Tethered olefin studies of alkene versus tetraphenylborate coordination and lanthanide olefin interactions in metallocenes

The tethered olefin cyclopentadienyl ligand, [(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](-), forms unsolvated metallocenes, [(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](2)Ln (Ln = Sm, 1; Eu, 2; Yb, 3), from [(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))]K and LnI(2)(THF)(2) in good yield. Each complex in the solid state has both tethered olefins oriented toward the Ln metal center with the Ln-C(terminal alkene carbon) distances 0.2-0.3 A shorter than the Ln-C(internal alkene carbon) distances. The olefinic C-C bond distances in 2 and 3, 1.328(4) and 1.328(5) A, respectively, are normal. Like its permethyl analogue, (C(5)Me(5))(2)Sm(THF)(2), complex 1 reductively couples CO(2) to form the oxalate-bridged dimer [[(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](2)Sm](2)(mu-eta(2):eta(2)-O(2)CCO(2)), 4, in which the tethered olefins are noninteracting substituents. Complex 1 reacts with AgBPh(4) to form an unsolvated cation that has the option of coordinating [BPh(4)](-) or a pendant olefin, a competition common in olefin polymerization catalysis. The structure of [[(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](2)Sm][BPh(4)], 5, shows that both pendant olefins are located near samarium rather than the [BPh(4)](-) counterion.     

Substituent effect on the tautomerism of 5-acetyl-2-methoxydihydropyrimidines: experimental NMR and computational DFT studies

Some selected 5-acetyl-2-methoxydihydropyrimidines (DHPMs) are synthesized and the electronic effect of the substituent on the 4-position of the heterocyclic ring on the ratio of the possible tautomeric forms is investigated using dynamic NMR experiments. The results of this study indicate that these compounds coexist in two tautomeric 1,4- and 3,4-forms in the solution and their tautomeric ratios depend on the electronic nature of the additional substituent on the 4-phenyl substituent, the nature of solvent used for NMR study and also on the temperature. B3LYP/6–31 ++G** computational studies in the gas and solution phases elucidate the effect of the additional substituent on the phenyl ring on the structural, electronic and bonding characteristics of the tautomeric forms in these compounds. Furthermore, solvation of these DHPMs determines the relative stabilities of the two tautomers in the solution phase, which cannot be probed by model solvents adopted in SCRF calculations.     

Z-Selectivity in olefin metathesis with chelated Ru catalysts: computational studies of mechanism and selectivity

The mechanism and origins of Z-selectivity in olefin metathesis with chelated Ru catalysts were explored using density functional theory. The olefin approaches from the "side" position of the chelated Ru catalysts, in contrast to reactions with previous unchelated Ru catalysts that favor the bottom-bound pathway. Steric repulsions between the substituents on the olefin and the N-substituent on the N-heterocyclic carbene ligand lead to highly selective formation of the Z product.     

Sequence dependence in base flipping: experimental and computational studies

Base flipping is the movement of a DNA base from an intrahelical, base-stacked position to an extrahelical, solvent-exposed position. As there are favorable interactions for an intrahelical base, both hydrogen bonding and base stacking, base flipping is expected to be energetically prohibitive for an undamaged DNA duplex. For damaged DNA bases, however, the energetic cost of base flipping may be considerably lower. Using a selective, non-covalent assay for base flipping, the sequence dependence of base flipping in DNA sequences containing an abasic site has been studied. The dissociation constants of the zinc-cyclen complex to small molecules and single strands of DNA as well as the equilibrium constants for base flipping have been determined for these sequences. Molecular dynamics simulations of the zinc-cyclen complex bound to both single- and double-stranded DNA have been performed in an attempt to rationalize the differences in the dissociation constants obtained for the two systems. The results are compared to previous studies of base flipping in DNA containing an abasic site.     

Hydrogen and deuterium atoms in octasilsesquioxanes: experimental and computational studies

The rate of detrapping of atomic hydrogen from several octasilsesquioxanes is the same for dissolved and solid samples and is independent of the presence of other species such as free radicals or oxygen; varying the cage substituents leads to only minor differences in the activation parameters. Hydrogen atoms are found to be more strongly stabilized in homosubstituted octasilsesquioxanes compared with singly Ge-substituted cages. A kinetic isotope effect observed for the detrapping of H and D from MeT(8) is ascribed to the difference in the zero-point energies of the trapped atoms. There is a secondary H/D isotope effect in the temperature dependence of the (29)Si-superhyperfine splitting constants in the range 228-353 K. Cage relaxation has a substantial effect on the detrapping barrier but little influence on the intracage potential. Calculations using a rigid cage approximation give satisfactory agreement with zero-point parameters extracted from experimental data. Different model chemistries yield qualitatively different pictures of the dependence of the hyperfine coupling constant of the trapped H atom upon the detrapping coordinate. Within an isotropic approximation of the vibrational displacements, the B3LYP data give fairly close agreement with the experimental temperature dependence, subject to a shift of the absolute value related to known weaknesses of the method. For the Si(7)Ge cage, it is found that the transition state in which the H atom passes through a Ge-containing face is strongly favored, accounting for the larger detrapping rate parameters observed experimentally for this species.     

