Efficient cross-coupling reactions of aryl chlorides and bromides with phenyl- or vinyltrimethoxysilane mediated by a Palladium/Imidazolium chloride system

A combination of palladium acetate and the imidazolium salt IPr.HCl (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) has proven to be highly efficient in the cross coupling reactions of aryl bromides and electron-deficient aryl chlorides with phenyltrimethoxysilane or vinyltrimethoxysilane. The catalytic performance of this system was found to be comparable to that of systems using PCy(3) and P(o-tol)(3).     

Chemistry surrounding monomeric copper(I) methyl, phenyl, anilido, ethoxide, and phenoxide complexes supported by N-heterocyclic carbene ligands: reactivity consistent with both early and late transition metal systems

Monomeric copper(I) alkyl complexes that possess the N-heterocyclic carbene (NHC) ligands IPr, SIPr, and IMes [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] react with amines or alcohols to release alkane and form the corresponding monomeric copper(I) amido, alkoxide, or aryloxide complexes. Thermal decomposition reactions of (NHC)Cu(I) methyl complexes at temperatures between 100 and 130 degrees C produce methane, ethane, and ethylene. The reactions of (NHC)Cu(NHPh) complexes with bromoethane reveal increasing nucleophilic reactivity at the anilido ligand in the order (SIPr)Cu(NHPh) < (IPr)Cu(NHPh) < (IMes)Cu(NHPh) < (dtbpe)Cu(NHPh) [dtbpe = 1,2-bis(di-tert-butylphosphino)ethane]. DFT calculations suggest that the HOMO for the series of Cu anilido complexes is localized primarily on the amido nitrogen with some ppi(anilido)-dpi(Cu) pi-character. [(IPr)Cu(mu-H)]2 and (IPr)Cu(Ph) react with aniline to quantitatively produce (IPr)Cu(NHPh)/dihydrogen and (IPr)Cu(NHPh)/benzene, respectively. Analysis of the DFT calculations reveals that the conversion of [(IPr)Cu(mu-H)]2 and aniline to (IPr)Cu(NHPh) and dihydrogen is favorable with DeltaH approximately -7 kcal/mol and DeltaG approximately -9 kcal/mol.     

(IPr)Pd(acac)Cl: an easily synthesized, efficient, and versatile precatalyst for C-N and C-C bond formation

A very straightforward synthesis of (IPr)Pd(acac)Cl from two commercially available starting materials, Pd(acac)2 and IPr.HCl [acac = acetylacetonate; IPr = N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], has been developed. The resulting complex, (IPr)Pd(acac)Cl (1), has proven to be a highly active PdII precatalyst in the Buchwald-Hartwig and the alpha-ketone arylation reactions. A wide range of substrates has been screened, including unactivated, sterically hindered, and heterocyclic aryl chlorides.     

Ligand-Facilitated Reductive Coupling of Benzyl Chlorides with Aryl Chlorides Catalyzed by Well-Defined Heteroleptic Ni (II)-NHC Complexes

Novel heteroleptic Ni (II) complexes bearing a highly hindered yet flexible IPr^* ligand, Ni (IPr^*)(PPh₃)Br₂ ( 1 ) and Ni (IPr^*)(PCy₃)Br₂ ( 2 ) (IPr^* = 1,3-bis(2,6-bis (diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene), were easily prepared in 78% and 89% yield, respectively. Both were characterized by elemental analysis and NMR spectroscopy, and 1 was subjected to X-ray crystallography. Compared with 2 and its analogue bearing a less sterically demanding IPr ligand (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), complex 1 exhibited superior catalytic activity in the magnesium-mediated reductive coupling of benzyl chlorides with aryl chlorides, featuring outstanding tolerance of both coupling partners with steric demand. This study discloses a ligand-facilitated reductive coupling of benzyl chlorides with aryl chlorides, which provides a new and practical synthetic tool for the synthesis of diarylmethanes.     

