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 109 毫秒
1.
The influence of twenty substituents R on the solvolysis rates and products of tertiary chlorides R - CH2CH2C(CH3)2Cl (7) has been studied. H, CH3, (CH3)2NCH2, ClCH2, Cl, CN and NO2 exert an inductive effect only, as the good correlation of the rates with the corresponding inductive substituent constants σ shows. Bulkier alkyl groups, i.e. isopropyl and t-butyl, lower the rate due to a Baker-Nathan effect, while the n-electron donors CH3S, CH3O, HO and (CH3)2N and the σ-electron donors (CH3)3Sn and HOCH2 cause rate increases based on σ constants. These accelerations are attributed to C,C-hyperconjugative and inductomeric effects which arise in the transition state for ionization. A comparison of the reactivity of the acyclic chlorides 7 and the corresponding 1-R-substituted 3-bromoadamantanes 1a shows that polar substituent effects are more strongly transmitted in the rigid bicyclic compounds 1a than in the flexible acyclic compounds 7.  相似文献   

2.
The rate constants (log k) for solvolysis of 2-, 3-, and 4-substituted bicyclo[2.2.2]octyl p-nitrobenzenesulfonates 10, 11 and 12 , respectively, correlate linearly with the corresponding inductive substituent constants s?. The formation of the ion pairs 9 is, therefore, controlled by the I effect of neighboring substituents. It follows from the corresponding reaction constants ρ1 of ?1.54, ?1.12, and ?1.22 that inductivity is highest at the positions α to C(1). It is lower and practically equal at the β- and γ-positions. Therefore, charge dispersal is similar to that previously observed in the quinuclidinium ion 7 .  相似文献   

3.
The rate constants (log k) for the solvolysis of 4e-substituted 2e- and 2a-adamantyl p-nitrobenzenesulfonates 14 and 15 , respectively, in 80% EtOH correlate linearly with the respective inductive substituent constants σ. Therefore, relative rates are controlled by the I effect of the substituents at C(4). The derived reaction constants, or inductivities, ρI of −0.80 and −0.64 for the series 14 and 15 , respectively, are far smaller than those previously determined for 6-substituted 2-norbornyl and 2-bicyclo[2.2.2]octyl sulfonates, in which the partial structure containing the substituent and the leaving group is the same. The ratio of the retained and inverted adamantanols obtained upon hydrolysis of the series 14 falls from 2.85 for R = CH3 to ca. 1 for R = CN, i.e. as the substituent at C(4) becomes more electron-attracting. In the 2a-series 15 this ratio is uniformly higher. These findings confirm that the 2-adamantyl cation is weakly bridged and that through-space induction in carbocations involves graded bridging of the cationic center by neighboring C-atoms.  相似文献   

4.
Three kinds of polar substitutent effects are observable in the solvolyses of 1-R-substituted 3-bromoadamantanes (VI). This follows from the relationship between products, rate constants k in 80% ethanol, and the inductive substituent constants σ of the substituent R. Alkyl groups and electron-attracting substituents at C (1) control the rate by their inductive effects alone, since logk correlates closely with σ. However, rates are higher than predicted on the basis of the respective σ values when conjugating (+ M)-substituents or electrofugal groups are attached to C(1). These exalted substituent effects are attributed to CC-hyperconjugative relay of positive charge from the cationic center at C(3) to the substituent at C(1). When the substituent is a strong electron donor (e.g. O? and S?), accelerated substitution gives way to heterolytic fragmentation, rates and products then being controlled by the frangomeric effect.  相似文献   

5.
The rate constants for 3-substituted adamantyl p-toluenesulfonates 3a - 3k in ethanol/water 80:20 correlate well with the respective inductive substituent constants σ. The reaction constant ρ for the toluenesulfonates 3 is 10% larger than for the corresponding bromides 2 , indicating somewhat more charge separation in the activation of the toluenesulfonates. Evidence is presented that stabilization of the resultant 1-adamantyl cations by induction involves graded 1,3-bridging, which is favored when the substituent is an electrofugal group, and that stabilization by n-electron donors involves C, C-hyperconjugation. Rate ratios for the toluenesulfonates 3 and the bromides 1 exceed 103 and are almost independent of the 3-substituents. The implications of this are discussed in the light of current hypotheses.  相似文献   

