Imidazole, catalysts

Acylation with acetic anhydride and imidazole catalyst ... 

Mixtures of PEI/epoxy monomer and PEI/BPACY monomer were prepared by dissolving PEI and thermoset monomers in methylene chloride at room temperature and evaporating the solvent. The epoxy monomer/PEI solution was heated on a hot plate to 140°C, and stoichiometric amounts of DDS were added while stirring for about 7 min. For the catalyzed system, 2-methyl imidazol catalyst was also added. 

Heat resistant resin compositions based on BMI/aminophenol-Epoxy blends are achieved by reacting a BMI/p-aminophenol 1 1 adduct with epoxy resin (62). Both the secondary amine and phenol functionality may react with the epoxy resin and subsequently cure through an imidazole catalyst. Imidazole catalysts promote both the epoxy/phenol reaction and the anionic maleimide crosslinking. The formation of a 1 2 BMI/aminophenol adduct, as in Fig. 20, is claimed in a patent (63). The hydroxy terminated BMI/aminophenol adduct is an advantageous curing agent for epoxy resins when high temperature performance is desired. 

Both primary and secondary hydroxyl groups react rapidly [389, 408-412] under classical Corey conditions [388] with rm-butylchlorodimethylsilane in DMF in the presence of imidazole catalyst. The primary hydroxyl group reacts preferentially, the ratio of products being dependent on the amount of imidazole present. For thymidine, the highest, ca. 75% yield of 5 -ether has been obtained [389,408] with a ratio of nucleoside /ert-butylchlorodimethylsilane imidazole near 1 1.1 2.2. Similar results were obtained with other deoxyribonucleosides [389, 408], as well as with uridine [410,411, 413], adenosine [410,411], cytidine [414], guanosine [414], and their... 

Latent imidazole catalysts have also been developed to provide cure rates considerably faster than those of dicyandiamide cured epoxy resins.18 They also exhibit excellent adhesive characteristics and heat and chemical resistance. 

A unique feature of these imidazole catalysts is that they do not have the high exotherm that dicyandiamide produces when cured in epoxy resins. Thus, they do not char or burn when exposed to high cure temperatures for fast cure. This is an important factor for adhesives that are cured via induction or dielectric heating. These adhesive systems are also much safer to ship via ah freight than conventional dicyandiamide catalyzed epoxy formulations due to their low exotherm. 

Piperazine (12) gave l,4-bis(nitroacetyl)piperazine (13) [Me02CCH2N02, imidazole (catalyst), EtOH, reflux, 90 min 46%].1113... 

Table 17. Hydiolyses of aiyl acetates by imidazole catalysts...

The small exotherm at 117°C preceding the main exotherm at 154°C is dependent on the concentration of phthalic anhydride it appearing at higher concentrations of phthalic anhydride. A mixture of phthalic anhydride and imidazole catalyst with no epoxy resin present shows a broad, low exotherm starting at 125°C. This suggests that anhydride ring opening occurs as the first step in the process. 

Typical resists include cyclized polyisoprene with a photosensitive crosslinking agent (ex bisazide) used in many negative photoresists, novolac resins with diazoquinone sensitizers and imidazole catalysts for positive photoresists, poly(oxystyrenes) with photosensitizers for UV resists, polysilanes for UV and X-ray resists, and polymethacrylates and methacrylate-styrenes for electron-beam resists (Clegg and Collyer, 1991). Also note the more recent use of novolac/diazonaphthoquinone photoresists for mid-UV resists for DRAM memory chips and chemically amplified photoacid-catalysed hydroxystyrene and acrylic resists for deep-UV lithography (Choudhury, 1997). 

In the hydrolysis of diethylphenyl orthoformate no general base catalysis is observed. The steric effects of substituents on the imidazole catalyst are very small, and the activation parameters have been shown to have large negative entropies. These observations, in conjunction with a solvent isotope effect, point to participation of water molecules in the transition state, i.e., at least one water molecule intervenes between imidazolium ion and the ortho ester (see (66)). General acid catalysis is assumed <87JCS(P2)669>. 

