Substituent effects on the stereochemistry of gas-phase intracomplex nucleophilic substitutions

The mechanism and stereochemistry of the intracomplex solvolysis of proton-bound complexes [Y...H...M]+ between M = CH3 (18)OH and Y = 1-arylethanol [(S)-1-(para-tolyl)ethanol (1S), (S)-1-(para-chlorophenyl)ethanol (2S), (S)-1-(meta-alpha,alpha,alpha-trifluoromethylphenyl)ethanol (3S), (S)-1-(para-alpha,alpha,alpha-trifluoromethylphenyl)ethanol (4S), (R)-1-(pentafluorophenyl)ethanol (5R), (R)-alpha-(trifluoromethyl)benzyl alcohol (6R), and (R)-1-phenylethanol (7R)] have been investigated in the gas phase (CH3F; 720 Torr) in the 25-140 degrees C temperature range. Gas-phase solvolysis of [Y...H...M]+ (Y=2S, 3S, 4S, and 7R) leads to extensive racemization above a characteristic temperature t(#) (e.g. at t(#)>60 degrees C for 7R), whereas below that temperature the reaction displays a preferential retention of configuration. Predominant retention of configuration is instead observed in the intracomplex solvolysis of [Y...H...M]+ (Y=1S, 4S, 5R, and 6R) with the temperature range investigated (25 相似文献   

Precise investigation of the axial ligand substitution mechanism on a hydrogenphosphato-bridged lantern-type platinum(III) binuclear complex in acidic aqueous solution

Detailed equilibrium and kinetic studies on axial water ligand substitution reactions of the "lantern-type" platinum(III) binuclear complex, [Pt(2)(mu-HPO(4))(4)(H(2)O)(2)](2)(-), with halide and pseudo-halide ions (X(-) = Cl(-), Br(-), and SCN(-)) were carried out in acidic aqueous solution at 25 degrees C with I = 1.0 M. The diaqua Pt(III) dimer complex is in acid dissociation equilibrium in aqueous solution with -log K(h1) = 2.69 +/- 0.04. The consecutive formation constants of the aquahalo complex () and the dihalo complex () were determined spectrophotometrically to be log = 2.36 +/- 0.01 and log = 1.47 +/- 0.01 for the reaction with Cl(-) and log = 2.90 +/- 0.04 and log = 2.28 +/- 0.01 for the reaction with Br(-), respectively. In the kinetic measurements carried out under the pseudo-first-order conditions with a large excess concentration of halide ion compared to that of Pt(III) dimer (C(X)()- > C(Pt)), all of the reactions proceeded via a one-step first-order reaction, which is a contrast to the consecutive two-step reaction for the amidato-bridged platinum(III) binuclear complexes. The conditional first-order rate constant (k(obs)) depended on C(X)()- as well as the acidity of the solution. From kinetic analyses, the rate-limiting step was determined to be the first substitution process that forms the monohalo species, which is in rapid equilibrium with the dihalo complex. The reaction with 4-penten-1-ol was also kinetically investigated to examine the reactivity of the lantern complex with olefin compounds.     

Gas-phase diastereoselectivity of secondary 5-substituted (X)-adamant-2-yl (X = F, Si(CH(3))(3)) cations

Secondary 5-X-adamant-2-yl cations IX (X = F, Si(CH3)3) have been generated in the gas phase (total pressure = 760 Torr) from protonation-induced defluorination of epimeric 2-F-5-X-adamantanes 1X and their kinetic diastereoselectivity toward CH318OH investigated in the 40-160 degrees C range. The experimental results indicate that the facial selectivity of IX is insensitive to the composition of the starting 1X epimers as well as to the presence and the concentration of a powerful base (N(C2H5)3). This kinetic picture, supported by B3LYP/6-31G* calculations, is consistent with a single stable pyramidalized structure for IX, that is, (Z)-5-F-adamant-2-yl (I(Z)F) and (E)-5-Si(CH3)3-adamant-2-yl cations (I(E)Si). The temperature dependence of the IX diastereoselectivity lends support to the intermediacy of noncovalent adducts [IX*CH318OH], characterized by a specific C2-H+...O18(H)CH3 hydrogen bonding interaction. Their conversion to the covalently bonded O-methylated (Z)- (II(Z)X) and (E)-5-X-adamantan-2-ols (II(E)X; X = F, Si(CH3)3) is governed by activation parameters, whose magnitude depends on the specific IX face accommodating CH318OH. The gas-phase diastereoselectivity of IX toward CH318OH is compared to that exhibited in related gas-phase and solution processes. The emerging picture indicates that the factors determining the diastereoselectivity of IX toward simple nucleophiles in the gaseous and condensed media are completely different.     

