摘要:

The reaction of Ph2Si(NHNHMe)2(1) with MeI results in a mixture of two six-membered ring isomers, 1,2,4,5-tetraaza-1,4-dimethyl-3,3,6,6-tetraphenyl-3,6-disilacyclohexane (2), 45%, and 1,2,4,5-tetraaza-1,5-dimethyl-3,3,6,6-tetraphenyl-3,6-disilacyclohexane (3), 40%. The reaction of1with HCl or Ph2SiCl2proceeds in a similar fashion. The thermolysis of1is studied from 25–600 °C. In the range of 250–300 °C, about 50% of1is converted into2,3, and 1,2,4-triaza-1-methyl-4-methylamino-3,3,5,5-tetraphenyl-3,5-disilacyclopentane,8. Possible pathways for the formation of2,3, and8in these cyclization reactions are discussed.

ISSN:0008-4042    

DOI:10.1139/v94-222

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The X-ray crystal structure of the title material has been determined and the results are compared to previous findings for benzo-9-crown-3 itself. The13C NMR spectrum in solution has been unambiguously assigned using COSY and HETCOR methods. The solid phase13C spectrum has been obtained and the 7, 10-d4derivative has been examined to aid in spectral assignments.

ISSN:0008-4042    

DOI:10.1139/v94-223

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

(2S, 2′R,RS)-2-(Fluoromethyl)-2-{[(4-methylphenyl)sulfinyl][phenyl]methyl}oxirane (4) was obtained in high chemical yield and with high diastereoselectivity by reacting diazomethane with pure (3R,RS)-1-fluoro-3-[(4-methylphenyl)sulfinyl]-3-phenylpropan-2-one (3), but also with its (3S,RS)-3-diastereoisomer, in the latter case through inversion of the configuration at carbon 3. The corresponding methyl enol ethers6were isolated as by-products. NMR experiments and molecular modelling calculations were performed in order to clarify the equilibria in solution and the preferred conformations of both ketones3. Several variously substituted, sulfur-free tertiary α-(fluoromethyl)carbinols were obtained from the main reaction product (2S, 2′R,RS)-4through appropriate elaborations of the sulfinyl group and of the oxirane ring. The structure of (2S, 2′R,RS)-4was established by X-ray analysis of the open-chain derivative (2S, 2′R,RS)-1-bromo-3-fluoro-2′-{[(4-methylphenyl)sulfinyl][phenyl]methyl}propan-2-ol (7).

ISSN:0008-4042    

DOI:10.1139/v94-224

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The proton chemical shifts and the proton spin–spin coupling constants are reported for 1-phenyl-1-butyne and 1-phenyl-1-pentyne dissolved in CS2/C6D12and acetone-d6. The long-range coupling constants between the methylene and ring protons are used to derive the twofold barriers to internal rotation in these molecules. They are 0.5 ± 0.1 kJ/mol; the perpendicular conformer is the most stable, the one in which the C(3)—C(4) bond of the side chain lies in a plane perpendicular to the phenyl group. This energetic preference is assigned to the difference between C—C and C—H hyperconjugative interactions with the aromatic π electron system, the C—C interaction being larger. Comparison of the twofold components of the internal rotational barriers in biphenyl and diphenylacetylene, that in the latter amounting to 40% of that in the former, implies that the hyperconjugative component of the internal barrier in ethylbenzene is 1.2 ± 0.3 kJ/mol, a minor component of the total magnitude. Molecular orbital computations of the conformational energies of 1 -phenyl-1-butyne all agree that the perpendicular conformer has the lowest energy but only to the extent of 0.1 kJ/mol at most.

ISSN:0008-4042    

DOI:10.1139/v94-225

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Dehydroacetic acid Schiff bases DAE (1) and DATS (2) were isolated from the reaction of dehydroacetic acid (2-acetyl-5-hydroxy-3-oxo-4-hexenoic acid-δ-lactone) with hydrazine-S-methylcarbodithioate and the corresponding acid amide, thiosemicarbazide, respectively. Reaction of such Schiff bases with nickel(II) and palladium(II) ions afforded the dimeric monoligand chelates [M(DAE-2H)]2and [M(DATS-2H)]2; M = Ni(II) or Pd(II). In the presence of monodentate Lewis bases, B, square-planar chelates [M(DAE-2H)B] and [M(DATS-2H)B], in which B = pyridine or triphenylphosphine, were isolated. The stoichiometry and structure of the isolated organic ligands and their chelates were confirmed through elemental analyses, molecular weight determination, and infrared, ultraviolet–visible, and1H and13C nuclear magnetic resonance spectroscopy.

