Organotitanium and niobium chemistry. I. Structure and reactivity of a titanium ethylene complex. II. Reactivity of decamethyl niobocene derivatives

The synthesis and X-ray diffraction study of bis(pentamethylcyclopentadienyl) ethylene titanium (I) are reported. This complex represents the first example of an isolable ethylene adduct of a group IV metal, a key intermediate in Ziegler-Natta olefin polymerization schemes. While treatment of I with ethylene leads to only traces of polymer after months, I participates in a wide range of stoichiometric and catalytic reactions. These include the catalytic conversion of ethylene specifically to butadiene and ethane and the catalytic isomerization of alkenes. Detailed studies have been carried out on the stoichiometric reactions of I with nitriles and alkynes. At low temperatures, nitriles react to form metallacycloimine species which more slowly undergo a formal 1,3-hydrogen shift to generate metallacycloeneamines. The lowest energy pathway for this rearrangement is an intramolecular hydrogen shift which is sensitive to the steric bulk of the R substituent. The reactions of I with alkynes yield metallacyclopentene complexes with high regioisomer selectivity. Carbonylation of the metallacyclopentene (η-C₅Me₅5)₂TiC(CH₃)=C(CH₃)CH₂ under relatively mild conditions cleanly produces the corresponding cyclopentenone and [C₅(CH₃)₅]₂Ti(CO)₂. Compounds derived from CO₂ and acetaldehyde have also been isolated.

The synthesis and characterization of bis-(η-pentamethylcyclopentadienyl) niobium(III) tetrahydroborate (II) are described and a study of its temperature-dependent proton NMR spectroscopic behavior is reported. The complex is observed to undergo a rapid intramolecular averaging process at elevated temperatures. The free energy of activation, ΔG^≠ = 16.4 ± 0.4 kcal/mol, is calculated. The reinvestigation of a related compound, bis(η-cyclopentadienyl)niobium(III) tetrahydroborate, established ΔG^≠ = 14.6 ± 0.2 kcal/mol for the hydrogen exchange process. The tetrahydroborate complex, II reacts with pyridine and dihydrogen to yield (η-C₅Me₅5)₂NbH₃ (III). The reactivity of III with CO and ethylene is reported.

Synthetic, kinetic and mechanistic studies in early transition metal systems. I. α- and β-hydrogen elimination reactions in derivatives of permethyltitanocene, -zirconocene, and -hafnocene. II. The reactions of aluminum alkyls with derivatives of permethylniobocene

The thermal decomposition of Cp*Ti(CH_3)_2 (Cp*≡ ƞ^5-C_5Me_5) toluene solution follows cleanly first-order kinetics and produces a single titanium product Cp*(C_5Me_4CH_2)Ti(CH_3) concurrent with the evolution of one equivalent of methane. Labeling studies using Cp*_2Ti- (CD_3)_2 and (Cp*-d_(15))_2Ti(CH_3)_2 show the decomposition to be intramolecular and the methane to be produced by the coupling of a methyl group with a hydrogen from the other TiCH_3 group. Activation parameters, ΔH^‡ and ΔS^‡, and kinetic deuterium isotope effects have been measured. The alternative decomposition pathways of α-hydrogen abstraction and a-hydrogen elimination, both leading to a titanium-methylidene intermediate, are discussed.