Ground state multiplicity of acylnitrenes: computational and experimental studies

The singlet—triplet energy splitting (ΔE _ST = E _S − E _T ) was calculated for formylnitrene (5) and for the syn- and anti-rotamers of carboxynitrene HOC(O)N (6) by the CCSD(T) method. Extrapolation of ΔE _ST to a complete basis set was calculated to be negative for 5 and strongly positive for 6. Similar results were obtained by the G2 procedure. The reason for the dramatic stabilization of the singlet state appeared to be a special bonding interaction between the nitrogen and oxygen atoms, which results in the structure intermediate between those of nitrene and oxazirene. It was found that the B3LYP/6-31G(d) method overestimates ΔE _ST by ∼9 kcal mol⁻¹ for 5 and by ∼7 kcal mol⁻¹ for 6. Taking into account this overestimation and the results of DFT calculations, it was concluded that benzoylnitrene has a singlet ground state. It was proved experimentally using photolysis of benzoyl azide in an argon matrix at 12 K that benzoylnitrene has a singlet ground state and its structure is similar to that of oxazirene. Nevertheless, these singlet intermediates have low barrier to the aziridine formation, which is traditionally considered to be a typical singlet nitrene reaction.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 519–526, March, 2005.     

Effect of substituents on the thermal decomposition of diazirines: experimental and computational studies

The thermal decomposition of phenylchlorodiazirine (1), phenyl-n-butyldiazirine (2), and 2-adamantane-2,3'-[3H]diazirine (3) has been studied in solution in the presence of C(60). The C(60) probe technique indicates that in the decomposition diazirine 1 yielded exclusively phenylchlorocarbene, diazirine 2 yielded mainly a diazo intermediate, and diazirine 3 yielded a mixture of carbene and diazo compound. In the case of diazirine 2, 13% of (E)-1-phenyl-1-pentene resulted from the direct thermal rearrangement of diazirine without the participation of a carbene. As well, the thermal decomposition of these diazirines has been studied theoretically with ab initio and density functional methods. The experimental results are broadly in agreement with the theoretical predictions. The calculations further indicate that the rebound reaction between carbene and molecular nitrogen leading to the formation of a diazo intermediate is an important reaction in the gas-phase decomposition of diazirine.     

N-centered radicals in self-terminating radical cyclizations: experimental and computational studies

Intermolecular addition of photochemically generated N-centered aminium and amidyl radicals 13a-d and 16a,b, respectively, to the cyclic alkyne 1 initiates a radical translocation/cyclization cascade, followed by an oxidative termination step that eventually leads to formation of the bicyclic ketones 7a and 8a. Computational studies were performed to gain insight into the mechanism of these reactions, which are an interesting modification of the recently discovered concept of self-terminating radical cyclizations.     

DNA binding studies of Vinca alkaloids: experimental and computational evidence

Fluorescence studies on the indole alkaloids vinblastine sulfate, vincristine sulfate, vincamine and catharanthine have demonstrated the DNA binding ability of these molecules. The binding mode of these molecules in the minor groove of DNA is non-specific. A new parameter of the purine-pyrimidine base sequence specificty was observed in order to define the non-specific DNA binding of ligands. Catharanthine had shown 'same' pattern of 'Pu-Py' specificity while evaluating its DNA binding profile. The proton resonances of a DNA decamer duplex were assigned. The models of the drug:DNA complexes were analyzed for DNA binding features. The effect of temperature on the DNA binding was also evaluated.     

Structure-property relationships in conjugated donor-acceptor molecules based on cyanoanthracene: computational and experimental studies

[reaction: see text] Two series of pi-conjugated bipolar compounds, namely, 9-phenyl-10-anthronitriles (PAN series) and 9-phenylethynyl-10-anthronitriles (PEAN series), having inherent redox centers have been synthesized and their electronic absorption, fluorescence emission, and electrochemical behavior have been studied. Electrochemiluminescence of these molecules bearing weak, strong, and spin-polarized donors is also studied. The observed electronic properties are explained with the help of results obtained from density functional theory (DFT- B3LYP/6-31G) calculations. The structure-property relationships of all the molecules are discussed.     

