Efficient Stille cross-coupling reaction catalyzed by the Pd(OAc)2/Dabco catalytic system

[reaction: see text] An efficient Pd(OAc)2/Dabco-catalyzed Stille cross-coupling reaction procedure has been developed. In the presence of Pd(OAc)2 and Dabco (triethylenediamine), various aryl halides including aryl iodides, aryl bromides, and activated aryl chlorides were coupled efficiently with organotin compounds to afford the corresponding biaryls, alkene, and alkynes in good to excellent yields. Furthermore, high TONs [turnover numbers, up to 980,000 TONs for the coupling reaction of 1-bromo-4-nitrobenzene and furan-2-yltributyltin] for the Stille cross-coupling reaction were observed.     

An efficient Stille cross-coupling reaction catalyzed by Pd(OAc)2/DAB-Cy catalytic system

An efficient palladium-catalyzed Stille cross-coupling reaction has been developed. In the presence of 3 mol% of Pd(dba)₂ and 6 mol% of DAB-Cy (1,4-dicyclohexyl-diazabutadiene), various aryl halides (iodides and bromides) were coupled with organotin compounds to afford the corresponding biaryls and alkyne in good to excellent yields. Furthermore, high TONs [turnover numbers, TONs up to 950,000 for the reaction of 1-iodo-4-nitrobenzene and tributyl(phenyl)stannane] for the Stille cross-coupling reaction were observed.     

Palladium supported on modified magnetic nanoparticles: a phosphine‐free and heterogeneous catalyst for Suzuki and Stille reactions

An efficient magnetic nanoparticle‐supported palladium (Fe₃O₄/SiO₂‐PAP‐Pd) catalyst is reported for the Suzuki cross‐coupling and Stille reactions. This method provides a novel and much improved modification of the Suzuki and Stille coupling reactions in terms of phosphine‐free catalyst, short reaction time, clean reaction and small quantity of catalyst. Another important feature of this method is that the catalyst can be easily recovered from the reaction mixture and reused with no loss of its catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

The mechanism of the Stille reaction investigated by electrospray ionization mass spectrometry

On-line monitoring of Stille reactions was performed via direct infusion electrospray ionization mass spectrometry (ESI-MS) and its tandem version (ESI-MS/MS). When operated in the positive ion mode, ESI(+)-MS was able to transfer, directly from solution to the gas phase, the species involved in all main steps of a Stille reaction, that is, the catalytically active palladium species Pd(PPh3)2, in its molecular ion form as well as the key cationic Pd(II) intermediates, including cyclic IPd-(CH2CH)Sn species. When searching for anionic species, ESI(-)-MS monitoring showed I- as the only anion detectable in the reaction medium. A detailed catalytic cycle for a Stille reaction was elaborated in which reaction intermediates and the previously elusive catalytically active Pd(0) species are shown in association with the respective ionic species intercepted by ESI-MS and further characterized by ESI-MS/MS.     

Dendrimer-encapsulated Pd nanoparticles as aqueous, room-temperature catalysts for the Stille reaction

We report that dendrimer-encapsulated Pd nanoparticles having a diameter of approximately 1.7 nm are effective and general catalysts for coupling aryl halides to organostannanes (the Stille reaction) under mild conditions. The significant results of this study are that the Stille reaction is catalyzed by dendrimer-encapsulated Pd nanoparticles in very good yield, in aqueous solution at 23 degrees C, and using only 0.100 atom % of Pd as catalyst.     

