Palladium compounds containing at least one metal-carbon bond intramolecularly stabilized by at least one donor atom, which are termed palladacycles, are one of the most popular class of ...organopalladum derivatives. The main achievements, potential, and limitations of palladacycles are discussed.
In the Heck reaction between aryl halides and n-butyl acrylate, the palladacycle {Pdκ1-C, κ1-N-C(C6H5)C(Cl)CH2NMe2(μ-Cl)}2, 1, is merely a reservoir of the catalytically active Pd(0) species 1 (Pd ...colloids or highly active forms of low ligated Pd(0) species) that undergoes oxidative addition of the aryl halide on the surface with subsequent detachment, generating homogeneous Pd(II) species. The main catalytic cycle is initiated by oxidative addition of iodobenzene to 1, followed by the reversible coordination of the olefin to the oxidative addition product. All the unimolecular subsequent steps are indistinguishable kinetically and can be combined in a single step. This kinetic model predicts that a slight excess of alkene relative to iodobenzene leads to a rapid rise in the Pd(0) concentration while when using a slight excess of iodobenzene, relative to alkene, the oxidative addition product is the resting state of the catalytic cycle. Competitive experiments of various bromoarenes and iodoarenes with n-butyl acrylate catalyzed by 1 and CS, CP, and NCN palladacycles gave the same ρ value (2.4−2.5 for Ar−Br and 1.7−1.8 for Ar−I) for all palladacycles employed, indicating that they generate the same species in the oxidative addition step. The excellent fit of the slope with the σ0 Hammett parameter and the entropy of activation of −43 ± 8 J mol-1 K-1 are consistent with an associative process involving the development of only a partial charge in the transition state for the oxidative step of iodobenzene.
Electrospray ionization mass spectrometry (ESI‐MS) is found to gently and efficiently transfer small to large as well as singly to multiply charged X+nA−m supramolecules of imidazolium ion (X+) ionic ...liquids to the gas phase, and to reveal “magic numbers” for their most favored assemblies. Tandem mass spectrometric experiments (ESI‐MS/MS) were then used to dissociate, via low‐energy collision activation, mixed and loosely bonded A‐ ‐ ‐ ‐X‐ ‐ ‐ ‐A′− and X‐ ‐ ‐ ‐A‐ ‐ ‐ ‐X′+ gaseous supramolecules, as well as their higher homologues, and to estimate and order via Cooks' kinetic method (CKM) and B3LYP/6‐311G(d,p) calculations the intrinsic solvent‐free magnitude of hydrogen bonds. For the five anions studied, the relative order of intrinsic hydrogen‐bond strengths to the 1‐n‐butyl‐3‐methylimidazolium ion X1+ is: CF3CO2− (zero) > BF4− (−3.1) > PF6− (−10.0) > InCl4− (−16.4) and BPh4− (−17.6 kcal mol−1). The relative hydrogen‐bond strength for InCl4− was measured via CKM whereas those for the other anions were calculated and used as CKM references. A good correlation coefficient (R=0.998) between fragment ion ratios and calculated hydrogen‐bond strengths and an effective temperature (Teff) of 430 K demonstrate the CKM reliability for measuring hydrogen‐bond strengths in gaseous ionic liquid supramolecules. Using CKM and Teff of 430 K, the intrinsic hydrogen‐bond strengths of BF4− for the three cations investigated is: 1‐n‐butyl‐3‐methyl‐imidazolium ion (0) > 1,3‐di‐(R)‐3‐methyl‐2‐butyl‐imidazolium ion (−2.4) > 1,3‐di‐(R)‐α‐methylbenzyl‐imidazolium ion (−3.0 kcal mol−1). As evidenced by “magic” numbers, greater stabilities are found for the (X1)2(BF4)3− and (X1)5A4+ supramolecules (A≠InCl4−).
Gentle and efficient transfer from small to large as well as singly to multiply charged X+nA−m supramolecules of imidazolium ion (X+) ionic liquids to the gas phase was achieved by electrospray ionization mass spectrometry (see figure for example). Tandem mass spectrometry, Cooks' kinetic method as well as B3LYP/6‐311G(d,p) calculations were then used to estimate the intrinsic solvent‐free magnitude of their hydrogen bonds. As evidenced by “magic” numbers, greater stability is found particularly for (X1)5A4+ supramolecules.
