An EPR study of the C-O bond activation of 1,2-epoxybutane by ground-state Al atoms

Group 13 metal atoms react with ethers under matrix isolation conditions to give a number of interesting products. This work has been extended to include the reaction of Al atoms with 1,2-epoxybutane (CH(3)CH(2)H(2)) and its isotopomers, 1,2-epoxybutane-1,1-d(2) (CH(3)CH(2)D(2)) and 1,2-epoxybutane-2-d(1) (CH(3)CH(2)H(2)). The paramagnetic species generated in the reaction have been studied by electron paramagnetic resonance (EPR) spectroscopy. Two divalent Al insertion products were spontaneously formed. Species A, with the magnetic parameters a(Al) = 855 MHz, a(H)(1) = 28.8 MHz, a(H)(2) = 13.6 MHz, and g = 2.0014, is the C(1)-O insertion radical CH(3)CH(2). Species B, thought to result from the insertion of Al atoms into the C(2)-O bond, CH(3)CH(2), has the magnetic parameters g = 2.0003, a(Al) = 739 MHz, a(H)(1) = 15.1 MHz, a(H)(2) = 18.5 MHz, and a(H)(1) = 37.8 MHz. Support for these assignments was obtained by comparing the experimental values of the Al and H hyperfine interaction (hfi) with those calculated using a DFT method. At temperatures < 150 K, there is evidence for the formation of the alkyl radical CH(3)CH(2)CH(O(-))CH(2)* due to ring opening at the C(1)-O bond, while at higher temperatures a radical with magnetic parameters similar to those reported for 1-methallyl was detected.     

Activation of C-Cl by ground-state aluminum atoms: an EPR and DFT investigation

The reaction of ground-state Al atoms with dichloromethane (CH(2)Cl(2)) in an adamantane matrix at 77 K yielded two mononuclear Al species. The magnetic parameters, extracted from the axial EPR spectrum of Species A/A' (g(1) = 2.0037, g(2) = g(3) = 2.0030, a(Al,1) = 1307 MHz, a(Al,2) = a(Al,3) = 1273 MHz, a(35Cl) = 34 MHz and a(37Cl) = 28 MHz) were assigned to the Al-atom insertion product, ClCH(2)AlCl. Density functional theory (DFT) calculations of the values of the Al and Cl hyperfine interaction (hfi) of the Cl(1)-Cl(2)gauche conformer were in close agreement with the experimental values of ClCH(2)AlCl. The second species, B/B', had identical magnetic parameters to those of ClCH(2)AlCl with the exception that the Al hfi was 15% smaller. Coordination of a ligand, possessing a lone pair of electrons, to the Al atom of the insertion product, [ClCH(2)AlCl]:X, could cause the a(Al) to decrease by 15%. Alternatively, it is possible that the Cl(1)-Cl(2) anti conformer of ClCH(2)AlCl is also isolated in the matrix. Support for the spectral assignments is given by calculation of the nuclear hfi of [ClCH(2)AlCl]:H(2)O and the Cl(1)-Cl(2) anti conformer of ClCH(2)AlCl using a DFT method. The potential energy hypersurface for an Al atom approaching CH(2)Cl(2), calculated at the B3LYP level, suggests that Al atom abstraction of Cl forming AlCl and CH(2)Cl is favoured in the gas phase. When produced in a matrix, the close proximity of AlCl and CH(2)Cl could account for the formation of ClCH(2)AlCl. EPR evidence was also found for the formation of the CHCl(2) radical.     

Alkylation of carbonyl compounds in water: formation of C-C and C-O bonds in the presence of surfactants

The formation of C-C and C-O bonds by the reaction of enolate intermediates with electrophilic substrates commonly requires strong bases, aprotic solvents and very low temperatures. A way of performing the same reactions with sodium hydroxide at moderate temperatures in aqueous surfactant solutions is presented. Different halides, ketones and surfactants (cationic, zwitterionic and anionic) have been used. The results obtained show that the amount of ketone alkylation is much higher and that the reactions are faster in the presence than in the absence of surfactant aggregates. The hydrolysis of the halide is minimised in the presence of cationic or zwitterionic surfactants.     

