ISSN:0947-6539    

DOI:10.1002/chem.19970030203

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe widespread application of the retro‐Diels‐Alder reaction in synthesis has been hampered by the high temperatures usually required to effect cycloreversion. The discovery of the anionic oxy‐Cope reaction was followed by predictions that the accelerating effect of the oxide anion should also be observed with other pericyclic reactions. Recently, such predictions have been confirmed for the retro‐Diels‐Alder reaction, which often proceeds rapidly at room temperature by oxide anion rate acceleration. Such mild retro‐Diels‐Alder reactions have now been employed in the synthesis of a range of molecular targets, including temperature‐sens

ISSN:0947-6539    

DOI:10.1002/chem.19970030204

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractAn asymmetric triangular FeIIIcomplex has been synthesized by an unusual FeII‐promoted activation of salicylaldoxime. Formation of the ligand 2‐(bis(salicylideneamino)methyl)phenol in situ is believed to occur through the reductive deoximation of salicylaldoxime by ferrous ions. The trinuclear ferric complex has been characterized on the basis of elemental analysis, IR, variable‐temperature magnetic susceptibility, and EPR and Mössbauer spectroscopies. The molecular structure established by X‐ray diffraction consists of a trinuclear structure with a [Fe3(μ3‐O)(μ2‐OPh)]6+core. Two iron ions are in a distorted octahedral environment having FeN2O4coordination spheres, and the five‐coordinated third iron ion, with an FeNO4coordination sphere, is in a trigonal bipyramidal environment. The magnetic susceptibility measurements revealed anSt= 5/2 ground state with the antiparallel exchange interactionsJ= – 34.3 cm−1,J'= – 4.7 cm−1, andD= ‐ 0.90 cm−1. The EPR results are consistent with a ground state ofS= 5/2 together with a negativeD5/2 value. The Mössbauer isomer shifts together with the quadrupole splitting also provide evidence for the high‐spin state of the three ferric sites. Magnetic Mössbauer spectra lead to the conclusion that the internal magnetic fields possibly lie in th

ISSN:0947-6539    

DOI:10.1002/chem.19970030205

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA TiO2nanocrystallite has been modified to recognise and selectively bind, by complementary hydrogen bonding, a uracil substrate incorporating a viologen moiety. Band‐gap excitation of the self‐assembled donor (TiO2nanocrystallite)‐acceptor (viologen) complex results in electron transfer. Some implications of these findings for the self‐assembly of functional nanostructures containing both condensed phase and molecular components are con

ISSN:0947-6539    

DOI:10.1002/chem.19970030206

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe reaction of hexakis(dibromomethyl)benzene with [(Bu3P)2‐Ni(COD)] (COD = 1,5‐cyclooctadiene) in DMF at 65–70°C yielded a mixture of the title compounds. The mixture was separated by column chromatography to yield hexabromotricyclobutabenzene (3 a) and hexabromohexaradialene (4) in 24 and 16% yields, respectively.1H and13C NMR spectroscopy suggest that3is obtained as thesyn‐all‐transisomer3 a, and the symmetricanti‐all‐transisomer3 bis not obtained at all. The X‐ray structures of3 aand4are reported. The hexaradialene4has a chair conformation, and deviates from planarity by 43.6°. Heat or radical impurities cause the clean transf

ISSN:0947-6539    

DOI:10.1002/chem.19970030207

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe Baeyer‐Villiger reaction ofp‐anisaldehyde with peroxyacetic acid in nonpolar solvents to givep‐anisylformate was examined on the basis of ab initio molecular orbital calculations. To explain the experimental observations, the free‐energy change was evaluated for each case in the absence and in the presence of an acid catalyst. It was found that, without catalysts, the rate‐determining step corresponds to the carbonyl addition of peroxyacetic acid top‐anisaldehyde and the reaction hardly occurs. Acetic acid was found to catalyze the carbonyl addition and change the rate‐determining step from the carbonyl addition to the migration of the carbonyl‐adduct intermediate. Trifluoroacetic acid was observed to catalyze both the carbonyl addition and migration, and the carbonyl addition was demonstrated to be a rate‐determining step. The results provided a convincing explanation of the complex kinetics seen experimentally. Further calculations were performed for the reaction of benzaldehyde with peroxyacetic acid to give phenylformate. Migratory aptitude was found to depend on the catalyst. Isotope effects were also investigated, and the exceptional isotope effect observed experimentally was shown to be due to the rate‐determining carbonyl addition caused by autocatalysis. It is concluded that the mechanism of the reaction varies with catalysis or

ISSN:0947-6539    

DOI:10.1002/chem.19970030208

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractTerrylenimides3and4represent a new class of blue colorants, exhibiting absorption maxima at 650 to 700 nm and fluorescence emissions in the NIR region (673 to 750 nm). The terrylenimides were synthesized by means of various organometallic coupling reactions, catalyzed by transition metal complexes (Nio, Pdo) and starting from the aromatic bromides, boronic acids, or organotin compounds. The terrylenimides have all the properties expected of excellent fluorescent dyes: high extinction coefficients, high fluorescence quantum yields, and very good thermal, chemical, and photochemical stabilities. Owing to its extended π system,3can reversibly accept four negative charges. By varying the substituents,3and4can be modified to serve either as soluble dyes or as insoluble pigments