The insertion of unactivated acetylenes into the metal-hydride bonds of Cp*_2MH_2 (M = Zr, Hf) proceeds rapidly at low temperature to form monoand/ or bisinsertion products, dependent upon the steric bulk of the acetylene substituents. Cp*_2M(H)(C(Me)=CHMe), Cp*_2M(H)(CH=CHCMe_3), Cp*_2M(H)-(CH=CHPh), Cp*_2M(CH=CHPh)_2, Cp*_2M(CH=CHCH_3)_2 and Cp*_2Zr- (CH=CHCH_2CH_3)_2 have been isolated and characterized. To extend the study of unsaturated-carbon ligands, Cp*_2M(C≡CCH_3)_2 have been prepared by treating Cp*_2MCl_2 with LiC≡CCH_3. The reactivity of many of these complexes with carbon monoxide and dihydrogen is surveyed. The mono(2- butenyl) complexes Cp*_2M(H)(C(Me)=CHMe) rearrange at room temperature, forming the crotyl-hydride species Cp*_2M(H)(ƞ^3-C_4H_7). The bis(propenyl) and bis(l-butenyl) zirconium complexes Cp*_2Zr(CH=CHR)_2 (R = CH_3, CH_2CH_3) also rearrange, forming zirconacyclopentenes. Labeling studies, reaction chemistry, and kinetic measurements, including deuterium isotope effects, demonstrate that the unusual 6-hydrogen elimination from an sp^2-hybridized carbon is the first step in these latter rearrangements but is not observed in the former. Details of these mechanisms and the differences in reactivity of the zirconium and hafnium complexes are discussed.

The reactions of hydride- and alkyl-carbonyl derivatives of permethylniobocene with equimolar amounts of trialkylaluminum reagents occur rapidly producing the carbonyl adducts Cp*_2Nb(R)(COAlR'_3) (R = H, CH_3, CH_2CH_3, CH_2CH_2Ph, C(Me)=CHMe; R' = Me, Et). The hydride adduct Cp*_2NbH_3•AlEt_3 has also been formed. In solution, each of these compounds exists in equilibrium with the uncomplexed species. The formation constants for Cp*_2Nb(H)(COA1R'_R) have been measured. They indicate the steric bulk of the Cp* ligands plays a deciding factor in the isolation of the first example of an aluminum Lewis acid bound to a carbonyl-oxygen in preference to a metalhydride. Reactions of Cp*_2Nb(H)CO with other Lewis acids and of the one:one adducts with H_2, CO and C_2H_4 are also discussed.

Cp*_2Nb(H)(C_2H_4) also reacts with equimolar amounts of trialkylaluminum reagents, forming a one:one complex that ^1H NMR spectroscopy indicates contains a Nb-CH_2CH_2-Al bridge. This adduct also exists in equilibrium with the uncomplexed species in solution. The formation constant for Cp*_2N+/b(H)(CH_2CH_2ĀlEt_3) has been measured. Reactions of Cp*_2Nb(H)(C_2H_4) with other Lewis acids and the reactions of Cp*_2N+b(H)- (CH_2CH_2ĀlEt_3) with CO and C_2H_4 are described, as are the reactions of Cp_*2Nb(H)(CH_2=CHR) (R = Me, Ph), Cp*_2Nb(H)(CH_3C≡CCH_3) and Cp*_2Ti-(C_2H_4) with AlEt_3.

Thiophene-fluorene derivatives with high three-photon absorption activities and their application to optical power limiting

The three-photon absorption (3PA) properties of two thiophene-fluorene derivatives (abbreviated as MOTFTBr and ATFTBr) have been determined by using a Q-switched Nd:YAG laser pumped wish 38ps pulses at 1064nm in DMF. The measured 3PA cross-sections are 152x10(-78)cm(6)s(2) and 139x10(-78)cm(6)s(2), respectively. The optimized structures were obtained by AM1 calculations and the results indicate that these two molecules show nonplanar structures, and attaching different donors has different effects on the molecular structure. The charge density distributions during the excitation were also systematically studied by using AM1 method. In addition, an obvious optical power limiting effect induced by 3PA has been demonstrated for both derivatives.

Theoretical investigation on the one- and two-photon absorption properties for a series of symmetrical charge transfer fluorene-phenylene and fluorene-thiophene derivatives

One- and two-photon absorption properties of a series of fluorene derivatives with symmetrical charge transfer D-IT-D and A-IT-A structural motifs have been theoretically investigated with ZINDO/S method. The optimized structures and the characterization of frontier molecular orbitals were obtained by using AMI calculations. Two-photon absorption properties of molecules have been studied using three-state model. The calculation results have shown that fluorene-thiophene derivatives exhibit larger two-photon absorption cross-section as compared with other studied molecules. To illustrate the results, the crucial effects of thiophene ring on fluorenethiophene derivatives and the net charge changes on the pi-conjugated bridges are analyzed theoretically. (c) 2006 Elsevier B.V. All rights reserved.

