Ch-stretching gas phase overtone spectrum of symmetric neopentane-d4

The overtone spectrum of neopentane-d₄, C(CH₂D)₄, is examined and the local mode model is used to assign the prominent peaks. Proposed coupling routes for intramolecular vibrational relaxation of CH-stretching overtones are discussed. The dominant vibrational relaxation pathway is identified by comparisons of neopentane-d₄, neopentane and benzene overtone bandwidths.     

Lewis Acid Coordination Redirects S-Nitrosothiol Signaling Output

S-Nitrosothiols (RSNOs) serve as air-stable reservoirs for nitric oxide in biology. While copper enzymes promote NO release from RSNOs by serving as Lewis acids for intramolecular electron-transfer, redox-innocent Lewis acids separate these two functions to reveal the effect of coordination on structure and reactivity. The synthetic Lewis acid B(C₆F₅)₃ coordinates to the RSNO oxygen atom, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials, B(C₆F₅)₃ coordination increases their reduction potential by over 1 V into the physiologically accessible +0.1 V vs. NHE. Outer-sphere chemical reduction gives the Lewis acid stabilized hyponitrite dianion trans-[LA-O-N=N-O-LA]²⁻ [LA=B(C₆F₅)₃], which releases N₂O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO-B(C₆F₅)₃] radical anion, which is susceptible to N−N coupling prior to loss of RSSR.     

Cu(0)/Selectfluor system-catalyzed intramolecular Csp2-H/Csp2-H cross-dehydrogenative coupling (CDC)

A Cu(0)/Selectfluor system-catalyzed intramolecular C_sp2-H/C_sp2-H bond cross-dehydrogenative coupling of 2-aryloxybenzaldehydes is described. A variety of substituted xanthone derivatives are synthesized in moderate to excellent yields. Reaction can also be extended to the synthesis of 9H-thioxanthen-9-one 10,10-dioxide and phenanthridin-6(5H)-ones, respectively.     

Oxidative Coupling of αω,-Di(cyclopentadienyl)alkane-diides

The Cu^II-induced oxidative coupling of αω,-di(cyclopentadienyl)alkane-diides 6 (n = 2–5) has been shown to proceed mainly by an intermolecular pathway to give polymers 8 , while the yield of intramolecular coupling 6 → 7 strongly decreases with increasing number n of C-atoms of the alkyl chain (Scheme 3). For n = 2, intramolecular coupling may be considerably enhanced by replacing the H-atoms of the CH₂CH₂ bridge of 6a (n = 2) by Me groups. In this case, intramolecular couplings 11 → 20 (Scheme 7) and 22 → 23 + 24 (Scheme 8) are accomplished with a total yield of 59% and 54%, respectively. All the intramolecular couplings investigated so far proceed stereoselectively to give the C₂-symmetrical cyclohexanes 7a, 20 and 23 with a fixed chair conformation. These results are easily explained, if a conformational equilibrium E ? F is operative in which large substituents R are assumed to enhance the gauche-conformation F which is the favored conformation for intramolecular couplings. Bridged dihydropentafulvalenes 20 and 23 are quantitatively rearranged to the thermodynamically favored bridged pentafulvenes 27 and 28 under base or acid catalysis, respectively (Scheme 9).     

Lewis Acid Coordination Redirects S‐Nitrosothiol Signaling Output

S‐Nitrosothiols (RSNOs) serve as air‐stable reservoirs for nitric oxide in biology. While copper enzymes promote NO release from RSNOs by serving as Lewis acids for intramolecular electron‐transfer, redox‐innocent Lewis acids separate these two functions to reveal the effect of coordination on structure and reactivity. The synthetic Lewis acid B(C₆F₅)₃ coordinates to the RSNO oxygen atom, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials, B(C₆F₅)₃ coordination increases their reduction potential by over 1 V into the physiologically accessible +0.1 V vs. NHE. Outer‐sphere chemical reduction gives the Lewis acid stabilized hyponitrite dianion trans‐[LA‐O‐N=N‐O‐LA]^2? [LA=B(C₆F₅)₃], which releases N₂O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO‐B(C₆F₅)₃] radical anion, which is susceptible to N?N coupling prior to loss of RSSR.     

Dynamical effects of mode specific excitation in unimolecular decomposition: A trajectory study of C2H6

Classical trajectories on a well coupled model potential energy surface have been used to study the effects of mode localized excitation on unimolecular decomposition in C₂H₆. The results display both apparent and intrinsic non-statistical effects that can be ascribed to restrictions in intramolecular energy transfer both among the CH vibrations and between these CC motion.     

Acetylenic Cyclophanes as Fullerene Precursors: Formation of C60H6 and C60 by Laser Desorption Mass Spectrometry of C60H6(CO)12

A logical precursor of macrocycle C ₆₀ H ₆ , cyclophane C₆₀H₆(CO)₁₂ ( 1 ) represents a building block in a possible total synthesis of C₆₀. In Fourier transform ion cyclotron resonance laser desorption mass spectroscopic experiments in the negative-ion mode, 1 fragments to C₆₀H₆ ( 2 ) under successive loss of CO. Further loss of six H atoms and rearrangement gives C₆₀ ions with a fullerenic structure.     

