Ring substitution, catalytic activity

Titanium Silicates. A number of titanium siUcate minerals are known (160) examples are Hsted in Table 19. In most cases, it is convenient to classify these on the basis of the connectivity of the SiO building blocks, eg, isolated tetrahedra, chains, and rings, that are typical of siUcates in general. In some cases, the SiO units may be replaced, even if only to a limited extent by TiO. For example, up to 6% of the SiO in the garnet schorlomite can be replaced by TiO. In general, replacement of SiO by TiO bull ding blocks increases the refractive indices of these minerals. Ti has also replaced Si in the framework of various zeofltes. In addition, the catalytic activity of both titanium-substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) has received attention (161), eg, the selective oxidation of phenol, with hydrogen peroxide, to hydroquinone and catechol over TS-1 has been operated at the 10,000 t/yr scale in Italy (162). 

The ease of dehalogenation of C H X by Ni(ll)/ IMes HCl 1/NaO Pr decreased in the order 1 > Br > Cl F. Subsequent work showed that a 1 1 combination of Ni and NHC in the presence of NaOCHEt resulted in enhanced reactivity towards aryl fluorides [6], Again, the A-mesityl substituted ligand IMes HCl 1 imparted the highest level of catalytic activity. Table 8.2 illustrates that hydrodefluorination is sensitive to both the nature of the substituents on the aromatic ring and the specific regioisomer. Thus, 2- or 4-fluorotoluene (Table 8.2, entry 2) proceeded to only 30% conversion after 15 h, whereas quantitative conversion of 2-fluoroanisole (Table 8.2, entry 3) and high conversion of 3-fluoropyridine (Table 8.2, entry 5) was achieved in only 2-3.5 h. The reactivity of 2-fluoropyridine was compromised by more efficient nucleophilic aromatic substitution. 

The percent ring substitution (% RS) of the polymer with active sites affects catalytic activity. Polystyrenes with < 25 % RS with lipophilic quarternary onium ions are swollen in triphase mixtures almost entirely by the organic phase. Water reduces the activity of anions by hydrogen bonding. In most triphase nucleophilic displacement reactions onium ion catalysts with <25% RS are highly active, and those with >40% RS, such as most commercial ion exchange resins, are much less active. However, low % RS is not critical for the reactions of hydroxide ion with active methylene compounds, as commericial ion exchange resins work well in alkylation of active nitriles. 

The % ring substitution of the polymer is a critical factor in catalytic activity. Its importance was demonstrated clearly in Regen s first full paper on triphase catalysis 89). Catalysts 2 and 13 (2% CL) were active for cyanide displacement on 1-bromooctane (Eq. (3)) only at 21 % or lower RS (Table 3). Commerical anion exchange resins, polystyrenes highly substituted as benzyltrimethylammonium ions 2 or benzyldimethyl-(2-hydroxyethyl)ammonium ions 14, were inactive. 

The modified polymer 70a with the larger concave pyridine was then tested in the base catalyzed addition of ethanol to diphenylketene (59a) and proved to be catalytically active [17]. To compare the polymer bound concave pyridine 70a to the corresponding free concave pyridine 3k (MeO-substituted in 4-position of the pyridine ring), the same quantities of pyridine units were used, in solution (3k) or in suspension (70a) respectively. The polymer bound catalyst 70a catalyzed 2-3 times slower than the analogous 4-methoxy-substituted... 

Substitution of the hydrogen atoms in the four benzene rings of Pc by chlorine produces only a slight change in the catalytic activity. 

In aromatic systems, the Lewis acids which activate via coordination are also capable of activating the aromatic system by the formation of a and ir complexes. There are a sufficient number of examples available to indicate that the activation via the latter processes is the more important of these, where all are present. Olivier (52) showed in 1913 that the kinetic behavior of such reactions consists of two portions. When the catalyst, say aluminum chloride, is present in less than the amount required to complex all the functional groups, the reaction is relatively slow and the catalytic activity is due to the small amount of Lewis acid resulting from the dissociation of the complex. As soon as all the functional groups are coordinated, any additional Lewis acid is found to accelerate the rate enormously. In these electrophilic substitutions it seems highly probable that the the activation involves the pi electron system of the benzene ring. Olivier studied the reaction sequence ... 

The effect of substituting a chloro, methyl, or ferf-butyl group for the para-hydrogens of the phenyl rings of Schiff base 26 has been studied with regard to their catalytic activities in the oxygenation of... 

The amine product plays a critical role, as it deprotonates the H2 complex 18 to regenerate the catalytically active 17. This also prevents side reactions based on nucleophilic ring opening of the aziridinium cations by the amine products. Alkyl substituted aziridinium cations react via a classical SN2-mechanism. 

From a mechanistic point of view (Scheme 3), a transition metal carbyne undergoes a [2+2] cydoaddition with an alkyne to form a metallacydobutadiene as an intermediate, or possibly as a transition state. The four-membered ring then cycloreverts, expelling a di-substituted alkyne with regeneration of a catalytically active carbyne complex. In a subsequent... 

---