Solid memory: structural preferences in group 2 dihalide monomers, dimers, and solids

The link between structural preferences in the monomers, dimers, and extended solid-state structures of the group 2 dihalides (MX(2): M = Be, Mg, Ca, Sr, Ba and X = F, Cl, Br, I) is examined theoretically. The question posed is how well are geometric properties of the gas-phase MX(2) monomers and lower order oligomers "remembered" in the corresponding MX(2) solids. Significant links between the bending in the MX(2) monomers and the D(2)(h)()/C(3)(v)() M(2)X(4) dimer structures are identified. At the B3LYP computational level, the monomers that are bent prefer the C(3)(v)() triply bridged geometry, while the rigid linear molecules prefer a D(2)(h)() doubly bridged structure. Quasilinear or floppy monomers show, in general, only a weak preference for either the D(2)(h)() or the C(3)(v)() dimer structure. A frontier orbital perspective, looking at the interaction of monomer units as led by a donor-acceptor interaction, proves to be a useful way to think about the monomer-oligomer relationships. There is also a relationship between the structural trends in these two (MX(2) and M(2)X(4)) series of molecular structures and the prevalent structure types in the group 2 dihalide solids. The most bent monomers condense to form the high coordination number fluorite and PbCl(2) structure types. The rigidly linear monomers condense to form extended solids with low coordination numbers, 4 or 6. The reasons for these correlations are explored.     

Theoretical studies of hetero-diels-alder reactions involving N-sulfinyl dienophiles

The gas-phase hetero-Diels-Alder reactions between butadiene and X-substituted sulfinyl dienophiles, O(-)-S(+)=N-X, are investigated theoretically at the B3LYP/6-31G level. The Z-forms of the dienophiles are found to be more stable (by 5-7 kcal mol(-)(1)) than the E-forms. Four modes of cycloadducts are considered: Z-endo; Z-exo; E(X)(-)(endo)(); E(X)(-)(exo)(). Five factors are responsible for the decreasing energetic preferences of the adducts in the order E(X)(-)(endo)() > E(X)(-)(exo)() > Z-endo > Z-exo: (i) The sigma-sigma proximate charge-transfer interactions in the TS; (ii) the relative sizes of the LUMO AO coefficients on S and N atoms; (iii) steric hindrance in the TS; (iv) the levels of the ground state and the LUMOs of the dienophile; (v) bond energies of the C-S and C-N bonds that are formed in the TS. All the reactions proceed concertedly, but the adduct formation is asynchronous. The endo-additions are favored over the exo-additions kinetically (lower DeltaG()) as well as thermodynamically (lower DeltaG degrees ). The major secondary orbital interaction determining the endo preference is that between the lone pair on N (n(N)) and the d(3) (C(3)-C(4)) sigma orbital (n(N)-sigma(d3)) interactions, whereas the larger AO lobe (LUMO) sizes on S favor a greater degree of d(5) (C-S) bond formation than d(6) (C-N) bond. The solvent, C(6)H(6), uniformly lowers the activation barriers so that the energetic preferences in the gas phase between various modes are maintained in solution.     

Influence of Sulfur Metalation on the Accessibility of the Ni(II/I) Couple in [N,N'-Bis(2-mercaptoethyl)-1,5-diazacyclooctanato]nickel(II): Insight into the Redox Properties of [NiFe]-Hydrogenase

