共查询到20条相似文献，搜索用时 15 毫秒
1.
K, Zn, Cs and La ion-exchanged Y zeolites were prepared and used as photocatalysts for the isomerization of norbornadiene.
The prepared catalysts were characterized by EDS, XRD, 27Al- and 29Si-NMR, UV-vis and IR. The La-exchanged sample showed the highest degree of exchange, but accompanied with some dealumination.
La-exchange generated both B-acid and L-acid sites, whereas other metals produced only L-acid sites. For the photocatalytic
isomerization reaction, the activity order was LaY > CsY > ZnY > KY. The heavy atom effect and B-acidity are expected to play
important roles in the reaction. 相似文献
2.
The photoisomerization of the 1,2-diphenylcyclopropanes in polar solvent sensitized by 1-cyano- and 1,4-dicyanonaphthalene proceeds via an electron-transfer mechanism. An analysis of photo-CIDNP results provide details regarding the intermediates. 相似文献
3.
H. Nishiguchi S. Okamoto M. Nishimura H. Yamashita M. Anpo 《Research on Chemical Intermediates》1998,24(8):849-858
The phosphorescence properties of benzophenone included within alkali metal cationexchanged ZSM-5 zeolites were investigated
to clarify the effects of the micro-environment of hostadsorbents on the electronic excited states of guest-molecules included
within the restricted void spaces. Benzophenone included within such cation-exchanged ZSM-5 zeolites was found to exist in
both a protonated and hydrogen-bonded form. It was found that exchanging the cations dramatically affects the ratio of their
contents. Photolysis of these systems revealed that both benzhydrol and benzpinacol were the main products, their yields strongly
depending on the kind of the cations exchanged. Especially, the protonated species was found to play a significant role in
the photoreactions observed with benzophenone included within zeolite cavities. 相似文献
4.
Masaya Matsuoka Woo-Sung Ju Hai-Jun Chen Yoshiaki Sakatani Masakazu Anpo 《Research on Chemical Intermediates》2003,29(5):477-483
Photoluminescence investigations of the Ag ion-exchanged ZSM-5 (Ag+ /ZSM-5) zeolite revealed that a Ag ion cluster (Agn
m
+) exists in the pore structure of ZSM-5 exhibiting photoluminesm cence at 380 nm upon excitation at 332 nm. UV irradiation ( = 285 nm) of Ag+ /ZSM-5 at 77 K leads to the transformation of Agn
m
+ into a different Ag ion cluster (Agm
(n-1)+) which exhibits photoluminescence at 465 nm upon excitation at 315 nm. This photo-transformation of the Ag ion clusters was found to be thermally reversible under vacuum. It was demonstrated that an electron transfer from the photo-excited Al3+ -O2- to Agn
m
+ plays a significant role in this process. In the presence of oxygen, UV irradiation of Ag+ /ZSM-5 leads to the formation of O2- instead of an Ag ion cluster (Agm
(n-1)+), suggesting that oxygen acts as an efficient electron scavenger, which interferes with the electron capture of Agn
m
+ under UV irradiation at 285 nm. 相似文献
5.
The potential surface of 1,2-dimethylcyclobutane is investigated with respect to fragmentation and isomerization by the semiempirical MO method SINDO1. Energy and geometry of eight transition states and four intermediates are determined for the nonconcerted fragmentations under optimization of all internal coordinates. We find that symmetric cleavage ofcis-dimethylcyclobutane is the most favored process leading to fragmentation. Whereas the symmetric elimination involves two different barrier heights, the asymmetric elimination involves only two equal barrier heights. The possibility of isomerization ofcis- totrans- dimethylcyclobutane was also studied and revealed two different barrier heights lower than the corresponding heights for fragmentation. 相似文献
6.
