Ab initio study of 4-monosubstituted pyrimidines in ground and excited n → π* states

Ab initio SCF and SCF -CI calculations have been performed to investigate substituent effects on ground- and excited-state properties of 4-R-pyrimidines, and to compare these with substituent effects in 2- and 4-R-pyridines, with R including the π donating and σ withdrawing groups CH₃, NH₂, OH, F, and C₂H₃ and the σ and π electron-withdrawing groups CHO and CN. Substitution leads to significant changes in the internal angles of the pyrimidine ring, which are independent of the nature of the substituent. The geometry of the pyrimidine ring is more sensitive to substitution in the 4 position than the pyridine ring geometry is to substitution in either the 2 or the 4 position. The isodesmic reaction energies for substituent transfer from the 4 position of pyrimidine to the 2 or 4 position of pyridine indicate that all R groups except CN have a relative stabilizing effect in pyrimidine. The presence of a π donating group leads to an increase in the n→π* transition energy of 4-R-pyrimidines, while the π withdrawing group CN leads to a decrease in the transition energy relative to pyrimidine. Orbital energy differences and virtual excitation energies tend to correlate with n→π* transition energies of 4-R-pyrimidines with saturated R groups, but such correlations are masked by π conjugation, n orbital interaction, and configurational mixing when the unsaturated groups C₂H₃, CHO, and CN are present. The electronic effects of a π donating group are stronger when the group is bonded to pyrimidine than to pyridine, but those of a π withdrawing group are weaker when the group is bonded to pyrimidine.     

NEW CYCLIC AND POLYMERIC PHOSPHAZENES DERIVED FROM N-SILYLPHOSPHORANIMINES

Silicon-nitrogen-phosphorus compounds of the type Me ₃ SiN═PR(R′)X(X= Cl, Br, OCH₂CF ₃ , OPh), known as N-silylphosphoranimines,are useful precursors to both cyclic and polymeric phosphazenes.Depending on the leaving group (X), thermolysis reactions afford either cyclic trimers, [N═PR(R′)] ₃ (when X = Cl, Br), or linear polymers,[N═PR(R′)]_n (when X = OCH ₂ CF ₃ or OPh). Treatment of the P-trifluoroethoxy and P-phenoxy derivatives, Me ₃ SiN═PR(R′)X (X = OCH ₂ CF ₃ , OPh), with alcohols at lower temperature usually results in the formation of cyclic phosphazene trimers via silyl ether elimination. Recently, we have applied these synthetic methods to the preparation of some new phosphazene systems including a series of 4-aryl-functionalized trimers and polymers and a variety of non-geminal, mixed-substituent cyclic trimers. Representative examples of the synthesis, structural characterization, and reactivity of these new phosphazenes and their Si─N─P precursors are reported here.     

The Evolution Aspect in the Crystallization Process of [5,5′‐(HCF2CF2CH2OCH2)2‐2,2′‐bpy]MX2 (M = Pt,Pd; X = I,Br): Role of the Intramolecular CF2─H…X(─M) Hydrogen Bonds

The general species (2,2′‐bpy)MX₂ (M = Pd, Pt; X = Br, I) in a crystallization process results in an isomorphous convergence in P2₁/c. Yet, with polyfluorinated side chains, the general [5,5′‐(HCF₂CF₂CH₂OCH₂)₂‐2,2′‐bpy]MX₂ species proceeds to crystallize the isomorphous structures of 5 (M = Pt; X = I) and 6 (M = Pd; X = I) in P2₁/c only; structure 7 (M = Pt; X = Br) crystallizes in P2₁/c but is not isomorphous with 5 and 6 , and structure 8 (M = Pd; X = Br) forms differently in P–1. The causes making the system nonlinear are (1) the intramolecular CF₂─H^…X(─M) hydrogen bonds found in 5–7 but not in 8, and (2) in response to the transition from I to Br, bifurcated [C─H^…]₂ F ─C hydrogen bonds that are formed in 5 and 6 and bifurcated C─ H [^…F─C]₂ hydrogen bonds in 7 . Additionally, the intramolecular CF₂─H^…X(─M) hydrogen bonding from compounds 5–7 could be affirmed by the IR studies.     

