首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
The proton magnetic resonance spectra of the three title carbanions have been observed in THF with potassium as a counter ion. The ortho-protons in the α-methylbenzyl carbanion are nonequivalent at room temperature. This shows that the α-carbon in this carbanion is in the near-sp2 configuration. The aromatic proton chemical shifts of the benzylpotassium obtained here are at higher shielding than those of benzyllithium reported previously by Sandel and Freedman. This seems to arise from the different ionic nature of the bonds between carbon and metal in the carbanions.  相似文献   

2.
The proton NMR spectra of the carbanions of xanthene [XH]? and thioxanthene [TxH]? have been recorded and interpreted. Paratropism in the central rings of [XH]? and [TxH]? is inferred from a comparison of the chemical shifts with those of the carbanion of 9,10-dihydroanthracene [AH]?. The contributions to the chemical shifts arising from n-electron excess charges, local dipoles and magnetic anisotropies are discussed. Numerical values for the various ring currents have been estimated by a least squares analysis of the observed chemical shifts after applying corrections for the excess charge effect. The results point to a strongly increasing paramagnetic ring current in the central ring in the order [AH]?, [TxH]?, [XH]?.  相似文献   

3.
The natural abundance C-13 NMR spectra of 1,1-diphenylethylene and α-methylstyrene dimer dianions have been obtained using the proton noise decoupling technique. The extra negative charge distributions in the carbanions have been discussed and compared with those obtained from the proton chemical shifts. The chemical shifts of the two ortho carbons in a phenyl ring are equivalent to each other for the former carbanion but not for the latter.  相似文献   

4.
Abstract

Continuing our studies on utilization of the Wittig-Horner reaction in organic synthesis, we have examined reactivity of α-lithium derivatives of several phosphonates towards carbonyl compounds, by 31P NMR at low temperature (from -100°C to r.t.). The chemical shifts δ [ppm] of a few representative phosphonates (1), carbanions (2) and betains (3) will be given.  相似文献   

5.
The PMR spectra of the titled carbanion have been observed well-resolved and suitable for first-order analysis. Excess charges localized in the phenyl rings are estimated from the chemical shifts and compared with those observed in other carbanions.  相似文献   

6.
Abstract

Carbanions stabilized with sulfur containing substituents are versatile intermedi-ates in organic synthesis. The great value and importance of such carbanions is connected with specific properties of the sulfur atom which is capable to exist in a variety of valent states and to form many functional groups. These various sulfur-containing groups exert different carbanion stabilizing effects and can also serve as leaving groups in nucleophilic substitution or elimination reactions. Taking into account numerous types of sulfur containing functional groups and a variety of reactions they can promote, it is well understandable that reactions of sulfur-containing carbanions have been thoroughly studied and widely exploited in organic synthesis. The reactions of such carbanions with aliphatic electrophilic partners: alkylating agents, carbonyl compounds and Michael acceptors consist a major section of this field and were subject of numerous studies as well as many monographs.1 Contrary to that, not very much was known about reactions of such carbanions with electrophilic aromatic compounds. Actually, there are only few reports on nitroarylation of sulfur-containing carbanions via replacement of halogen orfho- or para- to the nitro group in halonitrocornpounds.2 There are also some reports on the alkylation of nitroarenes and heterocyclic compounds in the reaction with sulfonium3 and sulfoxonium4 ylides, dimethylsulfoxide carbanion,5 and dialkyl or alkyl aryl sulfones carbanions.6  相似文献   

7.
The decomposition of polybromostyryl carbanions (PBS?), obtained by anionic polymerization of 4-bromostyrene in tetrahydrofuran (THF), was investigated in the dark in a temperature range of ?6–?21°C. It was accompanied by the evolution of bromine anions and by the formation of polymeric allylic carbanions (λmax = 575 nm; εmax = 6800 eq?1·L·cm?1). The reaction mechanism was elucidated. The rate constant of the unimolecular rate-determining step of the process was 1.3 × 10?5 s?1 and 9.7 × 10?5 s?1 at ?21 and ?6°C, respectively. Its apparent energy of activation Eapp = 18.38 Kcal/mol. The polybromostyrenes with allylic carbanions at their ends may decompose further. Their “dark” decomposition yielded 1,3-butadiene-1,3-diphenyl-macromers. The mechanisms of decomposition of the PBS? carbanions and the dark decomposition of the polybromostyryl allylic carbanions are analogous. The rate constant of the latter process was 2.5 × 10?6 s?1 at ?6°C. The anionic polymerization of prepared macromers can be initiated in THF at ?78°C by α-methylstyryl carbanions, which do not react, however, with PBS? carbanions. “Comblike” polymacromers were prepared in which each branch had a molecular weight of about 50,000. The overall molecular weight of the polymacromer was estimated to be about 1 × 106. It has been assumed that the 2–1 mode of addition to the diene group of the macromer is predominant during its polymerization. The 3–4 mode of addition followed by proton shift represents the termination step. The 4–3 mode of addition was ruled out on the basis of spectroscopic evidence.  相似文献   

