π-Bonding contribution to restricted internal rotations in saccharides

Extensive semiempirical SCF-MO calculations confirm that the exo-anomeric effect in methyl O-, N- and S-glycosides deals with an interaction of π-character along the C₁(SINGLE BOND)Y₁ bond in a X₅(SINGLE BOND)C₁(SINGLE BOND)Y₁(SINGLE BOND)Me moiety (where X = O, S; Y = O, NH, S). The bond-order between orbitals of pπ symmetry on C₁ and Y₁ serves as a measure of all significant molecular orbital interactions responsible for the exo-anomeric stabilization. The set of simpler compounds X(SINGLE BOND)CH₂(SINGLE BOND)Y (X = OH, SH, SeH, TeH; Y = OH, SH, SeH, TeH, NH₂) on which the anomeric effect has been well studied was also calculated and it is noticeable that the π-bond-orders accord with the results of other analyses of the ab initio wave function accounting for the anomeric effect. Although the AM1 and the PM3 parameterizations of MNDO do not accurately reproduce the anomeric effect energetic, they do reproduce accordingly the expected variations in the molecular conformations of complex carbohydrates, and thus it follows that there are maximal π-bond-orders for the synclinal arrangement around the C₁(SINGLE BOND)Y₁ bond. In addition, the π-bond-orders show the same behavior for conformational preferences around the C₁(SINGLE BOND)C′₁ and the C₅(SINGLE BOND)C₆ bonds in methyl C-glycosides and in the hydroxymethyl group of α-D -glucose, respectively. © 1996 by John Wiley & Sons, Inc.     

Gas-phase halo alkylation of C60-fullerene by ion-molecule reaction under chemical ionization

Chemical ionization (CI) mass spectra of C₆₀-fullerene were studied using 1,2-dibromoethane and 1,2-dichloroethane as CI reagents. The ion-molecule reaction between C₆₀ and C₂H₄X⁺ (X=Br and Cl) leads to the formation of (C₆₀+C₂H₄X)⁺ adducts. The collision-induced dissociation of the adducts reveal gas phase halo alkylation of C₆₀-fullerence involving the C?C bond formation.     

Corannulene derivatives as models for calculations of fullerene derivatives

Hypothetical derivatives of corannulene C₂₀H₁₀ (Cor), namely, CorX₅ ^• radicals, CorX₅ ⁻ anions (X=H, Cl, or Br), and their η⁵-π-complexes with SiCp, were calculated by the MNDO/PM3 method. The possibilities of using the results of these calculations for modeling the electronic structure and geometry of fragments of the analogous complexes of the fullerene derivatives C₆₀X₅ are discussed. Calculations of C₆₀X₅ ^• radicals and C₆₀X₅ ⁻ anions were also carried out. In all the compounds under study, the X atoms are attached to carbon atoms in α positions with respect to the same five-membered ring. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1268–1272, July, 1997.     

Chemical Reactivity of Oxo‐ and Aza‐γ‐lactam Rings

Different mechanisms for the alkaline hydrolysis of oxo and aza‐γ‐lactam rings have been studied by ab initio calculations at the MP2/6‐31+G*//MP2/6‐31+G* and B3LYP/6‐31+G*//B3LYP/6‐31+G* levels. The tetrahedral intermediate can undergo two different reactions, the cleavage of the C₂−N₂ bond (the classical mechanism) and the cleavage of the C₂−X₆ bond (X=O, N). Both compounds present similar energy barriers for the classical fragmentation, and show considerably lower barriers for the alternative mechanism. Because of this reactivity, the compounds studied are expected to be β‐lactamase inhibitors.     