Weak hydrogen and halogen bonding in 4‐[(2,2‐difluoroethoxy)methyl]pyridinium iodide and 4‐[(3‐chloro‐2,2,3,3‐tetrafluoropropoxy)methyl]pyridinium iodide

To enable a comparison between a C—H…X hydrogen bond and a halogen bond, the structures of two fluorous‐substituted pyridinium iodide salts have been determined. 4‐[(2,2‐Difluoroethoxy)methyl]pyridinium iodide, C₈H₁₀F₂NO⁺·I⁻, (1), has a –CH₂OCH₂CF₂H substituent at the para position of the pyridinium ring and 4‐[(3‐chloro‐2,2,3,3‐tetrafluoropropoxy)methyl]pyridinium iodide, C₉H₉ClF₄NO⁺·I⁻, (2), has a –CH₂OCH₂CF₂CF₂Cl substituent at the para position of the pyridinium ring. In salt (1), the iodide anion is involved in one N—H…I and three C—H…I hydrogen bonds, which, together with C—H…F hydrogen bonds, link the cations and anions into a three‐dimensional network. For salt (2), the iodide anion is involved in one N—H…I hydrogen bond, two C—H…I hydrogen bonds and one C—Cl…I halogen bond; additional C—H…F and C—F…F interactions link the cations and anions into a three‐dimensional arrangement.     

Preparation,spectral characterization,and single crystal X-ray structures of cis and trans isomers of 2,4,6-trifluoroethoxy-1,3,5-triethyl-1,3,5,2λ5, 4λ5, 6λ5-triazatriphosphorinane-2,4,6-trioxide, [ETNP(O)(OCH2CF3)]3

Oxidation of the cis isomer of the λ³-cyclotriphosphazane [EtNP(OCH₂CF₃)]₃ with trimethylamine-N-oxide (TMNO) gives the cis isomer of trioxo-λ⁵-cyclotriphosphazane [EtNP(O)(OCH₂CF₃)]₃; the trans isomer of [EtNP(O)(OCH₂CF₃)]₃ is obtained by the treatment of a cis and trans mixture of [EtNP(OCH₂CF₃)]₃ with aqueous H₂O₂. The two trioxocyclotriphosphazanes have been characterized by elemental analysis, IR, and NMR (¹H, ¹³C, ¹⁹F, and ³¹P) spectroscopy. The solid state structures of both the isomers have been determined by single crystal X-ray diffraction. The six-membered P₃N₃ ring in both the isomers exhibits a twist-boat conformation; in the cis isomer, the trifluoroethoxy substituents lie on the same side of the ring, whereas, in the trans isomer, two trifluoroethoxy groups are on one side of the ring and the third on the other side of the ring. Crystal data for cis-[EtNP(O)(OCH₂CF₃)]₃: monoclinic, P 2₁/ n , a = 13.593(3), b = 9.721(2), c = 17.539(3) Å, β = 99.49(2)°, V = 2286(1) ų, Z = 4, and Final R = 0.047. Crystal data for trans-[EtNP(O)(OCH₂CF₃)]₃: monoclinic, P 2₁/ n , a = 11.685(4), b = 15.115(5), c = 13.233(5) Å, - = 102.21(3)°, V = 2284(1) ų, Z = 4, and Final R = 0.078.     

Proton and boron-11 magnetic resonance studies of some potassium tetraarylborates

Proton and boron-11 magnetic resonance spectra for several potassium para-substituted tetraarylborate compounds [KB(C₆H₄-pX)₄, where X is H, OCH₃, CH₃, Br, Cl, F, CF₃] have been obtained. The chemical shift between the centers of the AA′ and XX′ multiplets for the ring proton multiplets, relative to a reference chemical shift of 0·39 ppm for potassium tetraphenylborate, correlated with the corresponding Hammett σ values for the para-substituent. Additionally, the boron-11 chemical shifts gave a good correlation with corresponding σ values for the substituents. Electronegativities of para-substituted phenyl rings were calculated and found to be approximately 2·70 for all compounds studied. It was shown that electronic substituent effects do not greatly influence the electron density surrounding the central boron atom in the tetraarylborate ions.     

