Deuterium kinetic isotope effects and mechanism of the thermal isomerization of bicyclo[4.2.0]oct-7-ene to 1,3-cyclooctadiene

The thermal conversion of cis-bicyclo[4.2.0]oct-7-ene to cis,cis-1,3-cyclooctadiene might involve a direct disrotatory ring opening, or it might possibly take place by way of cis,trans-1,3-cyclooctadiene. This cis,trans-diene might possibly form the more stable cis,cis isomer through a [1,5] hydrogen shift or a trans-to-cis isomerization about the trans double bond. Deuterium kinetic isotope effect determinations for the isomerizations of 2,2,5,5-d(4)-bicyclo[4.2.0]oct-7-ene and 7,8-d(2)-bicyclo[4.2.0]oct-7-ene rule out these two alternatives because the observed effects are much smaller than would be anticipated for these mechanisms: k(H)/k(D)(d(4)) at 250 degrees C is 1.17 (1.04 per D), and k(H)/k(D)(d(2)) at 238 degrees C is 1.20 (1.10 per D). The direct disrotatory ring opening route remains the preferred mechanism.     

Synthesis, characterization, and reactivity of Fe complexes containing cyclic diazadiphosphine ligands: the role of the pendant base in heterolytic cleavage of H2

The iron complexes CpFe(P(Ph)(2)N(Bn)(2))Cl (1-Cl), CpFe(P(Ph)(2)N(Ph)(2))Cl (2-Cl), and CpFe(P(Ph)(2)C(5))Cl (3-Cl)(where P(Ph)(2)N(Bn)(2) is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, P(Ph)(2)N(Ph)(2) is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane, and P(Ph)(2)C(5) is 1,4-diphenyl-1,4-diphosphacycloheptane) have been synthesized and characterized by NMR spectroscopy, electrochemical studies, and X-ray diffraction. These chloride derivatives are readily converted to the corresponding hydride complexes [CpFe(P(Ph)(2)N(Bn)(2))H (1-H), CpFe(P(Ph)(2)N(Ph)(2))H (2-H), CpFe(P(Ph)(2)C(5))H (3-H)] and H(2) complexes [CpFe(P(Ph)(2)N(Bn)(2))(H(2))]BAr(F)(4), [1-H(2)]BAr(F)(4), (where BAr(F)(4) is B[(3,5-(CF(3))(2)C(6)H(3))(4)](-)), [CpFe(P(Ph)(2)N(Ph)(2))(H(2))]BAr(F)(4), [2-H(2)]BAr(F)(4), and [CpFe(P(Ph)(2)C(5))(H(2))]BAr(F)(4), [3-H(2)]BAr(F)(4), as well as [CpFe(P(Ph)(2)N(Bn)(2))(CO)]BAr(F)(4), [1-CO]Cl. Structural studies are reported for [1-H(2)]BAr(F)(4), 1-H, 2-H, and [1-CO]Cl. The conformations adopted by the chelate rings of the P(Ph)(2)N(Bn)(2) ligand in the different complexes are determined by attractive or repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of the pendant amine and the C atom of the coordinated CO ligand for [1-CO]BAr(F)(4) is 2.848 ?, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes [1-H(2)](+), [2-H(2)](+), and [3-H(2)](+) carried out using H(2) and D(2) indicate that the relatively rapid H/D exchange observed for [1-H(2)](+) and [2-H(2)](+) compared to [3-H(2)](+) is consistent with intramolecular heterolytic cleavage of H(2) mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H(2). These mononuclear Fe(II) dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the distal Fe site of the active site of the [FeFe]-hydrogenase required for H-H bond formation and cleavage.     