A redox model study of [NiFe] hydrogenase has examined a series of five polymetallics based on the metalation of the dithiolate complex [1,5-bis(mercaptoethyl)-1,5-diazacyclooctane]Ni(II), Ni-1. Crystal structures of three polymetallics of the series have been reported earlier: [(Ni-1)(2)()Ni]Cl(2)(), [(Ni-1)(2)()FeCl(2)()](2)(), and [(Ni-1)(3)()(ZnCl)(2)()]Cl(2)(). Two are described here: [(Ni-1)(2)()Pd]Cl(2)().2H(2)()Ocrystallizes in the monoclinic system, space group P2(1)/c with cell constants a = 12.212(4) ?, b = 7.642(2) ?, c = 16.625(3) ?, beta = 107.69(2) degrees, V = 1443.230(0) ?(3), Z = 2, R = 0.051, and R(w) = 0.056. [(Ni-1)(2)()CoCl]PF(6)() crystallizes in the triclinic system, space group P&onemacr;, with cell constants a = 8.14(2) ?, b = 13.85(2) ?, c = 15.67(2) ?, alpha = 113.59(10) degrees, beta = 101.84(14) degrees, gamma = 94.0(2) degrees, V = 1561.620(0)?(3), Z = 2, R = 0.072, and R(w) = 0.077. In all Ni-1 serves as a bidentate metallothiolate ligand with a "hinge" angle in the range 105-118 degrees and Ni-M distances of 2.7- 3.7 ?. The most accessible redox event is shown by EPR and electrochemistry to reside in the N(2)S(2)Ni unit and is the Ni(II/I) couple. Charge neutralization of the thiolate sulfurs by metalation can (dependent on the interacting metal) stabilize the Ni(I) state as efficiently as methylation forming a thioether. The implication of these results for the heterometallic active site of [NiFe]-hydrogenase as structured from Desulfovibrio gigas (Volbeda, A., et al. Nature, 1995, 373, 580), the generality of the Ni(&mgr;-SR)(2)M hinge structure, and a possible explanation for the unusual redox potentials are discussed.     

Equilibrium and kinetics studies of reactions of manganese acetate, cobalt acetate, and bromide salts in acetic acid solutions

The oxidation of hydrogen bromide and alkali metal bromide salts to bromine in acetic acid by cobalt(III) acetate has been studied. The oxidation is inhibited by Mn(OAc)(2) and Co(OAc)(2), which lower the bromide concentration through complexation. Stability constants for Co(II)Br(n)() were redetermined in acetic acid containing 0.1% water as a function of temperature. This amount of water lowers the stability constant values as compared to glacial acetic acid. Mn(II)Br(n)() complexes were identified by UV-visible spectroscopy, and the stability constants for Mn(II)Br(n)() were determined by electrochemical methods. The kinetics of HBr oxidation shows that there is a new pathway in the presence of M(II)Br(n)(). Analysis of the concentration dependences shows that CoBr(2) and MnBr(2) are the principal and perhaps sole forms of the divalent metals that react with Co(III) and Mn(III). The interpretation of these data is in terms of this step (M, N = Mn or Co): M(OAc)(3) + N(II)Br(2) + HOAc --> M(OAc)(2) + N(III)Br(2)OAc. The second-order rate constants (L mol(-)(1) s(-)(1)) for different M, N pairs in glacial acetic acid are 4.8 (Co, Co at 40 degrees C), 0.96 (Mn, Co at 20 degrees C), 0.15 (Mn(III).Co(II), Co at 20 degrees C), and 0.07 (Mn, Mn at 20 degrees C). Following that, reductive elimination of the dibromide radical is proposed to occur: N(III)Br(2)OAc + HOAc --> N(OAc)(2) + HBr(2)(*). This finding implicates the dibromide radical as a key intermediate in this chemistry, and indeed in the cobalt-bromide catalyzed autoxidation of methylarenes, for which some form of zerovalent bromine has been identified. The selectivity for CoBr(2) and MnBr(2) is consistent with a pathway that forms this radical rather than bromine atoms which are at a considerably higher Gibbs energy. Mn(OAc)(3) oxidizes PhCH(2)Br, k = 1.3 L mol(-)(1) s(-)(1) at 50.0 degrees C in HOAc.     

烷烃同系物气相色谱保留指数的分子拓扑研究

定义了分子中原子的平衡电负性,并用原子的平衡电负性对分子图进行着色,在距离矩阵的基础上结合分子中各原子的支化度构建了一种新的拓扑指数N1,N2和N3。该拓扑指数对分子结构实现惟一性表征,具有优良的结构选择性。将拓扑指数N1,N2和N3与烷烃在固定相角鲨烷(柱温50　℃)及SE-30(柱温80　℃)上的气相色谱保留指数进行多元线性回归,结果表明烷烃的气相色谱保留指数可分别定量描述为I(Squalane)=23.97842N1-3.86562N2+0.787379N3+42.33061,I(SE-30)=23.83937N1-3.5687N2+0.939876N3+22.11952。用上述回归方程对烷烃的气相色谱保留指数进行预测,结果表明预测值与实验值的平均相对误差均为1.31%,预测结果误差在实验误差范围内。     

