Thermal decomposition mechanisms of methylamine, ethylamine, and 1-propylamine on Si(100)-2 × 1 surface

The thermal decomposition reactions of methylamine, ethylamine, and 1-propylamine absorbed on Si(100)-2 × 1 surface were theoretically investigated. Eight decomposition channels were found leading to desorption products of imine, H(2), alkyl cyanide, ammonia, aziridine, alkene, azetidine, and cyclopropane, which supports the experimental assignments. Our mechanistic studies strongly suggest that the alkyl cyanide (hydrogen cyanide in the case of methylamine) channel is coupled with the hydrogen desorption step. The β-hydrogen of ethylamine and 1-propylamine was found to undergo additional decomposition reactions producing aziridine and alkene, which were classified as γ- and β-eliminations, respectively. It was also found that the γ-hydrogen of 1-propylamine undergoes azetidine and cyclopropane producing decompositions, which were classified as δ- and γ-eliminations. In general, γ- and δ-hydrogen involved decomposition reactions are kinetically less favorable than β-hydrogen involved ones. Consequently, it is expected that the thermal decompositions of the primary alkyl amines with longer alkyl chains would not add additional favorable decomposition channels. Except alkyl cyanide and ammonia desorption channels, the decompositions occur in a concerted fashion.     

Stereo-isomerism controls surface reactivity: 1-chloropentane-pairs on Si(100)-2×1

Chloropentane forms asymmetric ('A') and symmetric ('S') pairs on Si(100)-2×1, differing in the direction of curvature of one pentane tail. Surprisingly this renders the rate of thermal reaction of 'A' fifteen times greater than 'S' in chlorinating room-temperature silicon. Correspondingly, for electron-induced reaction the energy threshold for A is 1 eV less than for S.     

Structure and stability of one-dimensional o-phthalaldehyde lines on the Si(100)-2 × 1:H surface

We present the structural models for the o-phthalaldehyde (OP) molecular lines on the H-terminated Si(100) surface which were recently observed by scanning tunnelling microscopy. Our first-principles density-functional theory calculations show that the formation of OP lines is not only kinetically more facile but also thermodynamically more stable than those of previously reported alkene lines.     

Enantioselective rhodium-catalyzed [4+2] cycloaddition of α,β-unsaturated imines and isocyanates

A [4+2] cycloaddition of α,β-unsaturated imines and isocyanates catalyzed by a phosphoramidite-rhodium complex provides pyrimidinones in good yields and high enantioselectivities.     

The isomeric effect on the adjacent Si dimer didechlorination of trans and iso-dichloroethylene on Si(100)-2×1

The dechlorination processes of isomers trans and iso-dichloroethylene (iso-DCE) on Si(100)-2×1 were investigated from first principles, to ascertain the isomeric effect on the adjacent Si dimer di-dechlorination of DCE on Si(100)-2×1. By comparing the feasible adspecies and their reaction barriers between trans and cis-DCE on Si(100)-2×1, we found that the isomeric effect of trans-DCE is negligible, which explained the similar C 1s peak locations in the X-ray photoelectron spectroscopy (XPS) experiment. In contrast, iso-DCE undergoes a more complicated reaction process, although the adjacent Si dimer di-dechlorination is still the dominant mechanism. Among the initial competitive reactions involving intra-, inter-cycloaddition and single C-Cl cleavage, the barrierless intra-cycloaddition is the most favorable reaction and precludes the single dechlorination that yields the mono-σ structure. Subsequent di-dechlorination undergoes a three-step reaction to yield the final product intra-dimer tetra-σ. In addition, the ionization energies of C 1s and Cl 2s electrons were calculated for the tentative assignment of the peaks observed in XPS.     

Intercyclic vs. intracyclic [Π2+σ2+σ2] / [Π2+Π2] cycloaddition reactions in sterically fixed norbornadiene/quadricyclane systems

In specifically designed substrates, the geometrical/stereoelectronic factors governing inter-/intracyclic thermal [Π2+σ2+σ2] and photochemical [Π2+Π2] cycloaddition reactions are studied.     