The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been studied in the solid ...state by X-ray diffraction analysis and in solution by 1H NMR spectroscopy, conductivity, and microcalorimetry. In the solid state, these compounds show an extended network of hydrogen-bonded cations and anions in which one cation is surrounded by at least three anions and one anion is surrounded by at least three imidazolium cations. In the pure form, imidazolium salts are better described as polymeric supramolecules of the type {(DAI)3(X)2+(DAI)(X)32-} n (where DAI is the dialkylimidazolium cation and X is the anion) formed through hydrogen bonds of the imidazolium cation with the anion. In solution, this supramolecular structural organization is maintained to a great extent, at least in solvents of low dielectric constant, indicating that mixtures of imidazolium ionic liquids with other molecules can be considered as nanostructured materials. This model is very useful for the rationalization of the majority of the unusual behavior of the ionic liquids.
The reduction of Pd(acac)(COD)BF 4 (acac = acetylacetonate; COD = 1,5-cyclooctadiene), dissolved in BMI·BF 4 , in the presence of P- or N-containing ionophilic ligands by H 2 yields “soluble” and ...stable Pd(0) n nanoparticles (NPs). These ionic liquid soluble NPs are active and selective catalysts for the hydrogenation of 1,3-dienes and alkynes under mild reaction conditions. Selectivities up to 87% to cis -2-pentene and 95% conversion were obtained using Pd NPs modified with 1-(3-(diphenylphosphino)propyl)-2,3-dimethyl-1 H -imidazol-3-ium N -bis(trifluoromethylsulfonyl)imide in 1- n -butyl-3-methylimidazolium tetrafluoroborate.
Phosphine ligands bearing an imidazolium fragment were easily prepared by one-step radical chain addition of secondary phosphines to allyl or vinyl imidazolium salts. These ligands were used to ...prepare new ionophilic second generation Grubbs-type catalysts. The catalyst immobilized in 1-butyl-3-methyl imidazolium ILs shows good catalytic activity in RCM reactions of several substrates and, depending on the media employed, is stable up to eight cycles.
trans-3-Hexene is converted to heavier linear olefins by a tandem Ru-catalysed biphasic isomerisation/metathesis sequence. The difference in olefin metathesis and isomerisation rates is modulated by ...keeping the ionophilic metathesis catalyst in an ionic phase whilst the isomerisation catalyst is in another organic non-polar phase.
The palladacycle Pd(k2‐C,N‐C(C6H5)C(Cl)CH2NMe2)(μ‐Cl)2 (1) derived from the chloropalladation of 3‐(dimethylamino)‐1‐phenyl‐1‐propyne and its phosphine adduct ...Pd(k2‐C,N‐C(C6H5)C(Cl)CH2NMe2)P(4‐CF3C6H4)3(Cl) (3) promote the alkynylation of bromo‐ and iodoarenes under relatively mild reaction conditions. The coupling of iodoarenes and activated bromoarenes with terminal alkynes can be conducted at room temperature. Turnover numbers have been achieved up to 105 with iodoarenes and up to 94 for deactivated bromoarenes but at higher temperatures (120 °C). Selective poisoning experiments (Hg, Collman and Crabtree tests) suggest that soluble Pd(0) species are the most probable catalytically active species involved in this Csp2–Csp coupling reaction.
The palladacycle {Pdk1-C, k1-N-C(C6H5)C(Cl)CH2NMe2(μ-Cl)}2 1 derived from the chloropalladation of 3-(dimethylamino)-1-phenyl-1-propyne promotes the arylation of olefins under relatively mild ...reaction conditions. The coupling of iodoarenes and activated bromoarenes with n-butylacrylate and styrene occurs at room temperature. Turnover numbers of up to 85 000 have been achieved with deactivated bromoarenes and up to 1000 for activated chloroarenes at higher temperatures (80−150 °C).
The mixed pincer palladacycles (Me2NCH2(Cl)CCCH2CH2Y‐κN,κC,κY)PdCl (1, Y=PPh2; 2, OPPh2) and (t‐BuSCH2CH2CC(Cl)(o‐NC5H4)‐κS,κC,κN)PdCl 3 have been obtained in high yields by chloropalladation of ...heterosusbstituted alkynes Me2NCH2CCCH2CH2PPh2, Me2NCH2CCCH2CH2OPPh2 and t‐BuSCH2CH2CC(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.
