Asymmetric transfer hydrogenation of benzaldehydes

A combined system of RuCl[(R, R)-YCH(C(6)H(5))CH(C(6)H(5))NH(2)](eta(6)-arene) (Y = NSO(2)C(6)H(4)-4-CH(3) or O) and t-C(4)H(9)OK catalyzes the asymmetric transfer hydrogenation of various benzaldehyde-1-d derivatives with 2-propanol to yield (R)-benzyl-1-d alcohols in 95-99% ee and with >99% isotopic purity. Reaction of benzaldehydes with a DCO(2)D-triethylamine mixture and the R,R catalyst affords the S deuterated alcohols in 97-99% ee.     

Mechanistical studies on the electron-induced degradation of polymers: polyethylene, polytetrafluoroethylene, and polystyrene

Mechanisms of the electron-induced degradation of three polymers utilized in aerospace applications (polyethylene (PE), polytetrafluoroethylene (PTFE), and polystyrene (PS)) were examined over a temperature range of 10 K to 300 K at ultra high vacuum conditions (～10(-11) Torr). These processes simulate the interaction of secondary electrons generated in the track of galactic cosmic ray particles in the near-Earth space environment with polymer material. The chemical alterations at the macromolecular level were monitored on-line and in situ by Fourier-transform infrared spectroscopy and mass spectrometry. These data yielded important information on the temperature dependent kinetics on the formation of, for instance, trans-vinylene groups (-CH=CH-) in PE, benzene (C(6)H(6)) production in PS, fluorinated trans-vinylene (-CF=CF-) and terminal vinyl (-CF=CF(2)) groups in PTFE together with molecular hydrogen release in PE and PS. Additional data on the radiation-induced development of unsaturated, conjugated bonds were collected via UV-vis spectroscopy. Temperature dependent G-values for trans-vinylene formation (G(-CH=CH-) ≈ 25-2.5 × 10(-4) units (100 eV)(-1) from 10-300 K) and molecular hydrogen evolution (G(H(2)) ≈ 8-80 × 10(-5) molecules (100 eV)(-1) from 10-300 K) for irradiated PE were calculated to quantify the degree of polymer degradation following electron irradiation. These values are typically two to three orders of magnitude less than G-values previously published for the irradiation of polymers with energetic particles of higher mass.     

Conductivities, volumes, fluorescence, and aggregation behavior of ionic liquids [C4mim][BF4] and [C(n)mim]Br (n = 4, 6, 8, 10, 12) in aqueous solutions

Densities, conductivities, and polarity indexes of pyrene for aqueous solutions of a series of ionic liquids [C(n)mim]Br (n = 4, 6, 8, 10, 12) and [C4mim][BF4] have been determined at 298.15 K as a function of ionic liquid concentrations. It was shown that possible aggregation appeared for the ionic liquids in aqueous solutions except for [C4mim]Br. The critical aggregation concentration (CAC) of the ionic liquids, the ionization degree of aggregates (beta), the standard Gibbs energy of aggregation (Delta G(m)(o)), the limiting molar conductivity (Lambda(m)(o)), and the standard partial molar volume (V(m)(o)) for the ionic liquids were derived from the experimental data. The dependence of the CAC, Delta G(m)(o), Lambda(m)(o), and V(m)(o) on the length of the alkyl chain of the cations was examined. It was further suggested from volumetric data that a micelle was formed for [C8mim]Br, [C10mim]Br, and [C12mim]Br in aqueous solutions. Their apparent molar volumes at the critical micelle concentration (V Phi,CMC), apparent molar volumes in the micelle phase (V(Phi)(mic)), and the change of their apparent molar volume upon micellization (Delta V Phi,m) were calculated by application of the pseudophase model of micellization. In addition, the average aggregation number of [C(n)mim]Br (n = 8, 10, 12) has been determined by the steady-state fluorescence quenching technique, and predicted from a simple geometrical mode. It is found that the experimental values are in good agreement with the predicted ones.     