The first example of [2+2] cycloaddition of Me4Ge2 to multiple bonds. The synthesis of Δ1,7-2,2,6,6,8,8,9,9-octa-methyl-4-thia-8,9-digermabicyclo[5.2.0]Nonene

The first example of a [2+2] cycloaddition reaction of Me₄Ge₂ with acetylene is described. A new derivative of 1,2-digermacyclobutene, namely Δ^1,7-2,2,6,6,8,8,9,9-octamethyl-4-thia-8,9-digermabicyclo[5.2.0]nonene, is prepared.     

Iridium-catalyzed [2+2+2] cycloaddition of α,ω-diynes with alkynyl ketones and alkynyl esters

We found that the combination of [Ir(cod)Cl]₂ and rac-BINAP served as an efficient catalyst for the [2+2+2] cycloaddition of 2,7-nonadiyne derivatives and related compounds with alkynyl ketones and alkynyl esters. The corresponding products were obtained in high yields under mild reaction conditions.     

Studies on the Cyclisation of 4-[2′-Cyclohexenyl]-3-hydroxy[1]benzopyran-2-one

The bicyclic coumarin derivative-1, 3 propano-2-bromo-1,2-dihydro-3H-pyrano [2, 3-c] [1]benzo-pyran-5-one (8) was synthesised by a sequence of reaction viz. acetylation of 4-[2′-cyclohexenyl] -3-hydroxy [1] benzopyran-2-one (4), addition of bromine to cyclohexenyl double bond and treating the resulting acetyldi-bromo derivative (7) with 4% alcoholic KOH. Benzofuro [2, 3-c] [1] -benzopyran-6-one (10) was synthesised from 4 via oxymercuration with mercuric acetate in methanol followed by dehydrogenative damercuration with Pd-C in refluxing diphenyl ether.     

Disulfide passivation of the Ge(100)-2 × 1 surface

Understanding the bonding of sulfur at the germanium surface is important to developing good passivation routes for germanium-based electronic devices. The adsorption behavior of ethyl disulfide (EDS) and 1,8-naphthalene disulfide (NDS) at the Ge(100)-2 × 1 surface has been studied under ultrahigh vacuum conditions to investigate both their fundamental reactivity and their effectiveness as passivants of this surface. X-ray photoelectron spectroscopy, multiple internal reflection-infrared spectroscopy, and density functional theory results indicate that both molecules adsorb via S-S dissociation at room temperature. Upon exposure to ambient air, the thiolate adlayer remains intact for both EDS- and NDS-functionalized surfaces, indicating the stability of this surface attachment. Although both systems resist oxidation compared to the bare Ge(100)-2 × 1 surface, the Ge substrate is significantly oxidized in all cases (17-57% relative to the control), with the NDS-passivated surface undergoing up to two times more oxidation than the EDS-passivated surface at the longest air exposure times studied. The difference in passivation capability is attributed to the difference in surface coverage on Ge(100)-2 × 1, where EDS adsorption leads to a saturation coverage 17% higher than that for NDS/Ge(100)-2 × 1.     

Regio- and stereoselective synthesis of 3- and 5-(C-glycosyl)-4-nitroisoxazolidines by nitrone–nitroalkene [3+2] cycloaddition reactions

Cycloaddition reactions of nitrones, including sugar nitrones, with nitroalkenes, including sugar nitroolefins, led with complete regioselectivity and stereospecificity to 4,5-trans-4-nitroisoxazolidines in 51–78% global yields. The endo/exo stereoselectivity depends on the type of sugar derivative used. As expected, the best π-diastereofacial selectivity was observed when both partners were sugar derivatives. Isomerisation of the first formed diastereomers by the action of silica gel was observed in some cases. Absolute configurations for two crystalline products were assigned by X-ray diffraction methods.     

