Iminopropadienethiones, Ar=N=C=C=C=S

Aryliminopropadienethiones 9 have been generated by flash vacuum thermolysis of isoxazolones of the type 5 and characterized by mass spectrometry and matrix isolation IR spectroscopy in conjunction with DFT calculations and chemical trapping.     

Chemistry of stable iminopropadienones, RN=C=C=C=O

The synthesis, spectroscopic properties, and chemical reactions of the stable (neopentylimino)-, (mesitylimino)-, and (o-tert-butylphenylimino)propadienones (6) are reported. Nucleophilic addition of amines affords the malonic amidoamidines 7 and 8. 3,5-Dimethylpyrazole reacts analogously to form 9b. Addition of 1,2-dimethylhydrazine produces pyrazolinones 10-12. Addition of N,N'-dimethyldiaminoethane, -propane, and -butane gives diazepine, diazocine, and diazonine derivatives 13-15, respectively (X-ray structures of 13c, 14a, and 15a are available). The mesoionic pyridopyrimidinium olates 18 are obtained by addition of 2-(methylamino)pyridine (X-ray structure of 18b available). Primary 2-aminopyridines afford the pyridopyrimidininones 20-29 and 31 (X-ray structure of 21a available), and 2-aminopyrimidines and 2-aminopyrazine afford pyrimidopyrimidinones and pyrazinopyrimidinones 33-35. Pyrimidoisoquinolinone 36 results from 1-aminoisoquinoline and pyridoquinolinone 40 from 8-aminoquinoline. 2-Aminothiazoline and 2-aminothiazole afford thiazolopyrimidinone derivatives 41-43 (X-ray structure of 43a available).     

Reaction of heterocumulenes R---N=C=N---R and R---P=C=P---R with a di-iron aminocarbene complex

Reaction of R---N=C=N---R (R=p-Me-C₆H₄) and R---P==C=P---R (R=2,4,6-^tBu₃C₆H₂) with the di-iron aminocarbene complex [Fe₂(CO)₇{1μ-C(Ph)C(NEt₂)}] (1c) gave corresponding complexes [Fe₂(CO)₆{C(Ph)C(NEt₂)C(NC₆H₄Me)N (C₆H₄Me)}] (2) and [Fe₂(CO)₆{C(Ph)C(NEt₂)C(PC₆H₂^tBu₃)P(C₆H₂^tBu₃)}] (4), resulting from a coupling reaction with carbon-carbon bond formation. [Fe₂(CO)₅(CNC₆H₄Me){C(Ph)C(NEt₂)N(C₆H₄Me)}], complex 3, obtained in the reaction with R---N=C=N---R, resulted from C=N bond rupture insertion of a nitrene fragment into the Fe=C bond. Complexes 2–4 were characterized by X-ray diffraction. The different geornetries of complexes 2 and 4 are discussed. The formation of these complexes may be explained by cycloaddition on the Fe =C metal-carbene bond.     

Volume parameters of some Diels-Alder reactions involving C=C,C=S,and N=N bonds

The effect of a hydrostatic pressure of up to 1000 kg cm⁻² on the rate constants of the Diels-Alder reactions of maleic anhydride with 1,2,3,4-tetraphenylcyclopentadiene and with 6,13-dichloropentacene, of 4-phenyl-1,2,4-triazoline-3,5-dione with hexachlorocyclopentadiene, and of thiobenzophenone with isoprene was studied at 25 °C. The volume parameters and ratios of the activation to reaction volumes make it possible to exclude electrostriction of the solvent during transition state solvation in all the reactions studied, which corresponds to the nonpolar nature of the transition state. Dedicated to Academician A. L. Buchachenko on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1973–1980, September, 2005.     

1,3-Digermacyclobutanes with exocyclic C=P and C=P=S double bonds

New 1,3-digermacyclobutanes, with two exocyclic C=PMes* bonds, and the corresponding first bis(methylenethioxo)phosphoranes with C=P(S)Mes* moieties have been synthesized.     

Reducing ability of supramolecular C60 dianion toward C=O, C=C and N-N bonds

Different from C60 dianion which readily reacts with electrophiles, supramolecular C60 dianion (2) generated from gamma-cyclodextrin-bicapped C60 (1) and NaBH4 (or diborate) in DMSO-H2O (9:1, v/v) is able to reduce N-N+, C=C-EWG and C=O bonds to provide the respective dihydro derivatives; 1-mediated reduction of acetophenone with NaBH4 in the presence of (Me2N)2CH2 and EtONa gives turn over frequency (TOF)/h of 400.     

