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John F Hartwig

Publications and source records attributed to John F Hartwig.

83 records · Page 5Linked to original sources
PubMed2002

Synthesis, characterization, and reactivity of monomeric, arylpalladium halide complexes with a hindered phosphine as the only dative ligand.

We report the isolation and structural characterization of several monomeric arylpalladium(II) halide complexes containing tri-tert-butyl phosphine, 1-adamantyl-di-tert-butylphosphine, or 2-adamantyl-di-tert-butylphosphine. X-ray diffraction, IR spectroscopy, and theoretical studies indicated that the complexes may be stabilized by agostic interactions. For example, the distance from the closest hydrogen atom to the palladium metal center in the X-ray structure of the 1-adamantyl-phenylpalladium bromide complex 1 was 2.26(3) A. The calculated Pd-H distance of 2.28 A and harmonic vibrational frequencies were in agreement with the measured distance, but Wiberg bond indices indicated only weak M-H-C interactions. Addition of 2-adamantyl-di-tert-butyl phosphine to 1 led to ligand exchange and formation of 2-adamantyl-di-tert-butyl phosphine complex 2. Addition of P(t-Bu)(3) generated free aryl bromide and Pd[P(t-Bu)(3)](2). Reactivity of complex 1 with nucleophiles provided evidence of the intermediacy of these complexes in palladium-catalyzed cross-coupling reactions. Complex 1 reacted with amine and base to form the corresponding arylamine, with tert-butoxide to form the corresponding ether, with boronic acid and fluoride to form the corresponding biaryl, and with styrene to form stilbene. This complex also catalyzed the reaction of bromobenzene with diphenylamine in the presence of base to produce 94% of the amination product in 45 min at room temperature. This rate is comparable to the reaction rate of in situ generated catalysts.

Halogens↗
PubMed2002

Synthesis, characterization, and reactivity of arylpalladium cyanoalkyl complexes: selection of catalysts for the alpha-arylation of nitriles.

A new coupling process, the palladium-catalyzed alpha-arylation of nitriles, was developed by exploring the structure and reactivity of arylpalladium cyanoalkyl complexes. Complexes of 1,2-bis(diphenylphosphino)benzene (DPPBz), 1,1'-bis(di-i-propylphosphino)ferrocene (D(i)()PrPF), racemic-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP), and diphenylethylphosphine (PPh(2)Et) were prepared. Coordination to palladium through the alpha-carbon was observed for DPPBz-ligated complexes and for complexes of primary and benzylic nitrile anions. However, the anion of isobutyronitrile was coordinated to palladium through the cyano-nitrogen when the complex was ligated by D(i)()PrPF. The isobutyronitrile anion displaced a phosphine ligand to form a C,N-bridged dimer when generated from PPh(2)Et-ligated palladium. These results suggest that the nitrile anion preferentially coordinates to palladium through the carbon atom in the absence of steric effects. Thermolysis of the arylpalladium cyanoalkyl complexes led to reductive elimination that formed alpha-aryl nitriles. The high yields and short reaction times observed for BINAP-ligated complexes suggested that BINAP-ligated palladium catalysts might be appropriate for the arylation of nitriles. Initial results on a palladium-catalyzed process for the direct coupling of aryl bromides and primary, benzylic, and secondary nitrile anions to form alpha-aryl nitriles in good yields are reported.

Catalysis↗
PubMed2002

Air stable, sterically hindered ferrocenyl dialkylphosphines for palladium-catalyzed C[bond]C, C[bond]N, and C[bond]O bond-forming cross-couplings.

Pentaphenylferrocenyl di-tert-butylphosphine has been prepared in high yield from a two-step synthetic procedure, and the scope of various cross-coupling processes catalyzed by complexes bearing this ligand has been investigated. This ligand creates a remarkably general palladium catalyst for aryl halide amination and for Suzuki coupling. Turnovers of roughly 1000 were observed for aminations with unactivated aryl bromides or chlorides. In addition, complexes of this ligand catalyzed the formation of selected aryl ethers under mild conditions. The reactions encompassed electron-rich and electron-poor aryl bromides and chlorides. In the presence of catalysts containing this ligand, these aryl halides coupled with acyclic or cyclic secondary alkyl- and arylamines, with primary alkyl- and arylamines, and with aryl- and primary alkylboronic acids. These last couplings provide the first general procedure for reaction of terminal alkylboronic acids with aryl halides without toxic or expensive bases. The ligand not only generates highly active palladium catalysts, but it is air stable in solution and in the solid state. Palladium(0) complexes of this ligand are also air stable as a solid and react only slowly with oxygen in solution.