Vanadium(V) complexes of a chelating dianionic [ONNO]-type amine bis(phenolate) ligand: synthesis and solid state and solution structures

The reaction between VO(OR)(3) (R = (i)()Pr, (t)()Bu, CH(2)CF(3)) and the chelating dianionic bis(phenoxy)amine ligand [ONNO]H(2) affords a mixture of two isomers (A and B in a ratio A:B approximately 3:1) formulated as VO(OR)[ONNO] (1a-c) (R = (i)()Pr (1a), (t)Bu (1b), CH(2)CF(3) (1c)). Multinuclear and NOESY NMR spectroscopy experiments were able to determine the structure in solution of the complexes. Both isomers have the symmetry-related phenolate groups in a trans configuration, the difference arising from the different configuration of the oxo and alkoxo ligands being located either cis (in isomer A) or trans (in isomer B) to the tripodal amino nitrogen donor atom and the (dimethylamino)ethyl sidearm respectively for the oxo and the alkoxo ligands. Crystals of isomer A (cis-1a) were obtained, and the structure determination confirms the arrangement of the ligands around the vanadium center. Analogue complexes VO(X)[ONNO] (X = Cl (2); X = N(3) (3)) were prepared by reacting equimolar amount of [ONNO]H(2) and VO(X)(n)(OR)(3-n) (X = Cl, R = Et, n = 1; X = N(3), R = (i)Pr, n = 2) at ambient temperature. Compounds 2 and 3 were further characterized by NMR spectroscopy experiments and X-ray structure determination. For both 2 and 3, a single isomer is obtained, having a trans-(O,O) configuration for the phenolate groups and a trans configuration of the oxo ligand in respect to the tripodal amino nitrogen donor atom. Finally, complex 2 could also be obtained by chlorination of 1a or 3 using a large excess of ClSiMe(3) in refluxing toluene.     

The First Mixed-Halide Zirconium Cluster Compounds: Zr(6)Cl(1.6)I(10.4)Be, Zr(6)Cl(1.3)I(10.7)B, and Zr(6)Cl(11.5)I(1.5)B. Matrix Effects and Halogen Substitution in Compact Network Structures

Investigations of the effect of halogen size on structure stability have been conducted in well-reduced and heavily interbridged zirconium chloride-iodide cluster systems. The title compounds are obtained in good yields from reactions of Zr, ZrCl(4), ZrI(4), and B or Be in sealed Ta tubes for approximately 4 weeks at 850 degrees C. Single-crystal diffraction at room temperature established these as Zr(6)Cl(1.65(4))I(10.35(4))Be and Zr(6)Cl(1.27(3))I(10.73(3))B [R&thremacr;, Z = 3, a = 14.3508(8), 14.389(1) ?, c = 9.8777(9), 9.915(2) ?, respectively] and Zr(6)Cl(11.47(2))I(1.53(2))B [P4(2)/mnm, Z = 2, a = 12.030(1) ?, c = 7.4991(8) ?]. These are derivatives of the Zr(6)I(12)C and orthorhombic Zr(6)Cl(13)B structures, respectively, the latter containing unusual linear chains of clusters interbridged by Cl(i-i) that are in turn interconnected by three-bonded Cl(a-a-a) atoms. The random substitution of fractional Cl at specific I sites in the first two, and I for certain Cl in the third, was positionally resolved in all cases. The replacement always occurs at two-bonded X(i), so that single types of halogen are left in sites that interconnect clusters and generate the three-dimensional array. Structural changes seen in both structures are specifically related to relief of X.X crowding in the parent structure (matrix effects). Substitution of Cl for I(i) in the Zr(6)I(12)C type greatly reduces intercluster I.I repulsions and allows, among other things, a 0.20 ? (5.8%) reduction in Zr-I(a-i) intercluster bond lengths. Increased Cl.I repulsions caused by I substitution in orthorhombic Zr(6)Cl(13)B (Pnnm) convert the twisted chains and angular Cl(a-a-a) interchain bridges to planarity in tetragonal Zr(6)Cl(11.5)I(1.5)B. Phase widths found are 0 相似文献   

Solvolysis of (Z)-5-trimethylstannyl 2-adamantyl p-bromobenzenesulfonate: mechanistic implications of a record-breaking secondary alpha-deuterium kinetic isotope effect for an SN1 substrate

The secondary alpha-deuterium kinetic isotope effect (alpha-kie) for the solvolysis of (Z)-5-trimethylstannyl 2-adamantyl p-bromobenzenesulfonate in 97% w/w aqueous 2,2,2-trifluoroethanol (97T) at 25 degrees C has been measured (k(H)/k(D) = 1.33). The alpha-kie is abnormally high compared to the value of 1.23 for the corresponding limiting S(N)1 solvolysis of 2-adamantyl p-bromobenzenesulfonate, which proceeds via an extended ion-pair mechanism. A novel mechanism for the solvolysis of the tin compound is proposed that accommodates not only the high alpha-kie but also the absence of internal return.     