Synthese verbrückter binuklearer Titanocenverbindungen – Kristallstruktur von Cl2Ti[(C5H4)(C5H4)(Me)Si–Si(Me)(C5H4)(C5H4)]TiCl2 · PhMe

Synthesis of Bridged Binuclear Titanocene Compounds – Crystal Structure of Cl₂Ti[(C₅H₄)(C₅H₄)(Me)Si–Si(Me)(C₅H₄)(C₅H₄)]TiCl₂ · PhMe Starting from Cp₂(Me)Si–Si(Me)Cp₂ 1 the complexes X₂Ti[(C₅H₄)(C₅H₄)(Me)Si–Si(Me)(C₅H₄)(C₅H₄)]TiX₂ (X = Cl ( 2 a ); X = Me ( 3 )) were synthesized. The compounds were characterized by means of their ¹H‐ and ¹³C‐n.m.r. and MS‐spectra. The crystal structure of 2 a · PhMe was determined.     

Intramolecular Interactions of Phosphor-Containing Groups with an Aromatic Fragment in Different Conformations

Abstract

Geometrical and electronic structure of conformers was studied by optical spectroscopy and quantum chemistry methods. The interaction mechanism of phosphor-containing groups with an aromatic fragment in YC₆H₄PX₂ (Y = H, Cl, Me, OMe, NMe₂, C(O)OR; × = Alk, OAlk, NAlk₂, Cl) and YC₆H₄P(Z)X₂ (Y = H, C1, Me, OMe, NMe₂; × = Alk, OAlk, NAlk₂, F, C1; Z = O, S) compounds is discussed. In case of bisector conformation (A), where the benzene ring plane coincides with a XPX-angle bisectrix, ll-acceptor action of phosphorus-containing groups increases with X varying in a series: Alk <NAlk₂ ≈ OAlk ≤ F4 << C1. These properties are displayed in ground and excited states of molecules, and are determined by interaction of PX₂ and P(Z)X₂ vacant group orbitals antisymmetric with respect to bisector plane with aromatic fragment π-orbitals. For phosphorchloride groups [sgrave]-π-conjugation dominates and the d-π-conjugation contribution is small. π-acceptor effect for P(0)X₂is weaker than for PX₂ and P(S)X₂ groups (especially in excited states) which is due to competitive transfer of electron density from oxygen to PX₂ fragment. In case of the gonal conformation (B) where benzene ring and bisector plane of PX₂ fragment are perpendicular, π-donor effect of PAlk₂ group is found to be 2–4 times weaker than for NAlk₂. According to quantum-chemical calculations with the MNDO method ArPX₂ the stabilization of conformer B for ArPX₂ increases in a series X: C1 <C=N <F <H <CH₃; and for ArP(0)X₂ the main conformation is (A).     

A theoretical investigation on the structures and stabilities of C<Subscript>60</Subscript>X<Subscript>18</Subscript> and C<Subscript>70</Subscript>X<Subscript>10</Subscript> (X = H,F, Cl,and Br)

A density functional theory study was performed on fullerene derivatives C₆₀X₁₈ and C₇₀X₁₀ (X = H, F, Cl, and Br). The calculated results show that the lowest energy isomers are IPR-satisfying for C₆₀X₁₈ (X = H, F, Cl, and Br). It is found that the addition patterns of X (X = Cl and Br) are different from those of X (X = H and F) for C₆₀, demonstrating that the stability of fullerene derivatives is partly attributed to the steric repulsion and electronegativity of added atoms. However, the lowest energy isomers are IPR-violating for C₇₀X₁₀ (X = H, F, and Cl), suggesting that many more fullerene derivatives may violate the isolated pentagon rule.     