ISSN:0947-6539    

DOI:10.1002/chem.19970030209

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA mixture of Na2MO4, M(CO)6, FeCl3·6H2O, and (EtCO)2O was heated at 120°C to produce the heterooctanuclear clusters Na2Fe2[M3O4‐(O2CEt)8]2(M = Mo,1; W,2). The bioxo‐capped clusters Na[M3O2(O2CEt)9] (M3= MoW2, W3) were treated with Fe‐(CO)5, Cr(CO)6, and Mo(CO)6in (EtCO)2O at 120°C to afford Na2M'2‐[MoW2O4(O2CEt)8]2(M' = Fe,3; Cr,4; Mo,5) and Na2Mo2[W3O4(O2CEt)8]2(6), respectively. The isomorphous clusters1, 2, 5, and6were characterized by X‐ray crystallography. The structures, similar to those of the analogues Na2Cr2[M3O4‐(O2CEt)8] (M = Mo,7; W,8) and Na2V2[M3O4(O2CEt)8]2(M = Mo,9; W,10), consist of two incomplete cubane‐type [M3O4(O2Et)8]4‐units centrosymmetrically bridged by two M'(III) metal ions through μ‐oxo and propionate groups; the resulting cluster dianions are linked by Na+ions into infinite chains. Clusters1–10do not dissolve in water and common organic solvents at room temperature. Unlike the chromium(III) clusters7and8, the iron(III) and vanadium(III) clusters1and9are soluble in aqueous acid solution, as a result of their decomposition into isolated [Mo3O4]4+cluster units. Temperature‐dependent magnetic susceptibilities of1, 2, and7–10were measured over the range 2–200 K and modeled by means of the spin‐HamiltonianH= – 2JS1S2to give spin exchange coupling constantsJ/kof –0.60, –0.72, –1.76, –1.31, –4.80, and –1.46 K, respectively. These figures show that antiferromagnetic spin exchange coupling interactions between the magnetic, bridging M'(III) ions are very weak. These species show characteristic bands in the IR spectra at 740–820 c

ISSN:0947-6539    

DOI:10.1002/chem.19970030210

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe 1, 2‐hydrogen shift isomers of neutral (singlet and triplet) thiazole(1)and its radical cation have been investigated by a combination of mass spectro‐metric experiments and hybrid density functional theory calculations. The latter were used to probe the structures and stabilities of selected C3H3NS and C3H3NS.+isomers and transition state structures. Although 3H‐thiazole‐2‐ylidene(2)is less stable than1, by 31.5 kcalmol−1, it is expected to be capable of independent existence, since the 1, 2‐hydrogen shift from carbon to nitrogen involves a very large energy barrier of 72.4 kcalmol−1. The other 1, 2‐hydrogen shift reaction from C(2) leads not to the expected cyclic 1H‐thiazole‐2‐ylidene structure(3), which is apparently unstable, but rather to the ring‐opened species HSCHCHNC(4), which is 34.5 kcalmol−1higher in energy than1. The barrier in this case is lower but still large (54.9 kcalmol−1). The triplet ground states of1,2and4are considerably destabilised (69.5, 63.2 and 58.7 kcalmol−1) relative to their singlet states. Interestingly, in addition to2.+and4.+, the cyclic radical cation3.+is predicted to be stable although it is substantially higher in energy than ionised thiazole1.+(by 53.9 kcalmol−1), whereas2.+and4.+are much closer in energy (only 10.2 and 27.0 kcalmol−1higher, respectively). Dissuading2.+and3.+from isomerising to1.+are energy barriers of 52.6 and 15.3 kcalmol−1, respectively. Experimentally, dissociative ionisation of 2‐acetylthiazole enabled the generation of2.+, which could be differentiated from1.+by collisional activation mass spectrometry. Reduction of the ylide ion2.+in neutralisation‐reionisation mass spectrometry experiments yielded the corresponding neutral molecule2. This direct observation of a thiazolium ylide provides support for postulates of such species as discrete intermediates

ISSN:0947-6539    

DOI:10.1002/chem.19970030211

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractAlkoxyvinylketenes4are generated by flash vacuum thermolysis (FVT) or photolysis of 3‐alkoxycyclobutenones3. The thermal interconversion of4and allene carboxylic acid esters5under FVT conditions is demonstrated by Ar matrix FTIR spectroscopy. In addition, ethoxy‐vinylketene4bundergoes thermal elimination of ethene with formation ofs‐cis‐ands‐trans‐acetylketene(8). An analogous aminovinylketene‐to‐allenecarbox‐amide conversion is observed on FVT of 3‐dimethylaminocyclobutenone3c. A facile 1, 3‐chlorine migration in 2, 3‐buta‐dienoyl chloride(5d)is also reported. Consistent with the experimental observations, 1, 3‐methoxy, 1, 3‐chloro, and 1, 3‐dimethylamino migrations in vinylketene are calculated (G2(MP2, SVP) level) to have moderate barriers of 169, 157, and 129 kJ mol−1, respectively, significantly less than the corresponding 1, 3‐H shift barrier (273 kJ mol−1). The stabilization of the four‐center transition structures is rationalized in terms of the donor‐acceptor interaction between the lone pair electrons of the migrating donor substituent and the vacant central carbon p orbital of the ketene LUMO. The predicted migratory aptitude in the series of substituted vinylketenes, R‐C(CH2)‐CHCO, is in the order N(CH3)2>SCH3>SH>Cl>NH2>OCH3>OH>F>H>CH3, and correlates wel

ISSN:0947-6539    

DOI:10.1002/chem.19970030212

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY