Ten π -electron nitrogen heterocyclic compounds:X the syntheses and structure determinations of some 1,2,4-triazolopyrimidines

It has been shown that the reaction of 2-hydrazinopyrimidines with ethyl orthoesters yields 1,2,4 -triazolo[4,3 -a]pyrimidines as well as 1,2,4-triazolo[1,5-a]pyrimidines. The structures of the parent compounds and of some methyl derivatives have been established by NMR spectroscopy.     

Charge calculations in molecular mechanics. V. Silicon compounds and π bonding

A previously published scheme for the calculation of partial atomic charges has been extended to include silicon, and has been parameterized for a range of Si? X bonds (X?C,H,O,F,Cl,Br). For the silicon–halogen and silicon–oxygen bonds, a comparison is made between charges calculated with and without the inclusion of π-bonding. An extensive data set consisting of experimental geometries and dipole moments for the silicon compounds considered is presented and this leads to the selection of standard Si? X bond lengths. The calculated dipole moments for the above compounds are in good agreement with those obtained experimentally only when the π charges are included. A comparison has also been made between the partial charges from this scheme and those obtained from computational methods using the Mulliken population analysis. There is considerable disagreement between the methods. Finally, the implications of the charges and structural data are considered in terms of application to zeolite systems.     

Reversible excited state metal-to-metal electron transfer across a heterocyclic σ, π-bridge

A laser flash photolysis study of a binuclear methyl (η¹-azaferrocene)cobaloxime is reported. Excitation at 355 nm (4 ns, approximately 60 mJ) results in the reversible electron transfer between cobalt and iron centres CO^III/Fe^II CO^II/Fe^III. This constitutes the first example of photoinduced metal-to-metal electron transfer across a heterocyclic σ, π-bridge     

Synthesis of heterocyclic compounds VIII. 4H-Pyrans from α-benzoylcinnamonitriles

A new synthesis of substituted 4H-pyrans is reported. The base catalyzed reaction of substituted α-benzoy]cinnamonitri]es (I) with malononitrile yields 6-amino-2,4-diaryl-3,5-dieyano-4H-pyrans (III). Similarly, 6-amirio-2,4-diaryl-5-carbethoxy-3-cyano-4H-pyrans (IV) have been prepared Irom ethyl cyanoacetate.     

Elektrochemische Synthesen. VIII. Darstellung von π-Cyclopentadienylmetallverbindungen

Electrochemical Syntheses. VIII. Preparation of π-Cyclopentadienyl Metal Compounds The electrochemical synthesis of π-cyclopentadienyl metal compounds M(π-Cp)₂ (M = Fe, Co, Ni) and of cyclopentadienyl carbonyl derivatives (π-Cp)_nM(CO)_m(M = Fe, Co, Ni, Ti, V) has been investigated. Most of the complexes can be prepared directly from the corresponding metal anodes, monomeric cyclopentadiene (and CO) in moderate or good yields.     

Semi-empirical π-electron calculations of proton,nitrogen and fluorine hyperfine-coupling constants

The application of the parameterization scheme of Beveridge and Hinze for π-electron molecular orbital calculations to spin density calculations is examined for a series of radicals including anions derived from pyridine derivatives, substituted styrenes, aromatic carboxylic acids, benzonitriles and fluoronitrobenzene derivatives and azabenzyl and fluorobenzyl radicals. The method correlates proton hyperfine coupling constants satisfactorily and gives a reasonable correlation of nitrogen coupling constants. The application to fluorine couplings is less satisfactory.     

The reaction of 2-(ω-chloroalkyl)benzimidazoles with thiourea. Studies on heterocyclic compounds. I

By reaction of the appropriate 2-(ω-chloroalkyl)benzimidazoles with thiourea, 2-(α-mercaptoalkyl)benzimidazoles (IV) have been prepared. The attempted preparation of 2-(β- and γ-mercaptoalkyl)benzimidazoles failed. β-Elimination was observed by alkaline treatment of S-[2-(2-benzimidazolyl)-propyl]isothiuronium chloride (VII), whereas 2-(γ-chlorobutyl)benzimidazoles did not give an isothiuronium chloride.     

