Alkyne Metatheses in Transition Metal Coordination Spheres: Convenient Tungsten‐ and Molybdenum‐Catalyzed Syntheses of Novel Metallamacrocycles

Reactions of (CO)₅Re(Br), (η⁵‐C₅H₅)Ru(Cl)(PPh₃)₂, and [Pt(μ‐Cl)(C₆F₅)(S(CH₂CH₂‐)₂)]₂ with the alkyne‐containing phosphine Ph₂P(CH₂)₆C≡CCH₃ give the bis(phosphine) complexes fac‐(CO)₃Re(Br)(Ph₂P(CH₂)₆C≡CCH₃)₂ ( 5 ), (η⁵‐C₅H₅)Ru(Cl)(Ph₂P(CH₂)₆C≡CCH₃)₂ ( 6 ), and trans‐(Cl)(C₆F₅)Pt(Ph₂P(CH₂)₆C≡CCH₃)₂ ( 7 ). Alkyne metatheses with the catalyst (t‐BuO)₃W(≡C‐t‐Bu) (10–15 mol %, chlorobenzene, 80 °C) give the seventeen‐membered metallamacrocycles fac‐(CO)₃Re(Br)(Ph₂P(CH₂)₆CC(CH₂)₆P Ph₂) ( 8 ), (η⁵‐C₅H₅)Ru(Cl)(Ph₂P(CH₂)₆CC(CH₂)₆P Ph₂) ( 9 ), and trans‐(Cl)(C₆F₅)Pt(PPh₂(CH₂)₆CC(CH₂)₆P Ph₂) ( 10 ). ³¹P NMR analyses show 90–75% conversions to 8 – 10 (59–47% isolated after chromatography). The identity of 8 was confirmed by a crystal structure, and 10 was hydrogenated over Pd/C to fac‐(CO)₃Re(Br)(Ph₂P(CH₂)₆CC(CH₂)₆P Ph₂) ( 12 , 87%), which was crystallographically characterized earlier. A catalyst derived from Mo(CO)₆/4‐chlorophenol effects a slower conversion of 7 to 10 at 140 °C. In the case of 5 , a mer, trans isomer of 8 is isolated ( 11 , 44%), as established by NMR and IR data. In 10 – 12 , the diphosphines span trans positions. These results, together with previous examples involving group VIII metallocenes, establish the wide viability of the title reaction.     

On the Use of Non‐Symmetrical Mixed PCN and SCN Pincer Palladacycles as Catalyst Precursors for the Heck Reaction

The mixed pincer palladacycles (Me₂NCH₂(Cl)CCCH₂CH₂Y‐κN,κC,κY)PdCl ( 1 , Y=PPh₂; 2 , OPPh₂) and (t‐BuSCH₂CH₂CC(Cl)(o‐NC₅H₄)‐κS,κC,κN)PdCl 3 have been obtained in high yields by chloropalladation of heterosusbstituted alkynes Me₂NCH₂CCCH₂CH₂PPh₂, Me₂NCH₂CCCH₂CH₂OPPh₂ and t‐BuSCH₂CH₂CC(o‐NC₅H₄), respectively. The molecular structures of 1 and 3 have been ascertained by means of X‐ray diffraction analysis. The catalytic properties of these mixed donor group pincer‐type palladacycles have been evaluated in the arylation of olefins (Heck reaction). The pincer palladacycle 1 is highly active for the coupling of aryl iodides and aryl bromides with n‐butyl acrylate. In contrast it is only moderately active for the coupling of aryl chlorides substituted with electron‐withdrawing groups and inactive for the coupling of electron neutral and electron deactivated aryl chlorides.     