Quantifying weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl: a combined computational and experimental investigation of NMR chemical shifts in the solid state

Weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl, for which single-crystal diffraction structures reveal close CH...O=C and C[triple bond]CH...N[triple bond]C distances, is investigated in a study that combines the experimental determination of 1H, 13C, and 15N chemical shifts by magic-angle spinning (MAS) solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route, including the isolation and characterization of intermediates, to 4-cyano-4'-ethynylbiphenyl at natural abundance and with 13C[triple bond]13CH and 15N[triple bond]C labeling is described. The difference in chemical shifts calculated, on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends on both intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the two effects are separated computationally by, first, determining the difference in chemical shift between that calculated for a plane (uracil) or an isolated chain (4-cyano-4'-ethynylbiphenyl) and that calculated for an isolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shifts that quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule to plane changes in the 1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved in CH...O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involved in conventional NH...O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxed uracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differences of no more than 0.002 A, which corresponds to changes in the calculated 1H chemical shifts of at most 0.1 ppm. For the C[triple bond]CH...N[triple bond]C weak hydrogen bonds in 4-cyano-4'-ethynylbiphenyl, the calculated molecule to chain changes are of similar magnitude but opposite sign for the donor 13C and acceptor 15N nuclei. In uracil and 4-cyano-4'-ethynylbiphenyl, the CH hydrogen-bonding donors are sp2 and sp hybridized, respectively; a comparison of the calculated changes in 1H chemical shift with those for the sp3 hybridized CH donors in maltose (Yates et al. J. Am. Chem. Soc. 2005, 127, 10216) reveals no marked dependence on hybridization for weak hydrogen-bonding strength.     

C−H Activation of Unbiased C(sp3)−H Bonds: Gold(I)-Catalyzed Cycloisomerization of 1-Bromoalkynes**

Selective functionalization of non-activated C(sp³)−H bonds is a major challenge in chemistry, so functional groups are often used to enhance reactivity. Here, we present a gold(I)-catalyzed C(sp³)−H activation of 1-bromoalkynes without any sort of electronic, or conformational bias. The reaction proceeds regiospecifically and stereospecifically to the corresponding bromocyclopentene derivatives. The latter can be readily modified, comprising an excellent library of diverse 3D scaffolds for medicinal chemistry. In addition, a mechanistic study has shown that the reaction proceeds via a so far unknown mechanism: a concerted [1,5]-H shift / C−C bond formation involving a gold-stabilized vinylcation-like transition state.     

Dihydroceramide delta(4) desaturase initiates substrate oxidation at C-4.

The intermolecular primary deuterium isotope effects on the individual C-H bond cleavage steps involved in dihydroceramide Delta(4) desaturation have been determined for the first time by incubating rat liver microsomes with 1:1 mixtures of nonlabeled substrate and the appropriate regiospecifically dideuterated analogue. Analysis of the enzymatic products via gas chromatography coupled to mass spectrometry showed that the introduction of the (E) double bond between C-4 and C-5 occurs in two discrete steps: cleavage of the C4-H bond was found to be very sensitive to isotopic substitution (k(H)/k(D) = 8.0 +/- 0.8), while a negligible isotope effect (k(H)/k(D) = 1.02 +/- 0.07) was observed for the C5-H bond-breaking step. According to a mechanistic model that we have previously proposed, these results suggest that initial oxidation for this desaturation reaction occurs at C-4. This finding correlates nicely with the observation that 4-hydroxylated products are produced from a similar substrate by a closely related oxidative enzyme in yeast.     

Formation and reactivity of Ir(III) hydroxycarbonyl complexes

Kinetic studies show that the reaction of [TpIr(CO)2] (1, Tp = hydrotris(pyrazolyl)borate) with water to give [TpIr(CO2H)(CO)H] (2) is second order (k = 1.65 x 10(-4) dm(3) mol(-1) s(-1), 25 degrees C, MeCN) with activation parameters DeltaH++= 46+/-2 kJ mol(-1) and DeltaS++ = -162+/-5 J K(-1) mol(-1). A kinetic isotope effect of k(H2O)/k(D2O) = 1.40 at 20 degrees C indicates that O-H/D bond cleavage is involved in the rate-determining step. Despite being more electron rich than 1, [Tp*Ir(CO)2] (1*, Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) reacts rapidly with adventitious water to give [Tp*Ir(CO2H)(CO)H] (2*). A proposed mechanism consistent with the relative reactivity of 1 and 1* involves initial protonation of Ir(I) followed by nucleophilic attack on a carbonyl ligand. An X-ray crystal structure of 2* shows dimer formation via pairwise H-bonding interactions of hydroxycarbonyl ligands (r(O...O) 2.65 A). Complex 2* is thermally stable but (like 2) is amphoteric, undergoing dehydroxylation with acid to give [Tp*Ir(CO)2H]+ (3*) and decarboxylation with OH- to give [TpIr(CO)H2] (4*). Complex 2 undergoes thermal decarboxylation above ca. 50 degrees C to give [TpIr(CO)H2] (4) in a first-order process with activation parameters DeltaH++ = 115+/-4 kJ mol(-1) and DeltaS++ = 60+/-10 J K(-1) mol(-1).     