Synthetic,Mechanistic, and Theoretical Studies on the Generation of Iridium Hydride Alkylidene and Iridium Hydride Alkene Isomers

Experimental and theoretical studies on equilibria between iridium hydride alkylidene structures, [(Tp^Me2)Ir(H){?C(CH₂R)ArO }] (Tp^Me2=hydrotris(3,5‐dimethylpyrazolyl)borate; R=H, Me; Ar=substituted C₆H₄ group), and their corresponding hydride olefin isomers, [(Tp^Me2)Ir(H){R(H)C? C(H)OAr}], have been carried out. Compounds of these types are obtained either by reaction of the unsaturated fragment [(Tp^Me2)Ir(C₆H₅)₂] with o‐C₆H₄(OH)CH₂R, or with the substituted anisoles 2,6‐Me₂C₆H₃OMe, 2,4,6‐Me₃C₆H₂OMe, and 4‐Br‐2,6‐Me₂C₆H₂OMe. The reactions with the substituted anisoles require not only multiple C? H bond activation but also cleavage of the Me? OAr bond and the reversible formation of a C? C bond (as revealed by ¹³C labeling studies). Equilibria between the two tautomeric structures of these complexes were achieved by prolonged heating at temperatures between 100 and 140 °C, with interconversion of isomeric complexes requiring inversion of the metal configuration, as well as the expected migratory insertion and hydrogen‐elimination reactions. This proposal is supported by a detailed computational exploration of the mechanism at the quantum mechanics (QM) level in the real system. For all compounds investigated, the equilibria favor the alkylidene structure over the olefinic isomer by a factor of between approximately 1 and 25. Calculations demonstrate that the main reason for this preference is the strong Ir–carbene interactions in the carbene isomers, rather than steric destabilization of the olefinic tautomers.     

Rate constants for addition of the OP·(OPri)2, Me3C·, and Me(CH2)3 ·CH2 radicals to methano[60]fullerenes C60CX1X2 (X1 = X2 = CO2Et; X1 = CO2Me,X2 = OP(OMe)2; X1 = X2 = OP(OEt)2)

The rate constants for the addition of the OP^·(OPrⁱ)₂, Me₃C^·, and Me(CH₂)₃ ^·CH₂ radicals to the methano[60]fullerenes C₆₀CX¹X² (X¹ = X² = CO₂Et; X¹ = CO₂Me, X² = OP(OMe)2; X¹ = X² = OP(OEt)₂) were determined by ESR spectroscopy. Methanofullerenes are more reactive toward these radicals than C₆₀ fullerene.     

Tritopic NHC Precursors: Unusual Nickel Reactivity and Ethylene Insertion into a C(sp3)–H Bond

The imidazolium chloride [C₃H₃N(C₃H₆NMe₂)N{C(Me)(=NDipp)}]Cl ( 1 ; Dipp=2,6‐diisopropyl phenyl), a potential precursor to a tritopic N^imineC^NHCN^amine pincer‐type ligand, reacted with [Ni(cod)₂] to give the Ni^I‐Ni^I complex 2 , which contains a rare cod‐derived η³‐allyl‐type bridging ligand. The implied intermediate formation of a nickel hydride through oxidative addition of the imidazolium C−H bond did not occur with the symmetrical imidazolium chloride [C₃H₃N₂{C(Me)(=NDipp)}₂]Cl ( 3 ). Instead, a Ni−C(sp³) bond was formed, leading to the neutral N^imineCHN^imine pincer‐type complex Ni[C₃H₃N₂{C(Me)(=NDipp)}₂]Cl ( 4 ). Theoretical studies showed that this highly unusual feature in nickel NHC chemistry is due to steric constraints induced by the N substituents, which prevent Ni−H bond formation. Remarkably, ethylene inserted into the C(sp³)−H bond of 4 without nickel hydride formation, thus suggesting new pathways for the alkylation of non‐activated C−H bonds.     