Metal Ion-Driven Constitutional Adaptation in Dynamic Covalent C=C/C=N Organo-Metathesis

Knoevenagel barbiturate derivatives and imines are able to undergo efficient component recombination through dynamic covalent C=C/C=N organo-metathesis in absence of a catalyst. A [2×2] dynamic covalent library (DCL) containing two Knoevenagel derivatives Kn1 and Kn2 and two imines A1 and A2 has been established and its adaptive features in response to the addition of metal cations have been investigated. Addition of Cu(I) triflate as an effector, induces fast and remarkable constitutional selection of the DCL towards amplification of the Cu(I)- A2 complex and its agonist Kn1 . This adaptation process could be reversed by addition of neocuproine as a competitive Cu(I) ligand. Furthermore, separate addition of five other metal cations as input agents, i. e. Ag(I), Fe(II), Zn(II), Cu(II) and Li(I), led to the generation of cation-specific distribution patterns as outputs, showing the ability of the present DCL to recognize different effectors.     

Unsaturated didehydrodeoxycytidine drugs. 1. Impact of C=C positions in the sugar ring

The chemical names of a pair of recently synthesized antitumor drugs are given in the present study as 1',2'-didehydro-3',4'-deoxycytidine and 3',4'-didehydro-2',4'-deoxycytidine. The order of stabilities, geometries, and ionization potentials of the unsaturated sugar-modified cytidine derivatives is investigated quantum mechanically. Our density functional theory calculations based on the B3LYP/6-311++G** model reveal that 3',4'-didehydro-2',4'-deoxycytidine (SD-C2) is slightly more stable than its isomer, 1',2'-didehydro-3',4'-deoxycytidine, by an energy of 5.28 kJ x mol(-1) in isolation. The isomers structurally differ by only the C=C location in the sugar ring. However, the compounds exhibit an unusual orientation with a less puckered sugar ring; that is, 3',4'-didehydro-2',4'-deoxycytidine is determined to be a beta-nucleoside, which is a C1'-endo, north conformer with an anticlinal sugar ring, whereas 1',2'-didehydro-3',4'-deoxycytidine is neither an alpha-nucleoside nor a beta-nucleoside but is a C4'-endo, south conformer with an antiperiplanar sugar ring. The present study further indicates that the C=C double bond location imposes significant effects on their ionization potentials (IPs) and other important molecular properties such as molecular electrostatic potential (MEP). In addition, inner shell binding energy spectral variations with respect to the C=C bond exhibit more site dependence. The valence shell binding energy spectral changes are, on the other hand, significant and delocalized. The latter indicates that such changes in valence space are not isolated effects but are within the entire nucleoside. Finally, the present study suggests that the nearly 0.6 eV difference in the first ionization potentials (highest occupied molecular orbital) of the isomers is sufficiently large to identify them by further spectroscopic measures.     

Terminal titanium-ligand multiple bonds. Cleavages of C=O and C=S double bonds with Ti imido complexes

Treatment of (t-)BuN=TiCl(2)Py(3) with 2 equiv lithium ketiminate compound, Li[OCMeCHCMeN(Ar)] (where Ar = 2,6-diisopropylphenyl), in toluene at room temperature gave (t-)BuN=Ti[OCMeCHCMeN(Ar)](2) (1) in high yield. The reaction of 1 with phenyl isocyanate at room-temperature resulted in imido ligand exchange producing PhN=Ti[OCMeCHCMeN(Ar)](2) (2). Compound 1 decomposed at 90 degrees C to form a terminal titanium oxo compound O=Ti[OCMeCHCMeN(Ar)](2) (3) and (t-)BuNHCMeCHCMeNAr (4). Also, the compound 3 could be obtained by reacting 1 with CO(2) under mild condition. Similarly, while 1 reacts with an excess of carbon disulfide, a novel terminal titanium sulfido compound S=Ti[OCMeCHCMeN(Ar)](2) (5) was formed via a C=S bond breaking reaction. A novel titanium isocyanate compound Ti[OCMeCHCMeN(Ar)](2)(NCO)(OEt) (6) was formed on heating 1 with 1 equiv of urethane, H(2)NCOOEt. Compounds 1-6 have been characterized by (1)H and (13)C NMR spectroscopies. The molecular structures of 1, 3, 5, and 6 were determined by single-crystal X-ray diffraction. A theoretical calculation predicted that the cleavage of the C-S double bonds for carbon disulfide with the Ti=N bond of compound 1 was estimated at ca. 21.8 kcal.mol(-1) exothermic.     