Three different organic bases, imidazole (IM), triazole (TZ) and hydrogenated 1,8-diaza-bicyclo(5.4.0) undecene-7 (DBU) were immobilized on mesoporous materials. Immobilized state of organic bases and their catalytic activities in Knoevenagel condensation between ethylcyanoacetate and benzaldehyde was studied. Organic bases immobilized mainly on mesopore still retained their catalytic activities with an inevitable loss due to immobilization. Imidazole catalyst immobilized on KIT-1 disordered mesoporous material with three-dimensional channels showed a sufficient activity for multi-repeated use. 

We also examined aldol condensations of the dialdehyde 17. Without the special catalysis afforded by the cyclodextrin i w-imidazoles there was an almost random reaction to form compounds 18 and 19, as either aldehyde acted as the enofizing group. However, the cyclodextrin imidazole catalysts directed the selective formation of products 19, with no selectivity among its stereoisomers. Interestingly, the least selective catalyst for 17 was the cyclodextrin mono-imidazole, the AD isomer of the fcw-imidazole was more selective, and the most selective was the AB isomer. Obviously these results indicate that the cyclodextrin imidazole catalysts promote enolization of the aldehyde group closest to the cyclodextrin, as expected, but the subtlety of preferences among the bw-imidazole isomers is not yet understood in this case. 

T. Kunitake, H. lhara, Y. Okahata, Phase-separation and reactivity changes of phenyl ester substrate and imidazole catalyst in the dialkylammonium bilayer-membrane, J. Am. Chem. Soc, 1983, 105, 6070-6078. 

Underfill adhesive (rapid cure) 30-60% Bisphenol F epoxy reaction product, 15% imidazole catalyst, 30-60% silica filler Loctite 3563 (Henkel)... 

Hoveyda and Snapper developed an excellent method for asymmetric silylation by employing a newly developed chiral imidazole catalyst, 45 (Scheme 22.10). By virtue of catalyst 45, silylative asymmetric desymmetrisation of meso-1,l-dioX 46 (Scheme 22.10A) and a-symmetric triol 48... 

Peptide-based imidazole catalyst 63 in the presence of L-proline as a cocatalyst promoted asymmetric Morita-Baylis-Hillman reactions between methyl vinyl ketone and aromatic aldehydes (Scheme 22.14). The combination... 

Scheme 22.10 Asymmetric silylation promoted by Hoveyda-Snapper s chiral imidazole catalyst 45...

The pH vs rate profile showed a bell-shaped curve indicating that this catalyst uses both B and BH+ in a bifunctional mechanism. As with the enzyme, the bis-imidazole catalyst can perform its bifunctional catalysis by a simultaneous mechanism, not the sequential mechanism of simple buffer catalysis. We saw that this was indeed the case, as revealed by the tool called "proton inventory." In this technique the reaction is performed in D2O, in H2O, and in mixtures of the two. If only one proton that can exchange with D2O is moving in the transition state, the points all lie on a straight line between the H2O and slower D2O points. If two (or more) protons are moving, the line is curved. It had been found for the enzyme ribonuclease A [10] that a curved line was seen corresponding to the movement of two protons, and we also saw a curved plot—with very similar data— for our cyclodextrin-6A,6B-bisimidazole catalyst 6 [11]. Controls established that indeed this was a reliable indication that our system is performing simultaneous bifunctional catalysis, just as the enzyme does. In particular, the... 

Polymeric imidazole catalysts are the synthetic prototype of esterolytic enzymes because, like the enzymes, they have binding centers for the substrate as well as for the catalytic groups. In case of the homopolymer, polyvinylimidazole, the partially protonated groups can act as binding sites, provided the substrate is negatively charged. Thus at pH 7.5, when... 

Portnoy and Goren immobilized M-alkylated imidazoles on polystyrene-bound polyether dendrons via three different synthetic routes (Scheme 15.43). All these systems catalyzed the Baylis-Hillman addition of methyl vinyl ketones to aromatic aldehydes, displaying a very strong positive dendritic effect and a significant enhancement effect of water as a cosolvent. Remarkably, substrates that did not undergo the reaction with the nondendritic immobilized or soluble M-alkyl imidazole catalysts underwent a smooth reaction with one of the catalytic second-generation dendrons. 

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