Electronic structure, molecular electrostatic potentials, vibrational spectra in substituted calix[n]arenes (n = 4, 5) from density functional theory

Electronic structure, molecular electrostatic potential, and vibrational frequencies of para-substituted calix[n]arene CX[n]-R (n = 4, 5; R = H, NH(2), t-Bu, CH(2)Cl, SO(3)H, NO(2)) and their thia analogs (S-CX[n]-R; with R = H and t-Bu) in which sulfur bridges two aromatic rings of CX[n] have been derived from the density functional theory. A rotation around CH(2) groups connecting the phenol rings engenders four, namely, cone, partial cone, 1,2-alternate, and 1,3-alternate CX[n]-R conformers. Of these, the cone conformer comprising of large number of O1-H1···O1' interactions turns out to be of lowest energy. Normal vibration analysis reveal the O1-H1 stretching frequency of unsubstituted CX[n] shifts to higher wavenumber (blue shift) on substitution of electron-withdrawing (NO(2) or SO(3)H) groups, while electron-donating substituents (NH(2), t-Bu) engender a shift of O1-H1 vibration in the opposite direction (red shift). The direction of frequency shifts have been analyzed using natural bond orbital analysis and molecular electrostatic potential (MESP) topography. Furthermore, calculated (1)H NMR chemical shift (δ(H)) in modified CX[n] hosts follow the order: H1 > H3/H5 > H7(a) > H7(b). The δ(H) values in CX[4] are in consonant with the observed (1)H NMR spectra.     

An ab initio study of substituent effects in [1,3]-hydrogen shifts

Reports in the literature place the TS for the [1,3]-H shift in propene comparable to or higher in energy than loss of the allylic H. However, [1,3]-H shifts have been repeatedly observed experimentally in enolates. We used GAUSSIAN 98 to examine the origin of this apparent contradiction. We found the first TS for an antarafacial [1,3]-H shift that is clearly lower in energy than simple dissociation of the migrating H. This occurs in the [1,3]-H shift in the acetone enolate. Symmetrical substituents (H, O(-), ethynyl) have TSs with C(2) symmetry, implying that they, and probably most [1,3]-H shift TSs, are antarafacial. Conjugating substituents at C2 lower the energy of [1,3]-H shifts and raise the energy of dissociation by loss of a hydrogen atom from C3, increasing the likelihood of the former type of reaction. Strongly electron-donating and electron-withdrawing substituents are more effective than neutral substituents in lowering the energy requirement of [1,3] shifts. Our best calculations predict that a [1,3]-H shift is lower in energy than dissociation by loss of the H by 27.8 kJ/mol in 2-methyl-1-butene-3-yne, by 36.8 kJ/mol in isoprene, by 55.9 kJ/mol in 2-aminopropene, by 114.5 kJ/mol in the acetone enolate, and by 120.8 kJ/mol in the 1-methylacryloyl cation. Thus, there is a chance of experimental observation of [1,3] shifts in conjugated alkenes and related species.     

Metal-assembled cobalt(II) resorc[4]arene-based cage molecules that reversibly capture organic molecules from water and act as NMR shift reagents

