Showing papers by "Anant R. Kapdi published in 2006"
TL;DR: Changing the type of dba-n,n'-Z ligand controls the concentration of L(n)Pd(0) and the rate of the oxidative addition step, and not other intimate steps within the catalytic cycle(s), for example, transmetallation and reductive elimination.
Abstract: The reactivity of palladium(0) complexes, [Pd(0) (2)(dba-n,n'-Z)(3)] (n,n'-Z=4,4'-F; 4,4'-CF(3); 4,4'-H; 4,4'-MeO) and [Pd(0)(dba-n,n'-Z)(2)] (n,n'-Z=4,4'-CF(3); 4,4'-H; 3,3',5,5'-OMe), used as precursor catalysts with suitable donor ligands (e.g. phosphines, N-heterocyclic carbenes), has been correlated in several palladium(0)-mediated cross-coupling processes. Increasing the electron density on the aryl moiety of the dba-n,n'-Z ligand increases the overall catalytic activity in the majority of these processes. This effect primarily derives from destabilization of the L(n)Pd(0)-eta(2)-dba interaction (in dpi-pi* synergic bonding, n=1 or 2), which ultimately increases the global concentration of catalytically active L(n)Pd(0) available for reaction with aryl halide in the first committed step in the general catalytic cycle(s) (oxidative addition). Decreasing electron density on the aryl moiety of the dba-n,n'-Z ligand stabilizes the Pd(0)-eta(2)-dba interaction, reducing catalytic activity. The specific type of dba-n,n'-Z ligand appears to also play a stabilizing role in the catalytic cycle, preventing Pd agglomeration, and increasing catalyst longevity. A subtle balance therefore exists between the L(n)Pd(0) concentration (and the associated catalytic activity) and catalyst longevity. Changing the type of dba-n,n'-Z ligand controls the concentration of L(n)Pd(0) and the rate of the oxidative addition step, and not other intimate steps within the catalytic cycle(s), for example, transmetallation (or carbopalladation) and reductive elimination. The role of dba-n,n'-Z ligands in Heck arylation is more convoluted and dependent on the alkene substrate employed, although trends have emerged. Changes in the structure of dba-n,n'-Z had a minimal affect on Buchwald-Hartwig aryl amination processes. A secondary Michael reaction of dba-n,n'-Z with amine and/or base effectively lessens its interference in the catalytic cycle.
86 citations
TL;DR: In this article, the Stille-cross-coupling of bromides was shown to proceed efficiently using a palladium-catalyst, possessing abridging(N,O)-imidateli-gands.
Abstract: Received:August 22,2005;Accepted: December 30,2005Abstract:TheStillecross-couplingofallylicandben-zyl bromides is shown to proceed efficiently usingphosphine-free dinuclear anionic palladacyclopenta-dienylcatalystspossessingbridging(N,O)-imidateli-gands.Thetypeofbridginganioninfluencesthecat-alytic activity considerably. Halide anions such aschloride, bromide or iodide also influence the cata-lyticactivitybuttoafarlesserextentthanthepseu-dohalideimidateanions(fromsuccinimideorphthal-imide). A Baldwin-type cooperative effect is seenwith 7a using CuI as a co-catalyst, in the presenceof two equivalents of CsF in DMFat 408C. In tol-uene, theseadditives slowdownsubstrate turnover.Keywords: C C bond formation; copper(I) salts;cross-coupling; halide effects; palladiumPalladium-catalysed cross-coupling processes havesolvednumeroussyntheticproblems,notablyinnaturalproduct synthesis,
51 citations
TL;DR: The Stille coupling of organostannanes and organohalides, mediated by air and moisture stable palladium(II) phosphine complexes containing succinimide or phthalimide (imidate) ligands, has been investigated.
Abstract: The Stille coupling of organostannanes and organohalides, mediated by air and moisture stable palladium(II) phosphine complexes containing succinimide or phthalimide (imidate) ligands, has been investigated. An efficient synthetic route to several palladium(II) complexes containing succinimide and phthalimide ligands, has been developed. cis-Bromobis(triphenylphosphine)(N-succinimide)palladium(II) [(Ph3P)2Pd(N-Succ)Br] is shown to mediate the Stille coupling of allylic and benzylic halides with alkenyl, aryl and allyl stannanes. In competition experiments between 4-nitrobromobenzene and benzyl bromide with a cis-stannylvinyl ester, (Ph3P)2Pd(N-Succ)Br preferentially cross-couples benzyl bromide, whereas with other commonly employed precatalysts 4-nitrobromobenzene undergoes preferential cross-coupling. Furthermore, preferential reaction of deactivated benzyl bromides over activated benzyl bromides is observed for the first time. The type of halide and presence of a succinimide ligand are essential for effective Stille coupling. The type of phosphine ligand is also shown to alter the catalytic activity of palladium(II) succinimide complexes.
1 citations
TL;DR: In this article, the Stille-cross-coupling of bromides was shown to proceed efficiently using a palladium-catalyst, possessing abridging(N,O)-imidateli-gands.
Abstract: Received:August 22,2005;Accepted: December 30,2005Abstract:TheStillecross-couplingofallylicandben-zyl bromides is shown to proceed efficiently usingphosphine-free dinuclear anionic palladacyclopenta-dienylcatalystspossessingbridging(N,O)-imidateli-gands.Thetypeofbridginganioninfluencesthecat-alytic activity considerably. Halide anions such aschloride, bromide or iodide also influence the cata-lyticactivitybuttoafarlesserextentthanthepseu-dohalideimidateanions(fromsuccinimideorphthal-imide). A Baldwin-type cooperative effect is seenwith 7a using CuI as a co-catalyst, in the presenceof two equivalents of CsF in DMFat 408C. In tol-uene, theseadditives slowdownsubstrate turnover.Keywords: C C bond formation; copper(I) salts;cross-coupling; halide effects; palladiumPalladium-catalysed cross-coupling processes havesolvednumeroussyntheticproblems,notablyinnaturalproduct synthesis,