Synthesis of Ruthenium Hydride Complexes Containing beta‐Aminophosphine Ligands Derived from Amino Acids and their use in the H2‐Hydrogenation of Ketones and Imines

The new complexes RuHCl(PPh₂CH₂CHRNH₂)₂ and RuHCl(PPh₂CH₂CHRNH₂)(R‐ binap), R=H (Pgly), R=Me [(R)‐Pala] were prepared by the substitution of the PPh₃ ligands in RuHCl(PPh₃)₃ or RuHCl(PPh₃)[(R)‐binap] with beta‐aminophosphines derived from amino acids. The complex trans‐RuHCl(Pgly)[(R)‐binap] has been characterized by X‐ray crystallography. The complex trans‐RuHCl[(S)‐Ppro]₂ where (S)‐Ppro is derived from proline was also prepared and characterized by X‐ray crystallography. These were used as catalyst precursors in the presence of a base (KOPr‐i or KOBu‐t) for the hydrogenation of various ketones and imines to the respective alcohols and amines with H₂ gas (1–11 atm) at room temperature. Acetophenone was hydrogenated to (S)‐1‐phenylethanol in low ee (up to 40%) when catalyzed by the enantiomerically pure complexes. These complexes are especially active in the hydrogenation of sterically congested and electronically deactivated ketones and imines and are selective for the hydrogenation of CO bonds over CC bonds.     

Single‐Pot Ethane Carboxylation Catalyzed by New Oxorhenium(V) Complexes with N,O Ligands

The oxorhenium(V) chelates [ReOCl(N,O‐L)(PPh₃)] [N,O‐L=(OCH₂CH₂)N(CH₂CH₂OH)(CH₂COO) ( 2 ), (OCH₂CH₂)N(CH₂COO)(CH₂COOCH₃) ( 3 )] and [ReOCl₂(N,O‐L)(PPh₃)] [N,O‐L=C₅H₄N(COO‐2) ( 4 ) C₅H₃N(COOCH₃‐2)(COO‐6) ( 5 )] have been prepared by reaction of [ReOCl₃(PPh₃)₂] ( 1 ), in refluxing methanol, with N,N‐bis(2‐hydroxyethyl)glycine [bicine; N(CH₂CH₂OH)₂(CH₂COOH)], N‐(2‐hydroxyethyl)iminodiacetic acid [N(CH₂CH₂OH)(CH₂COOH)₂], picolinic acid [NC₅H₄(COOH‐2)] or 2,6‐pyridinedicarboxylic acid [NC₅H₃(COOH‐2,6)₂], respectively, with ligand esterification in the cases of 3 and 5 . All these complexes have been characterized by IR and multinuclear NMR spectroscopy, FAB⁺‐MS, elemental and X‐ray diffraction structural analyses. They act as catalysts, in a single‐pot process, for the carboxylation of ethane by CO, in the presence of potassium peroxodisulfate K₂S₂O₈, in trifluoroacetic acid (TFA), to give propionic and acetic acids, in a remarkable yield (up to ca. 30%) and under relatively mild conditions, with some advantages over the industrial processes. The picolinate complex 4 provides the most active catalyst and the carboxylation also occurs, although much less efficiently, by the TFA solvent in the absence of CO. The selectivity can be controlled by the ethane and CO pressures, propionic acid being the dominant product for pressures about ca. 7 and 4 atm, respectively (catalyst 4 ), whereas lower pressures lead mainly to acetic acid in lower yields. These reactions constitute an unprecedented use of Re complexes as catalysts in alkane functionalization.     

Neutral Pd(II) and Ni(II) Acetylide Catalysts for the Polymerization of Methyl Methacrylate

Homogenous polymerization of methyl methacrylate using Pd(II)- and Ni(II)-based acetylide complexes as initiators has been investigated. M(PR'₃)₂(CCR)₂ (M=Pd, Ni; R'=PPh₃, Pn-Bu₃; R=Ph, CH₂OH, CH₂OOCCH₃, CH₂OOCPh, CH₂OOCPhOH-o) were found to be a novel type of effective initiators for the polymerization of methyl methacrylate. Among them, Pd(C CPh)₂(PPh₃)₂ (PPP) shows the highest activity in the MMA polymerization and the PMMA obtained is a syndiotactic polymer with high number-average molecular weight (M _n) of 14.1 × 10⁴. Some features and kinetic behavior of MMA polymerization initiated by PPP were studied in detail. The polymerization reaction is first-order with respect to both [PPP] and [MMA]. Radical polymerization mechanism is proposed.     