Stoichiometric and catalytic reactivity of the N-heterocyclic carbene ruthenium hydride complexes [Ru(NHC)(L)(CO)HCl] and [Ru(NHC)(L)(CO)H(eta2-BH4)] (L=NHC, PPh3)

Thermolysis of [Ru(AsPh3)3(CO)H2] with the N-aryl heterocyclic carbenes (NHCs) IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) or the adduct SIPr.(C6F5)H (SIPr=1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene), followed by addition of CH2Cl2, affords the coordinatively unsaturated ruthenium hydride chloride complexes [Ru(NHC)2(CO)HCl] (NHC=IMes , IPr , SIPr ). These react with CO at room temperature to yield the corresponding 18-electron dicarbonyl complexes . Reduction of and [Ru(IMes)(PPh3)(CO)HCl] () with NaBH4 yields the isolable borohydride complexes [Ru(NHC)(L)(CO)H(eta2-BH4)] (, L=NHC, PPh3). Both the bis-IMes complex and the IMes-PPh3 species react with CO at low temperature to give the eta1-borohydride species [Ru(IMes)(L)(CO)2H(eta1-BH4)] (L=IMes , PPh3), which can be spectroscopically characterised. Upon warming to room temperature, further reaction with CO takes place to afford initially [Ru(IMes)(L)(CO)2H2] (L=IMes, L=PPh3) and, ultimately, [Ru(IMes)(L)(CO)3] (L=IMes , L=PPh3). Both and lose BH3 on addition of PMe2Ph to give [Ru(IMes)(L)(L')(CO)H2](L=L'=PMe2Ph; L=PPh3, L'=PMe2Ph). Compounds and have been tested as catalysts for the hydrogenation of aromatic ketones in the presence of (i)PrOH and H2. For the reduction of acetophenone, catalytic activity varies with the NHC present, decreasing in the order IPr>IMes>SIMes.     

Modified (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes for room-temperature Suzuki-Miyaura and Buchwald-Hartwig reactions

A series of (NHC)Pd(R-allyl)Cl complexes [NHC: IPr = N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, SIPr = N,N'-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene; R = H, Me, gem-Me2, Ph] have been synthesized and fully characterized. When compared to (NHC)Pd(allyl)Cl, substitution at the terminal position of the allyl scaffold favors a more facile activation step. This translates into higher catalytic activity in the Suzuki-Miyaura and Buchwald-Hartwig reactions, allowing for the coupling of unactivated aryl chlorides at room temperature in minutes. In the Suzuki-Miyaura reaction, aryl triflates, bromides, and chlorides react with boronic acids using very low catalyst loading. In the N-aryl amination reaction, a wide range of substrates has been coupled efficiently; primary-, secondary-, alkyl-, or aryl-amines react in high yields with unactivated, neutral, and activated aryl chlorides and bromides. In both reactions, extremely hindered substrates such as tri-ortho-substituted biaryls and tetra-ortho-substituted diarylamines can be produced without loss of activity. Finally, the present catalytic system has proven to be efficient with as low as 10 parts-per-million (ppm) of precatalyst in the Buchwald-Hartwig reaction and 50 ppm in the Suzuki-Miyaura reaction.     

Generation of [(IPr)Pd(PR2Cl)] complexes via P-Cl reductive elimination

Treatment of [(IPr)Pd(Cl)(2)(PR(2)H)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; R = Cy, tBu, or 1-Ad) with NaN(SiMe(3))(2) generated isolable [(IPr)Pd(PR(2)Cl)] complexes (68-75%) that have been crystallographically characterized. The formation of these mixed-ligand Pd(0) species in this manner corresponds to an unusual net dehydrohalogenation/P-Cl reductive elimination sequence.     

Room-temperature activation of aryl chlorides in Suzuki-Miyaura coupling using a [Pd(micro-Cl)Cl(NHC)]2 complex (NHC = N-heterocyclic carbene)

A straightforwardly synthesised complex, [Pd(micro-Cl)Cl(NHC)](2) (NHC = bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr), has been employed to mediate Suzuki-Miyaura reactions involving aryl chlorides at very low catalyst loadings and at room temperature.     