STM and voltammetric studies on the structure of a 4-pyridinethiolate monolayer chemisorbed on Au(100)-(1×1) surface

The structure for a 4-pyridinethiolate monolayer chemisorbed on the Au(100)-(1×1) single crystal surface was characterized by in situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV). In situ STM observation showed a well-ordered p(√2 R 45°×5 R 53.1°) structure (abbreviated as √2×5) for the surface modified with either 4-pyridinethiol (4-PySH) or bis(4-pyridyl)disulfide (4,4′-PySSPy) in a 0.05 M HClO₄ solution. On the Au(100)-(1×1) surface, 4-PySH molecules formed a dimer structure with the S–S bond length of about 2.4 nm. The observed dimer structure is similar to that previously reported on the Au(111) surface, and the orientation of the pyridine ring is mostly perpendicular to the surface normal. However, the adsorbed molecules were more densely packed (√3/√2 times) on the Au(100) surface than on the Au(111) surface. The surface excess was estimated to be 5.8 (±0.2)×10⁻¹⁰ mol cm⁻² based on the voltammetric charge for the reductive desorption. This value is in good agreement with that (5.7×10⁻¹⁰ mol cm⁻²) calculated from the parallelogrammic (√2×5) unit cell. The Au(100)-(1×1) surface modified with 4-PySH gave a well-defined electrochemical response of cytochrome c.     

Regioselective [3+2] cycloaddition of nitrile oxides to 1Н-pyrrole-2,3-diones: synthesis of spiro[pyrroledioxazoles]

Chemistry of Heterocyclic Compounds - 1,3-Dipolar cycloaddition of 1H-pyrrole-2,3-diones to nitrile oxides generated in situ from N-hydroxybenzimidoyl chlorides by the action of triethylamine...     

Synthesis and structures of mercury and cadmium complexes: [Ph4Sb] 2 + [Hg2I6]2−·Ph2Hg, [Ph4Sb] 2 + [E2I6]2− (E = Hg, Cd), and [Ph4Sb] 2 + [Hg4I10]2−

The reaction of pentaphenylantimony with mercury iodide affords the ionic complex [Ph₄Sb] ₂ ⁺ [Hg₂I₆]^2?·Ph₂Hg (I). The [Ph₄Sb] ₂ ⁺ [Hg₂I₆]^2? (II) and [Ph₄Sb] ₂ ⁺ [Cd₂I₆]^2? (III) complexes are synthesized from tetraphenylantimony iodide and mercury and cadmium iodides. The [Ph₄Sb] ₂ ⁺ [Hg₄I₁₀]^2? complex (IV) is prepared from tetraphenylantimony 2,4-dimethylbenzenesulfonate and mercury iodide. According to the X-ray diffraction data, the Sb atom in the [Ph₄Sb]⁺ cations of complex I has virtually ideal tetrahedral coordination (the CSbC angles are 108.09°–109.64°). In the central square fragment Hg₂I₂ of the [Hg₂I₆]^2? anion, the Hg-I_br bond lengths are 2.825 and 3.075 Å, and the terminal iodine atoms are more strongly bonded to the mercury atoms (Hg-I_term 2.691 and 2.700 Å). The [Cd₂I₆]^2? anion in complex III has a similar structure (the Cd-I_bridg and Cd-I_term distances are 2.865, 2.872 and 2.723, 2.748 Å, respectively). The anions in complex IV are joined by I…Hg (3.651 Å) and I…I (4.058 Å) interactions into an infinite dimeric network.     

Influence of Lewis acids on the [4+2] cycloaddition of (2R,2′R)-N,N′-fumaroylbis[fenchane-8,2-sultam] to cyclopentadiene and cyclohexadiene

Compared to the analogous bornane-10,2-sultam derived dienophile (−)-1b, the reversed topology observed during the [4+2] cycloaddition of cyclopentadiene or cyclohexadiene to the (−)-1a–TiCl₄ chelate can be rationalised on the basis of IR studies of their complexes with different Lewis acids. According to X-ray analyses, the origin of this differentiation resides in the loss of masked C₂ symmetry, due to the pseudoequatorial ‘down’ orientation of the SO(1) bond in (−)-1a,c as compared to the pseudoequatorial ‘up’ direction adopted by the SO(2) bond in (−)-1b,d, associated with the steric influence of the apical Ti–Cl atoms. Dependent on the strength of the Lewis acid, the much higher constraint of the SO₂/CO syn-s-cis conformer diminishes the chelating properties of this type of fenchane-8,2-sultam derived dienophiles (−)-1a and 1c.     