A general strategy to enable the formal anti-hydrozirconation of arylacetylenes is reported that merges cis-hydrometallation using the Schwartz Reagent (Cp2ZrHCl) with a subsequent light-mediated geometric isomerization at λ = 400 nm. Mechanistic delineation of the contra-thermodynamic isomerization step indicates that a minor reaction product functions as an efficient in situ generated photocatalyst. Coupling of the E-vinyl zirconium species with an alkyne unit generates a conjugated diene: this has been leveraged as a selective energy transfer catalyst to enable E → Z isomerization of an organometallic species. Through an Umpolung metal–halogen exchange process (Cl, Br, I), synthetically useful vinyl halides can be generated (up to Z : E = 90 : 10). This enabling platform provides a strategy to access nucleophilic and electrophilic alkene fragments in both geometric forms from simple arylacetylenes.A general strategy to enable the formal anti-hydrozirconation of arylacetylenes is reported that merges cis-hydrometallation using the Schwartz Reagent (Cp2ZrHCl) with a subsequent light-mediated geometric isomerization at λ = 400 nm.The venerable Schwartz reagent (Cp2ZrHCl) is totemic in the field of hydrometallation,1 where reactivity is dominated by syn-selective M–H addition across the π-bond.2,3 This mechanistic foundation can be leveraged to generate well-defined organometallic coupling partners that are amenable to stereospecific functionalization. Utilizing terminal alkynes as readily available precursors,4 hydrozirconation constitutes a powerful strategy to generate E-configured vinyl nucleophiles that, through metal–halogen exchange, can be converted to vinyl electrophiles in a formal Umpolung process.5 Whilst this provides a versatile platform to access the electronic antipodes of the E-isomer, the mechanistic course of addition renders access to the corresponding Z-isomer conspicuously challenging. To reconcile the synthetic importance of this transformation with the intrinsic challenges associated with anti-hydrometallation and metallometallation,6 it was envisaged that a platform to facilitate geometric isomerization7 would be of value. Moreover, coupling this to a metal–halogen exchange would provide a simple Umpolung matrix to access both stereo-isomers from a common alkyne precursor (Fig. 1).Open in a separate windowFig. 1The stereochemical course of alkyne hydrometallation using the Schwartz reagent and an Umpolung platform to generate both stereo-isomers from a common alkyne precursor.Confidence in this conceptual blueprint stemmed from a report by Erker and co-workers, in which irradiating the vinyl zirconium species derived from phenyl acetylene (0.5 M in benzene) with a mercury lamp (Philips HPK 125 and Pyrex filter) induced geometric isomerization.8 Whilst Hg lamps present challenges in terms of safety, temperature regulation, cost and wavelength specificity, advances in LED technology mitigate all of these points. Therefore, a process of reaction development was initiated to generalize the anti-hydrozirconation of arylacetylenes. Crucial to the success of this venture was identifying the light-based activation mode that facilitates alkene isomerization. Specifically, it was necessary to determine whether this process was enabled by direct irradiation of the vinyl zirconium species, or if the E → Z directionality results from a subsequent selective energy transfer process involving a facilitator. Several accounts of the incipient vinyl zirconium species reacting with a second alkyne unit to generate a conjugated diene have been disclosed.9,10 It was therefore posited that the minor by-product diene may be a crucial determinant in driving this isomerization (Fig. 2).Open in a separate windowFig. 