Mass spectra of monosubstituted trans-stilbenes

The mass spectra of twenty-eight monosubstituted trans-stilbenes in ortho, meta, para and alpha positions with ? N(CH₃)₂, ? NH₂, ? OCH₃, ? OH, ? Br, ? Cl, ? F, ? CH₃, ? COOH, ? CN and ? NO₂ groups as substituents have been studied. A detailed fragmentation pathway is given for stilbene. This fragmentation, characteristic for most of the substituted stilbenes, takes place either from the molecular or from the [M - Substituent] ion. In o-nitro-, o-methoxy-,α-carboxyl- and α-methylstilbene, however, rearrangement reactions prior to fragmentation influence the fragmentation pattern.     

Substituent Effect on N-Benzylideneanilines by DFT Energy Partition

To investigate the substituent effect on x-electron delocalization of the N-benzylideneaniline （NBA）, the vertical resonance energies △E^V（θ） of eleven substituted NBAs were separated into n and a parts at the B3LYP/6-311G（d） level of the Density Functional Theory （DFT）. When substituted with an electron-releasing group --OH, the calculated △E^V（θ） of NBA was increased, indicative of more resonance destabilization than the mother molecule. However, when substituted with an electron-withdrawing group -NO2, the calculated △E^V（θ） values indicated less resonance destabilization. The most destabilizing effect was observed especially when the -OH group located at the ortho-position of the aromatic ring in the fragment -N=CH-Ar. For most of the substituted NBA molecules, it was the destabilized a framework that determined the destabilizing feature of the vertical resonance energy, instead of the stabilized n system. When the -NO2 substituent at the para-position of the aromatic ring of the -N=CH-Ar group, the π system had the highest stabilizing effect while the σ framework exhibited the highest destabilizing effect. While the -NO2 substituent was at the para-position of the left aromatic ring （At-）, the NBA had the least vertical resonance energy value.     

Unexpected preparation and X-ray crystal structure of the dinuclear complex Mo2(μ-P(O)(OCH3)2)2(CO)4(P(OCH3)3)2(CF3COO)2

Prolonged reaction of CF₃COOH with Mo(CO)₃(P(OCH₃)₃)₃ in CH₂Cl₂ yields the dimer [Mo(CO)₂(μ(O)P(OCH₃)₂)P(OCH₃)₃CF₃COO]₂ in low yield. The complex has been characterized by the usual spectroscopic methods and by an X-ray crystal structure determination. The molybdenum atoms are linked by two OP bridges, the six-membered dimetallacycle adopting a twisted-boat conformation. Each molybdenum is seven-coordinated and has a capped trigonal prism geometry, the capping position being occupied by an oxygen atom of the bridging OP(OCH₃)₂ group. A mechanism similar to the well-known Michaelis-Arbuzov rearrangement is proposed and substantiated by the concomitant apparition of CF₃COOCH₃ during the reaction.     

Substituenteneffekte auf azido-tetrazolo-gleichgewichte elektronische und sterische effekte bei s-triazolo[4,3-c]tetrazolo[1,5-a]pyrimidinen1

The azido tetrazolo valence isomerism of twenty 9-methyl-s-triazolo[4,3-c]tetrazolo[1,5-a]pyrimidines with different substituents at position 5 has been studied by means of ¹H NMR spectroscopy. All the compounds have been found to be tetrazoles in the solid state and in (CD₃)₂SO solution; in CF₃COOH azido and tetrazolo isomers are in equilibrium. From equilibrium constants K and thermodynamic data determined it is concluded that in this series K depends on both electronic effects and steric requirements of the 5-substituents. A linear relation between K and σ was found for 5-arylderivatives because ΔS° keeps approximately constant and the substituent mainly operates on ΔH°.     

Ortho effects in organic molecules on electron-impact II– Ortho effects in substituted α,α,α-trichloracetanilides

Elimination of trichloromethyl radical from the molecular ion is the abundant fragment in the mass spectra of most of the thirteen substituted trichloracetanilides studied. Unusual ortho effects of ? OCH₃, ? NO₂ and ? COOH groups are noticed only after the initial ejection of trichloromethyl radical from the molecular ion.     