Room-temperature Negishi cross-coupling of unactivated alkyl bromides with alkyl organozinc reagents utilizing a Pd/N-heterocyclic carbene catalyst

A high-yielding cross-coupling reaction of unactivated alkyl bromides possessing beta-hydrogens with alkylzinc halides utilizing a Pd/N-heterocyclic carbene (NHC) catalyst at room temperature is described. A variety of Pd sources, Pd2(dba)3, Pd(OAc)2, or PdBr2, with the commercially available ligand precursor 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IPr.HCl) successfully coupled 1-bromo-3-phenylpropane with n-butylzinc bromide in THF/NMP. An investigation of different NHC precursors showed that the bulky 2,6-diisopropylphenyl moiety was necessary to achieve high coupling yields (75-85%). The corresponding ethyl analogue was moderately active (11%). A range of unsymmetrical NHC precursors were prepared and evaluated. The ligand precursor containing one 2,6-diisopropylphenyl and one 2,6-diethylphenyl afforded the coupling product in 47% yield, clearly suggesting a direct relationship between the steric topography created by the flanking N-substituents and catalyst activity. Under optimal conditions, a number of alkyl bromides and alkylzinc halides possessing common functional groups (amide, nitrile, ester, acetal, and alkyne) were effectively coupled (61-92%). It is noteworthy that beta-substituted alkyl bromides and alkylzinc halides successfully underwent cross-coupling. Also, under these conditions alkyl chlorides were unaffected.     

Protonation of N-heterocyclic carbene ligand coordinated to copper(I): Coordination mode of imidazolium cation as a function of counterion as determined by solid-state structures

Reactions of (IPr)Cu(X) (X = Cl or trifluoromethanesulfonate, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) complexes with the strong acids HOTf or HCl result in protonation of the C2 carbon of the IPr ligand to form imidazolium cations. Coordination of the imidazolium to the resulting Cu^I system depends upon the identity of the two counterions (chloride or triflate). The copper complexes [(IPrH)Cu(OTf)(μ-OTf)]₂ and [IPrH][CuCl₂] as well as the imidazolium salt [IPrH][OTf] have been characterized by NMR spectroscopy and single crystal X-ray diffraction studies.     

Anti-Markovnikov hydroamination and hydrothiolation of electron-deficient vinylarenes catalyzed by well-defined monomeric copper(I) amido and thiolate complexes

Monomeric Cu(I) amido and thiolate complexes that are supported by the N-heterocyclic carbene ligand 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) catalyze the hydroamination and hydrothiolation of electron-deficient vinylarenes with reactivity patterns that are consistent with an intermolecular nucleophilic addition of the amido/thiolate ligand of (IPr)Cu(XR) (X = NH or S; R = Ph, CH2Ph) to free vinylarene.     

The synthesis and X-ray structure of trans-NiCl2(1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)2; attempts to polymerize olefins utilizing a nickel(II) complex of a sterically demanding N-heterocyclic carbene

The compound trans-NiCl₂(IPr)₂ (IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) is synthesized and characterized crystallographically. In combination with MAO, it is converted to an intermediate which catalyses the dimerization of ethylene but which not the polymerization of ethylene, propylene or 1-hexene.     

Efficient nickel-mediated intramolecular amination of aryl chlorides

The use of an in situ generated Ni(0) catalyst associated with 2,2'-bipyridine or N,N'-bis(2,6-diisopropylphenyl)dihydroimidazol-2-ylidene (SIPr) as a ligand and NaO-t-Bu as the base for the intramolecular coupling of aryl chlorides with amines is described. The procedure has been applied to the formation of five-, six-, and seven-membered rings. [reaction: see text]     

Suzuki-Miyaura, alpha-ketone arylation and dehalogenation reactions catalyzed by a versatile N-heterocyclic carbene-palladacycle complex

[reaction: see text] The activity of the complex (IPr)PdCl(eta2-N,C-C12H7NMe2), 1 [IPr = (N,N'-bis(2,6-diisopropylphenyl)imidazol)-2-ylidene], in the Suzuki-Miyaura cross-coupling reaction involving unactivated aryl chlorides and triflates with arylboronic acids at room temperature in technical grade 2-propanol is described. These conditions allow for the synthesis of di- and tri-ortho-substituted biaryls in very short reaction times. This complex also displays very high activity for alpha-ketone arylation and dehalogenation reactions of activated and unactivated aryl chlorides.     