Manifestation of stereoelectronic effects on the calculated carbon-hydrogen bond lengths and one-bond 1J(C-H) NMR coupling constants. Relative acceptor ability of the carbonyl (C=O), thiocarbonyl (C=S), and methylidene (C=CH2) groups toward C-H donor bonds

Theoretical examination [B3LYP/6-31G(d,p), PP/IGLO-III//B3LYP/6-31G(d,p), and NBO methods] of six-membered cyclohexane 1 and carbonyl-, thiocarbonyl-, or methylidene-containing derivatives 2-27 afforded precise structural (in particular, C-H bond distances) and spectroscopic (specifically, one-bond (1)J(C)(-)(H) NMR coupling constants) data that show the consequences of stereoelectronic hyperconjugative effects in these systems. Major observations include the following. (1) sigma(C)(-)(H)(ax)() -->(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() (Y = O, S, or CH(2)) hyperconjugation leads to a shortening (strengthening) of the equatorial C-H bonds adjacent to the pi group. This effect is reflected in smaller (1)J(C)(-)(H)(ax)() coupling constants relative to (1)J(C)(-)(H)(eq)(). (2) Comparison of the structural and spectroscopic consequences of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) hyperconjugation in cyclohexanone 2, thiocyclohexanone 3, and methylenecyclohexane 4 suggests a relative order of acceptor orbital ability C=S > C=O > C=CH(2), which is in line with available pK(a) data. (3) Analysis of the structural and spectroscopic data gathered for heterocyclic derivatives 5-12 reveals some additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y), pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)(), n(X) --> sigma(C)(-)(H)(ax)(), n(beta)(O) --> sigma(C)(-)(H)(eq)(), and sigma(S)(-)(C) --> sigma(C)(-)(H)(eq)() stereoelectronic effects that is, nevertheless, attenuated by saturation effects. (4) Modulation of the C=Y acceptor character of the exocyclic pigroup by conjugation with alpha-heteroatoms O, N, and S in lactones, lactams, and methylidenic analogues 13-24 results in decreased sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugation. (5) Additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugative effects is also apparent in 1,3-dicarbonyl derivative 25 (C=Y equal to C=O), 1,3-dithiocarbonyl derivative 26 (C=Y equal to C=S), and 1,3-dimethylidenic analogue 27 (C=Y equal to C=CH(2)).     

Rearrangement of the (6S,8R,11S) and (6R,8S,11R) exocyclic 1,N2-deoxyguanosine adducts of trans-4-hydroxynonenal to N2-deoxyguanosine cyclic hemiacetal adducts when placed complementary to cytosine in duplex DNA

trans-4-Hydroxynonenal (HNE) is a peroxidation product of omega-6 polyunsaturated fatty acids. The Michael addition of deoxyguanosine to HNE yields four diastereomeric exocyclic 1,N(2)-dG adducts. The corresponding acrolein- and crotonaldehyde-derived exocyclic 1,N(2)-dG adducts undergo ring-opening to N(2)-dG aldehydes, placing the aldehyde functionalities into the minor groove of DNA. The acrolein- and the 6R-crotonaldehyde-derived exocyclic 1,N(2)-dG adducts form interstrand N(2)-dG:N(2)-dG cross-links in the 5'-CpG-3' sequence context. Only the HNE-derived exocyclic 1,N(2)-dG adduct of (6S,8R,11S) stereochemistry forms interstrand N(2)-dG:N(2)-dG cross-links in the 5'-CpG-3' sequence context. Moreover, as compared to the exocyclic 1,N(2)-dG adducts of acrolein and crotonaldehyde, the cross-linking reaction is slow (Wang, H.; Kozekov, I. D.; Harris, T. M.; Rizzo, C. J. J. Am. Chem. Soc. 2003, 125, 5687-5700). Accordingly, the chemistry of the HNE-derived exocyclic 1,N(2)-dG adduct of (6S,8R,11S) stereochemistry has been compared with that of the (6R,8S,11R) adduct, when incorporated into 5'-d(GCTAGCXAGTCC)-3'.5'-d(GGACTCGCTAGC)-3', containing the 5'-CpG-3' sequence (X = HNE-dG). When placed complementary to dC in this duplex, both adducts open to the corresponding N(2)-dG aldehydic rearrangement products, suggesting that the formation of the interstrand cross-link by the exocyclic 1,N(2)-dG adduct of (6S,8R,11S) stereochemistry, and the lack of cross-link formation by the exocyclic 1,N(2)-dG adduct of (6R,8S,11R) stereochemistry, is not attributable to inability to undergo ring-opening to the aldehydes in duplex DNA. Instead, these aldehydic rearrangement products exist in equilibrium with stereoisomeric cyclic hemiacetals. The latter are the predominant species present at equilibrium. The trans configuration of the HNE H6 and H8 protons is preferred. The presence of these cyclic hemiacetals in duplex DNA is significant as they mask the aldehyde species necessary for interstrand cross-link formation.     

