Latent catalysts

Esters. Most acryhc acid is used in the form of its methyl, ethyl, and butyl esters. Specialty monomeric esters with a hydroxyl, amino, or other functional group are used to provide adhesion, latent cross-linking capabihty, or different solubihty characteristics. The principal routes to esters are direct esterification with alcohols in the presence of a strong acid catalyst such as sulfuric acid, a soluble sulfonic acid, or sulfonic acid resins addition to alkylene oxides to give hydroxyalkyl acryhc esters and addition to the double bond of olefins in the presence of strong acid catalyst (19,20) to give ethyl or secondary alkyl acrylates. 

Pyrovatex CP coreacts on cellulose with an amino resin in the presence of a latent acid catalyst, to produce finishes durable to laundering (125,126). A higher assay version, Pyrovatex (CP New, has also been introduced. 

Hexamethylenetetramine. Hexa, a complex molecule with an adamantane-type stmcture, is prepared from formaldehyde and ammonia, and can be considered a latent source of formaldehyde. When used either as a catalyst or a curative, hexa contributes formaldehyde-residue-type units as well as benzylamines. Hexa [100-97-0] is an infusible powder that decomposes and sublimes above 275°C. It is highly soluble in water, up to ca 45 wt % with a small negative temperature solubiUty coefficient. The aqueous solutions are mildly alkaline at pH 8—8.5 and reasonably stable to reverse hydrolysis. 

An important aspect of this procedure is the use of latent acid catalysts, such as phenyl hydrogen maleate, phenyl trifluoracetate, and butadiene sulfone. These catalysts reduce the peak exotherm from over 200°C to 130—160°C. The resin catalyst mixture has a working life of up to several days at RT. The elevated temperature of mol ding these latent catalysts generates the corresponding acids, namely, maleic, trifluoracetic, and phenolsulfonic, which cataly2e the resole reaction. Typically, a cycle time of 1—2 min is requited for a mold temperature of - 150° C. 

In order to obtain a sufficient rate of cure at moulding temperatures it is usual to add about 0.2-2.0% of a hardener (accelerator). This functions by decomposing at moulding temperatures to give an acidic body that will accelerate the cure rate. A very large number of such latent acid catalysts have been described in the literature, of which some of the more prominent are ammonium sulphamate, ammonium phenoxyacetate, ethylene sulphite and trimethyl phosphate. 

Urea-formaldehyde powders have a limited shelf-life but some improvement is made by incorporating a stabiliser such as hexamine into the moulding power. In some formulations the cure rate and the related time for flow are controlled by keeping the latent acid catalyst fixed and adjusting the stabiliser. 

Stork and Takahashi took -glyceraldehyde synthon from the chiral pool and condensed it with methyl oleate, using lithium diisopropyl amide as catalyst for the mixed aldol reaction, leading to The olefinic linkage is a latent form... 

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

The diazo function in compound 4 can be regarded as a latent carbene. Transition metal catalyzed decomposition of a diazo keto ester, such as 4, could conceivably lead to the formation of an electron-deficient carbene (see intermediate 3) which could then insert into the proximal N-H bond. If successful, this attractive transition metal induced ring closure would accomplish the formation of the targeted carbapenem bicyclic nucleus. Support for this idea came from a model study12 in which the Merck group found that rhodi-um(n) acetate is particularly well suited as a catalyst for the carbe-noid-mediated cyclization of a diazo azetidinone closely related to 4. Indeed, when a solution of intermediate 4 in either benzene or toluene is heated to 80 °C in the presence of a catalytic amount of rhodium(n) acetate (substrate catalyst, ca. 1000 1), the processes... 

Ladder polymer, synthesis of, 503 Lanthanide catalysts, 288 Lanthanide compounds, 73 Latent-reactive polymers, 455 Laurolactam, 136... 

When acidic or latent acidic excipients (anhydrides) are incorporated into the polymer to control erosion rate, the polymers become quite sensitive to moisture and heat and must be processed in a dry environment. A rigorous exclusion of moisture is particularly important with materials that are designed to erode in less than 24 hr. Such materials may contain up to 5 wt% of an acidic catalyst and are analogous to a "loaded gun" in that even the slightest amount of moisture will initiate hydrolysis at the elevated processing temperatures. ... 

These new sulfur bearing complexes can potentially be used as a thermo-switchable latent catalyst. 

