摘要:

The crystal structure of the title compound was determined at 250 K in space groupI4 bar 2m,a= 20.661(5) Å ,c= 6.764(7) Å ,Z= 8. The individual dithiadiazole rings form two sets of contrarotatory 4-member pinwheels clustered around a 4-fold rotation-inversion axis located halfway along the unit cell edges, describing an infinite channel lined with sulfur atoms but in which there are short intra-stack contacts through only one S atom of each CN2S2group. The double-layer stacking occurs in order to accommodate the bulk of thetBu group, and the spacing between layers is very regular, with short and long S cdot cdot cdot S contacts of 3.48(2) and 3.61(2) Å and considerable thermal motion in thecdirection. The title compound and its SbF6-salt are oxidized at + 0.81 V (in CH2Cl2) and at + 0.61 V (in CH3CN), while a reduction process is observed only in CH2Cl2at -0.73 V vs. SCE. Magnetic susceptibility data between 5 and 400 K demonstrate at very low temperature that the sample follows the Curie-Weiss law, θ = 0 K, and χ0= -156 ppm emu mol-1. The free-spin concentration atT= 0 K is approx 1.3%, due to paramagnetic defects in an essentially diamagnetic structure. The diamagnetism starts to lift above 210 K; above 260 K, a strong antiferromagnetic exchange is operative. These results are consistent with the lifting of the Peierls distortion in this structure, starting above approx 200 K. The crystal structure of the parent diamidine 5-tert-butyl-1,3-[(Me3Si)2NCNSiMe3]2C6H3was determined inC2/cwitha= 10.0788(3),b= 21.328(5),c= 20.876(5) Å , &bgr; = 99.41(2)°,Z= 4. The two amidine functional groups are equivalent by crystal symmetry.Key words: dithiadiazole, diradical, magnetic susceptibility, crystal structure, bulky substituent.

ISSN:0008-4042    

DOI:10.1139/v97-213

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The synthesis of ( ±)-9-deoxygoniopypyrone (1) from the &agr;-allenic alcohol5is described. Iodocyclofunctionaliztion of theN-tosyl carbamate derivative of5using I2and Ag2CO3provided, in a highly diastereoselective and regioselective fashion, the vinyl iodosyn-vicinal diol4. Two routes were explored in order to introduce the third stereogenic centre in the molecule. Reductive deiodination of the vinyl iodide and diastereoselective epoxidation of the derived acetonide14using mCPBA provided a mixture of epoxides15and16(2:1) in which the desiredthreodiastereomer predominated. Alternatively, dihydroxylation of acetonide14(OsO4, NMO) yielded a mixture of diols21and22(2:3) which were separated after monosilylation (TBDMSCl) of the primary alcohol. The major silyl ethererythrodiastereomer24was converted to the desired epoxide15by mesylation (MsCl, Et3N) and epoxide formation (TBAF) with inversion of stereochemistry. The minorthreodiastereomer23was also converted to the desired epoxide15(TBAF; ArSO2Cl; NaOMe). Epoxide opening was effected with lithium acetylide and the resulting alkyne27was carbonylated (MeLi, ClCO2Me) to afford the &agr; , &bgr;-acetylenic ester28. Semi hydrogenation over Lindlar's catalyst followed by protecting- group removal under acidic conditions provided ( ±)-8-epigoniodiol30. Finally, conversion of30to ( ±)-9-deoxygoniopypyrone1was effected under basic conditions (DBU).Key words: ( ±)-9-deoxygoniopypyrone, &agr;-allenic alcohol, iodocyclofunctionalization,syn-diol.

ISSN:0008-4042    

DOI:10.1139/v97-212

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Using ab initio molecular orbital theory mainly at the 3-21 + G level, intramolecular SN2 methyl transfer between two oxygens confined within a rigid template is found to proceed exclusively by a high energy retention mechanism when the oxygens are separated by three or four bonds, and by a high energy inversion mechanism when the oxygens are separated by six bonds. Both mechanisms exist when the oxygens are separated by five bonds. The CH3/CD3kinetic isotope effects are normal (1.21-1.34) in the retention processes and inverse (0.66-0.81) in the inversion reactions. In the case of inversion, compression of C-H bonds of the transition state by structural effects in the plane perpendicular to the O-C-O plane increases the inverse isotope effect. The retention barriers are high because retention is inherently unfavorable, even when pericyclic stabilization of the transition state is possible. The inversion barriers are high because a rigid template cannot accommodate a linear O-CH3-O structure, and the O-C-O bending vibration is stiff (the Eschenmoser effect). Using a novel design strategy, a nonrigid template has been found in which the barrier and the CH3/CD3kinetic isotope effect are the same as in an intermolecular reaction.Key words: Eschenmoser effect, isotope effect, compression, SN2, sigmatropic rearrangement.

ISSN:0008-4042    

DOI:10.1139/v97-215

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The experimental pseudo-first-order rate constant for the neutral hydration of acetone by H218O has been determined by taking advantage of the18O isotope shift in the13C NMR spectrum of 2-13C-labelled acetone. With the inclusion of this result, a previously reported computational procedure has been extended, with some refinements, to the neutral hydrations of formaldehyde, acetaldehyde, acetone, and cyclohexanone by different-sized water clusters in water solvent. Excellent agreement has been achieved between the experimental free energies of activation and the free energies of activation calculated for a cyclic mechanism in which the principal reaction channel contains four nonspectator water molecules. The computational procedure predicts that the cyclic water tetramer and the cubic water octamer are the principal constituents of bulk water, in agreement with Benson's tetramer-octamer model.Key words: isotope shift, basis set superposition error (BSSE), self-consistent reaction field (SCRF), water clusters, hydration.

ISSN:0008-4042    

DOI:10.1139/v97-216

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

A series of novel bistriazenes, the 1,2-bis(1-aryl-3-methyltriazen-3-yl)ethanes, Ar-N T N-NMe-CH2CH2-NMe-N T N-Ar, have been synthesized by diazonium coupling withN,N'-dimethylethylenediamine. These bistriazenes are stable crystalline compounds and have been unequivocally characterized by IR and NMR spectroscopy (1H and13C), and elemental analysis. The structures of two compounds in the series have been confirmed by X-ray crystallography. The1H NMR spectra show significant line broadening of the N-methyl resonances arising from the restricted rotation around the N2-N3 bond of the triazene units. The presence of strongly electron-withdrawing groups on the aryl ring restricts the rotation to the point where the N-methyl signals of the rotamers are distinct even at room temperature; four resonances of the N-methyl signal are clearly evident and these can be assigned to theanti-anti,syn-syn,andsyn-anticonformations of the bistriazene. Diazonium coupling withN,N'-diethylethylenediamine affords the N,N'-diethyl homologues of the bistriazenes, which have been similarly characterized. As model compounds to assist in spectroscopic analysis, a series of related triazenes, the 1-(1-aryl-3-methyltriazen-3-yl)-N,N-dimethyl-2-ethanamines, were prepared by diazonium coupling withN,N,N'-trimethylethylenediamine. These dialkyltriazenes exist mainly as oils, but characterization was achieved by IR,1H NMR, and13C NMR spectroscopy, also showing the presence of two rotamers in solution when strongly electron-withdrawing substituents are bonded to the aryl moiety.Key words: triazene, bistriazene, diazonium, ethylenediamine, molecular dynamics, NMR.

ISSN:0008-4042    

DOI:10.1139/v97-217

出版商:NRC Research Press    

年代:1998

数据来源: NRC