Synthesis, Characterization and Photophysical Properties of Metallopolyynes and Metallodiynes of Platinum(II) with Dibenzothiophene Derivatives

Soluble, thermally stable, and easily processable platinum(II) polyyne polymers of the form trans-[–Pt(PBu₃)₂C≡CRC≡C–]_n (R = 2,8-disubstituted dibenzothiophene, 2,8-disubstituted and 3,7-disubstituted dibenzothiophene-S,S-dioxide units) have been prepared in good yields by CuI-catalyzed polymerization involving the dehydrohalogenating coupling of trans-[PtCl₂(PBu₃)₂] and HC≡CRC≡CH. We report their optical absorption and photoluminescence spectra and compare the results with the monomeric model complexes trans-[Pt(Ph)(PEt₃)₂C≡CRC≡CPt(Ph)(PEt₃)₂]. The different electronic properties and linkage geometry of the central R group lead to new organometallic materials with distinct photophysical traits. The polymer with more conjugated 3,7-disubstituted dibenzothiophene-S,S-dioxide mainly displays fluorescence band, while its isomer with less conjugated 2,8-disubstituted spacer effectively enhances the intersystem crossing rate and strong phosphorescence emission can be detected even at room temperature.     

Helical and random coil conformations of N‐propargylamide polymer and copolymers

An N‐propargylamide monomer, CH?CCH₂NHCOC(CH₃)₂CH₂CH₃ (monomer 9), was polymerized in the presence of (nbd)Rh⁺B^?(C₆H₅)₄ (nbd represents norbornadiene) in CH₂Cl₂, CHCl₃, tetrahydrofuran or dimethylformamide, to provide polymers with moderate number‐average molecular weights (M_n = 8700–12 100 g mol^?1) in high yields (≥92%). The resulting poly(N‐propargylamide) (polymer 9) dissolves almost completely in CHCl₃ (>95%). According to the UV‐visible spectra, measured at various temperatures, polymer 9 forms relatively stable helices over a wide temperature range (35–65 °C). Moreover, it exhibits reversible conformational transitions from an ordered helix to a random coil. On copolymerization of monomer 9 with CH?CCH₂NHCO(CH₂)₃CH₃ (monomer 4) or CH?CCH₂NHCO(CH₂)₇CH₃ (monomer 8), the solubility of polymer 9 improves noticeably. All the copolymers form helices under the experimental conditions. From the viewpoint of monomers 4 and 8, copolymerization with monomer 9 is favorable in terms of the copolymers forming helices. These findings reveal that the helical content and thermodynamic stability of the helices formed in the copolymers are likely to be controlled by selecting a suitable comonomer and by adjusting the composition of the copolymer. Copyright © 2007 Society of Chemical Industry     

Synthesis and characterization of palladium (II) complex of Schiff base ligand: CS bond cleavage and catalytic activity

Reaction of ligand, L-CH₂Ph with Pd(CH₃CN)₂Cl₂ in acetonitrile in presence of PPh₃ and subsequent addition of NaClO₄ yielded Pd(II) complex, [Pd(L)(PPh₃)](ClO₄) 1 due to the SC (CH₂Ph) bond cleavage during complexation. Complex 1 was characterized by spectral analysis and the structure was authenticated by single crystal X-ray diffraction. The diffraction analysis revealed that the monoanionic ligand binds with palladium (II) in (N, N, S) fashion in a distorted square planar geometry where the fourth position is occupied by one tri phenyl phosphine group. One ClO₄⁻ ion satisfies the charge of the former aggregate, [Pd(L)(PPh₃)]⁺. The complex [Pd(L)(PPh₃)](ClO₄), 1 exhibits a very good catalytic activity towards CC bond formation.     