Guided-ion beam and theoretical study of the potential energy surface for activation of methane by W+

A guided-ion beam tandem mass spectrometer is used to study the reactions, W(+) + CH(4) (CD(4)) and [W,C,2H](+) + H(2) (D(2)), to probe the [W,C,4H](+) potential energy surface. The reaction W(+) + CH(4) produces [W,C,2H](+) in the only low-energy process. The analogous reaction in the CD(4) system exhibits a cross section with strong differences at the lowest energies caused by zero-point energy differences, demonstrating that this reaction is slightly exothermic for CH(4) and slightly endothermic for CD(4). The [W,C,2H](+) product ion reacts further at thermal energies with CH(4) to produce W(CH(2))(x)(+) (x = 2-4). At higher energies, the W(+) + CH(4) reaction forms WH(+) as the dominant ionic product with smaller amounts of WCH(3)(+), WCH(+), and WC(+) also formed. The energy dependent cross sections for endothermic formation of the various products are analyzed and allow the determination of D(0)(W(+)-CH(3)) approximately 2.31 +/- 0.10 eV, D(0)(W(+)-CH(2)) = 4.74 +/- 0.03 eV, D(0)(W(+)-CH) = 6.01 +/- 0.28 eV, and D(0)(W(+)-C) = 4.96 +/- 0.22 eV. We also examine the reverse reaction, [W,C,2H](+) + H(2) (D(2)) --> W(+) + CH(4) (CH(2)D(2)). Combining the cross sections for the forward and reverse processes yields an equilibrium constant from which D(0)(W(+)-CH(2)) = 4.72 +/- 0.04 eV is derived. Theoretical calculations performed at the B3LYP/HW+/6-311++G(3df,3p) level yield thermochemistry in reasonable agreement with experiment. These calculations help identify the structures and electronic states of the species involved and characterize the potential energy surface for the [W,C,4H](+) system.     

Carbon-sulfur bond cleavage and hydrodesulfurization of thiophenes by tungsten

The reactions of W(PMe(3))(4)(η(2)-CH(2)PMe(2))H, W(PMe(3))(5)H(2), W(PMe(3))(4)H(4) and W(PMe(3))(3)H(6) towards thiophenes reveal that molecular tungsten compounds are capable of achieving a variety of transformations that are relevant to hydrodesulfurization. For example, sequential treatment of W(PMe(3))(4)(η(2)-CH(2)PMe(2))H with thiophene and H(2) yields the butanethiolate complex, W(PMe(3))(4)(SBu(n))H(3), which eliminates but-1-ene at 100 °C. Likewise, sequential treatment of W(PMe(3))(4)(η(2)-CH(2)PMe(2))H with benzothiophene and H(2) yields W(PMe(3))(4)(SC(6)H(4)Et)H(3), which releases ethylbenzene at 100 °C. Moreover, W(PMe(3))(4)(η(2)-CH(2)PMe(2))H desulfurizes dibenzothiophene to form a dibenzometallacyclopentadiene complex, [(κ(2)-C(12)H(8))W(PMe(3))](μ-S)(μ-CH(2)PMe(2))(μ-PMe(2))[W(PMe(3))(3)].     

Steric control of organic transformation by a dendrimer cage: organocobalt dendrimer porphyrins as novel coenzyme B(12) mimics

Cobalt(II) complexes of poly(aryl ester) dendrimer porphyrins (m-[Gn]TPP)Co(II) and (p-[Gn]TPP)Co(II) (n = 0-3) underwent AIBN-initiated alkylation (AIBN = 2,2'-azobis(isobutyronitrile)) at the metal center with propargyl alcohol in CDCl(3) at 60 degrees C, where the dendritic substituents did not affect the overall conversion rate but selectivity of the alkylation. With the largest (m-[G3]TPP)Co(II), a single organocobalt(III) species (Co(III)-C(=CH(2))CH(2)OH, 4) was selectively formed in 91% yield, due to a steric protection of 4 by the large dendrimer cage from the access of another molecule of cobalt porphyrin species. In contrast, with other cobalt(II) porphyrins, isomerized compounds such as Co(III)-C(CH(3))=CHOH (5) and Co(III)-CH(CH(3))CHO (6) were formed in addition to 4. A stereochemical investigation with (m-[G3]TPP)Co(II) using AIBN-d(12), in place of nondeuterated AIBN, demonstrated that the alkylation (cobalt(III) hydride addition to propargyl alcohol) is selective to a trans adduct. Results also indicated that this addition step does not involve external activation of propargyl alcohol.     