Analysis of pπ → dπ bonding in halogen compounds of silicon, germanium and tin

Halogen lone pair ionization potentials for the main-group IV compounds MX₄ and MH₃Cl are corrected for the effects of potencial and relaxation energy using the corresponding halogen core binding energies. The corrected data indicate significant pπ --- dπ bonding in MX₄ (for M = Si, Ge and Sn), significant repulsion between the lone pairs and CH₃ group in CH₃Cl. and little or no pπ --- dπ bonding in SiH₃Cl and GeH₃Cl.     

Semi-empirical MO calculations of hydrocarbon radicals with the half electron method

A derivation for the “half electron method” is given. The half electron method is equivalent to a closed shell calculation of a radical dimer linked by an infinitely long bond with inclusion of the configuration interaction with the doubly excited degenerate state. The dissociation process of ethane into methyl radicals is discussed in some detail. Heats of atomization and equilibrium geometries, obtained with modified CNDO calculations within the half electron scheme, are presented for some hydrocarbon radicals.     

Organosilicon polymers with alternating σ- and π-conjugated systems

A series of organosilicon polymers containing polysilane and diethynylaryl units along the polymer backbone were synthesized and examined with respect to their optical absorptions. The results indicate that delocalization takes place through the σ–π conjugated system. Lengthening of π-conjugation leads to lower excitation energies while nearly identical UV–vis spectra are observed with increased Si–Si chain length. Introducing a thiophene unit into the π-system instead of a benzene unit leads to a bathochromic shift reflecting greater σ–π delocalization. The polymers undergo photodegradation, probably via cleavage of the Si–Si bonds, and thermal crosslinking by reaction at the C≡C triple bonds. When doped with iodine, these polymers become semiconducting with conductivity of the order of 10^?4 S cm^?1.     

Preparation of σ- and π-Benzyl Type Derivatives of Molybdenum

The reaction of -LiM₀(CO)₂Cp with α-chloromethylnaphthalene gives a yellow, crystalline σ-benzyl type derivative of molybdenum (1). Irridation of (1) in n-hexane gives a red π-benzyl type derivative (2). Both complexes (1) and (2) are stable in air.     

Bonding and reactivity in π-allyl—metal compounds

The π-allyl group of π-C₃H₅Co(CO)₃ has two angles of tilt, one of which (from semi-empirical molecular orbital calculations) is stabilised principally by the influence of the C(p_v) orbitals of the terminal carbon atoms, which form part of the σ-framework of the π-allyl group, and the other of which is stabilised by a balance between bonding orbital components of the central and terminal carbon atoms. The Co(CO)₃ moiety has asymmetric bonding, with one CO group more weakly bonded to the metal atom. The asymmetric bonding of the Co(CO)₃ moiety is primarily caused by the electronic character of the π-allyl group, but is significantly influenced by the magnitude of the τ-tilt angle of the π-allyl group. The relatively high reactivity of π-C₃H₅Co(CO)₃, compared with the reactivity of π-C₃H₅Fe(CO)₂NO, Co(NO)(CO)₃, or Ni(CO)₄, is explained by the relatively weak bonding of a CO group to the metal atom and a possible explanation of the anomalous relative rates of the reactions of π-C₃H₄RCo(CO)₃ (R = H, 1-CH₃, 2-CH₃, 1-Cl, 2-Cl) with P(C₆H₅)₃ is indicated.The angles of tilt of the π-allyl group and the asymmetric bonding of the π-cyclopentadienyl moiety in [π-C₃H₄Ni(π-C₅H₅)]₂ are caused by factors similar to those in π-C₃H₅Co(CO)₃.     

Hydrogenation of oxygen-containing heterocyclic compounds on group VIII metals

The hydrogenation of a number of oxygen-containing heterocyclic compounds on Pt, Pd, and Rh blacks in the liquid phase at atmospheric pressure was investigated. The results are compared with data on H-D exchange with D₂O and are interpreted from the point of view of the theory of -complex adsorption.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No.10, pp. 1332–1334, October, 1981.