Suzuki-Miyaura cross-coupling of aryl carbamates and sulfamates: experimental and computational studies

The first Suzuki-Miyaura cross-coupling reactions of the synthetically versatile aryl O-carbamate and O-sulfamate groups are described. The transformations utilize the inexpensive, bench-stable catalyst NiCl(2)(PCy(3))(2) to furnish biaryls in good to excellent yields. A broad scope for this methodology has been demonstrated. Substrates with electron-donating and electron-withdrawing groups are tolerated, in addition to those that possess ortho substituents. Furthermore, heteroaryl substrates may be employed as coupling partners. A computational study providing the full catalytic cycles for these cross-coupling reactions is described. The oxidative addition with carbamates or sulfamates occurs via a five-centered transition state, resulting in the exclusive cleavage of the aryl C-O bond. Water is found to stabilize the Ni-carbamate catalyst resting state, which thus provides rationalization of the relative decreased rate of coupling of carbamates. Several synthetic applications are presented to showcase the utility of the methodology in the synthesis of polysubstituted aromatic compounds of natural product and bioactive molecule interest.     

Ligand topology variations and the importance of ligand field strength in non-heme iron catalyzed oxidations of alkanes

A series of iron(II)-bis(triflate) complexes [Fe(L)(OTf)2] containing linear tetradentate bis(quinolyl)-diamine and bis(quinolylmethyl)-diamine ligands with a range of ligand backbones has been prepared. The coordination geometries of these complexes have been investigated in the solid state by X-ray crystallography and in solution by 1H and 19F NMR spectroscopy. Because of the labile nature of high-spin iron(II) complexes in solution, dynamic equilibria of complexes with different coordination geometries (cis-alpha, cis-beta, and trans) are observed with certain ligand systems. In these cases, the geometry observed in the solid-state does not necessarily represent the only or even the major geometry present in solution. The ligand field strength in the various complexes has been investigated by variable-temperature (VT) magnetic moment measurements and by UV-vis spectroscopy. The strongest ligand field is observed with the most rigid ligand that generates [Fe(L)(OTf)2] complexes with a cis-alpha coordination geometry, and the corresponding [Fe(L)(CH3CN)2]2+ complex displays spin crossover behavior. The catalytic properties of the complexes for the oxidation of cyclohexane have been investigated using hydrogen peroxide as the oxidant. An increased flexibility in the ligand results in a weaker ligand field, which increases the lability of the complexes. The activity and selectivity of the catalysts appear to be related to the strength of the ligand field and the stability of the catalyst.     

Study of the effect of the ligand structure on the catalytic activity of Pd@ ligand decorated halloysite: Combination of experimental and computational studies

Taking advantage of computational chemistry, the best diamine for the synthesis of a multi‐dentate ligand from the reaction with 3‐(trimethoxysilyl) propylisocyanate (TEPI) was selected. Actually, predictive Density Functional Theory (DFT) calculations provided the right diamino chain, i.e. ethylenediamine, capable to sequester a palladium atom, together with the relatively polar solvent toluene, and then undergo the experiments as a selective catalytic agent. The ligand was then prepared and applied for the decoration of the halloysite (Hal) outer surface to furnish an efficient support for the immobilization of Pd nanoparticles. The resulting catalyst exhibited high catalytic activity for hydrogenation of nitroarenes. Moreover, it showed high selectivity towards nitro functional group. The study of the catalyst recyclability confirmed that the catalyst could be recycled for several reaction runs with only slight loss of the catalytic activity and Pd leaching. Hot filtration test also proved the heterogeneous nature of the catalysis.     

"Inverse-electron-demand" ligand substitution: experimental and computational insights into olefin exchange at palladium(0)

The mechanism of olefin substitution at palladium(0) has been studied, and the results provide unique insights into the fundamental reactivity of electron-rich late transition metals. A systematic series of bathocuproine-palladium(0) complexes bearing trans-beta-nitrostyrene ligands (ns(X) = X-C(6)H(4)CH=CHNO(2); X = OCH(3), CH(3), H, Br, CF(3)), (bc)Pd(0)ns(X) (3(X)), was prepared and characterized, and olefin-substitution reactions of these complexes were found to proceed by an associative mechanism. In cross-reactions between (bc)Pd(ns(CH)()3) and ns(X) (X = OCH(3), H, Br, CF(3)), more-electron-deficient olefins react more rapidly (relative rate: ns(CF)()3 > ns(Br) > ns(H) > ns(OCH)()3). Density functional theory calculations of model alkene-substitution reactions at a diimine-palladium(0) center reveal that the palladium center reacts as a nucleophile via attack of a metal-based lone pair on the empty pi orbital of the incoming olefin. This orbital picture contrasts that of traditional ligand-substitution reactions, in which the incoming ligand donates electron density into an acceptor orbital on the metal. On the basis of these results, olefin substitution at palladium(0) is classified as an "inverse-electron-demand" ligand-substitution reaction.     