There are reactive softeners, some of which are N-methylol derivatives of long-chain fatty amides (10.241) while others are triazinyl compounds (10.242). The N-methylol compounds require baking with a latent acid catalyst to effect reaction, whereas dichloro-triazines require mildly alkaline fixation conditions. The N-methylol compounds are sometimes useful for combination with crease-resist, durable-press, soil-release and water-repellent finishes. In this context, the feasibility of using silane monomers such as methyltri-ethoxysilane (10.243), vinyltriethoxysilane (10.244), vinyl triace tylsilane (10.245) and epoxypropyltrimethoxysilane (10.246) in crosslinking reactions to give crease-resist properties and softness simultaneously has been investigated [492]. 

A more soluble derivative of compound 11.17, the tetrasulphonated analogue 11.18, has been recommended for application to cotton in combination with a resin finish. Unlike DAST-type FBAs under these conditions, compound 11.18 is compatible with resin formulations containing zinc nitrate as latent acid catalyst. The brightness achieved is not high, however. 

Many of the properties of a polymer depend upon the presence or absence of crystallites. The factors that determine whether crystallinity occurs are known (see Chapter 2) and depend on the chemical structure of the polymer chain, e.g., chain mobility, tacticity, regularity and side-chain volume. Although polymers may satisfy the above requirements, other factors determine the morphology and size of crystallites. These include the rate of cooling from the melt to solid, stress and orientation applied during processing, impurities (catalyst and solvent residues), latent crystallites which have not melted (this is called self-nucleation). 

The principles needed to design a polymer of low flammability are reasonably well understood and have been systematized by Van Krevelen (5). A number of methods have been found for modifying the structure of an inherently flammable polymer to make it respond better to conventional flame retardant systems. For example, extensive work by Pearce et al. at Polytechnic (38, 39) has demonstrated that incorporation of certain ring systems such as phthalide or fluorenone structures into a polymer can greatly increase char and thus flame resistance. Pearce, et al. also showed that increased char formation from polystyrene could be achieved by the introduction of chloromethyl groups on the aromatic rings, along with the addition of antimony oxide or zinc oxide to provide a latent Friedel-Crafts catalyst. 

An alternative approach has been formulated by Katz and co-workers (28). They consider the latent cis specificity in reactions of c/ s-olefins with metathesis catalysts to be quite high, based on their results with a catalyst having very low Lewis acidity. They further propose that the frequently observed lack of stereospecificity is mainly the consequence of cationic processes inherent in the metathesis step which mask stereospecificity. 

Enones have also served as latent enols in the hydroalkoxylation process. For example, an Au(m) catalyst has been used to effect the conjugate addition of alcohols (or other nucleophiles) to a,/ -unsaturated ketones, thereby triggering a hydroalkoxylation pathway of the resulting enol to furnish furans as products. (Equation (98)). 

For a comprehensive review of Sn(II) catalyst systems, see Nelson s review article, Catalyzed Enantioselective Aldol Additions of Latent Enolate Equivalents.51... 

Latchup mode, 22 252 Latent acid catalysts, in liquid-injection molding, 18 795... 

This procedure describes the preparation and use of an effective chiral catalyst for the asymmetric allylation of aldehydes. A previous synthesis of optically pure 1-(phenylmethoxy)-4-penten 2-ol requires seven steps from D-mannitol.4 This procedure has been employed successfully with other aldehydes,5 and also with methallyltributylstannane5 (see Table). Catalysts prepared from (R)- or (S)-BINOL and Ti(0-i-Pr)4 at 2 1 stoichiometry have also proven useful in these reactions.The olefinic products may be regarded as latent aldol products between aldehydes and the enolate of actetaldehyde or acetone. In all cases examined thus far, enantioselectivity... 

A review of enantioselective aldol additions of latent enolate equivalents covers a variety of Sn", boron, Ti, Cu, lanthanide, and Lewis base catalysts. Asymmetric aldol reactions using boron enolates have been reviewed (401 references). ... 

Coulembier O, Lohmeijer BGG, Dove AP, Pratt RC, Mespouille L, Culkin DA, Benight SJ, Dubois P, Waymouth RM, Hedrick J (2006) Alcohol adducts of N-heterocyclic carbenes latent catalysts for the thermally -controlled living polymerization of cyclic esters. Macromolecules 2006 5617-5628... 

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