2A working hypothesis for the light-mediated anti-hydrozirconation via selective energy transfer catalysis.To advance this working hypothesis and generalize the formal anti-hydrozirconation process, the reaction of Cp2ZrHCl with 1-bromo-4-ethynylbenzene (A-1) in CH2Cl2 was investigated (‡ for full details). This generates a versatile electrophile for downstream synthetic applications. Gratifyingly, after only 15 minutes, a Z : E-composition of 50 : 50 was reached (entry 1) and, following treatment with NBS, the desired vinyl bromide (Z)-1 was obtained in 76% yield (isomeric mixture) over the two steps. Further increasing the irradiation by 15 minute increments (entries 2–4) revealed that the optimum reaction time for the isomerization is 45 minutes (74%, Z : E = 73 : 27, entry 3). Extending the reaction time to 60 minutes (entry 4, 54%) did not lead to an improvement in selectivity and this was further confirmed by irradiating the reaction mixture for 90 minutes (entry 5). In both cases, a notable drop in yield was observed and therefore the remainder of the study was performed using the conditions described in entry 3. Next, the influence of the irradiation wavelength on the isomerization process was examined (entries 6–11). From a starting wavelength of λ = 369 nm, which gave a Z : E-ratio of 27 : 73 (entry 6), a steady improvement was observed by increasing the wavelength to λ = 374 nm (Z : E = 44 : 56, entry 7) and λ = 383 nm (Z : E = 53 : 47, entry 8). The selectivity reached a plateau at λ = 400 nm, with higher wavelengths proving to be detrimental (Z : E = 60 : 40 at λ = 414 nm, entry 9; Z : E = 26 : 74 at λ = 435 nm, entry 10). It is interesting to note that at λ = 520 nm, Z-1 was not detected by 1H NMR (entry 11).Reaction optimizationa
Open in a separate windowa(i) Cp2ZrHCl (62 mg, 0.24 mmol, 1.2 eq.), CH2Cl2 (1.5 mL), alkyne A-1 (36 mg, 0.2 mmol, 1.0 eq.) in CH2Cl2 (0.5 mL); (ii) irradiation; (iii) NBS (39 mg, 0.22 mmol, 1.1 eq.).baverage yield and Z : E ratio of two reactions determined by 1H-NMR with DMF as internal standard; isolated yield of the Z : E-mixture and Z : E-ratio in parentheses.Having identified standard conditions to enable a hydrozircononation/isomerization/bromination sequence, the scope and limitations of the method was explored using a range of electronically and structurally diverse phenylacetylenes (Fig. 3). This constitutes a net anti-Markovnikov hydrobromination of alkynes.11Open in a separate windowFig. 3Aromatic scope for the formal anti-hydrozirconation of terminal alkynes; reaction conditions: (i) Cp2ZrHCl (62 mg, 0.24 mmol, 1.2 eq.), CH2Cl2 (1.5 mL), alkyne A-1-17 (0.2 mmol, 1.0 eq.) in CH2Cl2 (0.5 mL), 15 min; (ii) irradiation (λ = 400 nm), 45 min; (iii) NBS (39 mg, 0.22 mmol, 1.1 eq.), 15 min; aisolated yield of Z : E-mixture as average of two reactions; b(i) Cp2ZrHCl (62 mg, 0.24 mmol, 1.2 eq.), CH2Cl2 (1.5 mL), alkyne A-15 (26 mg, 0.2 mmol, 1.0 eq.) in CH2Cl2 (0.5 mL); (ii) irradiation (λ = 400 nm), 45 min; (iii) PdPPh3 (7 mg, 0.006 mmol, 0.03 eq.) in THF (0.4 mL), BnBr (24 μL, 0.2 mmol, 1.0 eq.), rt, 18 h.12The introduction of halogen substituents in the 4-position proved to be compatible with the reaction conditions, enabling the formation of (Z)-1-4 in up to 81% yield (up to Z : E = 74 : 26). Interestingly, the introduction of the o-F (Z)-5 substituent led to a drop in the yield and selectivity: this is in stark contrast to cinnamoyl derivatives that have previously been examined in this laboratory.12 The m-Br proved to be less challenging enabling (Z)-6 to be generated smoothly (74%, Z : E = 67 : 33). The parent phenylacetylene (A-7) could be converted with a similar Z : E-ratio to (Z)-7 albeit less efficiently (36%, Z : E = 72 : 28). Electron donating groups in the para position such as (Z)-8-10 led to a general improvement in selectivity (up to 80%, Z : E = 81 : 19). Whereas methylation at the ortho-position compromised efficiency [(Z)-11, 37%, Z : E = 68 : 32], translocation