Size controlled synthesis of Pd nanoparticles in water and their catalytic application in C-C coupling reactions

Catalytically active Pd nanoparticles have been synthesized in water by a novel reduction of Pd(II) with a Fischer carbene complex where polyethylene glycol (PEG) was used as stabilizer. PEG molecules wrap around the nanoparticles to impart stability and prevent agglomeration, yet leave enough surface area available on the nanoparticle for catalytic activity. Our method is superior to others in terms of rapid generation and stabilization of Pd nanoparticles in water with a cheap, readily available PEG stabilizer. The size of the nanoparticles generated can be controlled by the concentration of PEG in water medium. The size decreased with the increase in the PEG: Pd ratio. This aqueous nano-sized Pd is a highly efficient catalyst for Suzuki, Heck, Sonogashira, and Stille reaction. Water is used as the only solvent for the coupling reactions.     

A unified approach to quinolizinium cations and related systems by ring-closing metathesis

[reaction: see text] The first example of an olefin ring-closing metathesis reaction on cationic heteroaromatic systems is described. Dihydroquinolizinium cations and a variety of related cationic systems are synthesized in an efficient approach from N-alkenyl alpha-vinyl azinium salts using Grubbs' catalysts.     

Palladium(II) complexes of unsymmetrical CNN pincer ligands

Unsymmetrical 1-(arylimino)-3-(2-hetarylimino)isoindolines have been prepared from 1,3-diiminoisoindoline, an arylamine (aniline, 2-methylaniline, 2-iodoaniline), and a heteroaromatic amine (2-amino-6-methylpyridine, 2-amino-4-methylthiazole) in a stepwise manner by two consecutive condensations. The metalation reactions of these compounds with palladium(II) acetate proceed upon cyclopalladation of the carbocyclic aryl moieties and yield unsymmetrical C, N, N pincer complexes in all cases. X-ray crystallographic analysis were performed on single crystals of hydrogen{acetato[1-phenylimino-3-(6-methylpyridylimino)isoindolinato]palladate(II)} H[(phpi)Pd(OAc)] and pyridine[1-(2-tolylimino)-3-(4-methylthiazolylimino)isoindolinato]palladium(II) [(2-tolti)Pd(py)] by which the coordination mode, the conformation, the protonation site, and the trans influence of the carbon donor were established. For one more C, N, N pincer complex, hydrogen{acetato[1-(2-iodophenylimino)-3-(6-methylpyridylimino)isoindolinato]palladate(II)} H[(2-Iphpi)Pd(OAc)], a similar mononuclear coordination mode was confirmed by (1)H NMR spectroscopy, whereas for the product of an oxidative addition reaction of a palladium(0) precursor to the iodoaryl derivative a product with exo coordination was found. First experiments showed the effectivity of one of these complexes as a precatalyst in CC coupling reactions (Heck and Stille coupling).     

Oxidative addition of N-halosuccinimides to palladium(0): the discovery of neutral palladium(II) imidate complexes, which enhance Stille coupling of allylic and benzylic halides

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.     

(BeDABCO)2Pd2Cl6 (1‐benzyl‐4‐aza‐1‐azoniabicyclo[2.2.2]octane chloride) as a highly active catalytic system for the Stille cross‐coupling reaction under microwave irradiation

An efficient catalytic system using (BeDABCO)₂Pd₂Cl₆ was developed for the Stille cross‐coupling reaction. The substituted biaryls were produced in excellent yields in short reaction times using a catalytic amount of this catalyst in DMF at 120 °C. The ionic character of homogeneous catalyst and microwave irradiation and also DMF as microwave‐active polar solvent gave higher yields and shorter reaction times than conventional heating. Benzyl DABCO as an efficient ligand and also a quaternary ammonium salt had an efficient stabilizing effect on the Pd(0) species. Copyright © 2013 John Wiley & Sons, Ltd.     