The complex Co4 1(2)8- is a tetranuclear cobalt(II) cage compound that assembles in aqueous solutions above pH 4 and is capable of encapsulating a variety of organic guest molecules, for example, benzene, hexane, chlorobutane, butanol, and ethyl acetate. Ligand 1 is a resorc[4]arene-based molecule with iminodiacetate moieties appended to its upper rim. 1H NMR studies of Co4 1(2)8-.guest complexes demonstrate inclusion of nonpolar hydrocarbons, substituted phenyls, alcohols, halogen-containing hydrocarbons, and polar organic molecules. The complex Co4 1(2)8- acts as an NMR shift reagent and causes substantial upfield isotropic hydrogen shifts (-30 to -40 ppm) in the guest molecule and separation of the guest hydrogen chemical shifts by typically 12 ppm. The complex Co4 1(2)8- will encapsulate molecules with fewer than eight atoms in a linear chain, mono- and disubstituted benzenes, and polar molecules with greater than two carbon atoms. The solid-state structure of Ba4[Co4 1(2).C6H5C2H5] shows a disordered guest molecule encapsulated within the cavity of Co4 1(2)8-. The cavity dimensions, bond lengths, and bond angles of Ba4[Co4 1(2).C6H5C2H5] are very similar to those determined in Ba4[Co4 1(2).6H2O].     

High sensitivity to transmission of attenuation in the metal activating effect of platinum on sequential H-D exchange at the four C8 sites of inosines in tetrakis(inosine)platinum(II) chloride

Hydrogen-deuterium exchange of the carbon-bound C(8)-H protons of the inosine residues in tetrakis(inosine)platinum(ii) chloride, S, with Pt binding at N(7), was studied in aqueous buffer solutions at 60 degrees C by (1)H NMR spectroscopy. The kinetics at all four C(8) sites as a function of pD of the D(2)O/OD(-) medium was measured through the disappearance of the C(8)-H signal, which yielded the pseudo first-order rate constant for exchange, k(obs). Plots of k(obs)versus [OD(-)] showed curvature reminiscent of saturation type kinetics and indicative of competitive deprotonation of N(1)-H sites. In contrast, the analogous N(1)-methylated cis-bis(1-methylinosine)diammineplatinum(ii) chloride leads to a linear k(obs)versus [OD(-)] plot. The potentiometrically determined macroscopic composite N(1)-H ionization constant was further dissected into the successive microscopic N(1)-H acidity constants of the four inosine residues of the complex S. The k(obs) values were also deconvoluted into individual rate constants k(ex) (i.e.k(0), k(1), k(2), k(3) for exchange of the successively deprotonated inosine moieties, S, S(1), S(2), S(3), it being assumed that S(4) where all four inosine ligands are deprotonated at N(1) is unreactive ("immunized") to exchange. The k(ex) values show a progressive attenuation in Pt activation of H-D exchange along the series, k(0), k(1), k(2), k(3). The k(ex) data thus generated, together with the deconvoluted individual pK(a) values allow the construction of the plot, log k(ex) [C(8)-H] vs. pK(a) [N(H)-1]. Remarkably, this plot exhibits good linearity (R(2) = 0.99), which accords this as a linear free energy relationship (LFER). The large negative slope value (-2.3) of this LFER reflects the high sensitivity of transmission of electron density from the ionized N(1) via Pt and/or through space to the remaining C(8)-H sites. This is to our knowledge the first instance in which a LFER is generated through modulation of a structure in a single molecule. One can anticipate that this approach may lead to: (1) predicting N-H acidity; (2) C-H H-D exchange susceptibility in a range of metal-biomolecule complexes; (3) their carbon acidity.     

Selective (15)N-labeling and analysis of (13)C-(15)N J couplings as an effective tool for studying the structure and azide-tetrazole equilibrium in a series of tetrazolo[1,5-b][1,2,4]triazines and tetrazolo[1,5-a]pyrimidines

Two general methods for the selective incorporation of an (15)N-label in the azole ring of tetrazolo[1,5-b][1,2,4]triazines and tetrazolo[1,5-a]pyrimidines were developed. The first approach included treatment of azinylhydrazides with (15)N-labeled nitrous acid, and the second approach was based on fusion of the azine ring to [2-(15)N]-5-aminotetrazole. The synthesized compounds were studied by (1)H, (13)C, and (15)N NMR spectroscopy in both DMSO and TFA solution, in which the azide-tetrazole equilibrium is shifted to tetrazole and azide forms, respectively. Incorporation of the (15)N-label led to the appearance of (13)C-(15)N J coupling constants (J(CN)), which can be measured easily using either 1D (13)C spectra with selective (15)N decoupling or with amplitude modulated 1D (13)C spin-echo experiments with selective inversion of the (15)N nuclei. The observed J(CN) patterns permit unambiguous determination of the type of fusion between the azole and azine rings in tetrazolo[1,5-b][1,2,4]triazine derivatives. Joint analysis of J(CN) patterns and (15)N chemical shifts was found to be the most efficient way to study the azido-tetrazole equilibrium.     