Reactivity of an N,N‐Chelated Germylene Toward Substituted Alkynes,Alkenes, and Allenes

Treatment of N,N‐chelated germylene [(iPr)₂NB(N‐2,6‐Me₂C₆H₃)₂]Ge ( 1 ) with ferrocenyl alkynes containing carbonyl functionalities, FcC≡CC(O)R, resulted in [2+2+2] cyclization and formation of the respective ferrocenylated 3‐Fc‐4‐C(O)R‐1,2‐digermacyclobut‐3‐enes 2 – 4 [R = Me ( 2 ), OEt ( 3 ) and NMe₂ ( 4 )] bearing intact carbonyl substituents. In contrast, the reaction between 1 and PhC(O)C≡CC(O)Ph led to activation of both C≡C and C=O bonds producing bicyclic compound containing two five‐membered 1‐germa‐2‐oxacyclopent‐3‐ene rings sharing one C–C bond, 4,8‐diphenyl‐3,7‐dioxa‐2,6‐digermabicyclo[3.3.0]octa‐4,8‐diene ( 5 ). With N‐methylmaleimide containing an analogous C(O)CH=CHC(O) fragment, germylene 1 reacted under [2+2+2] cyclization involving the C=C double bond, producing 1,2‐digermacyclobutane 6 with unchanged carbonyl moieties. Finally, 1 selectively added to the terminal double bond in allenes CH₂=C=CRR′ giving rise to 3‐(=CRR′)‐1,2‐digermacyclobutanes [R/R′ = Me/Me ( 7 ), H/OMe ( 8 )] bearing an exo‐C=C double bond. All compounds were characterized by ¹H, ¹³C{¹H} NMR, IR and Raman spectroscopy and the molecular structures of 3 , 4 , 5 , and 8 were established by single‐crystal X‐ray diffraction analysis. The redox behavior of ferrocenylated derivatives 2 – 4 was studied by cyclic voltammetry.     

Reactivity of 6-(2-tolyl)- and 6-(2,6-xylyl)-2,2′-bipyridines with palladium(II) derivatives. Selective C(sp)H vs. C(sp)H activation

Two new 6-substituted 2,2′-bipyridines, L, 6-(2-tolyl)bipy, L¹, and 6-(2,6-xylyl)bipy, L², have been synthesized. Their reactions with Na₂[PdCl₄] or {Pd(OAc)₂} afford either 1:1 adducts [Pd(L)X₂] (X=Cl, OAc) or five-membered cyclometallated derivatives [Pd(L¹-H)X] arising from C(sp²)H activation. From the chloro-alkyl intermediates [Pd(L)(Me)Cl], in the presence of Na[BAr′₄] (Ar′=3,5-(CF₃)₂C₆H₃), cationic species [Pd(L)(Me)(S)]⁺ (L=L¹, L²; S=CH₃CN) can be obtained. At variance, in less coordinating solvents, e.g. dichloromethane, unexpected activation of a C(sp³)H bond occurs with loss of methane, to afford 6-membered cyclometallated derivatives. The latter species were isolated as [Pd(L-H)(PPh₃)][BAr′₄].     

Theoretical studies of the oxidative addition of PhBr to Pd(PX3)2 and Pd(X2PCH2CH2PX2) (X = Me, H, Cl)

The density functional theory calculations were used to study the influence of the substituent at P on the oxidative addition of PhBr to Pd(PX₃)₂ and Pd(X₂PCH₂CH₂PX₂) where X = Me, H, Cl. It was shown that the C_ipso-Br activation energy by Pd(PX₃)₂ correlates well with the rigidity of the X₃P-Pd-PX₃ angle and increases via the trend X = Cl < H < Me. The more rigid the X₃P-Pd-PX₃ angle is, the higher the oxidative addition barrier is. The exothermicity of this reaction also increases via the same sequence X = Cl < H < Me. The trend in the exothermicity is a result of the Pd(II)-PX₃ bond strength increasing faster than the Pd(0)-PX₃ bond strength upon going from X = Cl to Me. Contrary to the trend in the barrier to the oxidative addition of PhBr to Pd(PX₃)₂, the C_ipso-Br activation energy by Pd(X₂PCH₂CH₂PX₂) decreases in the following order X = Cl > H > Me. This trend correlates well with the filled d_π orbital energy of the metal center. For a given X, the oxidative addition reaction energy was found to be more exothermic for the case of X₂PCH₂CH₂PX₂ than for the case of PX₃. This effect is especially more important for the strong electron donating phosphine ligands (X = Me) than for the weak electron donating phosphine ligands (X = Cl).     