Pd(0) complex of fuberidazole modified magnetic nanoparticles: A novel magnetically retrievable high-performance catalyst for Suzuki and Stille C-C coupling reactions

Fuberidazole has been successfully immobilized onto nano-Fe₃O₄ supported (3-chloropropyl)trimethoxysilane (3-CPTS) leading to a novel functionalized magnetic nanoparticle (FB/MNP). The Pd(0) complex, Pd-FB/MNP, was prepared by grafting Pd (OAc)₂ on FB/MNP and subsequent reduction of a synthesized Pd (II) complex using NaBH₄. Pd-FB/MNP has been characterized by FT-IR, SEM, TGA, XRD, ICP, EDS, BET and VSM. The Pd(0) complex proved to be an efficient phosphine- and halide-free recyclable heterogeneous catalyst for Suzuki as well as for Stille C-C coupling reactions showing high catalytic activity (up to 98%). Its catalytic activity in both reactions has been studied in PEG-400 as a green solvent. Besides, the selectivity of aryl iodide and aryl bromide over aryl chloride is observed during the C-C coupling reaction. The catalyst could be recovered easily from the reaction mixture using an external magnet device and recycled several times without considerable loss in activity. Additionally, the results of a palladium leaching test of the nano-catalyst demonstrate that no leaching of Pd took place during the C-C coupling process making the procedure environmentally friendly.     

Stille reactions catalytic in tin: a "Sn-F" route for intermolecular and intramolecular couplings

Polymethylhydrosiloxane (PMHS) made hypercoordinate by KF(aq) allows Me(3)SnH to be recycled during a Pd(0)-catalyzed hydrostannation/Stille cascade. Starting with a variety of alkynes, in situ vinyltin formation is followed by Stille reaction with aryl, styryl, benzyl, or vinyl electrophiles present in the reaction mixture. Both inter- and intramolecular versions of the process are possible with tin loads of approximately 6 mol %. Regeneration of the organotin hydride is believed to proceed through a Me(3)SnF intermediate. Given the aggregated nature of organotin fluorides and the ability to use these organotins in substoichiometric quantities, the hazards and purification problems associated with the removal of organotin wastes from reaction mixtures are minimized.     

Computational characterization of a complete palladium-catalyzed cross-coupling process: the associative transmetalation in the Stille reaction

[reaction: see text] The species presumably involved in the associative ligand substitution mechanism for the Stille cross coupling of vinyl bromide and trimethylvinyl stannane with Pd(PMe(3))(2)/PMe(3) as catalysts in DMF (as ligand and solvent) have been structurally and energetically characterized. The cyclic four-coordinate transition state for the rate-determining transmetalation step explains the retention of configuration in the Stille coupling of chiral nonracemic alkyl stannanes.     

Cine-substitution in the Stille coupling: evidence for the carbenoid reactivity of sp3-gem-organodimetallic iodopalladio-trialkylstannylalkane intermediates

Two complementary routes to sp3-gem-organodimetallic iodopalladio-trialkylstannanylalkanes are presented. Such intermediates have been proposed as Pd-carbenoid precursors in the Busacca-Farina cine-substitution mechanism in the Stille coupling. The decomposition of iodomethyltrialkylstannanes by Pd(0) catalysts was monitored by 1H, 2D, and 119Sn NMR. The formation of ethylene, trace formaldehyde, and iodotrialkylstannanes was detected. When the reaction was carried out in the presence of norbornene, the corresponding cyclopropane was produced in good yield. These observations are consistent with the intermediacy of a Pd-carbenoid species. sp3-gem-Organodimetallic iodopalladio-trialkylstannanylalkane complexes were also prepared under stoichiometric conditions via transmetalation from tin to Pd(II). Me3SnCH2Sn(CH2CH2CH2)3 reacted with [(D-t-BPF)PdI]+I-, yielding the (D-t-BPF)Pd(II)ICH2SnMe3 complex that dimerized to form ethylene and cyclopropanated norbornene. The carbenoid reactivity of iodopalladio-trialkylstannanylalkanes complexes validates the Busacca-Farina mechanism of the cine-substitution in the Stille coupling.     