Thermodynamic and kinetic studies of H atom transfer from HMo(CO)3(eta(5)-C5H5) to Mo(N[t-Bu]Ar)3 and (PhCN)Mo(N[t-Bu]Ar)3: direct insertion of benzonitrile into the Mo-H bond of HMo(N[t-Bu]Ar)3 forming (Ph(H)C=N)Mo(N[t-Bu]Ar)3

Synthetic studies are reported that show that the reaction of either H2SnR2 (R = Ph, n-Bu) or HMo(CO)3(Cp) (1-H, Cp = eta(5)-C5H5) with Mo(N[t-Bu]Ar)3 (2, Ar = 3,5-C6H3Me2) produce HMo(N[t-Bu]Ar)3 (2-H). The benzonitrile adduct (PhCN)Mo(N[t-Bu]Ar)3 (2-NCPh) reacts rapidly with H2SnR2 or 1-H to produce the ketimide complex (Ph(H)C=N)Mo(N[t-Bu]Ar)3 (2-NC(H)Ph). The X-ray crystal structures of both 2-H and 2-NC(H)Ph are reported. The enthalpy of reaction of 1-H and 2 in toluene solution has been measured by solution calorimetry (DeltaH = -13.1 +/- 0.7 kcal mol(-1)) and used to estimate the Mo-H bond dissociation enthalpy (BDE) in 2-H as 62 kcal mol(-1). The enthalpy of reaction of 1-H and 2-NCPh in toluene solution was determined calorimetrically as DeltaH = -35.1 +/- 2.1 kcal mol(-1). This value combined with the enthalpy of hydrogenation of [Mo(CO)3(Cp)]2 (1(2)) gives an estimated value of 90 kcal mol(-1) for the BDE of the ketimide C-H of 2-NC(H)Ph. These data led to the prediction that formation of 2-NC(H)Ph via nitrile insertion into 2-H would be exothermic by approximately 36 kcal mol(-1), and this reaction was observed experimentally. Stopped flow kinetic studies of the rapid reaction of 1-H with 2-NCPh yielded DeltaH(double dagger) = 11.9 +/- 0.4 kcal mol(-1), DeltaS(double dagger) = -2.7 +/- 1.2 cal K(-1) mol(-1). Corresponding studies with DMo(CO)3(Cp) (1-D) showed a normal kinetic isotope effect with kH/kD approximately 1.6, DeltaH(double dagger) = 13.1 +/- 0.4 kcal mol(-1) and DeltaS(double dagger) = 1.1 +/- 1.6 cal K(-1) mol(-1). Spectroscopic studies of the much slower reaction of 1-H and 2 yielding 2-H and 1/2 1(2) showed generation of variable amounts of a complex proposed to be (Ar[t-Bu]N)3Mo-Mo(CO)3(Cp) (1-2). Complex 1-2 can also be formed in small equilibrium amounts by direct reaction of excess 2 and 1(2). The presence of 1-2 complicates the kinetic picture; however, in the presence of excess 2, the second-order rate constant for H atom transfer from 1-H has been measured: 0.09 +/- 0.01 M(-1) s(-1) at 1.3 degrees C and 0.26 +/- 0.04 M(-1) s(-1) at 17 degrees C. Study of the rate of reaction of 1-D yielded kH/kD = 1.00 +/- 0.05 consistent with an early transition state in which formation of the adduct (Ar[t-Bu]N)3Mo...HMo(CO)3(Cp) is rate limiting.     

