30 research outputs found
Ortho-Fluoro Effect on the C–C Bond Activation of Benzonitrile Using Zerovalent Nickel
The effect of fluoro substitution on the C–C bond activation of aromatic nitriles has been studied by reacting a variety of fluorinated benzonitriles with the nickel(0) fragment [Ni(dippe)] and by locating the reaction intermediates and transition-state structures on the potential energy surface by using density functional theory calculations with the [Ni(dmpe)] fragment (dippe = 1,2-bis(diisopropylphosphino)ethane, dmpe = 1,2-bis(dimethylphosphino)ethane). As in the previous reports, the reaction of fluorinated benzonitriles with the [Ni(dippe)] fragment initially formed an η2-nitrile complex, which then converted to the C–CN bond activation product. Thermodynamic parameters for the equilibrium between these complexes have been determined experimentally in both a polar (tetrahydrofuran) and a nonpolar (toluene) solvent for 3-fluoro- and 4-fluorobenzonitrile. The stability of the C–C bond activation products is shown to be strongly dependent on the number of ortho-F substituents (−6.6 kcal/mol per o-F) and only slightly dependent on the number of meta-F substituents (−1.8 kcal/mol per m-F)
Experimental and Theoretical Mechanistic Investigation of the Iridium-Catalyzed Dehydrogenative Decarbonylation of Primary Alcohols
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DFT Investigation of para-Substituent Effects on the C—C Bond Activation of Benzonitrile by a Zerovalent Nickel Complex
C—C bond activation has been an active area of research due to its extensive range of applications in in industry and synthesis. Despite its significance, the cleavage of a C—C bond has been challenging due to the thermodynamic stability and steric hindrance of C—C σ-bonds. In this study, density functional theory (DFT) calculations on the C—CN bond activation of para-substituted benzonitriles, p-XC6H4CN, where X= NH2, OCH3, CH3, H, F, CO2CH3 CF3 and CN, with the [Ni(dmpe)] fragment as a model for [Ni(dippe)] fragment will be reported. A comparison of the computational results with the previously reported experimental results and the natural bond analysis (nbo) on the η2-nitrile complexes and the Ni(II) oxidative addition products will also be presented
Etheric C–O Bond Hydrogenolysis Using a Tandem Lanthanide Triflate/Supported Palladium Nanoparticle Catalyst System
Selective hydrogenolysis of cyclic and linear ether C–O
bonds is accomplished by a tandem catalytic system consisting of lanthanide
triflates and sinter-resistant supported palladium nanoparticles in
an ionic liquid. The lanthanide triflates catalyze endothermic dehydroalkoxylation,
while the palladium nanoparticles hydrogenate the resulting intermediate
alkenols to afford saturated alkanols with high overall selectivity.
The catalytic C–O hydrogenolysis is shown to have significant
scope, and the C–O bond cleavage is turnover-limiting
Rapid Ether and Alcohol C–O Bond Hydrogenolysis Catalyzed by Tandem High-Valent Metal Triflate + Supported Pd Catalysts
The thermodynamically leveraged conversion
of ethers and alcohols to saturated hydrocarbons is achieved efficiently
with low loadings of homogeneous MÂ(OTf)<sub><i>n</i></sub> + heterogeneous Pd tandem catalysts (M = transition metal; OTf =
triflate; <i>n</i> = 4). For example, HfÂ(OTf)<sub>4</sub> mediates rapid endothermic ether ⇌ alcohol and alcohol ⇌
alkene equilibria, while Pd/C catalyzes the subsequent, exothermic
alkene hydrogenation. The relative C–O cleavage rates scale
as 3° > 2° > 1°. The reaction scope extends to
efficient conversion of biomass-derived ethers, such as THF derivatives,
to the corresponding alkanes
Reaction Pathways and Energetics of Etheric C–O Bond Cleavage Catalyzed by Lanthanide Triflates
Efficient
and selective cleavage of etheric C–O bonds is
crucial for converting biomass into platform chemicals and liquid
transportation fuels. In this contribution, computational methods
at the DFT B3LYP level of theory are employed to understand the efficacy
of lanthanide triflate catalysts (LnÂ(OTf)<sub>3</sub>, Ln = La, Ce,
Sm, Gd, Yb, and Lu) in cleaving etheric C–O bonds. In agreement
with experiment, the calculations indicate that the reaction pathway
for C–O cleavage occurs via a C–H → O–H
proton transfer in concert with weakening of the C–O bond of
the coordinated ether substrate to ultimately yield a coordinated
alkenol. The activation energy for this process falls as the lanthanide
ionic radius decreases, reflecting enhanced metal ion electrophilicity.