(N-heterocyclic carbene)-Pd-catalyzed anaerobic oxidation of secondary alcohols and domino oxidation-arylation reactions

The use of commercially available (SIPr)Pd(cinnamyl)Cl (SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene) as a precatalyst for the anaerobic oxidation of secondary alcohols is described. The use of this complex allows for a drastic reduction in the reaction times and catalyst loading when compared to the unsaturated counterpart. This catalytic system is compatible with the use of microwave dielectric heating, decreasing even further catalyst loading and reaction times. Domino Pd-catalyzed oxidation-arylation reactions of secondary alcohols are also presented.     

Catalytic diboration of aldehydes via insertion into the copper-boron bond

Mesitaldehyde reacts cleanly with (IPr)CuB(pin) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene); pin = 2,3-dimethyl-2,3-butanediolate] to afford the product complex 1, the first well-defined product of carbonyl group insertion into a metal-boron bond. Analysis of 1 by NMR spectroscopy and single-crystal X-ray diffraction indicates the formation of a copper-carbon and a boron-oxygen bond. A copper(I) precatalyst supported by the less sterically demanding ligand ICy (1,3-dicyclohexylimidazol-2-ylidene) achieves the efficient 1,2-diboration of aryl-, heteroaryl-, and alkyl-substituted aldehydes at room temperature.     

Anhydrous TEMPO-H: reactions of a good hydrogen atom donor with low-valent carbon centres

In this paper, we report a novel synthesis of anhydrous 1-hydroxy-2,2,6,6-tetramethyl-piperidine (TEMPO-H). An X-ray crystal structure and full characterization of the compound are included. Compared to hydrated TEMPO-H, its anhydrous form exhibits improved stability and a differing chemical reactivity. The reactions of anhydrous TEMPO-H with a variety of low-valent carbon centres are described. For example, anhydrous TEMPO-H was reacted with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), an unsaturated NHC. Crystals of [CHNC(6)H(2)(CH(3))(3)](2)C···HO(NC(5)H(6)(CH(3))(4)), IMes···TEMPO-H, were isolated and a crystal structure determined. The experimental structure is compared to the results of theoretical calculations on the hydrogen-bonded dimer. Anhydrous TEMPO-H was also reacted with the saturated NHC, 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr), giving the product [CH(2)Ni-Pr(2)C(6)H(3)](2)CH···O(NC(5)H(6)(CH(3))(4)). In contrast, the reaction of hydrated TEMPO-H with 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene gave small amounts of the hydrolysis product, N-(2,6-diisopropylphenyl)-N-[2-(2,6-diisopropylphenylamino)ethyl]formamide. Finally, anhydrous TEMPO-H was reacted with (triphenylphosphoranylidene)ketene to generate Ph(3)PC(H)C(=O)O(NC(5)H(6)(CH(3))(4)). A full characterization of the product, including an X-ray crystal structure, is described.     