Ln4(CH2)4 cubane-type rare-earth methylidene complexes consisting of "(C5Me4SiMe3)LnCH2" units (Ln = Tm, Lu)

Tetranuclear cubane-type rare-earth methylidene complexes consisting of four "Cp'LnCH(2)" units, [Cp'Ln(μ(3)-CH(2))](4) (4-Ln; Ln = Tm, Lu; Cp' = C(5)Me(4)SiMe(3)), have been obtained for the first time through CH(4) elimination from the well-defined polymethyl complexes [Cp'Ln(μ(2)-CH(3))(2)](3) (2-Ln) or mixed methyl/methylidene precursors such as [Cp'(3)Ln(3)(μ(2)-Me)(3)(μ(3)-Me)(μ(3)-CH(2))] (3-Ln). The reaction of the methylidene complex 4-Lu with benzophenone leads to C═O bond cleavage and C═C bond formation to give the cubane-type oxo complex [Cp'Lu(μ(3)-O)](4) and CH(2)═CPh(2), while the methyl/methylidene complex 3-Tm undergoes sequential methylidene addition to the C═O group and ortho C-H activation of the two phenyl groups of benzophenone to afford the bis(benzo-1,2-diyl)ethoxy-chelated trinuclear complex [Cp'(3)Tm(3)(μ(2)-Me)(3){(C(6)H(4))(2)C(O)Me}] (6-Tm).     

Homoleptic rare earth dipyridylamides [Ln2(N(NC5H4)2)6], Ln = Ce, Nd, Sm, Ho, Er, Tm, Yb, and Sc: metal oxidation by the amine melt and in 1,2,3,4-tetrahydroquinoline with the focus of different metal activation by amalgams, liquid ammonia, and microwaves

Homoleptic dimeric dipyridylamide complexes of the rare earth elements are obtained by solvent-free oxidation reactions of the metals with melts of 2,2'-dipyridylamine. As the thermal stabilities of the ligand as well as the amide complexes are limiting factors in these high-temperature syntheses, several different metal activation procedures have been investigated: the formation of Ln amalgams and dissolution of the metals in liquid ammonia as well as coupling to microwaves. For comparison with a solvent that shows low solubility of the metals and products, reactions in 1,2,3,4-tetrahydroquinoline were also carried out. For all lanthanides and group 3 metals used homoleptic dimers of the formula [Ln(2)(Dpa)(6)], Ln = Ce (1), Nd (2), Sm (3), Ho (4), Er (5), Tm (6), Yb (7), and Sc (8) and Dpa- = (C5H4N)2N-, were obtained, all containing trivalent rare earth ions with a distorted square antiprismatic nitrogen coordination. Due to the large differences in the ionic radii of the metal ions, two different structure types are found that crystallize in the space groups P2(1)/c and P2(1)/n with the border of the two types being between Tm and Yb. The orientations of two 1,3/1,3-double chelating and linking dipyridylamide ligands (Dpa(-) = (C(5)H(4)N)(2)N(-)) result in different overall orientations of the dimers and thus two structure types. All compounds were identified by single-crystal X-ray analysis. Mid-IR, far IR, and Raman spectroscopy, microanalyses, and simultaneous DTA/TG as well as mass spectrometry regarding their thermal behavior were also carried out to characterize the products. Crystal data for the two types follow. Ce (1): P2(1)/n; T = 170(2) K; a = 1063.0(1), b = 1536.0(1), c = 1652.0(2) pm; beta = 101.60(1) degrees ; V = 2642.2(3) x 10(6) pm(3); R(1) for F(o) > 4sigma(F(o)) = 0.046, wR(2) = 0.120. Sc (8): P2(1)/c; T = 170(2) K; a = 1073.0(1), b = 1506.2(2), c = 1619.8(2) pm; beta = 103.16(9) degrees ; V = 2548.9(5) x 10(6) pm(3); R(1) for F(o) > 4sigma(F(o)) = 0.038, wR(2) = 0.091.     

Heterochiral DNA with Complementary Strands with α-d and β-d Configurations: Hydrogen-Bonded and Silver-Mediated Base Pairs with Impact of 7-Deazapurines Replacing Purines

Heterochiral DNA with hydrogen-bonded and silver-mediated base pairs have been constructed using complementary strands with nucleosides with α-d or β-d configuration. Anomeric phosphoramidites were employed to assemble the oligonucleotides. According to the T_m values and thermodynamic data, the duplex stability of the heterochiral duplexes was similar to that of homochiral DNA, but mismatch discrimination was better in heterochiral DNA. Replacement of purines by 7-deazapurines resulted in stable parallel duplexes, thereby confirming Watson–Crick-type base pairing. When cytosine was facing cytosine, thymine or adenine residues, duplex DNA formed silver-mediated base pairs in the presence of silver ions. Although the CD spectra of single strands with α-d configuration display mirror-like shapes to those with the β-d configuration, the CD spectra of the hydrogen-bonded duplexes and those with a limited number of silver pairs show a B-type double helix almost indistinguishable from natural DNA. Nonmelting silver ion–DNA complexes with entirely different CD spectra were generated when the number of silver ions was equal to the number of base pairs.     

