Accessible cations

The synthesis of new heterocyclic derivatives under conservation of a preformed cyclic structure is not only of particular importance for the synthesis of ionic 1,3,2-diazaphosphole or NHP derivatives but has also been widely apphed to prepare neutral species with reactive functional substituents. The reactions in question can be formally classified as 1,2-addition or elimination reactions involving mutual interconversion between 1,3,2-diazaphospholes and NHP, and substitution processes. We will look at the latter in a rather general way and include, beside genuine group replacement processes, transformations that involve merely abstraction of a substituent and allow one to access cationic or anionic heterocycle derivatives from neutral precursors. 

The active component for olefin oxidation is Pd2+, while Cu2+ acts as a promoter for the reoxidation of Pd. The sequence of ion exchange of Pd + and Cu2+ on the faujasite zeolite influences the catalytic performance. Best results seem to be obtained when Pd + is introduced in the second step of the ion exchange as it will then be located mainly at the more easily accessible cation sites II and/or III [23], The amount of exchanged Pd + determines the catalytic activity of Pd +Cu +Y, provided that Cu2+ is present in sufficient amounts to assure fast regeneration of Pd2+. A Pd/Cu atomic ratio of four is required here. Increasing acidity in Pd +Cu +NaY results in a decrease of both the activity and selectivity in the olefin oxidation [26]. 

Synthetic zeolites of various types differ in the number of cations in their voids which are accessible for direct interaction with the molecules adsorbed. Table I lists, for typical examples of zeolites, the numbers of accessible cations Na per zeolite void and their number Z in mmole/ gram for dehydrated zeolites. When passing from zeolite NaA to zeolite L, the number of accessible cations Z—i.e., the number of adsorption centers in the void—decreases almost by a factor of 10. Therefore, in the case of zeolite L, the relative role of interactions among cations and molecules adsorbed, conventionally called electrostatic, will be approximately one order lower than for zeolite NaA. In adsorption on this zeolite of substances with slightly pronounced nonuniformity of distribution of electron density in molecules— for instance, saturated hydrocarbons— one may expect that electrostatic interactions will not play the decisive role. As a result, we obtain the limiting case of adsorption on zeolites like zeolite L and erionite with a weak electrostatic interaction. 

In the robust, very easily accessible cationic complexes [FeCp(arene)][PF6]10 (Cp = h5-cyclopen-tadienyl), the benzyhc protons are more acidic than in the free arene because of the electron-withdrawing character of the 12-electron CpFe+ moiety. For instance, FeCp(C6Me6) Pl6 is more acidic by 15 pKa units (pKa = 28 in DMSO) (DMSO = dimethyl sulfoxide) than in the corresponding free arene (pKa = 43 in DMSO). Asa result, these complexes are much more easily deprotonated than the free arene.11 This key proton-reservoir property led us to synthesize stars and dendrimers in an easy way.12 Indeed, reaction of [FeCp(C6Me6)][PF6], with excess KOH (or i-BuOK) in THF (THF = tetrahydrofuran) or DME (DME = 1,2-dimethoxyethane) and excess methyl iodide, alkyl iodide, allyl bromide, or benzylbromide results in the one-pot hexasubstitution (Scheme 11. la).5,13,14 With allyl bromide (or iodide) in DME, the hexaallylated... 

We postulated that In the calcium form pure LSX should have about a 20% Increase In N2 capacity due to the Increased number of accessible calcium Ions. Figure 7 Illustrates a typical faujaslte structure Including the location of selected accessible and Inaccessible cation sites. The accessible cation content Increases 1n CaLSX. Standard CaX zeolite (S1/A1 = 1.25) has 43 calcium Ions per unit cell, whereas LSX has 48. In standard dehydrated CaX, 16 of the calcium Ions are In N2-1naccess1ble (site I) positions.(18) Assuming that the same siting occurs In LSX, there are 32 N2 accessible calcium Ions 1n LSX, compared to only 27 In a standard X. This results 1n an 18.5% Increase In the number of accessible cations. This Increase has a direct effect on the N2 equilibrium capacity which for pure LSX showed a 20% Increase over conventional X zeolites. Furthermore, compared to commercially available X adsorbents which all contain binders, the self-bound LSX adsorbents do not have any binder to "dilute" the active component and lower the gas capacity. 