Electroswitchable photoluminescence activity: synthesis, spectroscopy, electrochemistry, photophysics, and X-ray crystal and electronic structures of [Re(bpy)(CO)3(C[triple bond]C[bond]C6H4[bond]C[triple bond]C)Fe(C5Me5)(dppe)][PF6](n) (n = 0, 1)

A novel heterobimetallic alkynyl-bridged complex, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)Me(5))(dppe)], 1, and its oxidized species, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)Me(5))(dppe)][PF(6)], 2, have been synthesized and their X-ray crystal structures determined. A related vinylidene complex, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond](H)C[double bond]C)Fe(C(5)Me(5))(dppe)][PF(6)], 3, has also been synthesized and characterized. The cyclic voltammogram of 1 shows a quasireversible reduction couple at -1.49 V (vs SCE), a fully reversible oxidation at -0.19 V, and a quasireversible oxidation at +0.88 V. In accord with the electrochemical results, density-functional theory calculations on the hydrogen-substituted model complex Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)H(5))(dHpe) (Cp = C(5)H(5), dHpe = H(2)P[bond](CH(2))(2)[bond]PH(2)) (1-H) show that the LUMO is mainly bipyridine ligand pi* in character while the HOMO is largely iron(II) d orbital in character. The electronic absorption spectrum of 1 shows low-energy absorption at 390 nm with a 420 nm shoulder in CH(2)Cl(2), while that of 2 exhibits less intense low-energy bands at 432 and 474 nm and additional low-energy bands in the NIR at ca. 830, 1389, and 1773 nm. Unlike the related luminescent rhenium(I)-alkynyl complex [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C[bond]H)], 4, complex 1 is found to be nonemissive, and such a phenomenon is attributed to an intramolecular quenching of the emissive d pi(Re) --> pi*(bpy) (3)MLCT state by the low-lying MLCT and LF excited states of the iron moiety. Interestingly, switching on of the luminescence property derived from the d pi(Re) --> pi*(bpy) (3)MLCT state can be demonstrated in the oxidized species 2 and the related vinylidene analogue 3 due to the absence of the quenching pathway.     

Identification of histidine tautomers in proteins by 2D 1H/13C(delta2) one-bond correlated NMR

If the 13Cdelta2 chemical shift of neutral ("high pH") histidine is >122 ppm, primarily Ndelta1-H tautomer (2) is indicated; if it is <122 ppm, primarily Nepsilon2-H tautomer (1) is indicated. His resonances from the catalytic triad of active serine proteases, for example, are readily distinguished from those of denatured enzyme. The 13Cdelta2 chemical shifts increased by 6.2 ppm for the catalytic histidines in both alpha-lytic protease and subtilisin BPN' in raising the pH from that of imidazolium cation to that of tautomer 2. This tautomer identification method is easy to implement, requiring only bioincorporation of [U-13C] (or the more readily available [U-13C,15N])-histidine. Standard 1H/13C correlation HMQC or HSQC NMR pulse programs then yield the 13Cdelta2 chemical shifts with the benefit of high 1H sensitivity. Because of large one-bond spin-couplings (1JCH approximately 200 Hz), the method should extend to proteins having large 1H and 13C line widths, including very high molecular weights.     

Synthesis and Crystal Structure of 2[1-（1,1-Dimethylethyl）-5-methyl-1-H-pyrazol-4-ylcarbonyl]-N-phenyldrazinecarbothioamide

1INTRODUCTIONThiosemicarbazidespossessingNC=Sgroupareanessentialstructurefeatureresponsibleforvariousbiocidalproperties[1～7].Moreover,theyaretheconvenientandkeyintermediatesforthesyn-thesisofheterocycliccompoundssuchas1,3,4-thiadiazoles,1,2,4-triazoleand…     

Sulfoxide-induced stereoselection in [1,5]-sigmatropic hydrogen shifts of vinylallenes. A computational study

The sulfoxide-induced preference for a migrating trajectory in the vinylallene [1,5]-H sigmatropic shift (resulting in stereodefined trienes in the conceptual equivalent of torquoselectivity in electrocyclizations), originally reported by Okamura, has been computationally studied at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31++G(d,p) level. The face selectivity this group induces in the [1,5]-H shift is enhanced by bulky geminal substituents and is not reproduced by any of the other (more than 20) substituents tested. Analysis of transition-state geometries or charges and evaluation of steric effects did not show any correlation with the preferences. The origin of this selectivity is thought to lie in a secondary orbital interaction (SOI) involving the termini of the pericyclic array and the sulfinyl group which is only observed for this substituent. This secondary orbital interaction, arising from the favorable energies of the orbitals involved, is enhanced in the transition structure due to a better orbital overlap (piC2-C3-->sigma*C1-S), which correlates with a piC2-C3-->sigma*C6-H SOI, which is more important in the transition structure, that weakens the C-H bond, thus lowering the energy of the corresponding transition structure.     