Synthesis and reactivity of silyl ruthenium complexes: the importance of trans effects in C-H activation,Si-C bond formation,and dehydrogenative coupling of silanes

A series of octahedral ruthenium silyl hydride complexes, cis-(PMe(3))(4)Ru(SiR(3))H (SiR(3) = SiMe(3), 1a; SiMe(2)CH(2)SiMe(3), 1b; SiEt(3), 1c; SiMe(2)H, 1d), has been synthesized by the reaction of hydrosilanes with (PMe(3))(3)Ru(eta(2)-CH(2)PMe(2))H (5), cis-(PMe(3))(4)RuMe(2) (6), or (PMe(3))(4)RuH(2) (9). Reaction with 6 proceeds via an intermediate product, cis-(PMe(3))(4)Ru(SiR(3))Me (SiR(3) = SiMe(3), 7a; SiMe(2)CH(2)SiMe(3), 7b). Alternatively, 1 and 7 have been synthesized via a fast hydrosilane exchange with another cis-(PMe(3))(4)Ru(SiR(3))H or cis-(PMe(3))(4)Ru(SiR(3))Me, which occurs at a rate approaching the NMR time scale. Compounds 1a, 1b, 1d, and 7a adopt octahedral geometries in solution and the solid state with mutually cis silyl and hydride (or silyl and methyl) ligands. The longest Ru-P distance within a complex is always trans to Si, reflecting the strong trans influence of silicon. The aptitude of phosphine dissociation in these complexes has been probed in reactions of 1a, 1c, and 7a with PMe(3)-d(9) and CO. The dissociation is regioselective in the position trans to a silyl ligand (trans effect of Si), and the rate approaches the NMR time scale. A slower secondary process introduces PMe(3)-d(9) and CO in the other octahedral positions, most likely via nondissociative isomerization. The trans effect and trans influence in 7a are so strong that an equilibrium concentration of dissociated phosphine is detectable (approximately 5%) in solution of pure 7a. Compounds 1a-c also react with dihydrogen via regioselective dissociation of phosphine from the site trans to Si, but the final product, fac-(PMe(3))(3)Ru(SiR(3))H(3) (SiR(3) = SiMe(3), 4a; SiMe(2)CH(2)SiMe(3), 4b; SiEt(3), 4c), features hydrides cis to Si. Alternatively, 4a-c have been synthesized by photolysis of (PMe(3))(4)RuH(2) in the presence of a hydrosilane or by exchange of fac-(PMe(3))(3)Ru(SiR(3))H(3) with another HSiR(3). The reverse manifold - HH elimination from 4a and trapping with PMe(3) or PMe(3)-d(9) - is also regioselective (1a-d(9)() is predominantly produced with PMe(3)-d(9) trans to Si), but is very unfavorable. At 70 degrees C, a slower but irreversible SiH elimination also occurs and furnishes (PMe(3))(4)RuH(2). The structure of 4a exhibits a tetrahedral P(3)Si environment around the metal with the three hydrides adjacent to silicon and capping the P(2)Si faces. Although strong Si...HRu interactions are not indicated in the structure or by IR, the HSi distances (2.13-2.23(5) A) suggest some degree of nonclassical SiH bonding in the H(3)SiR(3) fragment. Thermolysis of 1a in C(6)D(6) at 45-55 degrees C leads to an intermolecular CD activation of C(6)D(6). Extensive H/D exchange into the hydride, SiMe(3), and PMe(3) ligands is observed, followed by much slower formation of cis-(PMe(3))(4)Ru(D)(Ph-d(5)). In an even slower intramolecular CH activation process, (PMe(3))(3)Ru(eta(2)-CH(2)PMe(2))H (5) is also produced. The structure of intermediates, mechanisms, and aptitudes for PMe(3) dissociation and addition/elimination of H-H, Si-H, C-Si, and C-H bonds in these systems are discussed with a special emphasis on the trans effect and trans influence of silicon and ramifications for SiC coupling catalysis.     