Binuclear platinum(I) complexes with bis(dimethylphosphino)methane ligands: evidence for both short- and long-range NMR trans-influences

The diplatinum(I) complexes or complex ions [Pt₂X₂(μ-dmpm)₂] (X = Cl or I), [Pt₂X(PPh₃)(μ-dmpm)₂]⁺] (X = I, Br or Me), and [Pt₂(PPh₃)₂(μ-dmpm)₂]²⁺, where dmpm = Me₂PCH₂PMe₂, have been prepared and characterized by ¹H and ³¹P NMR spectroscopy. In the linear X-Pt-Pt-Y unit the trans-influence of X is felt primarily at the PtPt bond, but groups X having a very high trans-influence (X = H or Me) can also exert a weaker long-range trans-influence on the PtY bond.     

金(I)配合物催化乙烯加氢的反应机理

采用密度泛函理论B3LYP方法, 对两类金(I)配合物AuX (X=F, Cl, Br, I)和AuPR₃⁺(R=F, Cl, Br, I, H, Me,Ph)催化C₂H₄加氢反应的机理进行了理论研究. 计算显示Au(I)配合物对C₂H₄氢化具有较好的催化效果, 其作用下的加氢反应存在“活化H―H键后再与C₂H₄反应”和“活化C＝C键后再与H₂反应”两种途径, 前者的活化能较后者低90-120 kJ·mol^-1, 因而具有明显的能量优势. 研究表明AuPR₃⁺ 的催化能力明显强于AuX. 此外, X/PR3基团供、吸电子能力的变化对配合物的催化能力也具有较为显著的影响. 电子结构分析显示Au(I)配合物在C₂H₄ 加氢反应中不仅能够削弱H―H、C＝C 键的强度, 还使H₂ σ_H―H^*、C₂H₄ π_C＝C^* 轨道能级下降, 从而缩小了π_C＝C-σ_H―H^*或σ_H―H-π_C＝C^*轨道间的能级差, 促进了C₂H₄-H₂反应中的电子离域, 从而降低禁阻反应发生的难度.σ_H―H^*、π_C＝C^*轨道能级改变量与加氢反应活化能E_a的降低值之间存在较好的一致性关系, 因此使上述轨道能级下降幅度越大的Au(I)配合物可以获得较好的催化效果.     

Synthese,Struktur und Eigenschaften von [nacnac]MX3‐Verbindungen (M = Ge,Sn; X = Cl,Br, I)

Synthesis, Structure, and Properties of [nacnac]MX₃ Compounds (M = Ge, Sn; X = Cl, Br, I) Reactions of [nacnac]Li [(2,6‐ⁱPr₂C₆H₃)NC(Me)C(H)C(Me)N(2,6‐ⁱPr₂C₆H₃)]Li ( 1 ) with SnX₄ (X = Cl, Br, I) and GeCl₄ in Et₂O resulted in metallacyclic compounds with different structural moieties. In the [nacnac]SnX₃ compounds (X = Cl 2 , Br 3 , I 4 ) the tin atom is five coordinated and part of a six‐membered ring. The Sn–N‐bond length of 3 is 2.163(4) Å and 2.176(5) Å of 4 . The five coordinated germanium of the [nacnac]GeCl₃ compound 5 shows in addition to the three chlorine atoms further bonds to a carbon and to a nitrogen atom. In contrast to the known compounds with the [nacnac] ligand the afore mentioned reaction creates a carbon–metal‐bond (1.971(3) Å) forming a four‐membered ring. The Ge–N bond length (2.419(2) Å) indicates the formation of a weakly coordinating bond.     