Alkylation↗
PubMed2002

Femtosecond excitation energy transport in triarylamine dendrimers.

The search for a model that can be used to describe the optical excitation migration in dendrimers has attracted great attention. In most cases in a dendrimer the conjugation is disrupted at the branching point; however, the excitation is delocalized. The strength of interactions among neighboring chromophores plays a key role in determining the energy migration mechanism. Conversely, having many identical chromophores held tightly together in an ordered macromolecular architecture will allow for many dipoles to be accessible for optical excitation. Therefore, the relative orientation of dipoles will be important in determining the mechanism of energy migration. Here we report the synthesis and photo-physical investigation of triarylamine-based dendrimers. Two important synthetic steps were utilized in the synthesis. First, we employed diphenylmethyl protective groups on the amines to assist in deprotective hydrogenolysis of the larger structures. Second, highly active catalysts for formation of both di- and triarylamines that are based on a 1:1 ratio of P(t-Bu)3 and Pd(dba)2 improved reaction yields of the C-N bond formation and decreased reaction times The energy migration processes in the dendrimers were investigated utilizing ultrafast time-resolved fluorescence anisotropy measurements. The fluorescence anisotropy of all three dendrimers decayed to a residual value within approximately 100 fs. This fluorescence anisotropy decay showed a general trend in decreasing with increasing dendrimer generation. The residual anisotropy value also showed a gradual decrease with an increase in the dendrimer generation. This fast energy depolarization is discussed through a coherent excitonic mechanism among dipoles oriented in different directions. We believe that the formation of coherent domains leads to fast energy migration extending over a large part of the dendrimer.

Journal Article↗
PubMed2002

A general nickel-catalyzed hydroamination of 1,3-dienes by alkylamines: catalyst selection, scope, and mechanism.

A simple colorimetric assay of various transition-metal catalysts showed that the combination of DPPF, Ni(COD)(2), and acid is a highly active catalyst system for the hydroamination of dienes by alkylamines to form allylic amines. The scope of the reaction is broad; various primary and secondary alkylamines react with 1,3-dienes in the presence of these catalysts. Detailed mechanistic studies revealed the individual steps involved in the catalytic process. These studies uncovered unexpected thermodynamics for the addition of amines to pi-allyl nickel complexes: instead of the thermodynamics favoring the reaction of a nickel allyl with an amine to form an allylic amine, the thermodynamics favored reaction of a nickel(0) complex with allylic amine in the presence of acid to form a Ni(II) allyl. The realization of these thermodynamics led us to the discovery that nickel and some palladium complexes in the presence or absence of acid catalyze the exchange of the amino groups of allylic amines with free amines. This exchange process was used to reveal the relative thermodynamic stabilities of various allylic amines. In addition, this exchange reaction leads to racemization of allylic amines. Therefore, the relative rate for C-N bond formation and cleavage influences the enantioselectivity of diene hydroaminations.

Alkenes↗
PubMed2002

Direct observation of aldehyde insertion into rhodium-aryl and -alkoxide complexes.

Several organorhodium(I) complexes of the general formula (PPh(3))(2)(CO)RhR (R = p-tolyl, o-tolyl, Me) were isolated and were shown to insert aryl aldehydes into the aryl-rhodium(I) bond. Under nonaqueous conditions, these reactions provided ketones in good yield. The stability of the arylrhodium(I) complexes allowed these reactions to be run also in mixtures of THF and water. In this solvent system, diarylmethanols were generated exclusively. Mechanistic studies support the formation of ketone and diarylmethanol by insertion of aldehyde into the rhodium-aryl bond and subsequent beta-hydride elimination or hydrolysis to form diaryl ketone or diarylmethanol products. Kinetic isotope effects and the formation of diarylmethanols in THF/water mixtures are inconsistent with oxidative addition of the acyl carbon-hydrogen bond and reductive elimination to form ketone. Moreover, the intermediate rhodium diarylmethoxide formed from insertion of aldehyde was observed directly during the reaction. Its structure was confirmed by independent synthesis. This complex undergoes beta-hydrogen elimination to form a ketone. This alkoxide also reacts with a second aldehyde to form esters by insertion and subsequent beta-hydrogen elimination. Thus, reactions of arylrhodium complexes with an excess of aldehyde formed esters by a double insertion and beta-hydrogen elimination sequence.

Journal Article↗
PubMed2002

Rhodium-catalyzed, regiospecific functionalization of polyolefins in the melt.