Quantum calculation of the intrinsic torsional barrier of C-C and C-O bonds

PCILO and ab initio calculations have been performed to investigate the energies associated to rotation about the central bond in n-butane and methyl ethyl ether. Quantum mechanical energies have been fit to a classical intramolecular force field, containing torsional and nonbonded (Lennard-Jones 6–12 plus Coulomb) contributions, with a standard deviation comprised between 0.03 and 0.09 kcal mol^–1. Two conditions have proved indispensable to reach such level of accuracy: (a) the use of a torsional potential with threefold periodicity, which corrects for the part of the rotation barrier not covered by van der Waals repulsions and may be interpreted as bond-bond repulsion; (b) the introduction in the force field for ethers of terms accounting for orbital interaction effects of different nature than the normal molecular mechanics nonbonded interactions; these terms are represented either by low order rotational potential functions or preferably by interactions of atoms simulating lone-pair orbitals and bonded to oxygen in such a way as to render it sp ³-hybridized. According to ab initio, the height of the threefold torsional potential about C-C and C-O bonds is comparable and is of the order of 3 kcal mol^–1. According to PCILO, it is larger for C-C (ca. 1.5 kcal mol^–1) than for C-O (ca. 0.5 kcal mol^–1).     

A computational study of the reaction of ground-state nitrogen atoms with chloromethyl radicals

A computational study of the N(4S) + CH2Cl reaction has been carried out. The first step of the reaction is the formation of an initial intermediate (NCH2Cl), which is relatively stable and does not involve any energy barrier. The two most exothermic products are those resulting from the release of a chlorine atom, H2C=N + Cl and trans-HC=NH + Cl. A kinetic study within the framework of the statistical theories suggests that the kinetically preferred product is also the most exothermic one. This is in contrast with the analogue reaction of nitrogen atoms with CH2F, where the preferred product from both thermodynamic and kinetic points of view is HFCN + H. Therefore, reactions of nitrogen atoms with chloromethyl radicals release chlorine atoms as major products. The rate coefficient for the title reaction is estimated to be about 3.09 x 10(-13) cm3 s(-1) molecule(-1) at 300 K, a value four times smaller than the rate coefficient for its fluorine analogue.     

A facile tandem construction of C-O and C-C bonds: a novel one-pot transformation of Baylis-Hillman adducts into 2-benzoxepines

A facile and one-pot synthesis of 2-benzoxepines, from Baylis-Hillman adducts that is, alkyl 3-aryl-3-hydroxy-2-methylenepropanoates, via treatment with HCHO in the presence of concd H₂SO₄, involving tandem construction of C-O and C-C bonds via Prins-type and Friedel-Crafts reactions, is described.     

An experimental and computational study of the reaction of ground-state sulfur atoms with carbon disulfide

The pulsed laser photolysis/resonance fluorescence technique was used to study the reaction of S((3)P(J)) with CS(2) in an Ar bath gas. Over 290-770 K pressure-dependent kinetics were observed and low- and high-pressure limiting rate constants were derived as k(0) = (11.5-0.0133 T/K) × 10(-31) cm(6) molecule(-2) s(-1) (error limits ± 20%) and k(∞) = (2.2 ± 0.6) × 10(-12) cm(3) molecule(-1) s(-1). Equilibration observed at 690-770 K yields a CS(2)-S bond dissociation enthalpy of 131.7 ± 4.0 kJ mol(-1) at 298 K. This agrees with computed thermochemistry for a spin-forbidden C(2V) adduct, estimated at the coupled-cluster single double triple level extrapolated to the infinite basis set limit. A pressure-independent pathway, assigned to abstraction, was observed from 690 to 1040 K and can be summarized as 1.14 × 10(-10) exp(-37.0 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) with error limits of ± 40%. The results are rationalized in terms of a computed potential energy surface and transition state theory and Troe's unimolecular formalism.     

A theoretical study of the competitive homolytic/heterolytic aniomesolytic cleavages of C-O alkyl ether bonds

Density functional theory electronic structure calculations of the homolytic/heterolytic aniomesolytic C-O fragmentations in the gas phase of a series of radical anions of substituted-phenyl benzyl ethers and substituted-benzyl phenyl ethers have been carried out. Along the series, the electron-withdrawing strength of the substituents increases. An intramolecular electron transfer from the pi system to the sigma molecular orbital of the scissile C-O bond is required to produce the fragmentation. As the electron-withdrawing strength of the substituents increases, the transition-state structures appear later with higher potential energy and Gibbs free energy barriers. The homolytic mesolytic cleavages are always thermodynamically favored versus the corresponding heterolytic mesolytic ones. The heterolytic mesolytic fragmentations in radical anions containing only weak electron-withdrawing groups are faster than the corresponding homolytic mesolytic ones. Conversely, in radical anions supporting strong electron-withdrawing groups the homolytic mesolytic fragmentations are faster in terms of potential energy barriers. However, the entropic contribution makes it comparable the homolytic and the heterolytic Gibbs free energy barriers in this case. The main factors that determine the relative rates of those kind of aniomesolytic cleavages are discussed.     