Dye-sensitized photooxygenation of the C=N bond. 5. substituent effects on the cleavage of the C=N bond of C-aryl-N-aryl-N-methylhydrazones

The title compounds are cleaved cleanly at the C=N bond by singlet oxygen ((1)O(2), (1)Delta(g)) yielding arylaldehydes and N-aryl-N-methylnitrosamines. These reactions take place more rapidly at -78 degrees C than at room temperature. The effects of substituent variation at both the C-aryl and N-aryl groups were studied using a competitive method. Good correlations of the resulting rate ratios with substituent constants (sigma(-) or sigma(+)) were obtained yielding small to very small rho values indicative of small to very small changes in charge distribution between the reactant and the rate determining transition state. Electron withdrawing groups on the C-aryl moiety retard reaction somewhat by preferential stabilization of the hydrazone. Electron donors on the other hand slightly stabilize the rate determining transition state. Substituents on the N-aryl group have almost no effect. Inhibition by 3,5-di-tert-butylphenol was not observed showing that free (uncaged) radical intermediates are not involved in the mechanism. We postulate a mechanism in which the initial event is exothermic electron transfer from the hydrazone to (1)O(2) leading to an ion-radical caged pair. Subsequent covalent bond formation between the hydrazone carbon and an oxygen atom is rate controlling. The transition state for this step also has a lower enthalpy than the starting reactants, but the free energy of activation is dominated by a large negative TDeltaS++term leading to the negative temperature dependence. Direct formation of a C-O bond in the initial step is not unambiguously ruled out. Subsequent steps lead to C-N cleavage.     

SYNTHESES OF AZETIDINES VIA PHOTOCYCLOADDITION OF C=C TO C=N DOUBLE BONDS

The photocycloaddition of 6-azauracil (AU) with olefins have been carried out. Four new compounds,1,2,4-triazabicyclo [4,2,0] octane and 1,2,4-triazatricyclo [4,2,0^1.6,0^7.12]dodecane derivatives,were obtained. Based on structure identificationof productsand photosensitization behavior of this syste,the regioselection and the multiplicity of thisreaction were discussed.     

Comparison of the kinetics and thermodynamics for methyl radical addition to C=C, C=O, and C=S double bonds

The barriers, enthalpies, and rate constants for the addition of methyl radical to the double bonds of a selection of alkene, carbonyl, and thiocarbonyl species (CH(2)=Z, CH(3)CH=Z, and (CH(3))(2)C=Z, where Z = CH(2), O, or S) and for the reverse beta-scission reactions have been investigated using high-level ab inito calculations. The results are rationalized with the aid of the curve-crossing model. The addition reactions proceed via early transition structures in all cases. The barriers for addition of methyl radical to C=C bonds are largely determined by the reaction exothermicities. Addition to the unsubstituted carbon center of C=C double bonds is favored over addition to the substituted carbon center, both kinetically (lower barriers) and thermodynamically (greater exothermicities). The barriers for addition to C=O bonds are influenced by both the reaction exothermicity and the singlet-triplet gap of the substrate. Addition to the carbon center is favored over addition to the oxygen, also both thermodynamically and kinetically. For the thiocarbonyl systems, addition to the carbon center is thermodynamically favored over addition to sulfur. However, in this case, the reaction is contrathermodynamic, addition to the sulfur center having a lower barrier due to spin density considerations. Entropic differences among corresponding addition and beta-scission reactions are relatively minor, and the differences in reaction rates are thus dominated by differences in the respective reaction barriers.     

THE EFFECT OF ACIDS ON THE INFRARED SPECTRA OF SCHIFF BASES—II. IMINES CONTAINING THE C=C-C=N AND C=C-C=C-C=N UNITS

Abstract— The Fourier-transform infrared spectra of chloroform-d solutions of conjugated imines CH₃CH=CHCH=NCH(CH₃)₂ and CH₃CH₂CH=CHCH=CHCH=NCH(CH₃)₂ and the related protonated species with HCl, HBr, HI, trichloro, dichloro, monobromo and monochloroacetic acids or propionic acid are presented. The effects of conjugation and protonation are examined. The results show that conjugation slightly increases the basicity of the Schiff bases. HCl, HBr and HI protonate the Schiff bases completely. The carboxylic acids protonate partially depending on their p K _a, values. When the Schiff base contains two (or more) C=C bonds conjugated with C=N, the main C=C stretching band undergoes a strong intensification showing that sizeable dipole moment variations occur along the conjugated chain.     