Transition-state variation in the nucleophilic substitution reactions of aryl bis(4-methoxyphenyl) phosphates with pyridines in acetonitrile

The kinetics and mechanism of the reactions of Z-aryl bis(4-methoxyphenyl) phosphates, (4-MeOC(6)H(4)O)(2)P(=O)OC(6)H(4)Z, with pyridines (XC(5)H(4)N) are investigated in acetonitrile at 55.0 degrees C. In the case of more basic phenolate leaving groups (Z = 4-Cl, 3-CN), the magnitudes of beta(X) (beta(nuc)) and beta(Z) (beta(lg)) indicate that mechanism changes from a concerted process (beta(X) = 0.22-0.36, beta(Z) = -0.42 to -0.56) for the weakly basic pyridines (X = 3-Cl, 4-CN) to a stepwise process with rate-limiting formation of a trigonal bipyramidal pentacoordinate (TBP-5C) intermediate (beta(X) = 0.09-0.14, beta(Z) = -0.08 to -0.28) for the more basic pyridines (X = 4-NH(2), 3-CH(3)). This proposal is supported by a large negative cross-interaction constant (rho(XZ) = -1.98) for the former and a positive rho(XZ) (+0.97) for the latter processes. In the case of less basic phenolate leaving groups (Z = 3-CN, 4-NO(2)), the unusually small magnitude of beta(Z) values is indicative of a direct backside attack TBP-5C TS in which the two apical sites are occupied by the nucleophile and leaving group, ap(NX)-ap(LZ). The instability of the putative TBP-5C intermediate leading to a concerted displacement is considered to result from relatively strong proximate charge transfer interactions between the pi-lone pairs on the directly bonded equatorial oxygen atoms and the apical bond (n(O)(eq) - sigma(ap)). These are supported by the results of natural bond orbital (NBO) analyses at the NBO-HF/6-311+G//B3LYP/6-311+G level of theory.     

New base pairing motifs. The synthesis and thermal stability of oligodeoxynucleotides containing imidazopyridopyrimidine nucleosides with the ability to form four hydrogen bonds

The synthesis and thermal stability of oligodeoxynucleotides (ODNs) containing imidazo[5',4':4,5]pyrido[2,3-d]pyrimidine nucleosides 1-4 (N(N), O(O), N(O), and O(N), respectively) with the aim of developing two sets of new base pairing motifs consisting of four hydrogen bonds (H-bonds) is described. The proposed four tricyclic nucleosides 1-4 were synthesized through the Stille coupling reaction of a 5-iodoimidazole nucleoside with an appropriate 5-stannylpyrimidine derivative, followed by an intramolecular cyclization. These nucleosides were incorporated into ODNs to investigate the H-bonding ability. When one molecule of the tricyclic nucleosides was incorporated into the center of each ODN (ODN I and II, each 17mer), no apparent specificity of base pairing was observed, and all duplexes were less stable than the duplexes containing natural G:C and A:T pairs. On the other hand, when three molecules of the tricyclic nucleosides were consecutively incorporated into the center of each ODN (ODN III and IV, each 17mer), thermal and thermodynamic stabilization of the duplexes due to the specific base pairings was observed. The melting temperature (T(m)) of the duplex containing the N(O):O(N) pairs showed the highest T(m) of 84.0 degrees C, which was 18.2 and 23.5 degrees C higher than that of the duplexes containing G:C and A:T pairs, respectively. This result implies that N(O)and O(N) form base pairs with four H-bonds when they are incorporated into ODNs. The duplex containing N(O):O(N) pairs was markedly stabilized by the assistance of the stacking ability of the imidazopyridopyrimidine bases. Thus, we developed a thermally stable new base pairing motif, which should be useful for the stabilization and regulation of a variety of DNA structures.     

Solid-phase synthesis of oligodeoxyribonucleoside boranophosphates by the boranophosphotriester method