Bimodal pore size distributions for carbons: experimental results and computational studies

It is well known that the bimodal shape of the pore size distribution (PSD) curves is typical for various carbonaceous materials (of different origin and/or treated thermally or chemically). A systematic investigation of this effect has been discussed using the Nguyen and Do method with proposed recently the ASA algorithm. A series of numerically generated adsorption isotherms (N(2), T=77 K) and experimental data were analyzed. We investigated various possible situations related to the shape of the PSD curves, i.e., the intensity of the both peaks, their mutual location and the vanishing of one of them. Moreover, the problem in the similarity of the local adsorption isotherms from the range of pore widths corresponding to the gap between peaks is discussed. The analysis of obtained results (as well as published by others) shows that the bimodal shape of the pore size distributions is a characteristic feature for many adsorbents possessing even a small amount of micropores. It is shown that this feature results from the similarity of the local adsorption isotherm in the range of the pore widths for which the gap between peaks (related to the primary and secondary micropore filling mechanism) exists.     

Ambident ethyl N-nitrosocarbamate anion: experimental and computational studies of alkylation and thermal stability

Alkylation of N-nitrosourethane tetrabutylammonium salt (2-Bu(4)N) with four electrophiles (MeI, EtI, i-PrI, and PhCH(2)Br) was studied by (1)H NMR in CD(2)Cl(2) and CD(3)CN solutions. The ratio of the three regioisomers N-alkyl-N-nitrosourethane 3, azoxy 4, and O-alkyldiazotate 5 was practically independent of solvent but dependent on the nature of the electrophile. The anion 2 and O-alkyl derivative 5 are thermally unstable and decompose to ethyl carbonates 9 and 10, respectively, with a first-order rate constant (2-Bu(4)N: k = 18.5 +/- 0.1 x 10(-5) s(-1); 5b (R = Et): k = 1.77 +/- 0.02 x 10(-5) s(-1); 5d (R = PhCH(2)): k = 4.78 +/- 0.08 x 10(-5) s(-1) at 35 degrees C in CD(2)Cl(2)). Further kinetic measurements gave activation parameters for the decomposition of 2 (E(a) = 24.2 +/- 0.3 kcal/mol and ln A = 30.9 +/- 0.1). Gas-phase calculations at the MP2(fc)/6-31+G(d)//MP2(fc)/6-31G(d) level showed that the alkylation of 2 involves the lone electron pairs of the N-N-O atoms, and the calculated activation energies correspond well to the observed ratio of regioisomers 3-5. The theoretical analysis of the decomposition processes supports a concerted mechanism with a four-center transition state in the first step for all four compounds. The calculated activation energy order (2 < 5 < 3 < 4) is consistent with the observed order of stability. Decomposition of 2 and 5 is a unimolecular process, giving carbonates 9 and 10 in a single step. In contrast, rearrangement of 3 and 4 leads to alkyl diazonium ions. A detailed theoretical analysis indicates that the rate-determining step for thermal decomposition of 2 is the loss of molecular nitrogen, while in 5 it is the trans-cis isomerization process. The nonconcerted process involving homolytic cleavage of the O-N bond in 5 was found to be significantly less favorable.     

Combined computational and experimental studies of the mechanism and scope of the retro-Nazarov reaction

Density functional theory calculations (B3LYP/6-31+G) demonstrate that conjugating and electron-donating substituents at carbons three and four of a cyclopentenyl oxyallylic cation should have a rate-accelerating effect on the retro-Nazarov reactions of these species. The retro-Nazarov reaction of these intermediates is predicted to exhibit significant torquoselectivity when carbon three is substituted with a methoxy and a methyl group. Experimental studies show that oxyallylic cations can undergo effective retro-Nazarov reactions when two alkyl and one aryl/vinyl groups are on carbons three and four. An equal number of alkyl substituents or a single aryl substituent is not effective in promoting the reaction. Interestingly, a single alkoxy substituent at carbon three is sufficient for the retro-Nazarov reaction to occur. The methodology developed was used in a total synthesis of the natural product turmerone.