Room-temperature phosphorescence and energy transfer in luminescent multinuclear platinum(II) complexes of branched alkynyls

A series of luminescent branched platinum(II) alkynyl complexes, [1,3,5-{RC[triple bond]C(PEt3)2PtC[triple bond]C-C6H4C[triple bond]C}3C6H3] (R=C6H5, C6H4OMe, C6H4Me, C6H4CF3, C5H4N, C6H4SAc, 1-napthyl (Np), 1-pyrenyl (Pyr), 1-anthryl-8-ethynyl (HC[triple bond]CAn)), [1,3-{PyrC[triple chemical bond]C(PEt3)2PtC[triple bond]CC6H4C[triple bond]C}2-5-{(iPr)3SiC[triple bond]C}C6H3], and [1,3-{PyrC[triple bond]C(PEt3)2PtC[triple bond]CC6H4C[triple bond]C}2-5-(HC[triple bond]C)C6H3], was successfully synthesized by using the precursors [1,3,5-{Cl(PEt3)2PtC[triple bond]CC6H4C[triple bond]C}3C6H3] or [1,3-{Cl(PEt3)2PtC[triple bond]CC6H4C[triple bond]C}2-5-{(iPr)3SiC[triple bond]C}C6H3]. The X-ray crystal structures of [1,3,5-{MeOC6H4C[triple bond]C(PEt3)2PtC[triple bond]CC6H4C[triple bond]C}3C6H3] and [1,8-{Cl(PEt3)2PtC[triple bond]C}2An] have been determined. These complexes were found to show long-lived emission in both solution and solid-state phases at room temperature. The emission origin of the branched complexes [1,3,5-{RC[triple bond]C(PEt3)2PtC[triple bond]CC6H4C[triple bond]C}3C6H3] with R=C6H5, C6H4OMe, C6H4Me, C6H4CF3, C5H4N, and C6H4SAc was tentatively assigned to be derived from triplet states of predominantly intraligand (IL) character with some mixing of metal-to-ligand charge-transfer (MLCT) (dpi(Pt)-->pi*(C[triple bond]CR)) character, while the emission origin of the branched complexes with polyaromatic alkynyl ligands, [1,3,5-{RC[triple bond]C(PEt3)2PtC[triple bond]CC6H4C[triple bond]C}3C6H3] with R=Np, Pyr, or HC[triple bond]CAn, [1,3-{PyrC[triple bond]C(PEt3)2PtC[triple bond]CC6H4C[triple bond]C}2-5-{(iPr)3SiC[triple bond]C}C6H3], [1,3-{PyrC[triple bond]C(PEt3)2PtC[triple bond]CC6H4C[triple bond]C}2-5-(HC[triple bond]C)C6H3], and [1,8-{Cl(PEt3)2PtC[triple bond]C}2An], was tentatively assigned to be derived from the predominantly 3IL states of the respective polyaromatic alkynyl ligands, mixed with some 3MLCT (d(pi)(Pt)-->pi*(C[triple bond]CR)) character. By incorporating different alkynyl ligands into the periphery of these branched complexes, one could readily tune the nature of the lowest energy emissive state and the direction of the excitation energy transfer.     