Quantum-chemical study of manifestations of conjugation in molecules containing conjugated C=C and C=O bonds

Based on the MNDO calculations of the electronic structure of the molecules of acrolein, glyoxal, and butadiene, possible mechanisms of the conjugation in systems containing conjugated C=C and C=O bonds have been analyzed. In the electronic ground state ofs-trans-acrolein, the , -conjugation is very small, whereas in the first excited electronic state, the conjugation is substantial, In the ground state ofs-trans-glyoxal, the ,-conjugation should manifest itself clearly but should be weaker than in butadiene, whereas in the first excited electronic state, this conjugation should be more pronounced, Alternation of double and single bonds in the classic structural formula of a molecule does not ensure that this molecule exhibits the properties of a -conjugated system even in planar conformations.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1648–1652, July, 1996.     

Novel Triplet Ground State Silylenes: H–N = C = Si, CN–N = C = Si, and MeO–N = C = Si at DFT Levels

Summary.  DFT calculations predict the existence of three new triplet ground state silylenes: [(imino)-methylene]silylene, [(cyanoimino)methylene]silylene, and [(methoxyimino)methylene]silylene, with CNSiX formula (X = H, CN, and OMe, respectively). Discrepancies are found between DFT and some ab initio results.     

Energy of the C = C Double Bond

Determination of the bond energy as the difference between the experimental energy of the double bond and standard value of the C-C bond energy does not take into account changes in the energy of the C-C bond as it becomes shorter. The change in the single bond energy upon its shortening by 0.2 Å was evaluated from the compressibilities of diamond and polyethylene, and also by quantum-chemical calculations. With this value taken into account, the energies of the and bonds in ethylene become close, which proves the equivalence of the traditional and banana models of the C = C bond.     

Gallium “Shears” for C=N and C=O Bonds of Isocyanates

Digallane [L¹Ga−GaL¹] ( 1 , L¹=dpp-bian=1,2-[(2,6-iPr₂C₆H₃)NC]₂C₁₂H₆) reacts with RN=C=O (R=Ph or Tos) by [2+4] cycloaddition of the isocyanate C=N bonds across both of its C=C−N−Ga fragments to afford [L¹(O=C−NR)Ga−Ga(RN−C=O)L¹] (R=Ph, 3 ; R=Tos, 4 ). The reactions with both isocyanates result in new C−C and N−Ga single bonds. In the case of allyl isocyanate, the [2+4] cycloaddition across one C=C−N−Ga fragment of 1 is accompanied by insertion of a second allyl isocyanate molecule into the Ga−N bond of the same fragment to afford compound [L¹Ga−Ga(AllN− C=O)₂L¹] ( 5 ) (All=allyl). In the presence of Na metal, the related digallane [L²Ga−GaL²] ( 2 ; L²=dpp-dad=[(2,6-iPr₂C₆H₃)NC(CH₃)]₂) is converted into the gallium(I) carbene analogue [L²Ga:]⁻ ( 2 A ), which undergoes a variety of reactions with isocyanate substrates. These include the cycloaddition of ethyl isocyanate to 2 A affording [Na₂(THF)₅]{L²Ga[EtN−C(O)]₂GaL²} ( 6 ), cleavage of the N=C bond with release of 1 equiv. of CO to give [Na(THF)₂]₂[L²Ga(p-MeC₆H₄)(N−C(O))₂−N(p-MeC₆H₄)]₂ ( 7 ), cleavage of the C=O bond to yield the di-O-bridged digallium compound [Na(THF)₃]₂[L²Ga-(μ-O)₂-GaL²] ( 8 ), and generation of the further addition product [Na₂(THF)₅][L²Ga(CyNCO₂)]₂ ( 9 ). Complexes 3 – 9 have been characterized by NMR (¹H, ¹³C), IR spectroscopy, elemental analysis, and X-ray diffraction analysis. Their electronic structures have been examined by DFT calculations.     

A Linear C=Ge=C Heteroallene with a Di-coordinated Germanium Atom**

Allenes (>C=C=C<) are classified as cumulated dienes with a linear structure and an sp-hybridized central carbon atom. We have synthesized and isolated a stable 2-germapropadiene with bulky silyl substituents. The 2-germapropadiene allene moiety adopts a linear structure both in the solid state and in solution. An X-ray diffraction electron-density-distribution (EDD) analysis of this 2-germapropadiene confirmed the linear C=Ge=C geometry with a formally sp-hybridized germanium atom that bears two orthogonal C=Ge π-bonds. Based on detailed structural and computational studies, we concluded that the linear geometry of the isolated 2-germapropadiene most likely arises from the negative hyperconjugation of the silyl substituents at the terminal carbon atoms. The 2-germapropadiene reacts rapidly with nucleophiles, indicating that the linearly oriented germanium atom is highly electrophilic.