Details of the reaction mechanism for YbÂ(OTf)<sub>3</sub>-catalyzed
ring opening are explored in depth, and for 1-methyl-<i>d</i><sub>3</sub>-butyl phenyl ether, the computed primary kinetic isotope
effect of 2.4 is in excellent agreement with experiment (2.7), confirming
that etheric ring-opening pathway involves proton transfer from the
methyl group alpha to the etheric oxygen atom, which is activated
by the electrophilic lanthanide ion. Calculations of the catalytic
pathway using eight different ether substrates indicate that the more
rapid cleavage of acyclic versus cyclic ethers is largely due to entropic
effects, with the former C–O bond scission processes increasing
the degrees of freedom/particles as the transition state is approached
Palladium-Catalyzed C8-Selective C–H Arylation of Quinoline <i>N</i>‑Oxides: Insights into the Electronic, Steric, and Solvation Effects on the Site Selectivity by Mechanistic and DFT Computational Studies
We
report herein a palladium-catalyzed C–H arylation of
quinoline <i>N</i>-oxides that proceeds with high selectivity
in favor of the C8 isomer. This site selectivity is unusual for palladium,
since all of the hitherto described methods of palladium-catalyzed
C–H functionalization of quinoline <i>N</i>-oxides
are highly C2 selective. The reaction exhibits a broad synthetic scope
with respect to quinoline <i>N</i>-oxides and iodoarenes
and can be significantly accelerated to subhour reaction times under
microwave irradiation. The C8-arylation method can be carried out
on a gram scale and has excellent functional group tolerance. Mechanistic
and density functional theory (DFT) computational studies provide
evidence for the cyclopalladation pathway and describe key parameters
influencing the site selectivity
Axial Ligand Exchange of <i>N</i>‑heterocyclic Cobalt(III) Schiff Base Complexes: Molecular Structure and NMR Solution Dynamics
The kinetic and thermodynamic ligand exchange dynamics
are important
considerations in the rational design of metal-based therapeutics
and therefore, require detailed investigation. CoÂ(III) Schiff base
complex derivatives of bisÂ(acetylacetone)Âethylenediimine [acacen]
have been found to be potent enzyme and transcription factor inhibitors.
These complexes undergo solution exchange of labile axial ligands.
Upon dissociation, CoÂ(III) irreversibly interacts with specific histidine
residues of a protein, and consequently alters structure and causes
inhibition. To guide the rational design of next generation agents,
understanding the mechanism and dynamics of the ligand exchange process
is essential. To investigate the lability, pH stability, and axial
ligand exchange of these complexes in the absence of proteins, the
pD- and temperature-dependent axial ligand substitution dynamics of
a series of <i>N</i>-heterocyclic [CoÂ(acacen)Â(X)<sub>2</sub>]<sup>+</sup> complexes [where X = 2-methylimidazole (2MeIm), 4-methylimidazole
(4MeIm), ammine (NH<sub>3</sub>), <i>N</i>-methylimidazole
(NMeIm), and pyridine (Py)] were characterized by NMR spectroscopy.
The pD stability was shown to be closely related to the nature of
the axial ligand with the following trend toward aquation: 2MeIm >
NH<sub>3</sub> ≫ 4MeIm > Py > Im > NMeIm. Reaction
of each
[CoÂ(III)Â(acacen)Â(X)<sub>2</sub>]<sup>+</sup> derivative with 4MeIm
showed formation of a mixed ligand CoÂ(III) intermediate via a dissociative
ligand exchange mechanism. The stability of the mixed ligand adduct
was directly correlated to the pD-dependent stability of the starting
CoÂ(III) Schiff base with respect to [CoÂ(acacen)Â(4MeIm)<sub>2</sub>]<sup>+</sup>. Crystal structure analysis of the [CoÂ(acacen)Â(X)<sub>2</sub>]<sup>+</sup> derivatives confirmed the trends in stability
observed by NMR spectroscopy. Bond distances between the CoÂ(III) and
the axial nitrogen atoms were longest in the 2MeIm derivative as a
result of distortion in the planar tetradentate ligand, and this was
directly correlated to axial ligand lability and propensity toward
exchange