6.
The solvolysis rates and products of the 6-exo-substituted 2-exo- 1a - 1u , and 2-endo-norbornyl p-toluenesulfonates 2a - 2u , have been determined. In general, the rate constants for 1 and 2 (log k) correlate well with the inductive constants σ of the substitutents at C(6); however, their sensitivity to σ is much larger in the 2-exo-series 1 than in the 2-endo-series 2 . This differential transmission of polar effects is the cause of decreasing 2-exo/2-endo rate ratios from 2388 for R = t-C4H9 to 0.37 for R = Br, i. e. with increasing electron attraction by the substituent. The high sensitivity of the rate constants for the 2-exo-p-toluenesulfonates 1 to σ indicates an unusually strong inductive interaction between C(6) and the incipient cationic center at C(2). This interaction is ascribed to the participation of the pentacoordinate C(6)-atom, i. e. to 1,3-bridging, a consequence of steric hindrance of nucleophilic solvent participation in norbornanes. Donor substituents enhance 1,3-bridging, lead to faster reactions and to the formation of 2-exo substitution products. Conversely, acceptor substituents reduce 1,3-bridging, decrease rates and facilitate the formation of 2-endo substitution products. Graded 1,3-bridging is discussed in the light of Winstein's nonclassical ion concept.  相似文献   

7.
13C-NMR. sepctra of 37 4-substituted quinuclidinium perchlorates, 15 4-substituted quinuclidines and the corresponding 1-methylquinuclidinium iodides have been measured. The chemical shifts δ for all compounds lie in the expected range. No correlation is found between δ and the inductive substituent constant σ of the substituent. Abnormal shift differences between quinuclidines bearing a nucleofugal group and the corresponding protonated or N-methylated quinuclidinum salt are observed for the bridgehead carbon C(4). These differences are ascribed to incipient fragmentation, i.e. C, C-hyperconjugation in the ground state.  相似文献   

8.
The solvolysis rates and products of the 6endo-R-substituted 2endo-norbornyl toluenesulfonates 6a – 6i have been determined. The rates of 6a – 6g correlate with the inductive constants σ the 6endo-substituents and are not related to the size of the latter. It is therefore concluded that polar rather than steric effects control the exo/endo-rate ratios of norbornyl sulfonates. Products are derived mainly from rearranged 6exo-R-norbornyl cations when the substituent is an electron donor and from unrearranged 6endo-R-substituted cations when the substituent is an electron acceptor.  相似文献   

9.
Calculations were carried out on several data sets to study the mechanism of hydrogen abstraction from phenols by peroxy radicals: (1) Rate constants, k values, were collected for the reactions of cumyl-, 1-phenylethyl- and tert-butyl-peroxy radicals with ortho-para-substituted phenol inhibitors. The rate constants were recalculated for the same temperature. Solvent effects were neglected because the solvents used were similar in nature. The phenol ortho substituents were characterized by an indicator variable ItBu accounting for the presence or absence of di-tert-butyl groups. The phenol para substituents were characterized by Charton's σI, σR, and σ substituent constants. The dependence of log k values on Itbu, σI, σR, σ was investigated using stepwise linear regression analysis. The combined data set of 32 reactions gives: and The results suggest that hydrogen abstraction from phenols by peroxy radicals proceeds by an electrophilic mechanism, and that neither the peroxy-radical nor the ortho-di-tert-butyl groups have considerable effect on the rate of reaction (1).  相似文献   

10.
The solvolysis rates and products of 4- and 5-exo-substituted 2-exo- and 2-endo-norbornyl tosylates 9 and 10 , respectively, are reported. The logarithms of the rate constants (log k) correlate linearly with the inductive constants σ for the substituents. A comparison of the reaction constants p1 for the 4-, 5-, 6-, and 7-substituted 2-exo- and 2-endo-tosylates 9 , 10 , 1 , and 2 respectively, indicates that inductivity is higher for 2-exo-ionization than for 2-endo-ionization in all series. This observation is attributed to the more favorable alignment of neighbouring C-atoms for dorsal participation in exo-ionization, especially, in the case of C(6).  相似文献   