Oligodeoxyribonucleoside boranophosphates (BH3-ODNs), containing four kinds of nucleobases, were synthesized by the solid-phase boranophosphotriester method. The 2'-deoxyribonucleoside 3'-boranophosphate monomers having 2-cyanoethyl (CE) groups as the phosphorus protecting groups were synthesized in good yields. A new condensing reagent, 1,3-dimethyl-2-(3-nitro-1,2,4-triazol-1-yl)-2-pyrrolidin-1-yl-1,3,2-diazaphospholidinium hexafluorophosphate, was found to be highly effective for the condensation reaction on the solid support. We also found that 1,8-bis(N,N-dimethylamino)naphthalene could accelerate the condensation reaction without causing beta-elimination of the CE groups from the boranophosphate triesters. The internucleotidic CE groups were selectively removed by treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) under anhydrous conditions. The acetylation of the terminal 5'-hydroxy group was found to be effective to suppress the decomposition of the BH3-ODNs during the DBU treatment on the solid support. Under optimized conditions for the solid-phase synthesis and the deprotection reactions, BH3-ODNs (4mers and 12mers) containing four kinds of nucleobases were synthesized in good yields. The hybridization properties of the BH3-ODN 12mers with the complementary native DNAs and RNAs were determined by the thermal denaturing studies. In contrast to the low thermal melting (Tm) value of the duplex composed of T((PB)T)11 and native dA12 (12.8 degrees C), the duplex consisting of d(C(PB)A(PB)G(PB)T)3 and d(ACTG)3 showed a higher Tm value (44.7 degrees C) under high-salt conditions. Furthermore, d(C(PB)A(PB)G(PB)T)3 formed a more stable duplex with the complementary RNA, r(ACUG)3 with a Tm value of 50.5 degrees C. Thus, we first demonstrated that the binding affinity of BH3-ODN to a complementary DNA or RNA is dramatically increased, owing to the inclusion of the four kinds of nucleobases.     

Synthetic and structural studies of monocyclopentadienyl cyclometalated aryl tantalum(V) compounds

Cyclometalated aryl tetra- or trichlorido cyclopentadienyl tantalum complexes [TaXCl(3){C(6)H(4)(2-CH(2)NMe(2))-κ(2)C,N}] (X = Cl 1, η(5)-C(5)H(5)2, η(5)-C(5)H(4)(SiMe(3)) 3, η(5)-C(5)Me(5)4) containing a five-membered TaC(3)N chelate ring were synthesized by reaction of the TaXCl(4) (X = Cl, η(5)-C(5)H(5), η(5)-C(5)H(4)(SiMe(3)), η(5)-C(5)Me(5)) with the appropriate lithium aryl reagent [Li{C(6)H(4)(2-CH(2)NMe(2))}]. The reported complexes were studied by IR and NMR spectroscopy and the X-ray molecular structures of compounds 2, 3 and 4 were determined by diffraction methods. These compounds were theoretically analyzed by the DFT method and their structures were rationalized. The preferential coordination of the 2-{(dimethylamino)methyl}phenyl ligand was justified by an analysis of the molecular orbitals of the Ta(η(5)-C(5)H(5))Cl(3) and C(6)H(4)(2-CH(2)NMe(2)) fragments. In addition, the exchange pathways that account for the NMR equivalency of the Me(2)N- methyl groups and -CH(2)- hydrogen atoms of the coordinated C(6)H(4)(2-CH(2)NMe(2))-κ(2)C,N ligand were theoretically studied.     

Expanding the world of endohedral fullerenes--the Tm3N@C2n (39< or =n< or =43) clusterfullerene family

A new family of endohedral fullerenes, based on an encaged trithulium nitride (Tm(3)N) cluster, was synthesised, isolated and characterised by HPLC, mass spectrometry, and visible-NIR and FTIR spectroscopy. Tm(3)N clusterfullerenes with cages as small as C(76) and as large as C(88) were prepared and six of them were isolated. Tm(3)N@C(78) is a small clusterfullerene. The two isomers of Tm(3)N@C(80) (I and II) were the most abundant structures in the fullerene soot. Tm(3)N@C(82), Tm(3)N@C(84), and Tm(3)N@C(86) represent a new series of higher clusterfullerenes. All six isolated Tm(3)N clusterfullerenes were classified as large energy-gap structures with optical energy gaps between approximately 1.2 and approximately 1.75 eV. Tm(3)N@C(80) (I) and Tm(3)N@C(80) (II) were assigned to the C(80) cages C(80):7 (I(h)) and C(80):6 (D(5h)). For Tm(3)N@C(78), the analysis pointed to an elliptical carbon cage with C(78):1 (D(3)) or C(78):4 (D(3h)) being the probable structures.     