1,3-dialkyl- and 1,3-diaryl-3,4,5,6-tetrahydropyrimidin-2-ylidene rhodium(i) and palladium(II) complexes: synthesis, structure, and reactivity

The synthesis of novel 1,3-diaryl- and 1,3-dialkylpyrimidin-2-ylidene-based N-heterocyclic carbenes (NHCs) and their rhodium(i) and palladium(II) complexes is described. The rhodium compounds bromo(cod)[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (7), bromo(cod)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (8) (cod=eta(4)-1,5-cyclooctadiene, mesityl=2,4,6-trimethylphenyl), chloro(cod)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (9), and chloro(cod)[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (10) were prepared by reaction of [[Rh(cod)Cl](2)] with lithium tert-butoxide followed by addition of 1,3-dimesityl-3,4,5,6-tetrahydropyrimidinium bromide (3), 1,3-dimesityl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate (4), 1,3-di-2-propyl-3,4,5,6-tetrahydropyrimidinium bromide (6), and 1,3-di-2-propyl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate, respectively. Complex 7 crystallizes in the monoclinic space group P2(1)/n, and 8 in the monoclinic space group P2(1). Complexes 9 and 10 were used for the synthesis of the corresponding dicarbonyl complexes dicarbonylchloro(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (11), and dicarbonylchloro[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (12). The wavenumbers nu(CO I)/nu(CO II) for 11 and 12 were used as a quantitative measure for the basicity of the NHC ligand. The values of 2062/1976 and 2063/1982 cm(-1), respectively, indicate that the new NHCs are among the most basic cyclic ligands reported so far. Compounds 3 and 6 were additionally converted to the corresponding cationic silver(i) bis-NHC complexes [Ag(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2)]AgBr(2) (13) and [Ag[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene](2)]AgBr(2) (14), which were subsequently used in transmetalation reactions for the synthesis of the corresponding palladium(II) complexes Pd(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2) (2+)(Ag(2)Br(2)Cl(4) (4-))(1/2) (15) and Pd[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2)]Cl(2) (16). Complex 15 crystallizes in the monoclinic space group P2(1)/c, and 16 in the monoclinic space group C(2)/c. The catalytic activity of 15 and 16 in Heck-type reactions was studied in detail. Both compounds are highly active in the coupling of aliphatic and aromatic vinyl compounds with aryl bromides and chlorides with turnover numbers (TONs) up to 2000000. Stabilities of 15 and 16 under Heck-couplings conditions were correlated with their molecular structure. Finally, selected kinetic data for these couplings are presented.     

Nickel-catalyzed cycloadditions of unsaturated hydrocarbons, aldehydes, and ketones

The nickel-catalyzed cycloaddition of unsaturated hydrocarbons and carbonyls is reported. Diynes and enynes were used as coupling partners. Carbonyl substrates include both aldehdyes and ketones. Reactions of diynes and aldehydes afforded the [3,3] electrocyclic ring-opened tautomers, rather than pyrans, in high yields. The cycloaddition reaction of enynes and aldehydes afforded two distinct products. A new carbon-carbon bond is formed, prior to a competitive beta-hydrogen elimination of a nickel alkoxide, between the carbonyl carbon and either one of the carbons of the olefin or the alkyne. The steric hindrance of the enyne greatly affected the chemoselectivity of the cycloaddition of enynes and aldehydes. In some cases, dihydropyran was also formed. The scope of the cycloaddition reaction was expanded to include the coupling of enynes and ketones. No beta-hydrogen elimination was observed in cycloaddition reaction of enynes and ketones. Instead, C-O bond-forming reductive elimination occurred exclusively to afford dihydropyrans in excellent yields. In all cases, complete chemoselectivity was observed; only dihydropyrans where the carbonyl carbon forms a carbon-carbon bond with a carbon of the olefin, rather than of the alkyne, were observed. All cycloaddition reactions occur at room temperature and employ nickel catalysts bearing the hindered 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or its saturated analogue, 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolin-2-ylidene (SIPr).     

Kumada-Tamao-Corriu Coupling of Heteroaromatic Chlorides and Aryl Ethers Catalyzed by (IPr)Ni(allyl)Cl

The complex (IPr)Ni(allyl)Cl (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidene) catalyzes the cross-coupling reactions of heteroaromatic chlorides with aryl Grignard reagents. Catalyst loadings as low as 0.1 mol % have been used to afford the products in excellent yields. This nickel-based catalytic system also promotes the activation of the C(Ar)-O bond of anisoles in the Kumada-Tamao-Corriu reaction under fairly mild conditions.