Synthesis, Characterization and Photoluminescence of Dimeric and Polymeric Metallaynes of Group 10–12 Metals Containing Conjugation-breaking Diphenylmethane Unit

A novel approach based on conjugation interruption was developed for a luminescent and thermally stable platinum(II) polyyne polymer trans-[–Pt(PBu₃)₂C≡C(C₆H₄)CH₂(C₆H₄)C≡C–] _n (1) containing the diphenylmethane chromophoric spacer. Particular attention was focused on the photophysical properties of this group 10 polymetallayne and comparison was made to its binuclear model complex trans-[Pt(Ph)(PEt₃)₂C≡C(C₆H₄)CH₂(C₆H₄)C≡CPt(Ph)(PEt₃)₂] (2) and their closest group 11 gold(I) and group 12 mercury(II) neighbors, [MC≡C(C₆H₄)CH₂(C₆H₄)C≡CM] (M = Au(PPh₃) (3), HgMe (4)). The regiochemical structures of these angular-shaped compounds were studied by various spectroscopic analyses. Upon photoexcitation, each of them emits an intense purple-blue fluorescence emission in the near UV region in dilute fluid solutions at room temperature. Harvesting of organic triplet emissions harnessed through the strong heavy-atom effects of group 10–12 transition metals was examined. These metal-containing phenyleneethynylenes spaced by the conjugation-breaking CH₂ unit were found to have high optical gaps and high-energy triplet states. The influence of metal and sp³-hybridized methylene conjugation-interrupters on the intersystem crossing rate and the spatial extent of the lowest singlet and triplet excitons was fully elucidated. Our investigations indicate that high-energy triplet states in these materials intrinsically give rise to very efficient phosphorescence with fast radiative decays. Dedicated to Professor Didier Astruc in recognition of his outstanding contribution to metallodendrimers and polymers.     

Metallkomplexe mit biologisch wichtigen Liganden. CXXIII. Darstellung von Isophthaloyl‐ und Terephthaloyl‐di‐[(β,γ,δ)‐amino‐α‐aminocarboxylat]‐verbrückten zweikernigen Ruthenium‐, Rhodium‐ und Iridium‐Halbsandwich‐Komplexen

The reaction of the Cu(II) bis N,O‐chelate‐complexes of L‐2,4‐diaminobutyric acid, L‐ornithine and L‐lysine {Cu[H₂N–CH(COO)(CH₂)_nNH₃]₂}²⁺(Cl^–)₂ (n = 2–4) with terephthaloyl dichloride or isophthaloyl dichloride gives the polymeric complexes {‐OC–C₆H₄–CO–NH–(CH₂)_n–CH(nh₂)(COO)Cu(OOC)(NH₂)CH–CH₂)_n–NH‐}_x 1 – 5 . From these the metal can be removed by precipitation of Cu(II) with H₂S. The liberated ω,ω′‐N,N′‐diterephthaloyl (or iso‐phthaloyl)‐diaminoacids 6 – 10 react with [Ru(cymene)Cl₂]₂, [Ru(C₆Me₆)Cl₂]₂, [Cp*RhCl₂]₂ or [Cp*IrCl₂]₂ to the ligand bridged bis‐amino acidate complexes [L_n(Cl)M–(OOC)(NH₂)CH–(CH₂)_nNH–CO]₂–C₆H₄ 11 – 14 .     