Activation of dimethyl ether with barium and strontium atoms

Organometallic compounds are produced when barium and strontium atoms are cocondensed with dimethyl ether at ?196°C; hydrolysis of the cocondensation products from both reactions yields mainly C₁—C₈ alkanes, alkenes, and alkynes.     

Temperature dependence of the rate coefficients for the reactions of Br atoms with dimethyl ether and diethyl ether

The rate constants for the reactions of atomic bromine with dimethyl ether and diethyl ether were measured from approximately 300 to 350 K using the relative rate method. Both isooctane and isobutane were used as the reference reactants, and the rate constants for the reactions of these hydrocarbons were measured relative to each other over the same temperature range. The kinetic measurements were made by photolysis of dilute mixtures of bromine, the reference reactant, and the test reactant in mixtures of argon and oxygen at a total pressure of 1 atm. The resulting ratios of rate constants were combined with the absolute rate constant as a function of temperature for the reference reaction of Br with isobutane to calculate absolute rate constants for the reactions of Br with isooctane, dimethyl ether, and diethyl ether. The absolute rate constant, in the units cm3 molecule(-1) s(-1), for the reaction of Br with dimethyl ether was given by k = (3.8 +/- 2.4) x 10(-10) exp(-(3.54 +/- 0.21) x 10(3)/T) while for the reaction of Br with diethyl ether the rate constant is given by k = (2.8 +/- 2.7) x 10(-10) exp(-(2.44 +/- 0.32) x 10(3)/T). On the same basis, the rate constant for the reaction of Br with isooctane is given by k = (3.34 +/- 0.59) x 10(-12) exp(-(1.80 +/- 0.11) x 10(3)/T). In each case, the activation energy of the reaction is significantly smaller than the endothermicity of the reaction. This is discussed in terms of a complex mechanism for these reactions.     

New methods of C-C and C-O bond formation in the synthesis of conjugated polymers

New synthetic developments towards polymers containing aromatic building blocks in the polymer backbone are described. The synthesis of polyphenylene-related systems with an improved solubility due to their alkyl substitution is discussed. Via bridging of the phenylene rings a twisting of the aromatic subunits is avoided and the conjugation improved. This concept is extended to ribbon-type polymers containing 6- or 6- and 5- membered rings. It is demonstrated that during a polycondensation process anthracene can be incorporated into various polymers via C-O bond formation as well as via C-C connection. Cycloaddition reactions are presented, allowing for modification and grafting of these polymers. Additionally, the use of anthracene derivatives as initiators in homo and block copolymerisation is shown.     

An efficient intermolecular [Pd]-catalyzed C-C and intramolecular [Cu]-catalyzed C-O bonds formation: synthesis of functionalized flavans and benzoxepine

An efficient three-step strategy for the synthesis of functionalized flavans, starting from readily available 2-bromoiodobenzenes and aryl vinyl alcohols, is presented and successfully extended to benzoxepine. An intermolecular [Pd]-catalyzed C-C and an intramolecular [Cu]-catalyzed C-O bond formations have been employed as key transformations of the strategy.     

A domino palladium-catalyzed C-C and C-O bonds formation via dual O-H bond activation: synthesis of 6,6-dialkyl-6H-benzo[c]chromenes

An efficient Pd-catalyzed domino reaction of α,α-dialkyl-(2-bromoaryl)methanols to 6,6-dialkyl-6H-benzo[c]chromenes is presented. Their formation can be explained via a five membered Pd(II)-cycle that efficiently involves a domino homocoupling with the second molecule, β-carbon cleavage, and finally intramolecular Buchwald-Hartwig cyclization. This domino process effectively involves breaking of five σ-bonds (2C-Br, 2O-H, and a C-C) and formation of two new σ-bonds (C-C and C-O). This mechanistic pathway is unprecedented and further illustrates the power of transition metal catalysis.     