Mother structures related to the hexagonal and cubic close packing in Cu24 clusters: solvent-influenced derivatives

Artículo científico CrystEngComm

Synthesis and properties of a new series of low-molar-mass organo-siloxane derivatives

Two series of ferroelectric liquid crystalline organo-siloxanes containing a laterally attached halogen on the phenyl ring have been synthesised and characterised to determine the impact of the siloxane group and the halogen on the mesomorphism and electro-optic switching properties. Both monomesogenic and bimesogenic compounds have been studied. The monomesogenic derivatives were found to be ferroelectric with high tilt and Ps. The tilt angle of 45° and the Ps of 95nC/cm2 are almost temperature independent. The bimesogenic bromo substituted derivatives showed mainly ferroelectric phases about 60°C wide. Maximum values for the spontaneous polarisation and the tilt angle were only slightly influenced by the length of the siloxane spacer. Altering the halogen to a fluorine shifted the liquid cystalline phase slightly to higher temperatures whilst maintaining the mesophase range of 60°C.

Dichotomains A and B: Two new highly oxygenated phenolic derivatives from Dicranopteris dichotoma

Dichotomains A (1) and B (2), two new highly oxygenated phenolic derivatives that feature a spirodilactone moiety in their structures, were isolated from the fronds of Dicranopteris dichotoma. Their structures were elucidated on the basis of NMR and MS sp

Synthesis and anti-HIV-1 activities of novel podophyllotoxin derivatives

In order to explore the range of biological activities of the podophyllotoxin compound class, a novel series of derivatives of podophyllotoxin, which were conjugates containing stavudine and different structural podophyllotoxin analogues, were designed, s

Synthesis and anti-HIV-1 activity of S-dihydro(alkyloxy)benzyloxypyrimidine derivatives

Several 2-heteroaryl-, 2-heteroarylcarbonylmethyl-, 2-arylcarbonylmethyl, and 2-arylethyl derivatives of S-dihydro(alkyloxy)benzyloxypyrimidines have been synthesized and the anti-HIV activities of these compounds were tested in C8166 cell and against RT

Synthesis and Anti-Human Immunodeficiency Virus Type 1 Activity of (E)-N-Phenylstyryl-N-alkylacetamide Derivatives

A series of (E)-N-phenylstyryl-N-alkylacetamides, 5, were synthesized by direct reduction-acetylation of beta-arylnitroolefins, followed by N-alkylation. The title compounds were characterized by H-1-NMR, EIMS and IR analysis. All the synthesized compound

Synthesis of Zidovudine Derivatives with Anti-HIV-1 and Antibacterial Activities

Twelve novel zidovudine derivatives were prepared by modifying 5 '-hydroxyl group of sugar moiety (1-8) and 5-methyl group of thymidine nucleus (9-12) and characterized spectrally. The compounds were evaluated for anti-HIV-1, antitubercular and antibacter

Molecular docking and 3D-QSAR study of pyranmycin derivatives against 16S rRNA A site

To understand pharmacophore properties of pyranmycin derivatives and to design novel inhibitors of 16S rRNA A site, comparative molecular field analysis (CoMFA) approach was applied to analyze three-dimensional quantitative structure-activity relationship (3D-QSAR) of 17 compounds. AutoDock 3.0.5 program was employed to locate the orientations and conformations of the inhibitors interacting with 16S rRNA A site. The interaction mode was demonstrated in the aspects of inhibitor conformation, hydrogen bonding and electrostatic interaction. Similar binding conformations of these inhibitors and good correlations between the calculated binding free energies and experimental biological activities suggest that the binding conformations of these inhibitors derived from docking procedure were reasonable. Robust and predictive 3D-QSAR model was obtained by CoMFA with q(2) values of 0.723 and 0.993 for cross-validated and noncross-validated, respectively. The 3D-QSAR model built here will provide clear guidelines for novel inhibitors design based on the Pyranmycin derivatives against 16S rRNA A site. (c) 2005 Elsevier B.V. All rights reserved.