Polymerization of N-substituted 2-propynamides with transition metal catalysts

Summary Polymerization of N-substituted 2-propynamides proceeded in the presence of Pd(II) catalysts such as [Pd(CH₃CN)₄](BF₄)₂, PdCl₂(PhCN)₂-n-BuLi, and PdCl₂(nbd)-n-BuLi. The molecular weights of the obtained polymers ranged 8,500 to 14,000, depending on the catalysts. From the ¹H NMR spectra, these polymers were found to have alternating double bonds in the main chain. Received: 25 December 2000/Revised version: 17 January 2001/Accepted: 19 January 2001     

A novel phosphine Pd2Te2 dimer. Synthesis and X-ray characterization of the complex [(N(CH2CH2PPh2)3)PdTe]2

The novel phosphine Pd/Te dimer [(np₃)PdTe]₂, np₃=N(CH₂CH₂PPh₂)₃, has been synthesized by reaction of (np₃)Pd with TePEt₃ and characterized by X-ray diffraction studies.     

Hydrogenation of methyl sorbate and soybean esters with polymer-bound metal catalysts

New polymer-bound hydrogenation catalysts were made by complexing PdCl₂, RhCl₃·3H₂O, or NiCl₂ with anthranilic acid anchored to chloromethylated polystyrene. The Pd(II) and Ni(II) polymers were reduced to the corresponding Pd(O) and Ni(O) catalysts with NaBH₄. In the hydrogenation of methyl sorbate, these polymer catalysts were highly selective for the formation of methyl 2-hexenoate. The diene to monoene selectivity decreased in the order: Pd(II), Pd(O), Rh(I), Ni(II), Ni(O). Kinetic studies support 1,2-reduction of the Δ4 double bond of sorbate as the main path of hydrogenation. In the hydrogenation of soybean esters, the Pd(II) polymer catalysts proved superior because they were more active than the Ni(II) polymers and produced lesstrans unsaturation than the Rh(I) polymers. Hydrogenation with Pd(II) polymers at 50~100 C and 50 to 100 psi H₂ decreased the linolenate content below 3% and increasedtrans unsaturation to 10~26%. The linolenate to linoleate selectivity ranged from 1.6 to 3.2. Reaction parameters were analyzed statistically to optimize hydrogenation. Recycling through 2 or 3 hydrogenations of soybean esters was demonstrated with the Pd(II) polymers. In comparison with commercial Pd-on-alumina, the Pd(II) polymers were less active and as selective in the hydrogenation of soybean esters but more selective in the hydrogenation of methyl sorbate. Presented at ISF-AOCS Meeting, New York, April 1980.     

Nickel(II) Chloride‐Catalyzed Regioselective Hydrothiolation of Alkynes

Regioselective Markovnikov‐type addition of PhSH to alkynes (HC≡C‐R) has been performed using easily available nickel complexes. The non‐catalytic side reaction leading to anti‐Markovnikov products was suppressed by addition of γ‐terpinene to the catalytic system. The other side reaction leading to the bis(phenylthio)alkene was avoided by excluding phosphine and phosphite ligands from the catalytic system. It was found that catalytic amounts of Et₃N significantly increased the yield and selectivity of the catalytic reaction. Under optimized conditions high product yields of 60–85% were obtained for various alkynes [R=n‐C₅H₁₁, CH₂NMe₂, CH₂OMe, CH₂SPh, C₆H₁₁(OH), (CH₂)₃CN]. The X‐ray structure of one of the synthesized products is reported.     

Synthesis of stimuli-responsive ionic cyclopolymers in search of phosphorous-free antiscalants