Admission of hydrogen at 575 K to a dehydrated sample causes a strong increase of both acidic OH groups due to the reduction of Cu(II) ions to Cu(I) accompanied by the formation of protons (Fig. 3). Simultaneously the amount of Lewis acid sites decreases, observable by the corresponding IR bands of adsorbed pyridine (Fig. 2, intermediate spectrum). The low-frequency band of the 1455 cnT doublet is reduced to a shoulder, indicating a loss of accessible cations, probably the Cu(II) ions. 

By using supported NaX zeolite membranes Caro and eo-workers" found that considering equimolar mixtures at 23 °C, CO2 is the less permeable gas with respect to nitrogen. The lower permeance of the earbon dioxide ean be explained by the electrostatic interaction of the quadrupolar molecules with the accessible cations of the NaX zeolite. NaX and NaY zeolites have the same topology but different Si/Al ratios (NaX from 1 to 1.5 NaY higher than 1.5) and as a result the X structure has a number of Na" cations higher than NaY." ... 

The fraction of accessible cations was only about 5% for the li-sample and 3.6% for the Na-sample. The two figures being not far from each other, a comparable number of cationic sites are actually available on the two samples, notwithstanding the different cationic content. It thus appears that a remarkable fraction of cations are not readily reached by incoming molecules, as it happens with some zeolitic systems, e.g., cages of which are not readily accessed. 

In the original method Trj was 650 K, Tr2 was varied from 730 K to 830 K and r was 4 K min l (ref.26). NaY zeolites were thus dealuminated to contain between 4 and 9 Al atoms per unit cell. In another study (ref.27), Trj was about 520 K. A temporary temperature rise between 30 and 70 K occurred during the first minutes of the contact of NaY zeolite with SiCl4 vapor. Only when the reactor temperature had reached its original value of 520 K, the zeolite bed was heated at a rate of 10 K min l to Tr2 values between 600 K and 745 K. This way samples with Al contents between 16 and 2 Al/UC were produced (Table 2, Nos. 24-29). In the same work (ref.27) it was stated that in order to avoid structural damage to the zeolite, the heating to Tr2 should only be started when the exothermicity is over. In view of the reaction mechanism advanced earlier, this means that a simultaneous attack of aluminium atoms associated with hidden and accessible cations causes a too violent reaction and, consequently, lattice destruction and has to be avoided. 

In literature samples have been reported with very high Si/AI ratios (refs.1,4 and 5). Such samples can be obtained if after the first step, which necessarily has to be performed at low reaction temperature, the temperature is gradually increased in presence of SiCl4 to values of 720 or 730 K. Under such conditions the less accessible cations and associated framework aluminium atoms probably become reactive. 

Table 7. Number of accessible cations and of aluminium atoms extracted with...

Another possible explanation is that 2-methyl-2-propyl cation allows better access to solvent than adamantyl cation. Examine hydrates of 2-methyl-2-propyl and adamantyl cations. How many water molecules does each accomodate Calculate hydration energies for the two cations. (The energy of water is provided at left.)... 

Additional control of the nucleophilic substitution pathways a and b should be possible by varying the properties of the heteroarylium moiety in 33 as well as the substituent R and, to a minor extent, by the nature of the C-bonded halogen. Tire cation of 7a appeared to be an especially useful model compound and was thus selected in order to systematically study these influences and to define a standard situation. Structure 7a is easily accessible in excellent yield, and its molecular size allowed high-level MO calculations. 

By far the most common starting material is l-methylimidazole. This is readily available at a reasonable cost, and provides access to the majority of cations likely to be of interest to most researchers. There is only a limited range of other N-substituted imidazoles commercially available, however, and many are relatively expensive. The synthesis of l-alkylimidazoles may be achieved without great difficulty, though, as indicated in Scheme 2.1-2. 

Fig. 11. Peptide moiety indicating the monopoles (net charges) on each of the atoms 54 551. This demonstrates that, on the basis of size, charge and accessibility, a polypeptide could more effectively provide selectivity for cations...

The key features of Pirrung s synthesis of isocomene are outlined retrosynthetically in Scheme 1. Working one step back from 1 gives the tertiary carbocation intermediate 2. The intermediacy of 2 should be brief, for it should readily participate in an Ei-type reaction, in the forward sense, to give isocomene (1). Inspired by the observation that bicyclo[3.3.0] frameworks can be accessed from bicyclo[4.2.0] frameworks through cyclobutyl carbinyl cation rear-... 

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