Stereochemistry of cyclopropane formation involving group IV organometallic complexes

The reaction of (Z)-HDC=CHCH(OCH(3))C(6)H(5) (1) with Cp(2)Zr(D)Cl followed by BF(3).OEt(2) gave phenylcyclopropanes 3a and 3b, both having cis deuterium. This stereochemical outcome requires inversion of configuration at the carbon bound to zirconium and is consistent with a "W-shaped" transition state structure for cyclopropane formation. In a Kulinkovich hydroxycyclopropanation, trans-3-deutero-1-methyl-cis-2-phenyl-1-cyclopropanol (5) was formed stereospecifically from Ti(O-i-Pr)(4), ethyl acetate, EtMgBr, and trans-beta-deuterostyrene. This stereochemistry requires retention of configuration at the carbon bound to titanium and is consistent with frontside attack of the carbon-titanium bond on a carbonyl group coordinated to titanium. In a de Meijere cyclopropylamine synthesis, a 3:1 mixture of N,N-dimethyl-N-(trans-3-deutero-trans-2-phenylcyclopropyl)amine (6a) and N,N-dimethyl-N-(cis-3-deutero-cis-2-phenylcyclopropyl)amine (6b) was formed from Ti(O-i-Pr)(4), DMF, Grignard reagents, and trans-beta-deuterostyrene. This stereochemistry requires inversion of configuration at the carbon bound to titanium and is consistent with a W-shaped transition structure for ring closure.     

Synthesis and Characterization of a New Alkenyldecaborane and Alkenyl Monocarbon Carboranes

Decaborane(14) reacts with 1-(CH(3))(3)SiC&tbd1;CC(4)H(9) in the presence of dimethyl sulfide to give the new alkenyldecaborane 5-(S(CH(3))(2))-6-[(CH(3))(3)Si(C(4)H(9))C=CH]B(10)H(11) (I). Crystal data for 5-(S(CH(3))(2))-6-[(CH(3))(3)Si(C(4)H(9))C=CH]B(10)H(11): space group P2(1)/n, monoclinic, a = 9.471(1) ?, b = 13.947(3) ?, c = 17.678(3) ?, beta = 100.32(1) degrees. A total of 3366 unique reflections were collected over the range 2.0 degrees /= 3sigma(F(o)(2)) and were used in the final refinement. R(F)() = 0.083; R(w)(F)() = 0.094. The single-crystal X-ray structure of 5-(S(CH(3))(2))-6-[((CH(3))(3)Si)(2)C=CH]B(10)H(11) (A) is also reported. Crystal data for 5-(S(CH(3))(2))-6-[((CH(3))(3)Si)(2)C=CH]B(10)H(11): space group, P2(1)2(1)2(1), orthorhombic, a = 9.059 (3) ?, b = 12.193(4) ?, c = 21.431(3) ?. A total of 4836 unique reflections were collected over the range 6 degrees /= 3sigma(F(o)(2)) and were used in the final refinement. R(F)() = 0.052; R(w)(F)() = 0.059. The reactions of 5-(S(CH(3))(2))6-[(CH(3))(3)Si(C(4)H(9))C=CH]B(10)H(11) and 5-(S(CH(3))(2))6-[((CH(3))(3)Si)(2)C=CH]B(10)H(11) with a variety of alkyl isocyanides were investigated. All of the alkenyl monocarbon carboranes reported are the result of incorporation of the carbon atom from the isocyanide into the alkenyldecaborane framework and reduction of N&tbd1;C bond to a N-C single bond. The characterization of these compounds is based on (1)H and (11)B NMR data, IR spectroscopy, and mass spectrometry.     