Synthesis and properties of ionic liquid-type Gemini imidazolium surfactants

A series of ionic liquid-type Gemini imidazolium surfactants with four-methylene spacer groups were synthesized ([C(n)-4-C(n)im]Br(2), n=10, 12, 14). The surface activity and thermodynamic properties of micellization between the Gemini imidazolium surfactants and their corresponding monomers ([C(n)mim]Br, n=10, 12, 14) were compared by means of surface tension and electrical conductivity measurements. The values of cmc, gamma(cmc), pc(20), Gamma(max), and A(min) derived from surface tension measurement at 25 degrees C suggest that the surface activity of [C(n)-4-C(n)im]Br(2) is higher than that of [C(n)mim]Br. While the thermodynamic parameters of micellization (DeltaG(m)(o), DeltaH(m)(o), DeltaS(m)(o)) derived from electrical conductivity indicate that the micellization of [C(n)-4-C(n)im]Br(2) is entropy-driven, aggregation of [C(n)mim]Br is entropy-driven at low temperature but enthalpy-driven at high temperature. Finally, the activation energy of conductance (E(a)) that is associated with the effective charge is also obtained for [C(n)-4-C(n)im]Br(2) and it is constant below the cmc, but it increases above the cmc.     

Hydrophobic, Non-Hydrogen-Bonding Bases and Base Pairs in DNA

We report the properties of hydrophobic isosteres of pyrimidines and purines in synthetic DNA duplexes. Phenyl nucleosides 1 and 2 are nonpolar isosteres of the natural thymidine nucleoside, and indole nucleoside 3 is an analog of the complementary purine 2-aminodeoxyadenosine. The nucleosides were incorporated into synthetic oligodeoxynucleotides and were paired against each other and against the natural bases. Thermal denaturation experiments were used to measure the stabilities of the duplexes at neutral pH. It is found that the hydrophobic base analogs are nonselective in pairing with the four natural bases but selective for pairing with each other rather than with the natural bases. For example, compound 2 selectively pairs with itself rather than with A, T, G, or C; the magnitude of this selectivity is found to be 6.5-9.3 °C in Tm or 1.5-1.8 kcal/mol in free energy (25 °C). All possible hydrophobic pairing combinations of 1, 2, and 3 were examined. Results show that the pairing affinity depends on the nature of the pairs and on position in the duplex. The highest affinity pairs are found to be the 1-1 and 2-2 self-pairs and the 1-2 heteropair. The best stabilization occurs when the pairs are placed at the ends of duplexes rather than internally; the internal pairs may be destabilized by imperfect steric mimicry which leads to non-ideal duplex structure. In some cases the hydrophobic pairs are significantly stabilizing to the DNA duplex; for example, when situated at the end of a duplex, the 1-1 pair is more stabilizing than a T-A pair. When situated internally, the affinity of the 1-1 pair is the same as, or slightly better than, the analogous T-T mismatch pair, which is known to have two hydrogen bonds. The studies raise the possibility that hydrogen bonds may not always be required for the formation of stable duplex DNA-like structure. In addition, the results point out the importance of solvation and desolvation in natural base pairing, and lend new support to the idea that hydrogen bonds in DNA may be more important for specificity of pairing than for affinity. Finally, the study raises the possibility of using these or related base pairs to expand the genetic code beyond the natural A-T and G-C pairs.     

Zirconium complexes incorporated with asymmetrical tridentate pincer type mono- and di-anionic pyrrolyl ligands: mechanism and reactivity as catalytic precursors

The reactions of Zr(NR(2))(4) (1, R = Me; 2, R = Et) with an asymmetrical tridentate pincer type pyrrole ligand precursor [C(4)H(2)NH(2-CH(2)NH(t)Bu)(5-CH(2)NMe(2))] and treatment of the derivatives with either PhNCS or PhNCO have been carried out and characterized. Reacting Zr(NR(2))(4) (1, R = Me; 2, R = Et) with [C(4)H(2)NH(2-CH(2)NH(t)Bu)(5-CH(2)NMe(2))] generates Zr[C(4)H(2)N(2-CH(2)N(t)Bu)(5-CH(2)NMe(2))](NR(2))(2) (3, R = Me; 4, R = Et) in high yield along with the elimination of 2 equiv of dimethylamine or diethylamine, respectively. Interestingly, while changing the solvent from Et(2)O to CH(2)Cl(2), the complex Zr[C(4)H(2)N(2-CH(2)N(t)Bu)(5-CH(2)NMe(2))][C(4)H(2)N(2-CH(2)NH(t)Bu)(5-CH(2)NMe(2))]Cl (5) is produced by undergoing C-Cl bond cleavage. Furthermore, reaction of either 3 or 4 with 1 or 2 equiv of PhNCS or PhNCO yields Zr[C(4)H(2)N(2-CH(2)N(t)Bu)(5-CH(2)NMe(2))](NMe(2))[PhNC(NMe(2))S] (6), Zr[C(4)H(2)N(2-CH(2)N(t)Bu)(5-CH(2)NMe(2))](NEt(2))[PhNC(NEt(2))O] (7) and Zr[C(4)H(2)N(2-CH(2)NH(t)Bu)(5-CH(2)NMe(2))][PhNC(NEt(2))O](3) (8), respectively. All the aforementioned complexes were characterized by (1)H and (13)C NMR spectrometry and the molecular structures of 5, 6, and 8 have been determined by single-crystal X-ray diffractometry. Complexes 4, 5, and 7 initiated the ethylene polymerization in the presence of MAO as the co-catalyst.     