8.
Five new thiophenoxyketinimines have been synthesized. 1H and 13C NMR spectra as well as deuterium isotope effects on 13C chemical shifts are determined, and spectra are assigned. DFT and MP2 calculations of both structures, chemical shifts, and isotope effects on chemical shifts are done. The combined analysis reveals that the compounds are primarily on a zwitterionic form with an NH+ and a S group and with a little of the neutral form mixed in. Very strong intramolecular hydrogen bonding is found and very high NH chemical shifts are observed. The theoretical calculations show that calculations at the MP2 level are best to obtain correct “C═S” chemical shifts.  相似文献   

9.
Proton and carbon-13 chemical shifts of para-substituted stilbenes have been measured. 1H-1H, 1H-13C COSY spectra were obtained to analyze unambiguously the chemical shifts of protons and carbons. A long range coupling between 2-H and α-H was observed in a 1H-1H COSY spectrum. The observed chemical shifts have been correlated with Hammett substituent parameters. Among ethenyl protons and carbons, all but the chemical shifts of α-H show good correlation with both dual substituent parameters and single substituent parameters. In addition to this finding, the excellent linear correlations of C-l, and 4′-H of 4-substituted trans-stilbenes are also reported. Besides the correlations of chemical shifts with Hammett parameters, a good correlation between the chemical shifts and the calculated charges of position C-4′ are reported.  相似文献   

10.
The 13C and 1H chemical shifts of the ferrocene moiety, as well as the carbonyl carbons and styrene moiety, of substituted 2-benzylidene[3]ferrocenophane-1,3-diones have been assigned. Correlations of 13C substituent chemical shifts of both carbonyl carbons with the Hammett constants have been found, and the effect of the transmission of substituent effects on these chemical shifts through the styrene moiety is discussed. An explanation is given for the different sensitivities of the carbonyl carbon chemical shifts to the electronic effect of substituents in mono- and dicarbonyl derivatives.  相似文献   

11.
Institute of Mass Spectrometry, University of Amsterdam, Amsterdam, The Netherlands By using the method of Fourier transform ion cyclotron resonance mass spectrometry, substituent stabilization energies of homologous series of cycloalkyl carbanions, Ξ-c-CnH2n?2 (n = 3, 4, 5, 6, 7) with π-accepting substituents (Ξ = Ph, CN, COOMe, NO2) have been determined experimentally in the gas phase as the difference between the proton affinity of the substituted and corresponding unsubstituted (Ξ = H) cycloalkyl carbanions. The stabilization energy data have been analyzed in terms of Taft’s parametrization of polarizability, field/inductive, and resonance effects. The linear regression analyses show excellent correlations within the ΞCH2 ? Ξ-c-CnH2n?2 ? (n = 4, 5, 6, 7), and Ξ-c-C3H4 ? carbanion series, from which it appears that the contributions of polarizability effects are independent of the above type of carbanions and only depend on the nature of the substituent. Further, it follows that inductive stabilization is more effective in the substituted methyl, ΞCH2 ?, than in the substituted cycloalkyl, Ξ-c-CnH2n?2 ? (n = 4, 5, 6, 7) carbanions. This result suggests that inductive stabilization is counteracted by the electron releasing effect of alkyl groups. Resonance stabilization is significantly more effective in the substituted cycloalkyl, Ξ-c-CnH2n?2 ? (n = 4, 5, 6, 7), than in the substituted methyl, ΞCH2 ?, carbanions, which suggests that m contrast to inductive stabilization, resonance stabilization is assisted by the electron releasing effect of alkyl groups. Finally, it appears that substitutent stabilization in the geometrically restricted substituted cyclopropyl carbanions, Ξ-c-C3H4 ?, is dramatically less effective than in the corresponding geometrically unrestricted larger substituted cycloalkyl carbanions, Ξ-c-CnH2n?2 ? (n = 4, 5, 6, 7). The linear regression analyses of the substituted cycloalkyl carbanions indicate that reduction of the stabilization energy is caused not exclusively by a geometrically hindered resonance stabilization, but also to a smaller extent by a less efficient inductive stabilization in the substituted cyclopropyl carbanions.  相似文献   

12.
S. Braun  J. Kinkeldei  L. Walther 《Tetrahedron》1980,36(10):1353-1360
In order to study the substituent influence on the 13C chemical shifts in polycyclic conjugated π-electron systems with five and seven membered rings we have studied the aceheptylene system with markedly alternating bond length— in contrast to azulene investigated earlier—as well as 5-azaazulene as a hetero-analogue of azulene. The substituent induced 13C chemical shifts Δδc observed in the monomethylazulenes, monomethylaceheptylenes and in 5-azaazulene are correlated with the atom-atom polarizibilities πij of the corresponding unsubstituted compound. For 5-aza-azulene it was further tested, if the 13C chemical shifts could be predicted using the 13C chemical shifts of 4-substituted derivatives (RCN, Cl, OEt) and the correlations given in the literature for the substituent induced 13C chemical shifts in the system benzene/butadiene. The results show that the influence of the π-electrons is markedly overshadowed in the methylazulenes and -aceheptylenes by other effects, but is clearly discernible in 5-azaazulene as a derivative of azulene; so the data can be used to predict the 13C chemical shifts of other azaazulenes.  相似文献   