to the meta-position led to a recovery in terms of yield and Z : E-ratio [(Z)-12, 71%, Z : E = 75 : 25]. Extending the π-system from phenyl to naphthyl enabled the generation of (Z)-13 90% and with a Z : E-ratio of 77 : 23. To enable a direct comparison of strongly and weakly donating groups on the reaction outcome the p-CF3 and p-OMe derivatives were examined. In the trifluoromethyl derivative (Z)-14 a decrease in yield (31%) and selectivity (Z : E = 48 : 52) was noted. In contrast, the para methoxy group in (Z)-15 led to an enhanced Z : E ratio of 86 : 14 (68% yield). This behavior was also observed with the trimethoxy derivative (Z)-16 (Z : E-ratio of 81 : 19). The piperonyl derivative performing similarly to the para methoxy derivative thereby enabling the formation of (Z)-17 with a Z : E-ratio of 85 : 15 (67% yield). Finally, to demonstrate the utility of the method, a direct transmetallation protocol was performed to intercept the Z-vinyl zirconium species with benzyl bromide.13 This enabled the synthesis of (Z)-18 in 67% yield.To demonstrate the compatibility of this platform with other common electrophiles, the deuterated, chlorinated and iodinated systems (Z)-19, -20 and -21 were prepared (Fig. 4). Yields and selectivities that are fully comparable with Fig. 3 were observed (up to 80% yield and Z : E = 80 : 20). Finally, to augment the photostationary composition further, a process of structural editing was conducted. It was envisaged that integrating a stabilizing non-covalent interaction in the Z-vinyl zirconium species may bias isomerization selectivity. Recent studies from this laboratory have established that a stabilizing interaction between the boron p-orbital and an adjacent non-bonding electron pair can be leveraged to induce a highly selective geometric isomerization of β-borylacrylates (Fig. 5, top).14Open in a separate windowFig. 4Scope of electrophiles for the formal anti-hydrozirconation; reaction conditions: (i) Cp2ZrHCl (62 mg, 0.24 mmol, 1.2 eq.), CH2Cl2 (1.5 mL), A-9 (36 mg, 0.2 mmol, 1.0 eq.) in CH2Cl2 (0.5 mL); (ii) irradiation (λ = 400 nm), 45 min; (iii) E+ (DCl, NCS or NIS) (0.22 mmol, 1.1 eq.), 15 min; isolated yields of the Z : E-mixture are reported.Open in a separate windowFig. 5Enhancing the selectivity of anti-hydrozirconation by leveraging a postulated nS → Zr interaction. Reaction conditions: (i) Cp2ZrHCl (62 mg, 0.24 mmol, 1.2 eq.), CH2Cl2 (1.5 mL), alkyne A-22-24 (0.2 mmol, 1.0 eq.) in CH2Cl2 (0.5 mL), rt, 15 min; (ii) irradiation (λ = 400 nm), 45 min; (iii) NBS (39 mg, 0.22 mmol, 1.1 eq.), rt, 15 min.Gratifyingly, the 5-bromo thiophenyl derivative (Z)-22 was generated with a Z : E ratio of 87 : 13 in 73% yield, and the unsubstituted derivative (Z)-23 was obtained in 41% yield higher selectivity (Z : E = 90 : 10). As a control experiment, the regioisomeric product (Z)-24 was prepared in which the sulfur atom is distal from the zirconium center. This minor alteration resulted in a conspicuous drop of selectivity (Z : E = 78 : 22), which is in line with the phenyl derivatives. Given the prominence of Frustrated-Lewis-Pairs (FLPs) in small molecule activation,15 materials such as (Z)-22 and (Z)-23 may provide a convenient starting point for the development of future candidates.To provide structural support for the formation of a Z-vinyl zirconium species upon irradiation at λ = 400 nm, the standard experiment was repeated in deuterated dichloromethane and investigated by 1H NMR spectroscopy. The spectra shown in Fig. 6 confirm the formation of transient E- and Z-vinyl zirconium species (E)-Zr1 and (Z)-Zr1 and are in good agreement with literature values.8 Diagnostic resonances of (E)-Zr1 include H1 at 7.76 ppm, whereas the analogous signal in (Z)-Zr1 is high field shifted to 6.33 ppm (Δδ(H1Z−E) = −1.43 ppm). In contrast, the H2 signal for (Z)-Zr1 appears at 7.56 ppm, which is at lower field compared to the H2 signal for (E)-Zr1 at 6.64 ppm (Δδ(H2Z−E) = 0.92 ppm). In the 13C-NMR spectra (see the ESI‡) the carbon signal of C1 and C2 are both low field shifted for (Z)-Zr1 compared to (E)-Zr1 (Δδ(C1Z−E) = 10.5 ppm and Δδ(C1Z-E) = 5.6 ppm).Open in a separate windowFig. 