11.
When the Hammett-Taft equation log (k/ko)=ρq · σ is applied to the solvolysis of the 3-substituted propyl bromides 6a-6i in ethanol/water 4:1 (v/v) log k correlates linearly with σ except in cases where R exerts an anchimeric effect. The reaction constant ρq for 6 is ? 0.12 and is typical for a nucleophilic solvent-assisted ks process at a primary C-atom. The tertiary halides 1 and 3 , however, which react with little or no nucleophilic solvent assistance, i.e. by kc processes, lead to larger ρq values of ?0.71 and ?1.14, respectively. The reaction constant pq is therefore a sensitive gauge for charge development in the transition state for solvolysis of saturated compounds.  相似文献   

12.
A simple criterion for estimating the extent of π delocalization in the five-membered ring of pentafulvenes and pentafulvalenes is described. It is based on the fact that changes of bond lengths (induced by exocyclic substituents R1-R2 of 1 ) are reflected by systematic changes of 3J(H,H) values, so that linear correlations of σ vs, 3J(H,H)are obtained. Plots of that type (Fig. 1) are very useful for determining the extent of π delocalization of various pentafulvalenes 2 – 5 (Fig. 3) which show a very similar behavior to pentafulvenes. In principle, these plots could additionally be used for estimating substituent constants σ or for approximating the extent of π overlap between exocyclic substituents and the π system of pentafulvenes. Charge-density effects of pentafulvenes and pentafulvalenes are observed by substituen-induced shifts of the ring C-atoms (Fig. 4).  相似文献   

13.
The solvolysis rates and products of several 1-substituted 2exo- and 2-endo-norbornyl p-toluenesulfonates 7 and 8 , respectively, have been determined. Hydrolyses of these epimeric tosylates yielded rearranged products in varying amounts, except when the substituent was COOCH3 or CN. The logarithms of the rate constants (log k) for the endo-series 8 correlated linearly with the corresponding inductive constants σ with a reaction constant ρI of ?1.24. On the other hand, log k values for the exo-series 7 appear to fit two regression lines, the first line (ρI = ?1.90) defined by the tosylates that ionize, with rearrangement, to the tertiary cations 11 , the second (ρI = ?1.86) by the tosylates 7 (R = H, COOCH3, and CN) that ionize to an asymmetrically bridged secondary cation 19 . These results confirm the unique participation of C(6) with a ρI of ?2.00 in the ionization of 2-exo-nor-bornyl tosylate.  相似文献   

14.
It is shown that 1,1-dimethyl-1,2,3,4-tetrahydroquinolinium ions undergo, under direct irradiation through quartz in CH3OH and independent of the nature of the counterion (I?, BF), a reductive cleavage of the N(1)–C(8a) bond (photo-Emde degradation). The corresponding N,N-dimethyl-3-phenylpropylamines are formed in high yields and without contamination by Hofmann degradation products of the tetrahydroquinolinium salts. Me groups at C(2) as well as substituents at C(6) (CH3, Cl, CH3O) favour the photo-Emde degradation. The aromatic Cl-substituent is reductively split off in the course of the photoreaction.  相似文献   

15.
Silaheterocycles. III. Synthesis and Reactivity of Di-tbutylneopentylsilaethene, Bu Si?CHCH2But The three di-tbutylvinylsilanes BuSi(X)CH?CH2 (X = H 5 , X = F 9 , X = Cl 22 ) are prepared by the reaction of their SiCl precursors with vinyl lithium. In the treatment with LiBut the first step is the generation of the α-lithio compound BuSi(X)CH(Li)CH2But, the following reactions are governed by the nature of the substituent X and the reaction conditions (solvent, concentration, temperature). For X = H 2,3-LiH elimination leads to BuSi(H)CH?CHBut ( 7 ), with X = F or Cl Si?C formation by 1,2-LiX elimination competes with intermolecular Si-C-coupling producing BuSi(H)CH(SiBuCH?CHBut)CH2But ( 13 ) as the main product. BuSi?CHCH2But ( 1 ) probably coordinates to LiBut and reacts to yield BuSiCH?CHBut ( 3 ) and 7 , forms tetrabutyl-dineopentyl-1,3-disilacyclobutane 2 by cyclodimerization and 13 by addition of BuSi(X)CH(Li)CH2But.  相似文献   