Reversible Morphological Evolution of Responsive Giant Vesicles to Nanospheres by the Self‐Assembly of Crystalline‐b‐Coil Polyphosphazene Block Copolymers

The preparation of long‐term‐stable giant unilamellar vesicles (GUVs, diameter ≥1000 nm) and large vesicles (diameter ≥500 nm) by self‐assembly in THF of the crystalline‐b‐coil polyphosphazene block copolymers [N=P(OCH₂CF₃)₂]_n‐b‐[N=PMePh]_m ( 4 a : n=30, m=20; 4 b : n=90, m=20; 4 c : n=200, m=85), which combine crystalline [N=P(OCH₂CF₃)₂] and amorphous [N=PMePh] blocks, both of which are flexible, is reported. SEM, TEM, and wide‐angle X‐ray scattering experiments demonstrated that the stability of these GUVs is induced by crystallization of the [N=P(OCH₂CF₃)₂] blocks at the capsule wall of the GUVS, with the [N=PMePh] blocks at the corona. Higher degrees of crystallinity of the capsule wall are found in the bigger vesicles, which suggests that the crystallinity of the [N=P(OCH₂CF₃)₂] block facilitates the formation of large vesicles. The GUVs are responsive to strong acids (HOTf) and, after selective protonation of the [N=PMePh] block, they undergo a morphological evolution to smaller spherical micelles in which the core and corona roles have been inverted. This morphological evolution is totally reversible by neutralization with a base (NEt₃), which regenerates the original GUVs. The monitoring of this process by dynamic light scattering allowed a mechanism to to be proposed for this reversible morphological evolution in which the block copolymer 4 a and its protonated form 4 a⁺ are intermediates. This opens a route to the design of reversibly responsive polymeric systems in organic solvents. This is the first reversibly responsive vesicle system to operate in organic media.     

Synthesis,characterization and antimicrobial activity of triorganotin(IV) derivatives of some bioactive Schiff base ligands

Triorganotin(IV) complexes of the type Me₃Sn[OC(R¹):CH(CH₃)C:NR²OH] and Ph₃Sn[OC(R′):CH(CH₃)C:NR″OH] (R′ = ─CH₃, ─C₆H₅; R″ = ─(CH₂)₂─, ─(CH₂)₃─) have been synthesized by the reactions of trimethyl/phenyltin(IV) chloride with the sodium salt of corresponding Schiff base ligands in unimolar ratio in refluxing tetrahydrofuran. All these compounds have been characterized using elemental analyses and their probable structures have been proposed on the basis of infrared, ¹H NMR, ¹³C NMR, ¹¹⁹Sn NMR and mass spectroscopic studies. In the trimethyltin(IV) derivatives the central tin atom is tetracoordinated, whereas in the analogous triphenyltin(IV)derivatives the central tin atom is pentacoordinated. All these ligands, metal precursors and corresponding triorganotin(IV) complexes have been screened for antimicrobial activities. A comparison of activities of the ligands and their corresponding triorganotin(IV) derivatives has been made. Attempts have also been made to relate the activity to the structure of these compounds.     

Molecular structures of 3‐[(2,2,3,3‐tetrafluoropropoxy)methyl]‐ and 3‐[(2,2,3,3,3‐pentafluoropropoxy)methyl]pyridinium saccharinates