61H-NMR of the transient vinylzirconium species (E)-Zr1 (top) and (Z)-Zr1 (bottom).A computational analysis of the vinyl zirconium isomers (E)-Zr1 and (Z)-Zr1 revealed two low energy conformers for each geometry (Fig. 7. For full details see the ESI‡). These optimized structures served as a basis for more detailed excited state calculations using a time-dependent density functional theory (TDDFT) approach. These data indicate that isomerization of the styrenyl zirconium species by direct irradiation is highly improbable using λ = 400 nm LEDs. However, upon measuring the absorption spectrum of the reaction mixture (Fig. 8, bottom), the shoulder of a band reaching to the visible part of the spectrum is evident (for more details see the ESI‡). Furthermore, the fluorescence spectrum (Fig. 8, top) clearly shows light emission from the reaction mixture. Collectively, these data reinforce the working hypothesis that a minor reaction product functions as a productive sensitizer, thereby enabling the isomerization to occur via selective energy transfer.Open in a separate windowFig. 7A comparative analysis of (E)-Zr1 and (Z)-Zr1.Open in a separate windowFig. 8(Top) Fluorescence spectra of the reaction mixture before and after irradiation, and the diene 25 (c = 0.1 mm, irradiation at λ = 350 nm). (Bottom) Absorption spectra of the reaction mixture before and after irradiation (c = 0.1 mm), the alkyne A-1 and the diene 25 (c = 0.05 mm).As previously highlighted, phenylacetylenes are known to dimerize in the presence of Cp2Zr* based complexes.9,16 Therefore, to provide support for the involvement of such species, diene 25 was independently prepared and its absorption and emission spectra were compared with those of the reaction mixture (Fig. 8). The emission spectra of the reaction mixture and of diene 25 are closely similar. It is also pertinent to note that diene 25 was also detected in the crude reaction mixture by HRMS (see the ESI‡).Whilst the spectral measurements in Fig. 8 are in line with diene 25 functioning as an in situ photocatalyst, more direct support was desirable. Frustratingly, efforts to subject (E)-Zr-1 and (Z)-Zr-1 to standard Stern–Volmer quenching studies were complicated by difficulties in removing diene 25 from the samples. It was therefore envisaged that doping reactions with increasing quantities of diene 25 might be insightful. To that end, the hydrozirconation/isomerization sequence was performed with 0.5, 1.0 and 2.5 mol% of diene 25 and the reactions were shielded from light after 5 minutes. Analysis of the mixture by 1H NMR spectroscopy revealed a positive impact of 25 on the Z : E selectivity, (Z : E = 23 : 77, 24 : 76 and 30 : 70, respectively. Fig. 9, top). To further demonstrate the ability of diene 25 to act as an energy transfer catalyst for geometric isomerization, two model alkenes containing the styrenyl chromophore were exposed to the standard reaction conditions and the photostationary composition was measured after 45 min. Exposing trans-stilbene (E)-26 to the isomerization conditions furnished a Z : E photostationary composition of 44 : 56. Similarly, trans-β-methyl styrene (E)-27 could be isomerized to the cis-β-methyl styrene (Z)-27 with a Z : E ratio of 47 : 53. No isomerization was observed at λ = 400 nm in the absence of the catalyst. Whilst direct comparison with the isomerization of vinyl zirconium species must be made with caution, these experiments demonstrate that dienes such as 25 have the capacity to act as photosensitizers with styrenyl chromophores.Open in a separate windowFig. 