16.
Using a new mathematical treatment, the nature and stability constants of the simple and mixed complex-species of copper(II) with hydroxyde and ammonia as ligands have been determined. The solubility curves of CuO in heterogeneous equilibrium have been identified in function of pH only and in function of pH and pNH3tot at 25° and unit ionic strength (NaClO4). The predominent species in the relatively dilute system limited by the ionic strength are [Cu2+], [Cu(OH)2], [Cu(OH)], [Cu(OH)], [Cu(NH3)], [Cu(NH3)], [Cu(NH3)], [Cu(NH3) (OH)+], [Cu(NH3)3(OH)+] and [Cu(NH3)2(OH)2].  相似文献   

17.
The products of the metastable decompositions of ionized ethyl formate (a) are characterized. The loss of water from a produces ·CH2CH2CO+, a rarely reported product. Loss of H appears to produce CH2=CHC(OH). The third decomposition is an unusual formation of C2H. This work demonstrates that a previous supposition that isomerization to different intermediates is involved in the losses of ethene and of water from a is correct.  相似文献   

18.
Carbon-13 chemical shifts and J(PC) coupling constants of 29 vinyl phosphate derivatives are presented. In the series of compounds (R1O)2P(O)OC1(R)?C2X2 (where 3 in R indicates the first carbon of the R2 substituent) large differences were found between the 3J(P, O, C-1, C-3) and 3J(P, O, C-1, C-2) coupling constants of the chlorinated (X?CI) and the unsubstituted (X?H) derivatives. A possible explanation of this phenomenon is given on the basis of Jameson's s bond character theory. Strong stereospecificity of 3J(P, O, C-1, C-3) coupling constants was observed in the series of compounds (R1O)2 P(O)OC1(R)?C2HR3. Coupling constants varied between 3.2–4.9 Hz in the E isomers, while peaks could not be resolved in the Z isomers. The 3J(P, O, C-1, C-2) coupling constants were regularly 20–30% greater in the Z than in the E isomers.  相似文献   

19.
Preparation of Fluorophosphates, Difluorophosphates, Fluorophosphonates, and Fluorophosphites in Fluoride-containing Urea Melts Phosphoric acid, phosphonic acid, and organylphosphonic acid react on heating in fluoride-containing urea melts in high yields to fluorophosphates, MPHO2F, organylfluorophosphonates, M1RPO2F, organylpolyfluorophosphonates, MR1CX(PO2F)2, MN(CH2PO2F)3, and phosphonoorganylfluorophosphonates, MR1CX(PO3)PO2F (M1 = K, NH4; R = organic substituent; R1 = H, organic substituent; X = OH, NH2, NR2). The reaction mechanism of the formation of fluorophosphate ions in fluoride containing urea melts is discussed.  相似文献   

20.
Polymerizations of ethylene by the MgCl2/ethylbenzoate/p-cresol/AlEt3 TiCl4-AlEt3/methyl-p-toluate (CW-catalyst) have been studied. The initially formed active site concentration, [Ti] has a maximum value of 50% of total titanium at 50°C and lower values at other temperatures. The Ti decays rapidly to Ti sites with conc. ca. 10 mol %/mol Ti. The rate constants for four chain transfer processes have been obtained at 50°C: for transfer with AlEt3, k = 2.1 × 10?4 s?1 and k = 4.8 × 10?4 s?1; for transfer with monomer, k = 3.6 × 10?3 (M s)?1 and K = 8.3 × 10?3 (M s)?1; for β-hydride transfer, k = 7.2 × 10?4 s?1 and k = 4.9 × 10?4 s?1; and transfer with hydrogen, k = 4.0 × 10?3 torr1/2 s? and k = 5.1 × 10?3 torr1/2 s?1. The rate constants for the termination assisted by hydrogen is k = 1.7 (M1/2 torr1/2 S)?1. If monomer is assisting termination as was observed for propylene polymerization, then k = 7.8 (M3/2 s)?1. Values of all the rate constants can be higher or lower at other temperatures. Detailed comparisons were made with the results of propylene polymerizations. There are more than four times as many Ti active sites for ethylene polymerization than there are for stereospecific polymerization of propylene; the difference is more than a factor of two for the Ti sites. Certain rate constants are nearly the same for both monomers while others are markedly different. Some of the differences can be explained by stereoelectronic effects.  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