The salts 3‐[(2,2,3,3‐tetrafluoropropoxy)methyl]pyridinium saccharinate, C₉H₁₀F₄NO⁺·C₇H₄NO₃S⁻, (1), and 3‐[(2,2,3,3,3‐pentafluoropropoxy)methyl]pyridinium saccharinate, C₉H₉F₅NO⁺·C₇H₄NO₃S⁻, (2), i.e. saccharinate (or 1,1‐dioxo‐1λ⁶,2‐benzothiazol‐3‐olate) salts of pyridinium with –CH₂OCH₂CF₂CF₂H and –CH₂OCH₂CF₂CF₃meta substituents, respectively, were investigated crystallographically in order to compare their fluorine‐related weak interactions in the solid state. Both salts demonstrate a stable synthon formed by the pyridinium cation and the saccharinate anion, in which a seven‐membered ring reveals a double hydrogen‐bonding pattern. The twist between the pyridinium plane and the saccharinate plane in (2) is 21.26 (8)° and that in (1) is 8.03 (6)°. Both salts also show stacks of alternating cation–anion π‐interactions. The layer distances, calculated from the centroid of the saccharinate plane to the neighbouring pyridinium planes, above and below, are 3.406 (2) and 3.517 (2) Å in (1), and 3.409 (3) and 3.458 (3) Å in (2).     

The effect of polar substituents on the heterocyclic benzoxazoles

The synthesis, characterization, and mesomorphic properties of a new type of heterocyclic compounds 1, 2 derived from benzoxazole are reported. In order to understand the relationship between the structure and the mesomorphic behavior, compounds containing a variety of polar substituents (i.e., X=H, F, Cl, Br, CH₃, CF₃, OCH₃, NO₂, CN, OH, NMe₂, COOCH₃) on the terminal end were prepared. The phase behavior of these mesogenic compounds was characterized and studied by differential scanning calorimetry (DSC) and polarization optical microscopy. The formation of mesophases was strongly dependent on the electronic and/or the steric factors of the substituents. In general, a mesophase was better induced by introduction of a polar substituent. Compounds (X=H) formed a crystalline phase, however, other compounds, except for X=OH, exhibited nematic or smectic A phases. Interestingly all compounds with electron-donating substituents (X=CH₃, OCH₃, NMe₂) exhibited nematic phases, however, other compounds with electron-withdrawing substituents (X=F, Cl, Br, CF₃, NO₂, CN, COOCH₃) formed smectic A phases. Compounds (X=NO₂, CN, COOCH₃) have higher clearing temperatures than those of other homologues, and the higher T_cl was attributed to an enhanced conjugative interaction. However, no linear correlation between the clearing temperature or the temperature range of mesophases with Hammett σ_p constants was found. The fluorescent properties of the compounds were examined. All λ_max peaks of the absorption and photoluminescence spectra of compounds occurred at ca. 348-381 and 389-478 nm, respectively. Whereas, the quantum yields of some compounds were relatively low.     

Easy Access to the Copper(III) Anion [Cu(CF3)4]−

CuCl or pre‐generated CuCF₃ reacts with CF₃SiMe₃/KF in DMF in air to give [Cu(CF₃)₄]^? quantitatively. [PPN]⁺, [Me₄N]⁺, [Bu₄N]⁺, [PhCH₂NEt₃]⁺, and [Ph₄P]⁺ salts of [Cu(CF₃)₄]^? were prepared and isolated spectroscopically and analytically pure in 82–99 % yield. X‐ray structures of the [PPN]⁺, [Me₄N]⁺, [Bu₄N]⁺, and [Ph₄P]⁺ salts were determined. A new synthetic strategy with [Cu(CF₃)₄]^? was demonstrated, involving the removal of one CF₃^? from the Cu atom in the presence of an incoming ligand. A novel Cu^III complex [(bpy)Cu(CF₃)₃] was thus prepared and fully characterized, including by single‐crystal X‐ray diffraction. The bpy complex is highly fluxional in solution, the barrier to degenerate isomerization being only 2.3 kcal mol^?1. An NPA study reveals a huge difference in the charge on the Cu atom in [Cu(CR₃)₄]^? for R=F (+0.19) and R=H (+0.46), suggesting a higher electron density on Cu in the fluorinated complex.     