“Expanded” local mode approach for XY2 (C2v) molecules

For XY₂ (C_2v symmetry) molecules, the local mode method is modified on the basis of a careful analysis of the transformation coefficients l_Nαλ and the properties of the intramolecular potential function, and an expanded local mode approach is derived. It is shown that relationships between known spectroscopic parameters, found in the spectroscopic literature, can be improved. New relationships between the centrifugal distortion coefficients are obtained. It is shown that knowledge of the centrifugal distortion coefficients of the ground vibrational state allows the possibility of predicting values of both the band centres and the spectroscopic parameters of deformational bands, even under conditions of a total absence of initial information on these deformational bands. The prediction possibility of the derived expanded local mode approach is analyzed.     

Synthetic studies on bafilomycin A1: stereoselective synthesis of the C12-C17 fragment and its coupling with the C1-C11 subunit

Stereoselective addition of (E)-1-lithio-2-tributylstannylethylene on a chiral cyclic di-t-butylsilyleneketal C₁₄-C₁₇ aldehyde afforded the required Felkin-Anh adduct for the synthesis of the C₁₂-C₁₇ fragment of bafilomycin A₁, the configuration of which was assigned unambiguously. After appropriate coupling with the enantiopure C₁-C₁₁ fragment, the C₁₂-C₁₇ subunit obtained here can be used for the study of the 16-membered macrolide formation either by an acyl activation or an intramolecular Stille reaction. Intermolecular esterification of the 15-OH with an acyl activation of the carboxylic acid of the C₁-C₁₁ fragment, in modified Yamaguchi's conditions, affords here an intermediate for examining an intramolecular Stille coupling.     

Nickel (II) complexes supported by a fluorinated β‐diketiminate backbone ligand: synthesis,catalytic activity toward norbornene polymerization,and the oxygenated species

The reaction of the lithium salt of backbone fluorinated β‐diketiminate ligands, ArNC(CF₃)CHC(CF₃) NArLi, with trans‐[NiCl(Ph)(PPh₃)₂] gives nickel (II) complexes, ArNC(CF₃)CHC(CF₃)NAr(Ph) (PPh₃)Ni (Ar = 2, 6‐Me₂C₆H₃: 1 ; 2, 6‐ⁱPr₂C₆H₃: 2 ). When activated by methylaluminoxane (MAO), both complexes polymerize norbornene rapidly via a vinyl‐type polymerization mechanism. Treatment of nickel complex 1 with oxygen gives rise to intramolecular aerobic hydroxylation. The oxygenated species 3 was characterized by X‐ray crystallography. Copyright © 2006 John Wiley & Sons, Ltd.     

Carboxylic acid to amide hydrogen bonding. 10-Oxo-semirubins

Using their amide (and pyrrole) groups, dipyrrinones act as hydrogen bonding receptors for carboxylic acids, as found in a large number of 10-oxo-semirubins (1-6). The latter can be synthesized readily by Friedel-Crafts coupling of 9-H dipyrrinones with half-ester acid chlorides or diacid dichlorides of α,ω-dicarboxylic acids, ranging from C₂ to C₁₀. With ω-oxo-alkanoic acid chains of C₅ or ≥C₅, intramolecular hydrogen bonding is observed. With acid chains <C₅ hydrogen bonding is not observed. Uncharacteristically (for dipyrrinones), 10-oxo-dipyrrinone acids (1-6) and their corresponding esters (1e-6e) remain monomeric in hydrogen bond promoting solvents.     

Dissociation and IVR pathways for the CF3H(H2O)3 cluster

Classical trajectory simulations are used to study the intramolecular dynamics of isolated CF₃H and the CF₃H(H₂O)₃ cluster, by either exciting the CH stretch local mode to then=6 level or by adding an equivalent amount of energy to an OH stretch normal mode. Energy transfer from the CH local mode is statistically the same for CF₃H(H₂O)₃ as for isolated CF₃H, and agrees with previous experimental studies. Clusters excited with 6 quanta in the CH local mode are remarkably stable. Though the CF₃H-(H₂O)₃ intermolecular potential is only 1.5 kcal/mol, only 1 of 26 clusters excited with 6 quanta in the CH local mode dissociate within 10 ps. The absorption linewidth for the CH local mode in CF₃H(H₂O)₃ is related to IVR within CF₃H and not to the unimolecular lifetime of the cluster. When an OH stretch normal mode of the cluster is excited, energy transfer to CF₃H is negligible and nearly one half of the clusters dissociate within 10 ps.     