9(Top) Exploring the impact of adding diene 25 as an external photocatalyst. (Bottom) Validating photosensitization of the styrenyl chromophore using diene 25.Collectively, these data support the hypothesis that isomerization does not result from direct irradiation alone,17 but that conjugated dienes, which are produced in small amounts, function as in situ energy transfer catalysts (Fig. 10). This antenna undergoes rapid inter-system crossing (ISC)18 to generate the triplet state and, upon energy transfer to the alkene fragment, returns to the ground state.19 This mechanistic study has guided the development of an operationally simple anti-hydrozirconation of alkynes that relies on inexpensive LED irradiation. Merging this protocol with a sequential metal–halogen exchange enables the formal anti-Markovnikov hydrobromination of alkynes11 and provides a sterodivergent platform to access defined alkene vectors from simple alkynes. This complements existing strategies to isomerize vinyl bromides,20 and circumvents the risks of vinyl cation formation and subsequent degradation.21 Finally, the selectivity of this geometric isomerization can be further augmented through the judicious introduction of stabilizing non-covalent interactions (up to Z : E = 90 : 10). It is envisaged that this selective, controlled geometric isomerization of an organometallic species will find application in contemporary synthesis. Furthermore, it contributes to a growing body of literature that describes the in situ formation of photoactive species upon irradiation.22Open in a separate windowFig. 10Postulated energy transfer catalysis cycle predicated on in situ formation of a conjugated diene photocatalyst. 相似文献
| Entry | λ [nm] | Time [min] | Yieldb | Z : E ratiob |
|---|---|---|---|---|
| 1 | 400 | 15 | 76% | 50 : 50 |
| 2 | 400 | 30 | 72% | 68 : 32 |
| 3 | 400 | 45 | 74% (74%) | 74 : 26 (73 : 27) |
| 4 | 400 | 60 | 54% | 73 : 27 |
| 5 | 400 | 90 | 49% | 73 : 27 |
| 6 | 369 | 45 | 66% | 27 : 73 |
| 7 | 374 | 45 | 61% | 44 : 56 |
| 8 | 383 | 45 | 64% | 53 : 47 |
| 9 | 414 | 45 | 67% | 60 : 40 |
| 10 | 435 | 45 | 72% | 26 : 74 |
| 11 | 520 | 45 | 67% | <5 : 95 |
7.
《Tetrahedron letters》1992,33(5):611-614
Upon electron transfer 1,2-distyrylbenzene2 undergoes a valence isomerization giving rise to an indane derivative. This process is controlled by the ion-pair structures of the anionic intermediates and differs from the photochemically induced rearrangement. 相似文献
8.
Ramamurthy V Shailaja J Kaanumalle LS Sunoj RB Chandrasekhar J 《Chemical communications (Cambridge, England)》2003,(16):1987-1999
The alkali ions present in the supercages of zeolites X and Y interact with included guest molecules through quadrupolar (cation-pi), and dipolar (cation-carbonyl) interactions. The presence of such interactions can be inferred through solid-state NMR spectra of the guest molecules. Alkali ions, as illustrated in this article, can be exploited to control the photochemical and photophysical behaviors of the guest molecules. For example, molecules that rarely phosphoresce can be induced to do so within heavy cation-exchanged zeolites. The nature (electronic configuration) of the lowest triplet state of carbonyl compounds can be altered with the help of light alkali metal ions. This state switch (n pi*-pi pi*) helps to bring out reactivity that normally remains dormant. Selectivity obtained during the singlet oxygen oxidation of olefins within zeolites illustrates the remarkable control that can be exerted on photoreactions with the help of a confined medium that also has active sites. The reaction cavities of zeolites, like enzymes, are not only well-defined and confined, but also have active sites that closely guide the reactant molecule from start to finish. The examples provided here illustrate that zeolites are far more useful than simple shape-selective catalysts. 相似文献
9.