Ammonium persulfate-initiated cyclopolymerization of maleic acid (HO₂CC=CCO₂H) (MA) with diallylamine (–NR₂, R = CH₂ = CH-CH₂) derivatives (DADs): R₂NH⁺CH₂CO₂⁻ ( I ), R₂NCH₂CO₂⁻Na⁺ ( II ), R₂NCH₂CO₂Et ( III ), R₂NH⁺(CH₂)₃CO₂⁻ ( IV ), R₂N(CH₂)₃CO₂Et ( V ), R₂NH⁺(CH₂)₃SO₃⁻ ( VI ) and R₂N(CH₂)₃SO₃⁻Na⁺ ( VII ) gave a series of new pH-responsive alternate copolymers: –[(DAD-alt-MA)]_n– VIII - XIV , respectively. Homopolymers XV and XVI of the corresponding monomers IV and V ·HCl were also synthesized. The evaluation of the synthesized polymers as potential antiscalants in reverse osmosis (RO) plants was examined. An effective scale inhibitor must arrest the formation of scale for a residence time (≈15 min) of feed water in the RO chamber. R₂NH⁺(CH₂)₃CO₂⁻ ( IV )-alt-MA copolymer XI at a concentration of 5 ppm imparted 99% scale inhibition for 360 min, while at 2.5 and 1 ppm concentrations, the CaSO₄ scale inhibitions were found to be 99% and ≈ 100% during 120 and 20 min, respectively, at 40°C. Homopolymers XV and XVI , at a 20-ppm concentration, demonstrated ≈ 95% efficiency in inhibiting mild steel corrosion in 1 M HCl. In some important oilfield applications, requirement of simultaneous scale and corrosion inhibition make these polymers potential candidates for desalination and oilfield applications.     

Reactivity of mononuclear dithiolato palladium and platinum metalloligands towards the formation of homo- and heterobimetallic complexes

The reactivity of mononuclear platinum and palladium dithiolato species as building blocks for homo- and heterobimetallic complexes is described. The heterobimetallic cationic complex [(PPh₃)₂Pt(μ-S(CH₂)₂S)Pd(dppb)](BF₄)₂ was obtained by reaction of [Pt(S(CH₂)₂S)(PPh₃)₂] with [PdCl₂(Ph₂P(CH₂)₄PPh₂)] in presence of AgBF₄.     

New soluble azophenol polymers prepared by oxidative polycondensation

New azophenol polymers (P₁, P₂ and P₃) were synthesised by the oxidative polycondensation (OP) reaction of three different azophenol monomers in aqueous alkaline medium with NaOCl as the oxidant. The monomers and the polymers were characterised by elemental analyses, and UV‐visible, Fourier transform infrared, ¹H NMR and ¹³C NMR spectroscopic studies, which revealed that the polymers synthesised by OP are composed of oxyphenylene (C? O? C) and phenylene (C? C) units. The polymers obtained are soluble in dimethylformamide and dimethylsulfoxide. Average molecular weights of the polymers were determined by gel permeation chromatography. Additionally, P₂ and P₃ are soluble in water and methanol. On the basis of thermogravimetric analyses, 5 and 50% weight‐loss temperatures of the polymers were found to be 218, 700 (P₁), 263, 609 (P₂) and 100, 809 °C (P₃), respectively, suggesting a high thermal stability. Thermal analyses using differential scanning calorimetry revealed that the azophenol polymers are highly amorphous, and melting peaks were not observed in the heating cycles. This suggests that all the polymers are highly amorphous. The azophenol polymers show a reversible trans–cis–trans isomerisation process. These properties of the polymer could be promising for their technological usage. Copyright © 2007 Society of Chemical Industry     

Synthesis,Characterization, and Reactivity of trans-Ru(X)(Cl)(nbd)(dppb) (X = H,Cl; nbd = 2,5-norbornadiene; dppb = 1,4-bis(diphenylphosphino)butane), Including the X-ray Crystal Structure of the Dichloride