Aryl‐Copper(III)‐Acetylides as Key Intermediates in CCsp Model Couplings under Mild Conditions

The mechanism of copper‐mediated Sonogashira couplings (so‐called Stephens–Castro and Miura couplings) is not well understood and lacks clear comprehension. In this work, the reactivity of a well‐defined aryl‐Cu^III species ( 1 ) with p‐R‐phenylacetylenes (R=NO₂, CF₃, H) is reported and it is found that facile reductive elimination from a putative aryl‐Cu^III‐acetylide species occurs at room temperature to afford the C_aryl?C_sp coupling species ( I_R ), which in turn undergo an intramolecular reorganisation to afford final heterocyclic products containing 2H‐isoindole ( P , P , P_Ha ) or 1,2‐dihydroisoquinoline ( P_Hb ) substructures. Density Functional Theory (DFT) studies support the postulated reductive elimination pathway that leads to the formation of C?C_sp bonds and provide the clue to understand the divergent intramolecular reorganisation when p‐H‐phenylacetylene is used. Mechanistic insights and the very mild experimental conditions to effect C_aryl?C_sp coupling in these model systems provide important insights for developing milder copper‐catalysed C_aryl?C_sp coupling reactions with standard substrates in the future.     

Modulating the preferred O—H...O hydrogen‐bonding motif in a conformationally constrained environment through hydroxy‐group derivatization

The crystal structures of three conformationally locked esters, namely the centrosymmetric tetrabenzoate of all‐axial perhydronaphthalene‐2,3,4a,6,7,8a‐hexaol, viz.trans‐4a,8a‐dihydroxyperhydronaphthalene‐2,3,6,7‐tetrayl tetrabenzoate, C₃₈H₃₄O₁₀, and the diacetate and dibenzoate of all‐axial perhydronaphthalene‐2,3,4a,8a‐tetraol, viz. (2R*,3R*,4aS*,8aS*)‐4a,8a‐dihydroxyperhydronaphthalene‐2,3‐diyl diacetate, C₁₄H₂₂O₆, and (2R*,3R*,4aS*,8aS*)‐4a,8a‐dihydroxyperhydronaphthalene‐2,3‐diyl dibenzoate, C₂₄H₂₆O₆, have been analyzed in order to examine the preference of their supramolecular assemblies towards competing inter‐ and intramolecular O—H...O hydrogen bonds. It was anticipated that the supramolecular assembly of the esters under study would adopt two principal hydrogen‐bonding modes, namely one that employs intermolecular O—H...O hydrogen bonds (mode 1) and another that sacrifices those for intramolecular O—H...O hydrogen bonds and settles for a crystal packing dictated by weak intermolecular interactions alone (mode 2). Thus, while the molecular assembly of the two crystalline diacyl derivatives conformed to a combination of hydrogen‐bonding modes 1 and 2, the crystal packing in the tetrabenzoate preferred to follow mode 2 exclusively.     

Anharmonic oscillator model of overtone spectra for D3h symmetry molecules

The bond stretching vibrations of XF₅ molecules with D_3h symmetry are treated computationally on the Morse oscillator model in which the bond oscillators are coupled harmonically. Each calculation involves four parameters for two types of Morse potential and three parameters for the kinetic-energy, potential coupling terms. The eigenvalue formula for overtone and combination states up to three are presented and can be used to predict all the vibrational energy levels from local mode molecules through normal mode molecules. For PF₅, AsF₅ and VF₅, the coupled Morse oscillator model gives a prediction in good agreement with the experimental data.     

Unusual Rate Acceleration in Brønsted Acid Catalyzed Dehydration Reactions: Local Hydrophobic Environment in Aggregated N‐(2,6‐diphenylphenyl)‐N‐mesitylammonium Pentafluorobenzenesulfonates

Bulky diarylammonium pentafluorobenzenesulfonates effectively promote dehydration reactions, such as condensation reactions to give esters and the dehydrative cyclization of 1,3,5‐triketones. In particular, N‐(2,6‐diphenylphenyl)‐N‐mesitylammonium pentafluorobenzenesulfonate shows much higher catalytic activity than C₆F₅SO₃H under reaction conditions without the removal of generated water, even though the former is a weaker acid. Its crystallization gives an aggregated cyclic ion pair, which is composed of two diarylammonium cations, four pentafluorobenzenesulfonate anions, and two oxonium cations. This ion pair is strongly stabilized by four intermolecular and two intramolecular π–π attractive interactions and 10 hydrogen bonds. The extremely high catalytic activity of N‐(2,6‐diphenylphenyl)‐N‐mesitylammonium pentafluorobenzenesulfonate in the dehydration reactions may be ascribed to the local hydrophobic environment of the tightly aggregated ammonium salts.     

Synthetic experiments in the limonin-series III. Stereospecific alkoxy-and dialkoxy-alkylation of 19-hydroxy-testosterone acetate]

Three different procedures for intramolecular alkoxy-alkylation of 19-hydroxy-testosterone-acetate ( 4 ) are described each yielding a mixture of two products monoalkylated either at position C(2), C( 4 ) or C(6). In all cases the alkoxy groups are introduced stereospecifically from the β-side. Treatment of 4 with I₂/(CH₃)₂CO leads to 5 and 6. Reaction of 4 with HgCl₂/ClCH₂SCH₃ gives rise to 12 and 13 whereas treatment with HC(OCH₃)₃/CH₃C ₆ H ₄ SO₃H affords 15 , 16 and 17 .