13.
The 17O nmr chemical shifts of 20 acetyl and formylthoiphenes, furans, and pyrroles are reported. The chemical shifts qualitatively correlate with the electronic character of the heterocyclic rings. The effect of steric factors are noted for the 17O chemical shifts of alkyl substituted acetylthiophenes and for N-methylpyr-roles.  相似文献   

14.
The NMR chemical shifts of two azoles and one benzazole whose crystal structures present polymorphism have been computed using the GIPAW approach. 15N and 13C nuclei have been studied. Statistical analysis of the computed 13C and 15N chemical shifts indicates that the GIPAW chemical shifts reproduce with a high degree of accuracy those experimentally reported. This methodology can be used to identify other polymorphic crystal structures.  相似文献   

15.
A linear scaling of the calculated chemical shifts is used in order to improve the accuracy of the DFT predicted 13C NMR chemical shifts. The widely applied method of GIAO B3LYP/6-311+G(2d,p) using the B3LYP/6-31G(d) optimized geometries is chosen, which allows cost-effective calculations of the 13C chemical shifts in the molecular systems with 100 and more atoms. A set of 27 13C NMR chemical shifts determined experimentally for 22 simple molecules with various functional groups is used in order to determine scaling factors for reproducing experimentally measured values of 13C chemical shifts. The results show that the use of a simple relationship (δscalc = 0.95 δcalc + 0.30, where δcalc and δscalc are the calculated and the linearly scaled values of the 13C chemical shifts, respectively) allows to achieve a three-fold improvement in mean absolute deviations for 27 chemical shifts considered. To test the universal applicability of the scaling factors derived, we have used complex organic molecules such as taxol and a steroid to demonstrate the significantly improved accuracy of the DFT predicted chemical shifts. This approach also outperforms the recently recommended usage of the Hartree-Fock optimized geometries for the GIAO B3LYP/6-311+G(2d,p) calculations of the 13C chemical shifts.  相似文献   

16.
The effect of trifluoroacetylation on the 13C chemical shifts of 2-acetamido-2-deoxyhexoses was examined. Studies of the 2-acetamido derivatives of glucose, galactose and mannose established that no regular trend in the 13C shifts occurred on trifluoroacetylation. This was in marked contrast to the results obtained for the 1H chemical shifts.  相似文献   

17.
The wave functions for calculating gas phase 19F chemical shifts were optimally selected using the factorial design as a multivariate technique. The effects of electron correlation, triple-ξ valance shell, diffuse function, and polarization function on calculated 19F chemical shifts were discussed. It is shown that of the four factors, electron correlation and the polarization functions affect the results significantly. B3LYP/6-31 + G(df,p) wave functions have been proposed as the best and the most efficient level of theory for calculating 19F chemical shifts. An additional series of fluoro compounds were used as a test set and their predicted 19F chemical shifts values confirmed the validity of the approaches.  相似文献   

18.
Y. Senda  S. Imaizumi 《Tetrahedron》1975,31(23):2905-2908
13C NMR spectra of menthol stereoisomers have been determined. The correlations of chemical shifts of these ring carbons with those of stereoisomeric 2-isopropylcyclohexanols are examined. Observed chemical shifts of 1-Me carbons are compared with those predicted from the chemical shifts of stereoisomeric 1-methyl-4-t-butylcyclohexanes. 13C NMR spectra of menthyl acetates, and cis and trans p -menthanes have also been examined.  相似文献   

19.
The 13C n.m.r. spectra of fourteen chlorocyclohexanones have been recorded to examine the variation of the 13C chemical shifts as a function of the position and the number of the chlorine substituents. Additivity relationships were found which enable reasonable prediction of the chemical shifts of chlorinated cyclohexanones. Comparison between the observed and calculated chemical shifts of mono- and dichlorinated flexible molecules shows that the chlorine effects are additive.  相似文献   

20.
The 15N and 13C chemical shifts of 6‐(fluoro, chloro, bromo, and iodo)purine 2′‐deoxynucleoside derivatives in deuterated chloroform were measured. The 15N chemical shifts were determined by the 1H? 15N HMBC method, and complete 15N chemical‐shift assignments were made with the aid of density functional theory (DFT) calculations. Inclusion of solvation effects significantly improved the precision of the calculations of 15N chemical shifts. Halogen‐substitution effects on the 15N and 13C chemical shifts of purine rings are discussed in the context of DFT results. The experimental coupling constants for 19F interacting with 15N and 13C of the 6‐fluoropurine 2‐deoxynuleoside are compared with those from DFT calculations. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号