Studies on the Extension of Sequence‐independence and the Enhancement of DNA Triplex Formation

A more elaborate sequence‐independent triple‐helix formation viability study was carried out and extended from a recombination‐like triple‐helical DNA motif of a previous study (J. Mol. Recognition 14, 122–139 (2001)). The intended triple‐helix was formed by mixing one part of a DNA hairpin duplex and one part of a single (or third) strand identical to one of the duplex strands and complementary to the other strand. In contrast to the common purine and pyrimidine motifs in triple‐stranded DNA, the strands of the recombination‐like motif are not monotonously built from pyrimidine only, or purine only, in the sequence. The stability of the recombination‐like motif triplexes with varying sequences was monitored by UV thermal melting curves. The results showed that the order of the stability of the R‐form DNA base triads (J. Mol. Biol., 239, 181–200 (1994)) is G*(G ○ C) > C*(C ○ G) > A*(A ○ T) >T*(T ○ A) (the Watson‐Crick base pair is denoted in the parentheses) in 200 mM NaCl, at pH 7. In an attempt to increase the stability of the triplex in the recombination‐like motif, we replaced cytidine by 5‐methylcytidine (^mC) of the third strand. There is a general trend that ^mC modification stabilizes the complex (<2 °C per ^mC). The complex is furthermore stabilized by Mg²⁺ ion. The Tm increases from 7 to 2 °C from less stable to highly stable triplex by 20 mM Mg²⁺ ion in solution.     

A homologous series of alkaline earth phosphanides: syntheses,crystal structures,and unusual dynamic behavior of (THF)(n)M[P(CH(SiMe(3))(2))(C(6)H(4)-2-CH(2)NMe(2))](2) (M = Mg,Ca, Sr,Ba)

The secondary phosphine R(Me(2)NCH(2)-2-C(6)H(4))PH reacts with Bu(2)Mg to give the homoleptic complex Mg[PR(C(6)H(4)-2-CH(2)NMe(2))](2) (1) [R = CH(SiMe(3))(2)]. The analogous heavier alkaline earth metal complexes (THF)(n)Ae[PR(C(6)H(4)-2-CH(2)NMe(2))](2) [Ae = Ca (2), n = 0; Ae = Sr (3), Ba (4), n = 1] have been synthesized by metathesis reactions between K[PR(C(6)H(4)-2-CH(2)NMe(2))] and 0.5 equiv of the respective alkaline earth metal diiodide. Compounds 1-4 have been characterized by X-ray crystallography and multielement NMR spectroscopy. In the solid state, compounds 1-4 are monomeric, complexes 1 and 2 adopting a distorted tetrahedral geometry and complexes 3 and 4 adopting a distorted square pyramidal geometry (1: orthorhombic, P2(1)2(1)2(1), a = 11.413(3) A, b = 12.072(3) A, c = 32.620(11) A, Z = 4. 2: monoclinic, P2(1)/c, a = 9.5550(4) A, b = 17.4560(7) A, c = 24.5782(10) A, beta = 91.673(2) degrees, Z = 4. 3: monoclinic, C2/c, a = 15.0498(9) A, b = 13.0180(8) A, c = 24.3664(14) A, beta = 104.593(2) degrees, Z = 4. 4: monoclinic, C2/c, a = 15.2930(10) A, b = 13.0326(9) A, c = 24.6491(17) A, beta = 105.542(2) degrees, Z = 4). In toluene solution, compounds 2-4 are subject to dynamic processes which are attributed to a monomer-dimer equilibrium for which bridge-terminal exchange of the phosphanide ligands in the dimer may be frozen out at low temperatures.