The rhodium-catalyzed, terminal-selective borylation of alkanes has been used to modify polyolefins. The functionalization of two materials, polyethylethylene (PEE) of molecular weights 1200 and 37 000, was conducted by combining bis-pinacoldiboron and 2.5 mol % [Cp*RhCl2]2 in neat polymer and heating at 150 degrees C. This procedure causes the polymer and boron reagent to melt, the catalyst to dissolve, and the reaction to form material with boryl groups at the terminal position of the polymer side chains. Oxidation of the borylated material generated polymers with hydroxyl groups at the terminal position of the side chains. The functionalization was conducted at various ratios of boron reagent to monomer. The resulting borylated and subsequent hydoxylated materials were characterized by 1H and 13C NMR spectroscopy, as well as MALDI-MS and GPC. Little change in polymer molecular weight and polydispersity was observed, and these data indicate that scission of the main chain does not occur. Measurements of the Tg of the polymers showed in increase in Tg of up to 50 degrees C after the modification. Thus, homogeneous, catalytic, selective alkane functionalization can be used to modify polymer properties.

Journal Article↗
PubMed2002

A new pathway for hydroamination. Mechanism of palladium-catalyzed addition of anilines to vinylarenes.

The mechanism of the hydroamination of vinylarenes with anilines catalyzed by phosphine-ligated palladium triflates was uncovered. eta3-Arylethyl diphosphine palladium triflate complexes, which result from migratory insertion of vinylarene into a palladium hydride triflate, were shown to be the resting state of the catalytic cycle. A series of these complexes has been isolated and fully characterized. The [(R)-Tol-BINAP][1-(2-naphthyl)ethyl]palladium triflate derivative 1a was crystallographically characterized. This complex reacted with aniline to form the N-phenethylaniline product in 83% yield. Reaction of the benzylic derivative [(R)-Tol-BINAP](eta3-benzyl)palladium triflate with aniline also formed the N-benzylaniline product in a high 87% yield. Predominant inversion of configuration from the reaction between 1a, which is enantiopure, and aniline showed that external attack of the amine on the eta3-arylethyl ligand occurred to form the product. This product from reaction of aniline with 1a is the opposite enantiomer to that obtained from the catalytic process. Thus, a minor diastereomer gives the major enantiomer in the catalytic cycle, and the major diastereomer gives the minor enantiomer. Consistent with this assertion, kinetic studies showed that the major diastereomer formed product with a rate constant roughly 3.5 times slower than the rate constant for the catalytic process that contains all diastereomers.

Amination↗
PubMed2002

Palladium-catalyzed arylation of malonates and cyanoesters using sterically hindered trialkyl- and ferrocenyldialkylphosphine ligands.

Palladium-catalyzed reactions of aryl bromides and chlorides with two common stabilized carbanions-enolates of dialkyl malonates and alkyl cyanoesters-are reported. An exploration of the scope of these reactions was conducted, and the processes were shown to occur in a general fashion. Using P(t-Bu)(3) (1), the pentaphenylferrocenyl ligand (Ph(5)C(5))Fe(C(5)H(4))P(t-Bu)(2) (2), or the adamantyl ligand (1-Ad)P(t-Bu)(2) (3), reactions of electron-poor and electron-rich, sterically hindered and unhindered aryl bromides and chlorides were shown to react with diethyl malonate, di-tert-butyl malonate, diethyl fluoromalonate, ethyl cyanoacetate, and ethyl phenylcyanoacetate. Although alkyl malonates and ethyl alkylcyanoacetates did not react with aryl halides using these catalysts, the same products were formed conveniently in one pot from diethylmalonate by cross-coupling of an aryl halide in the presence of excess base and subsequent alkylation.

Acetates↗
PubMed2002

Mild iridium-catalyzed borylation of arenes. High turnover numbers, room temperature reactions, and isolation of a potential intermediate.

The borylation of arenes leads to the formation of synthetically versatile products from unactivated arene reagents. We report that Ir(I) precursors in conjunction with bipyridine ligands catalyze in high yields the borylation of arenes under mild conditions. These reactions encompase arenes bearing both electron-withdrawing and electron-donating substituents. The temperatures required for the transformation are much lower than those previously reported for direct arene borylation. The combination of [Ir(COE)2Cl]2 and (4,4-di-t-butyl)bipyridine even allows for reaction at room temperature. The same catalyst system at 100 degrees C provides remarkably high turnover numbers for a hydrocarbon functionalization process. Mechanistic studies show that the reactions involve uncommon, Ir(II) tris-boryl complexes. An example of this type of complex ligated by di-t-butylbipyridine was isolated and structurally characterized. It reacted rapidly at room temperature to produce aryl boronate esters in high yields.

Journal Article↗