C. Bezouhanova H. Lechert G. Taralanska A. Meyer 《Reaction Kinetics and Catalysis Letters》1989,40(2):209-214
In the IR spectra of 1-hexene adsorbed on HZSM-5 zeolites with different Si/Al ratios, a new band at 1510 cm–1 has been observed. The cis/trans-2-hexene and trans-2-hexene/1-hexene ratios were determined.
1-, HZSM-5 Si/Al, 1510 cm–1. /-2- -2-/1-.相似文献
10.
[reaction: see text] Effects of metal cation recognition on photosensitized E-Z isomerization of 1,2-dichloroethylene have been investigated with two types of m,p- and o,o'-podand (oligo-oxyethylene unit) benzophenones 2 and 3. With increasing cation size, the %Z ratio at the photostationary state was changed from 67 to 91% for 2 but remained almost unchanged (86-88%) for 3. These results were interpreted in terms of the variation of excited triplet energy, E(T), associated with the cation complexation of these photosensitizers 2 and 3. 相似文献
11.
改性Y沸石上的氢转移反应 总被引：1，自引：0，他引：1
本文试图用环已烯和邻二甲苯两典型反应考察改性Y沸石的氢转移反应性能。发现在改性Y沸石上环已烯的氢转移, 异构化和聚合反应时发生。在反应开始时SY沸石的氢转移反应和异构化反应活性与铝含量之间的关系与沸石强酸量和铝含量的关系一致, 呈火山形曲线, 极大值在Al/(Al+Si)为0.15左右, 结焦以后反应总转化迅速下降, 且氢转移活性和异构化活性随铝含量减少而单调下降。USY沸石的氢转移活性大幅度降低。在改性Y沸石上邻二甲苯异构化和歧化反应同时进行, SY沸石的歧化反应变化规律与环乙烯氢转移反应相似。当沸石样品中有大量非质子酸位存在时, 两个反应的数据会出现予盾。 相似文献
12.
Natarajan A Joy A Kaanumalle LS Scheffer JR Ramamurthy V 《The Journal of organic chemistry》2002,67(24):8339-8350
Irradiation of 2-benzoyladamantane derivatives in zeolites yields the endo-cyclobutanols as the only photoproduct via a gamma-hydrogen abstraction process. The cyclobutanols readily undergo retro-aldol reaction to give delta-ketoesters. The enantiomeric excess (ee) in the endo-cyclobutanols is measured by monitoring the ee in the ketoesters. Whereas in solution the ee in the product ketoester is zero, within achiral NaY zeolite, in the presence of a chiral inductor such as pseudoephedrine, ee's up to 28% have been obtained. The influence of zeolite on several chiral esters of 2-benzoyladamantane-2-carboxylic acids has also been examined. Whereas in solution the diastereomeric excess is <15%, in zeolite the delta-ketoesters are obtained in 79% de (best examples). Ab initio computations suggest that enhancement of chiral induction within zeolites is likely to be due to cation complexation with the reactant ketone. Alkali ion-organic interaction, a powerful tool, is waiting to be fully exploited in photochemical and thermal reactions. In this context zeolites could be a useful medium as one could view them as a reservoir of "naked" alkali ions that are only partially coordinated to the zeolite walls. 相似文献
13.