[2+2+2] cycloaddition reactions of α,β-unsaturated N-arylaldimines,acetylenedicarboxylates and 2-arylidene-1,3-indanediones

The domino [2+2+2] cycloaddition reaction of α,β-unsaturated N-arylaldimines, dialkyl acetylenedicarboxylates and 2-arylidene-1,3-indanediones in dry methylene dichloride at room temperature afforded isomeric 2′-styryl-1,2′,3,4′-tetrahydro-1′H-spiro[indene-2,3′-pyridines] in good yields. Under similar conditions, the reaction of α,β-unsaturated N-arylaldimines with two molecules of dialkyl acetylenedicarboxylates resulted in 1-aryl-6-styryl-1,6-dihydropyridine-2,3,4,5-tetracarboxylates. Additionally, the three-component reaction of α,β-unsaturated N-arylaldimines, dimethylacetylenedicarboxylate and benzoyl cyanide also gave isomeric 1,3-oxazine-4,5-dicarboxylates.     

Intramolecular [8+2] cycloaddition and 10π-electron electrocyclization reactions of an 8-acylheptafulvene

Compound 8-(trans-3-ethoxycarbonylallylacetyl)heptafulvene,8, undergoes both an intramolecular [8+2] cycloaddition and a 10π-electron electrocyclization followed by 1,5-sigmatropic hydrogen shifts to give10 and₁₂, respectively.     

Formal [4+2] cycloaddition of cyclobutanones bearing alkyne–cobalt complex at their 3-positions

Cyclobutanones bearing an alkyne–cobalt complex at their 3-positions reacted with aldehydes to give formal [4+2] cycloadducts by using tin(IV) chloride as a Lewis acid. Highly substituted tetrahydropyrone derivatives were stereoselectively prepared by this method.     

Multiple pathways of dissociative attachment: CH3Br on Si(100)-2×1

We describe the dissociative attachment (DA) of methyl bromide to form chemisorbed CH(3) and Br on a Si(100)-2×1 surface at 270 K. The patterns of DA were studied experimentally by ultra-high vacuum scanning tunneling microscopy (STM) and interpreted by ab initio theory. The parent molecules were found to dissociate thermally by breaking the C-Br bond, attaching the resulting fragments CH(3) and Br at adjacent Si-atom sites. The observed DA resulted in three distinct attachment geometries: inter-row (IR, 88%), inter-dimer (ID, 11%), and on-dimer (OD, 1%). Ab initio computation agreed in predicting these three DA reaction pathways, with yields decreasing down the series, in accord with experiment. The three computed physisorption geometries, each of which correlated with a preferred outcome, IR, ID, or OD, exhibited similar heats of adsorption, the choice of pathway being governed by the energy barriers to DA chemisorption predicted to increase along the series: E(IR) = 0.48 eV, E(ID) = 0.57 eV, and E(OD) = 0.63 eV.     

Structural and metallo selectivity in the assembly of [2 × 2] grid-type metallosupramolecular species: mechanisms and kinetic control

An unsymmetrical bis(tridentate) ligand LH in which one binding site can be readily deprotonated forms a kinetically inert [Co(III)L(2)](+) complex which can be used as a "corner" species for the "Coupe du Roi" assembly of trans,trans-[Co(2)M(2)L(4)](6+) metallogrids (M = Fe(II), Co(II), Cu(II), Zn(II)). In the mixed Co(III)/Co(II) species, the oxidation states appear to be localised. In solution, the ligand LH forms octacationic, homometallic [2 × 2] grids with the individual labile metal ions Fe(II), Co(II), Cu(II), Zn(II), seemingly as mixtures of all possible isomers arising from the unsymmetrical nature of the ligand. In the solid state, however, [Zn(4)L(4)](CF(3)SO(3))(8)·4CH(3)CN crystallises as a single species where the cation has S(4) symmetry. This stereoselectivity in the crystalline lattice is associated with interactions between the cation and triflate anions which can again be analysed in terms of the Coupe du Roi concept.