Cascade cyclization: an easy access to highly unsaturated polycyclic ring systems through a tandem stille/[4 + 2] reaction under mild conditions

The synthesis of several polycyclic compounds 1a-c, 2, and 3 has been performed through a tandem Stille/[4 + 2] cascade reaction from cyclic bis(enoltrifluomethanesulfonate) 4a-c, 5, and 6, respectively. The reaction proceeds very efficiently in a one-pot operation at roomtemperature in DMF in the presence of a catalytic amount of Pd(CH(3)CN)(2)Cl(2) and LiCl.[reaction: see text]     

Pd2(dba)3/P(i-BuNCH2CH2)3N-catalyzed Stille cross-coupling of aryl chlorides

[reaction: see text] The Pd(2)(dba)(3)/P(i-BuNCH(2)CH(2))(3)N (1d) catalyst system is highly effective for the Stille cross-coupling of aryl chlorides with organotin compounds. This method represents only the second general method for the coupling of aryl chlorides. Other proazaphosphatranes possessing benzyl substituents also generate very active catalysts for Stille reactions. Noteworthy features of the method are: (a) commercial availability of ligand 1d, (b) the wide array of aryl chlorides that can be coupled, and (c) applicability to aryl, vinyl, and allyl tin reagents.     

PEG-400 promoted Pd(OAc)2/DABCO-catalyzed cross-coupling reactions in aqueous media

PEG-400 [poly(ethylene glycol-400)] was found to improve the Pd(OAc)₂/DABCO-catalyzed aqueous Suzuki-Miyaura and Stille cross-coupling reactions. In the presence of Pd(OAc)₂, DABCO, and PEG-400, a variety of aryl halides were coupled with arylboronic acids or organotin compounds efficiently to afford the corresponding cross-coupled products in moderate to excellent yields. The turnover numbers was up to 900,000 for the Suzuki-Miyaura reaction and up to 9800 for the Stille reaction. The catalyst system was also effective for Heck and Sonogashira cross-coupling reactions to some extent.     

Pd/P(t-Bu)(3): a mild and general catalyst for Stille reactions of aryl chlorides and aryl bromides

Pd/P(t-Bu)(3) serves as an unusually reactive catalyst for Stille reactions of aryl chlorides and bromides, providing solutions to a number of long-standing challenges. An unprecedented array of aryl chlorides can be cross-coupled with a range of organotin reagents, including SnBu(4). Very hindered biaryls (e.g., tetra-ortho-substituted) can be synthesized, and aryl chlorides can be coupled in the presence of aryl triflates. The method is user-friendly, since a commercially available complex, Pd(P(t-Bu)(3))(2), is effective. Pd/P(t-Bu)(3) also functions as an active catalyst for Stille reactions of aryl bromides, furnishing the first general method for room-temperature cross-couplings.     

A simple phenylation of heteroaromatic compounds using diphenyliodonium triflate

In this study a novel method for direct synthesis of 2-phenyl heteroaromatic arenes was successfully developed. The title compounds were synthesized by reaction of heteroaromatic compounds with diphenylene iodonium trifluoromethanesulfonate in the presence of 5?mol% Pd(OAc)₂ under mild reaction conditions (THF, 60?°C, 24?h). The proposed reaction mechanism was studied by HPLC.     

Dual effect of halides in the Stille reaction: in situ halide metathesis and catalyst stabilization

Halide anions can increase or decrease the transmetallation rate of the Stille reaction through in situ halide metathesis. Although the influence of the halogen present in oxidative addition complexes on the transmetallation rate with organostannanes was already known, the application of in situ halide metathesis to accelerate cross-coupling reactions with organometallic reagents is not described in the literature yet. In addition a second unprecedented role of halides was discovered. Halide anions stabilize the [Pd(0)(L)(2)] catalyst in Stille reactions, by means of [Pd(0)X(L)(2)](-) formation (X=Cl, I), hereby preventing its leaching from the catalytic cycle. Both arene (iodobenzene) and azaheteroarene (2-halopyridine, halopyrazine, 2-halopyrimidine) substrates were used.