Cleavage of carbon-carbon bonds of diphenylacetylene and its derivatives via photolysis of Pt complexes: tuning the C-C bond formation energy toward selective C-C bond activation

Carbon-carbon bond activation of diphenylacetylene and several substituted derivatives has been achieved via photolysis and studied. Pt0-acetylene complexes with eta2-coordination of the alkyne, along with the corresponding PtII C-C activated photolysis products, have been synthesized and characterized, including X-ray crystal structural analysis. While the C-C cleavage reaction occurs readily under photochemical conditions, thermal activation of the C-C bonds or formation of PtII complexes was not observed. However, the reverse reaction, C-C reductive coupling (PtII --> Pt0), did occur under thermal conditions, allowing the determination of the energy barriers for C-C bond formation from the different PtII complexes. For the reaction (dtbpe)Pt(-Ph)(-CCPh) (2) --> (dtbpe)Pt(eta2-PhCCPh) (1), DeltaG was 32.03(3) kcal/mol. In comparison, the energy barrier for the C-C bond formation in an electron-deficient system, that is, (dtbpe)Pt(C6F5)(CCC6F5) (6) --> (dtbpe)Pt(eta2-bis(pentafluorophenyl)acetylene) (5), was found to be 47.30 kcal/mol. The energy barrier for C-C bond formation was able to be tuned by electronically modifying the substrate with electron-withdrawing or electron-donating groups. Upon cleavage of the C-C bond in (dtbpe)Pt(eta2-(p-fluorophenyl-p-tolylacetylene) (9), both (dtbpe)Pt(p-fluorophenyl)(p-tolylacetylide) (10) and (dtbpe)Pt(p-tolyl)(p-fluorophenylacetylide) (11) were obtained. Kinetic studies of the reverse reaction confirmed that 10 was more stable toward the reductive coupling [the term "reductive coupling" is defined as the formation of (dtbpe)Pt(eta2-acetylene) complex from the PtII complex] than 11 by 1.22 kcal/mol, under the assumption that the transition-state energies are the same for the two pathways. The product ratio for 10 and 11 was 55:45, showing that the electron-deficient C-C bond is only slightly preferentially cleaved.     

Kinetics of the grignard reaction with silanes in diethyl ether and ether-toluene mixtures

Kinetics of the reactions of butylmagnesium chloride and phenylmagnesium bromide with tetraethoxysilane and methyltrichlorosilane was investigated in diethyl ether and diethyl ether-toluene mixtures. Replacement of ether by toluene significantly accelerates the reaction with alkoxysilanes, while no effect was found for the reaction with chlorosilanes. We established that the reaction with alkoxysilanes consists of replacement of a donor molecule at the magnesium center by the silane followed by subsequent rearrangement of the complex to products through a four-center transition state. Chlorosilanes react differently without solvent molecule replacement but also via a four-center transition state. Large negative activation entropies are consistent with formation of cyclic transition states. Small activation enthalpy values together with remarkable exothermicity point to early transition states of the reactions.     

TiCl4 catalyzed tandem construction of C-C and C-O bonds: a simple and one-pot atom-economical stereoselective synthesis of spiro-oxindoles

An atom-economical stereoselective synthesis of [{1-acetyl-5-methyl-6,8-dioxabicyclo(3.2.1)octane}-7-spiro-3'-(indolin-2'-one)] derivatives, containing both the oxindole and 6,8-dioxabicyclo(3.2.1)octane moieties via TiCl(4) catalyzed coupling of 2-acetyl-6-methyl-2,3-dihydro-4H-pyran with isatin derivatives is described.     

Borrowing hydrogen: iridium-catalysed reactions for the formation of C-C bonds from alcohols

Alcohols have been employed as substrates for C-C bond-forming reactions which involve initial activation by the temporary removal of hydrogen to form an aldehyde. The intermediate aldehyde is converted into an alkene via a Horner-Wadsworth-Emmons reaction, nitroaldol and aldol reactions. The 'borrowed hydrogen' is then returned to the alkene to form a C-C bond.     

Gold-catalyzed tandem C-C and C-O bond formation: a highly diastereoselective formation of cyclohex-4-ene-1,2-diol derivatives

We have reported an efficient gold(I)-catalyzed tandem cyclization of tert-butyl carbonate derivatives of hex-1-en-5-yn-3-ol where nucleophilic participation of the O-Boc group appears to intercept a carbocationic (or cyclopropyl carbene) Au intermediate. This novel protocol leads to densely functionalized cyclohexene-3,4-diol derivatives where 1,2- or 1,2,3-stereocenters are controlled in a highly diastereoselective fashion.