Gas-phase identity nucleophilic substitution reactions of cyclopropenyl halides

The gas-phase identity nucleophilic substitution reactions of halide anions (X = F, Cl, and Br) with cyclopropenyl halides, X(-) + (CH)(3)X <= => X(CH)(3) + X(-), are investigated theoretically at four levels of theory, B3LYP/6-311+G**, MP2/6-311+G**, G2(+)MP2//MP2/6-311+G**, and G2(+)//MP2/6-311+G**. Four types of reaction paths, the sigma-attack S(N)2, pi-attack S(N)2'-syn, and S(N)2'-anti and sigmatropic 1,2-shift, are possible for all the halides. In the fluoride anion reactions, two types of stable adducts, syn- and anti-1,2-difluorocyclopropyl anions, can exist on the triple-well-type potential energy surface of the identity substitution reactions with rearrangement of double bond (C=C), S(N)2'-syn, and S(N)2'-anti processes. The TSs for the sigma-attack S(N)2 paths have "open" (loose) structures so that the ring positive charges are high rendering strong aromatic cyclopropenyl (delocalized) cation-like character. In contrast, in the pi-attack S(N)2' paths, a lone pair is formed at the unsubstituted carbon (C3), which stabilizes the 1,2-dihalocyclopropyl (delocalized) anion-like TS by two strong n(C)-sigma*(C-F) vicinal charge-transfer delocalization interactions. The barrier height increases in the order S(N)2'-anti < sigma-attack S(N)2 < S(N)2'-syn for X = Cl and Br, whereas for X = F the order is changed to S(N)2'-anti < S(N)2'-syn < sigma-attack S(N)2 due to the stable difluoro adduct formation. The sigmatropic 1,2-shift (circumambulatory) reactions have high activation barriers and cannot interfere with the substitution reactions.     

Isolation, structural characterization, and synthesis of a naturally occurring bisfuranopseudopterane ether: biskallolide A. Evidence for a carbocation intermediate during the facile conversion of kallolide A and isokallolide A into various solvolysis products

The West Indian alcyonacean Pseudopterogorgia bipinnata (Verrill, 1864) is shown to contain a novel bisditerpenoid ether: biskallolide A (2). The structural assignment of 2 was mainly based on 1D and 2D NMR and MS spectral data and was further confirmed by synthesis. The 2-C-alkoxylation of furanopseudopteranes kallolide A (1) and isokallolide A (8) occurs spontaneously in some solvents and involves replacement of the C2 hydroxyl with an alkoxyl group to yield solvolysis products that display net retention of configuration. The facile solvolytic 2-C-acyloxylation of kallolide A was achieved readily under similar circumstances to afford kallolide A acetate (4) as the sole product. Mechanistic details in conversion of alcohols 1 and 8 into various solvolysis products, including dimeric ethers 2 and 9, were investigated in this study. Solvolysis of kallolide A and isokallolide A in [(18)O]-labeled solvent demonstrated that the C2 alkoxyl of the solvolysis products originated from the solvent, suggesting that these conversions may proceed through an S(N)1 mechanism with generation of a carbocation intermediate. The chemical structures of kallolide A derivatives 3-7and those of isokallolide A congeners 9-11 were established by detailed analysis of the spectral data.     

High-Valent Manganese in Polyoxotungstates. 3. Dimanganese Complexes of gamma-Keggin Anions

Dimanganese-substituted gamma-Keggin heteropoly tungstates have been synthesized by reaction of the lacunary species gamma-[(SiO(4))W(10)O(32)](8)(-) with appropriate mixtures of Mn(II) and MnO(4)(-). The crystal structure of [(CH(3))(3)(C(6)H(5))N](4)[(SiO(4))W(10)Mn(III)(2)O(36)H(6)].2CH(3)CN.H(2)O (anion 1) was determined by X-ray diffraction. Crystallographic data: space group P&onemacr;, a = 12.951(3) ?, b = 14.429(3) ?, c = 20.347(4) ?, alpha = 81.95(3) degrees, beta = 88.92(3) degrees, gamma = 67.48(3) degrees, V = 3475.2(13) ?(3), and Z = 2. The final R value is 7.29% for 15861 reflections with I > 2sigma(I). The anion has the anticipated gamma-Keggin structure with virtual C(2)(v)() symmetry. The two Mn cations occupy adjacent, edge-shared octahedra with bridging hydroxo and terminal aqua ligands. Anion 1 can be oxidized and reduced to the corresponding Mn(III)Mn(IV) (2) and Mn(II)(2) (3) species respectively. The magnetic susceptibility of 1 between 2 and 300 K indicates that the Mn(III) cations are antiferromagnetically coupled, with J = -17.0 cm(-)(1) and g = 1.965. No simple magnetic behavior was observed for 2 or 3.     