V. Ramamurthy Nicholas J. Turro 《Journal of inclusion phenomena and macrocyclic chemistry》1995,21(1-4):239-282
In this article, we illustrate how one can utilize the cation embedded in a zeolite matrix to control the photophysical and photochemical behavior of guest molecules included in zeolite cages/cavities. Three aspects of cation-guest interaction are highlighted. Strong electronic interaction between the cation and the guest leads to mobilization of guests within zeolite supercages, alteration of the lowest electronic configuration (n*, *) and restriction of conformation of molecules. Less obvious to a non-photochemist is the ability of cations, depending on their atomic weight, to induce spin conversion (singlet-triplet) in molecules. Photophysical studies carried out with aromatics and photodimerization of acenaphthylene highlight this point. The power of the heavy-atom cation effect in zeolites has been demonstrated by recording phosphorescence from several olefins whose phosphorescence has not previously been recorded. Cations by their sheer size can influence the mobility of molecules within zeolites. Restriction of motion of reactive intermediates results in product selectivity in reactions as illustrated with a few examples. Properties of cations and consequently their influence can change depending on whether they are hydrated or uncoordinated to water. Aggregation of dyes and aromatics and reactivities of carbonyl compounds are influenced by water present within zeolites. We have shown in this article that zeolites can be used as unique reaction vessels for photochemical reactions. 相似文献
14.
B. Sulikowski 《Reaction Kinetics and Catalysis Letters》1981,16(1):39-42
Dehydroxylation and catalytic conversion of m-xylene over dealuminated zeolites were studied.
- .相似文献
15.
The kinetics of the gas-phase decomposition of 1,2-epoxycyclohexane has been studied over the temperature range 680–740 K at pressures between 1.6 and 6 torr. Isomerization to cyclohexanone and 2-cyclohexen-1-ol accounts for ca. 97% of the primary reaction products and occurs by first-order, homogeneous, nonradical processes: 相似文献
16.
Kyoichi Oki Seiji Yamada Sachiko Tsuchida Hiromasa Nishikiori Nobuaki Tanaka Tsuneo Fujii 《Research on Chemical Intermediates》2003,29(7-9):827-837
The photocatalytic degradation and isomerization of trans- and cis-1,2-dichloroethenes (1,2-DCEs) by TiO2 photocatalyst have been investigated using gas chromatography. The reaction half-life of 1,2-DCEs in nitrogen was longer than in dry air and oxygen, and the initial concentration of them affects the extent of the isomerization. The results indicate that the oxygen molecule and chlorine atoms play an important role in the degradation and isomerization of 1,2-DCE. It is also apparent that the photocatalytic degradation of 1,2-DCEs occurs on the TiO2 surface. 相似文献
17.
Two types of paramagnetic centers have been revealed in the composition of coke formation products on zeolite surfaces. The oxygen effect on the paramagnetic properties of these systems is determined by the concentration of these centers, which in turn characterizes the structure of coke precursors on their surface.
, . , , , .相似文献
18.
Conversion of paraffins on TsVK and Y zeolites follow a similar mechanism yielding the same intermediates. Low selectivity of Y zeolites with respect to aromatic hydrocarbons is attributed to the strong adsorption of these hydrocarbons on zeolites. It is assumed that the centers of strong adsorption are Al3+ ions in cationic positions in zeolite.
, . . , Al3+ .相似文献
19.
The presence of the regenerable visual pigment rhodopsin has been shown to be primarily responsible for the acute photodamage to the retina. The photoexcitation of rhodopsin leads to isomerization of its chromophore 11-cis-retinal to all-trans-retinal (ATR). ATR is a potent photosensitizer and its role in mediating photodamage has been suspected for over two decades. However, there was lack of experimental evidence that free ATR exists in the retina in sufficient concentrations to impose a risk of photosensitized damage. Identification in the retina of a retinal dimer and a pyridinium bisretinoid, so called A2E, and determination of its biosynthetic pathway indicate that substantial amounts of ATR do accumulate in the retina. Both light damage and A2E accumulation are facilitated under conditions where efficient retinoid cycle operates. Efficient retinoid cycle leads to rapid regeneration of rhodopsin, which may result in ATR release from the opsin "exit site" before its enzymatic reduction to all-trans-retinol. Here we discuss photodamage to the retina where ATR could play a role as the main toxic and/or phototoxic agent. Moreover, we discuss secondary products of (photo)toxic properties accumulating within retinal lipofuscin as a result of ATR accumulation. 相似文献