Synthesis and Characterization of 1,3,2‐Bis (arylamino) phospholidine, 2‐oxide Derivatives: Crystal Structures of $\rm Cl(O)\overline{PN(p-Me-C_{6}H_{4})CH_{2}CH_{2}N}(p-Me-C_{6}H_{4})$ and $\rm ((CH_{2}C_{6}H_{5})(O)\overline{PN(p-Me-C_{6}H_{4})CH_{2}CH_{2}N}(p-Me-C_{6}H_{4})$

Using phosphoryl chloride as a substrate, a family of 1,3,2‐bis(arylamino) phospholidine, 2‐oxide of the general formula ; (X=Cl, 6a ; X=NMe₂, 1b ; X=N(CH₂C₆H₅)(CH₃), 2b ; X=NHC(O)C₆H₅, 3b ; X=4Me‐C₆H₄O, 4b ; X=C₆H₅O, 5b ; X=NHC₆H₁₁, 6b ; X=OC₄H₈N, 7b ; X=C₅H₁₀N, 8b ; X=NH₂, 9b ; X=F, 10b and Ar=4Me‐C₆H₄) was prepared and characterized by ¹H, ¹⁹F, ³¹P and ¹³C NMR and IR spectroscopy, and elemental analysis. A general and practical method for the synthesis of these compounds was selected. The structures of 6a and 2b were determined by single‐crystal X‐ray diffraction techniques. The low temperature NMR spectra of 2b revealed the restricted rotation of P‐N bond according to two independent molecules in crystalline lattice.     

Experimental and ab initio structural study of the ketotin compounds,X3SnCR2CH2COMe: crystal structures of X3SnCMe2CH2COMe (X = Cl and I)

The MeCOCH₂CMe₂ ligand in X₃SnCMe₂CH₂COMe ( 2 ; X = halide) acts as a C,O‐chelating group both in the solid state and in non‐coordinating solutions. The intramolecular Sn? O bond lengths in trigonal bipyramidal 2 (X = Cl and I), as determined by X‐ray crystallography, indicate that the stronger interaction occurs in 2 X = Cl. Comparisons with the Sn? O bond lengths in the estertin trihalides, X₃SnCH₂CH₂CO₂R ( 1 ; R = Me), suggest that the latter form stronger chelates than do 2 . In chlorocarbon solution, 2 (X = Cl, I) undergoes exchange reactions, as shown by NMR spectra, to give all possible halide derivatives, ∑(Cl_nI_3?nSnCMe₂CH₂COMe) (n = 0–3). Various ab initio calculations on 2 and X₃SnCH₂CH₂COMe ( 3 ) have been carried out. Comparisons of the theoretical and experimental structures of 2 for X = Cl or I are reported. Copyright © 2005 John Wiley & Sons, Ltd.     

Theoretical study of mechanism of extraction reaction between silylene carbene and its derivatives and ethylene oxide

The mechanism of the oxide extraction reaction between singlet silylene carbene and its derivatives [X₂Si = C: (X = H, F, Cl, CH₃)] and ethylene oxide has been investigated with density functional theory, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by B3LYP/6‐311G(d,p) method. From the potential energy profile, it can be predicted that the reaction pathway of this kind consists two steps, the first step is the two reactants firstly form an intermediate (INT) through a barrier‐free exothermic reaction; the second step is the INT then generates a product via a transition state (TS). This kind reaction has similar mechanism, when the silylene carbene and its derivatives [X₂Si = C: (X = H, F, Cl, CH₃)] and ethylene oxide close to each other, the shift of 2p lone electron pair of O in ethylene oxide to the 2p unoccupied orbital of C in X₂Si = C: gives a p → p donor–acceptor bond, thereby leading to the formation of INT. As the p → p donor–acceptor bond continues to strengthen (that is, the C? O bond continues to shorten), the INT generates product (P + C₂H₄) via TS. It is the substituent electronegativity, which mainly affects the extraction reactions. When the substituent electronegativity is greater, the energy barrier is lower, and the reaction rate is greater. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011