Kinetische und mechanistische untersuchungen von übergangsmetallkomplex-reaktionen : VI. Kinetik und mechanismus der einschiebung von dimethylcyanamid in die metall—carbenkohlenstoff-bindung von pentacarbonyl(arylphenylcarben)wolfram

Pentacarbonyl(arylphenylcarbene)tungsten complexes, (CO)₅W[C(p-C₆H₄R)C₆H₅] (Ia, R = OCH₃; Ib, R = CH₃; Ic, R = H; Id, R = Br; Ie, R = CF₃) react with dimethylcyanamide (II) via insertion of the CN group into the metalcarbene bond. The formation of pentacarbonyl[dimethylamino(imino)carbene]tungsten(0) (IIIa–IIIe) follows a second-order rate law: d[III]/dt = k[I][II]. Replacement of R = H by electron-withdrawing substituents (Br, CF₃) results in an increase, by electron-donating groups (CH₃, OCH₃) in a decrease of the reaction rate. The rate constants correlate well with Hammett's σ-constants. The activation enthalpies ΔH^≠ are low (37.3–41.6 kJ mol^?1), the activation entropies ΔS^≠ strongly negative (?119 to ?133 J mol^?1 K^?1). The results are discussed on the basis of an associative stepwise mechanism with a nucleophilic attack of the CN group of II at the carbene carbon in the first reaction step.     

The influence of sodium trifluoroacetate on the acidity function of solutions of trifluoroacetic acid in N,N-dimethylformamide

The influence of CF₃COONa on the acidity function (H ₀) and IR spectra of the CF₃COOH-N,N-dimethylformamide (DMF)-CF₃COONa system ([CF₃COOH] ≤ [DMF], [CF₃COONa] = 0–3 M) and H ₀ of solutions of the salt in 100% CF₃COOH ([CF₃COONa] = 0–1.2 M) was studied. The addition of the salt to 100% CF₃COOH insignificantly changed solution acidity; H ₀ passed a minimum at [CF₃COONa] = 0.7 M because of the formation of (CF₃COO…H…OOCCF₃)^? anions and destruction of acid associates in the presence of the salt. Changes in the acidity of solutions of CF₃COOH in DMF caused by the addition of the salt depended on the n = [CF₃COOH]/[DMF] ratio. At n = 1, the salt almost did not influence H ₀. At n < 1, a substantial decrease in the acidity of solutions was observed, because the salt increased the degree of proton transfer in CF₃COO…H…OC(H)N(CH₃)₂ quasi-ion pairs solvated by DMF molecules.     

Reaction of para-substituted aromatic acids with methyl thiocyanate in trifluoroacetic acid

We have investigated the reactions of aromatic carboxylic acids (p-XC₆H₄COOH, in which X=F, Cl, Br, I, NO₂, CH₃, H, OCH₃) with methyl thiocyanate in trifluoroacetic acid. It was shown that the starting acids are converted to S-methyl benzothioates [p-XC₆H₄COSCH₃ (40–70%)] and the nitrile of the acid [p-XC₆H₄CN (20–50%)]. In addition to starting acid, in the reaction mixtures products of the reaction of CF₃COOH with methyl thiocyanate were identified.Institute of Oil and Gas Problems, Russian Academy of Sciences, I. M. Gubkin State Academy of Oil and Gas, 117917 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 412–416, February, 1992.     

A theoretical investigation of low energy band gap polymers: polythiophene systems

Recently, the organic synthesis and electronic device applications of π-conjugated polymer-based materials with low energy band gap (below 2 eV) and high values of incident photon to current efficiency have been presented. In the present study, the physical properties of polythiophene (PTH) and its derivative systems (PTs) were investigated as π-conjugated low energy band gap polymers. Density functional theory with periodic boundary condition (PBC), the B3LYP functional, and the 6-31G(d) basis set was applied to determine their geometric and electronic structures and corresponding energies (E _HOMO, E _LUMO, and E _g = E _LUMO ? E _HOMO) from the monomer of thiophene and its derivatives for one-dimensional (1D) extension to polymer. The effects of 3-substitution in PTs including electron-donating (CH₃–, C₆H₁₃–, OH–, Cl–, OCH₃–, and CHO–) and electron-withdrawing groups (Cl–, CHO–, CN–, NO₂–, CF₃–, and COOH–) compared with PTH were investigated. According to the calculation results, PTs with electron-donating and electron-withdrawing substituents should exhibit red- and blue-shifts, respectively, compared with PTH. These calculation results show good agreement with experimental data and provide further information for molecular design considerations.