ISSN:0008-4042    

DOI:10.1139/v94-226

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

While deuterium kinetic isotope effects for solvolyses have been extensively studied, other nucleophilic substitutions have received less attention, and identity processes, that is, substitutions where the nucleophile and leaving group are the same, have rarely been examined. Identity reactions must pass through a truly symmetrical stage, a transition state or an intermediate, so that data will be of interest to both theoretical and experimental chemists. Values ofkH/kDhave been determined by polarimetry for bromide exchange– racemization at ArCHBrCH3/CD3(Ar = C6H5,4-Br- and 4-Me-C6H4, and 3,4,-dimethyl-C6H3) in acetone, acetonitrile, and nitromethane. Observed values are analogous to values seen in solvolyses. They range from 1.01 to 1.35 and, in some cases, increase markedly as the concentration of Bu4NBr decreases. Solvolyses are either first order or pseudo first order whereas plotting observed racemization rate versus [Bu4NBr] allows separation of first- and second-order components; those species giving more stable carbocations in the more dipolar solvents, the systems showingkH/kDvariation with Br−concentration, alone show an appreciable first-order component. The second-orderkH/kDratio averages 1.062 ± −0.018 at temperatures ranging from 25 to 50 °C for all substrates in the three solvents, very analogous to the values seen for racemization of 1,1,1-d3-2-bromooctane or solvolysis of ethyl substrates but considerably lower than the typical solvolysis values of 1.15–1.25 forsecondary, and 1.35–1.5 fortertiarysubstrates. The first-orderkH/kDvalues obtained are higher, 1.1–1.5. These and other results are discussed.

ISSN:0008-4042    

DOI:10.1139/v94-227

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The nature of the first coordination sphere of the La(III) cation in acetonitrile solutions of La(NO3)3.6H2O was investigated by139La and17O NMR. All three nitrate anions are coordinated to La(III). There is a competition between water and acetonitrile molecules for the occupancy of the La(III) first coordination sphere. The139La and17O NMR results, obtained for various concentrations of La(NO3)3.6H2O, were quantitatively interpreted through a chemical equilibrium model. It was shown that the presence of only two La(III) species, {La(III) (NO3−)3} and {La(III) (NO3−)3.H2O}, in rapid exchange in solution could account for all the NMR results. The variation of the139La and17O NMR parameters (chemical shifts and relaxation rates) with the concentration of La(NO3)3.6H2O results from the population increase of the species {La(III) (NO3−)3.H2O}. The chemical exchange between free and coordinated water molecules is also fast on the17O NMR time scale. The17O NMR signal of water is considerably broadened when it coordinates to the La(III) cation. This broadening was attributed to both direct and indirect interactions between the two quadrupolar nuclei17O and139La. The direct interaction comes from an increase of the electric field gradient at the17O nucleus when a water molecule coordinates to the La(III) cation. The indirect interaction results from an17O–139La scalar coupling. The detailed analysis of the17O NMR spectra also allowed the determination of the indirect coupling constant between coordinated17OH2and139La(III),88 ± 13 Hz.

ISSN:0008-4042    

DOI:10.1139/v94-228

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The possibility of complexation between an acidic promoter like BF3and the alcoholic aglycone has been studied as a key mechanistic intermediate in glycosylation reactions operating via an electron-deficient anomeric carbon. The BF3complexes at O3′,O4′,O5′, and N3 of the model compound of 3′-,4-N-diacetyl-2′-deoxyribocytosine (1) have been studied. The last regioisomer of the four BF3complexes was found to have the weakest hydrogen bond and yet was the most stable isomer energetically. Furthermore, the O5′ nucleophilicity of this isomer was the most pronounced, as evaluated by Bader-type charge density topological analysis at the HF/6-31G**//HF/3-21G level of theory. This indicates that such a complex will make glycosylation at the O5′ oxygen possible.

ISSN:0008-4042    

DOI:10.1139/v94-229

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Reaction of alkyl 2-alkynoates with sodium iodide (1.6–5.5 equiv.) in acetic acid (6.2–13 equiv.) at 115 °C provides good to excellent yields of the corresponding alkyl (Z)-3-iodo-2-alkenoates.

ISSN:0008-4042    

DOI:10.1139/v94-230

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Transannular Diels–Alder reactions of four 14-membered macrocyclic trienes (27,28,43, and44) possessing a dimethyl tetrasubstituted dienophile have been investigated. Macrocycles having acis-trans-cis(CTC) (27), a CTT (43), and a TTT (44) geometry produced the predictedtrans-syn-cis(TSC) (45),cis-anti-cis(CAC) (48), and TAC (49) tricycles, respectively. The TTC macrocycle (28) gave exclusively the TST tricycle (46), no CSC tricycle (47) being observed. TTT macrocycle (44) underwent macrocyclization at a lower temperature than the TTC isomer (28).

ISSN:0008-4042    

DOI:10.1139/v94-231

出版商:NRC Research Press    

年代:1994

数据来源: NRC