The ruthenium (II) diene complexes [Ru(X)(Cl)(nbd)(dppb)] (X = Cl, H; nbd = 2,5-norbornadiene; dppb = PPh₂(CH₂)₄PPh₂) have been prepared and characterized spectroscopically. The X-ray crystal structure of RuCl₂(nbd)(dppb) (crystal data at 22°C: space group P1, a = 10.896 (1) Å, b = 15.168(2) Å, c = 10.829 (1) Å, α = 103.02(1)°, β = 107.08(1)°, γ = 81.65(1)°, Z = 2, R = 0.054 for 6420 reflections) shows an octahedral geometry at Ru, with the chloro ligands slightly distorted from a trans configuration (Cl)(1)-Ru-C1(2) = 168.4°); the unit cell contains two molecules of the complex and one molecule of benzene. Reaction of this complex with H₂, in presence of Proton Sponge (PS, 1,8-bis(dimethylamino)naphthalene) as base, is complicated by initial dissociation of nbd, and [Ru₂Cl₅(dppb)₂]⁻-PSH⁺ is the major product. A minor product, the hydrido(diene) complex trans-RuCl(nbd)(dppb) 5 , characterized spectroscopically, is more effectively synthesized from (a) trans-Ru(H)Cl(nbd)(PPh₃)₂, 1 , and dppb, or (b) reaction of RuCl₂(dppb)-(PPh₃) with H₂ in presence of nbd and PS. Complex 5 is unreactive toward H₂ or CO while 1 has been shown previously to give η²-H₂ and norbornenoyl derivatives, respectively; the differences in reactivity are discussed.     

Synthesis of Silica-Supported Platinum(II) and Nickel(II) Complexes of Bis(Diphenylphosphinomethyl)Amino Ligand: Applications as Catalysts for the Synthesis of 2-Methyl-1,4-Naphthoquinone (Vitamin K3)

Pt(II) and Ni(II) complexes of N,N-bis(diphenylphosphinomethyl)aminopropyltriethoxysilane [(CH₃CH₂O)₃Si(CH₂)₃N(CH₂PPh₂)₂] (DIPAPTES) and silica supported [SiO₂–(DIPAPES)] ligands have been synthesized under nitrogen atmosphere using Schlenk method and characterized by using atomic absorption, FT-IR, NMR (¹H and ³¹P) and elemental analysis techniques. All the complexes were used as catalysts for the oxidation of 2-methyl naphthalene (2MN) to 2-methyl-1,4-naphthoquinone (vitamin K₃, menadione, 2MNQ) using hydrogen peroxide as a clean and cheap oxidant. [Pt–(DIPAPTES)Cl₂] and supported complex [SiO₂–(DIPAPES)–PtCl₂] showed medium catalytic activity whereas Ni(II) complexes did not show any catalytic activity for the selective oxidation of 2-methyl naphthalene to 2-methyl-1,4-naphthoquinone.     

A Density Functional Theory Study of the Stille Cross‐Coupling via Associative Transmetalation. The Role of Ligands and Coordinating Solvents

An associative mechanism has been computationally characterized for the Stille cross‐coupling of vinyl bromide and trimethylvinylstannane catalyzed by PdL₂ (L=PMe₃, AsMe₃) with or without dimethylformamide as coordinating ligand. All the species along the catalytic cycles that start from both the cis‐ and the trans‐PdL(Y)(vinyl)Br complexes (Y=L or S; L=PMe₃, AsMe₃ or PH₃; S=DMF) have been located in the gas phase and in the presence of polar solvents. Computations support the central role of species trans‐PdL(DMF)(vinyl)Br which react by ligand dissociation and stannane coordination in the rate‐limiting transmetalation step via a puckered four‐coordinate (at palladium) transition state comprised of Pd, Br, Sn and sp² C atoms. A donating solvent may enter the catalytic cycle assisting isomerization of cis‐PdL₂(vinyl)Br to trans‐PdL(DMF)(vinyl)Br complexes via a pentacoordinate square pyramidal Pd intermediate. In keeping with experimental observations, the activation energies of the catalytic cycles with arsines as Pd ligands are lower than those with phosphines. Polytopal rearrangements from the three‐coordinate T‐shaped Pd complexes resulting from transmetalation account for the isomerization and the C C bond formation on the reductive elimination step.