea nd efficient method for the access of phenanthridinones.F urthermore,r elatedh eterobiaryls and biaryl heterocyclic compounds as wella sb iaryl sultams have been successfullyp repared under equivalent direct arylation procedures. [6a, 7a-c] Therefore,i tw ould be desirable to develop an ovel palladium-catalyzed approachf or the direct functionalization of arenes using very low catalyst loadings, [7d-e] which would provide ac ost-effective and environmentally very attractivep rocedure for the preparation of such compounds.M oreover, considering the problems associated to the minuteq uantities of metal particles usually detected in the productso f transitionm etal catalyzed reactions, [8] such method wouldp revent metal contamination of the product and therefore be of addedi nterest regarding its potential applicationinm edicinal chemistry.Our group hasa mple experience in the synthesis and applicationo fp incer-type palladium complexes as ve...

Cascade reactions provide a straightforward access to many valuable compounds and reduce considerably the number of steps of a synthetic sequence. Among the domino and multicomponent processes that involve alkynes, the cascade reaction between alkynoic acids and C-, N-, O- and S-aminonucleophiles stands out as a particularly powerful tool for the one-pot construction of libraries of nitrogen-containing heterocyclic compounds with scaffold diversity and molecular complexity. This reaction, based on an initial metal-catalyzed cycloisomerization that generates an alkylidene lactone intermediate, was originally catalyzed by gold(I) catalysts, along with silver salts or Brönsted acid additives, but other alternative metal catalysts have emerged in the last decade as well as different reaction media. This review examines the existing literature on the topic of metal-catalyzed cascade reactions of acetylenic acids and dinucleophiles and discusses aspects concerning substrate/catalyst ratio for every catalyst system, nature of the aminonucleophile involved and substrate scope. In addition, alternative solvents are also considered, and an insight into the pathway of the reaction and possible intermediates is also provided.