Singlet-triplet gap in triplet ground-state biradicals is modulated by substituent effects

Three S = 1 bis(semiquinone) complexes have been prepared. To ensure ferromagnetic intramolecular exchange coupling, the two semiquinones are attached 1,3 to a 5-substituted phenylene ring. The biradical complexes differ in their meta-substituents: 1-NMe(2)(), X = N,N-dimethylamino; 1-t-Bu, X = tert-butyl; 1-NO(2)(), X = nitro. All three structures have been determined by X-ray crystallography. Results of structural studies indicate that the biradical ligands of all three complexes have nearly identical conformations with average semiquinone ring torsions of 32 degrees +/- 2 degrees relative to the 5-substituted phenylene ring. The exchange parameter, J (Eta = -2JS(1).S(2)), ranges from +31.0 +/- 0.6 cm(-)(1) for 1-NO(2)() to +59.3 +/- 1.2 cm(-)(1) for 1-t-Bu, with J = +34.9 +/- 0.7 cm(-)(1) for 1-NMe(2)(). Since the conformations are nearly identical, the differences in exchange coupling parameter J are due to substituent effects. The experimental results are supported by Hückel theory arguments and previous computational work.     

Chalcogenide-halides of niobium (V). 1. Gas-phase structures of NbOBr(3), NbSBr(3), and NbSCl(3). 2. Matrix infrared spectra and vibrational force fields of NbOBr(3), NbSBr(3), NbSCl(3), and NbOCl(3)

The molecular structures of NbOBr(3), NbSCl(3), and NbSBr(3) have been determined by gas-phase electron diffraction (GED) at nozzle-tip temperatures of 250 degrees C, taking into account the possible presence of NbOCl(3) as a contaminant in the NbSCl(3) sample and NbOBr(3) in the NbSBr(3) sample. The experimental data are consistent with trigonal-pyramidal molecules having C(3)(v)() symmetry. Infrared spectra of molecules trapped in argon or nitrogen matrices were recorded and exhibit the characteristic fundamental stretching modes for C(3)(v)() species. Well resolved isotopic fine structure ((35)Cl and (37)Cl) was observed for NbSCl(3), and for NbOCl(3) which occurred as an impurity in the NbSCl(3) spectra. Quantum mechanical calculations of the structures and vibrational frequencies of the four YNbX(3) molecules (Y = O, S; X = Cl, Br) were carried out at several levels of theory, most importantly B3LYP DFT with either the Stuttgart RSC ECP or Hay-Wadt (n + 1) ECP VDZ basis set for Nb and the 6-311G basis set for the nonmetal atoms. Theoretical values for the bond lengths are 0.01-0.04 A longer than the experimental ones of type r(a), in accord with general experience, but the bond angles with theoretical minus experimental differences of only 1.0-1.5 degrees are notably accurate. Symmetrized force fields were also calculated. The experimental bond lengths (r(g)/A) and angles ( 90 degree angle (alpha)()/deg) with estimated 2sigma uncertainties from GED are as follows. NbOBr(3): r(Nb=O) = 1.694(7), r(Nb-Br) = 2.429(2), 90 degree angle (O=Nb-Br) = 107.3(5), 90 degree angle (Br-Nb-Br) = 111.5(5). NbSBr(3): r(Nb=S) = 2.134(10), r(Nb-Br) = 2.408(4), 90 degree angle (S=Nb-Br) = 106.6(7), 90 degree angle (Br-Nb-Br) = 112.2(6). NbSCl(3): r(Nb=S) = 2.120(10),r(Nb-Cl) = 2.271(6), 90 degree angle (S=Nb-Cl) = 107.8(12), 90 degree angle (Cl-Nb-Cl) = 111.1(11).