硼氢二茂铁季铵盐的结构化学研究——Ⅰ、十硼十氢双(二茂铁甲基三甲基)铵晶体结构

本文报告了(C_5H_5FeC_5H_4CH_2N(CH_3)_3)_2B_(10)H_(10)晶体的结构。该晶体属三斜晶系、空间群为P_1~-。晶胞参数为:a=10.043(2),b=10.513(7),C=15.094(10),a=85.01(6),β=97.58(3),Υ=94.87(3)°,V=1625.7~3,Z=2。晶体在室温下用CAD-4四圆衍射仪收集衍射强度数据(MoKa),用重原子法解出铁原子坐标,综合应用E图、Fourief合成和差Fourier合成解出其他非氢原子坐标,按SDP加氢程序等求解阳离子中40个氢原子位置。各原子坐标及热振动参数经全矩阵最小二乘方修正,对于2258个独立衍射点(I≥3σI)],偏离因子R=0.043。 结构分析表明,B_(10)H_(10)~(2+)阴离子和C_5H_5FeC_5H_4CH_2N(CH_3)_3~+阳离子分别按畸变的八面体和畸变的三角形配位。两个二茂铁季铵阳离子中,两对戊二稀环平面间距为3.271(5)。阳离子中主要键长平均值为:Fe-C=2.026,C—C(环)=1.401,C—N=1.503。B_(10)H_(10)~(2+)阴离子为四方(三角)十六面体笼状结构,主要键长平均值为:B—B(顶点到正方形平面原子)1.706A,B—B(其它原子间)=1.830。     

(3h)J((15)N-(31)P) spin-spin coupling constants across N[bond]H....O[bond]P hydrogen bonds

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to evaluate three-bond (15)N-(31)P coupling constants ((3h)J(N[bond]P)) across N[bond]H....O[bond]P hydrogen bonds in model cationic and anionic complexes including NH(4)(+):OPH, NH(4)(+):OPH(3), NH(3):(-)O(2)PH(2), NFH(2):(-)O(2)PH(2), and NF(2)H:(-)O(2)PH(2). Three-bond coupling constants can be appreciable when the phosphorus is P(V), but are negligible with P(III). (3h)J(N[bond]P) values in complexes with cyclic or open structures are less than 1 Hz, a consequence of the nonlinear arrangement of N, H, O, and P atoms. For complexes with these structures, (3h)J(N[bond]P) may not be experimentally measurable. In contrast, complexes in which the N, H, O, and P atoms are collinear or nearly collinear have larger values of (3h)J(N[bond]P), even though the N[bond]P distances are longer than N[bond]P distances in cyclic and open structures. In linear complexes, (3h)J(N[bond]P) is dominated by the Fermi-contact term, which is distance dependent. Therefore, N[bond]P (and hydrogen-bonding N[bond]O) distances in these complexes can be determined from experimentally measured (15)N-(31)P coupling constants.     

Mechanistic studies of ethylene hydrophenylation catalyzed by bipyridyl Pt(II) complexes

Cationic platinum(II) complexes [((t)bpy)Pt(Ph)(L)](+) [(t)bpy =4,4'-di-tert-butyl-2,2'-bipyridyl; L = THF, NC(5)F(5), or NCMe] catalyze the hydrophenylation of ethylene to generate ethylbenzene and isomers of diethylbenzene. Using ethylene as the limiting reagent, an 89% yield of alkyl arene products is achieved after 4 h at 120 °C. Catalyst efficiency for ethylene hydrophenylation is diminished only slightly under aerobic conditions. Mechanistic studies support a reaction pathway that involves ethylene coordination to Pt(II), insertion of ethylene into the Pt-phenyl bond, and subsequent metal-mediated benzene C-H activation. Studies of stoichiometric benzene (C(6)H(6) or C(6)D(6)) C-H/C-D activation by [((t)bpy)Pt(Ph-d(n))(THF)](+) (n = 0 or 5) indicate a k(H)/k(D) = 1.4(1), while comparative rates of ethylene hydrophenylation using C(6)H(6) and C(6)D(6) reveal k(H)/k(D) = 1.8(4) for the overall catalytic reaction. DFT calculations suggest that the transition state for benzene C-H activation is the highest energy species along the catalytic cycle. In CD(2)Cl(2), [((t)bpy)Pt(Ph)(THF)][BAr'(4)] [Ar' = 3,5-bis(trifluoromethyl)phenyl] reacts with ethylene to generate [((t)bpy)Pt(CH(2)CH(2)Ph)(η(2)-C(2)H(4))][BAr'(4)] with k(obs) = 1.05(4) × 10(-3) s(-1) (23 °C, [C(2)H(4)] = 0.10(1) M). In the catalytic hydrophenylation of ethylene, substantial amounts of diethylbenzenes are produced, and experimental studies suggest that the selectivity for the monoalkylated arene is diminished due to a second aromatic C-H activation competing with ethylbenzene dissociation.