Cross-condensation products

The cross-condensation reaction of benzyl benzoate (46) and 44 was carried out under solvent-free conditions. Treatment of a 1 1 mixture of 46 and 44a with Bu OK at 120 °C for 1 h gave the cross-condensed product 47a in 42% yield (Scheme 7). Similar reaction of 46 with 44b gave 47b in 45% yield. Because heating of46,44, and Bu OK in toluene under reflux for 16 h did not give any product, it is clear that the solvent-free reaction is again effective for the cross-condensation. In these cases, self-condensation of 44 itself did not occur probably because of the high reactivity of 46 [9]. 

Another important example is the cross-aldol condensation of citral and acetone, which yields pseudoionone (Scheme 14), an intermediate in the commercial production of vitamin A. Numerous commercial routes to the preparation of pseu-doionones are based on the aldol condensation using conventional homogeneous catalysts, such as aqueous alkali metal hydroxide solutions, alcoholates in alcohol or benzene solvents (126-129). The yields of the cross-condensation product vary between 50% and 80%, depending on the type of catalyst and conditions such as catalyst concentration, ratio of reagents, and temperature. 

The nitridation of phosphate precursors (AIPO4 and Alo.5Gao.5PO4) has a positive effect on the selectivity to cross-condensation product. Maximum selectivities are obtained over samples with intermediate nitrogen contents. Selective poisoning of the acid and the basic sites, by adding tripropylamine and benzoic acid respectively in the reactor showed that both acid and basic sites are responsible for the catalytic activity of Al(Ga)PO(N) catalysts. 

Similar effects have been described in the literature for the condensation of formaldehyde with acetaldehyde. (1) Dimitriu et al. [9] concluded that strong acid catalysts favor the acetaldehyde selfcondensation (but-2-enal). (2) Ai et al. showed that catalysts possessing weak acid-base conjugated sites favor the cross condensation product (propenal) [10]. 

Without this control, each aldehyde would self-condense and would condense with the other aldehyde giving four products in unpredictable amounts. One of the cross-condensation products is, of course, the enal we are trying to make. 

A unique feature of in situ encapsulation technology is that polymerization occurs ia the aqueous phase thereby produciag a condensation product that deposits on the surface of the dispersed core material where polymerization continues. This ultimately produces a water-iasoluble, highly cross-linked polymer capsule shell. The polymerization chemistry occurs entirely on the aqueous phase side of the iaterface, so reactive agents do not have to be dissolved ia the core material. The process has been commercialized and produces a range of commercial capsules. 

Uses. About 35% of the isophthahc acid is used to prepare unsaturated polyester resins. These are condensation products of isophthahc acid, an unsaturated dibasic acid, most likely maleic anhydride, and a glycol such as propylene glycol. The polymer is dissolved in an inhibited vinyl monomer, usually styrene with a quinone inhibitor. When this viscous hquid is treated with a catalyst, heat or free-radical initiation causes cross-linking and sohdification. A range of properties is possible depending on the reactants used and their ratios (97). 

Epoxy cross-linking is cataly2ed by TYZOR TPT and TYZOR TBT, alone or with piperidine, and by TYZOR TE. The soHd condensation product from 3 TPT 4 TEA (triethanolamine) has also been appHed to epoxy curing (490). Titanate curing is accelerated by selected phenoHc ethers and esters at 150°C the mixtures have along pot life at 50°C (491) (see Epoxyresins). 

Amino Resins. Amino resins (qv) include both urea- and melamine—formaldehyde condensation products. They are thermosets prepared similarly by the reaction of the amino groups in urea [57-13-6] or melamine [108-78-1] with formaldehyde to form the corresponding methylol derivatives, which are soluble in water or ethanol. To form plywood, particle board, and other wood products for adhesive or bonding purposes, a Hquid resin is mixed with some acid catalyst and sprayed on the boards or granules, then cured and cross-linked under heat and pressure. 

Trimethylolpropane (TMP), the reduced crossed aldol condensation product of //-butyraldehyde and formaldehyde, competes in many of the same markets as glycerol (qv) and pentaerythritol. The largest market for TMP is as a precursor in unsaturated polyester resins, short-oil alkyds, and urethanes for surface coatings (see Alkyd resins). 

The principal markets for neopentyl glycol (NPG), the hydrogenated, crossed aldol condensation product of isobutyraldehyde and formaldehyde, are in water-borne and alkyd-surface coatings. 

Isobutyl isobutyrate, the Tischenko condensation product of two molecules of isobutyraldehyde, is a slow evaporating ester solvent that has been promoted as a replacement for ethoxyethyl acetate. Although produced primarily by the acetylation of isobutyl alcohol, some isobutyl acetate is produced commercially by the crossed Tischenko condensation of isobutyraldehyde and acetaldehyde. Isobutyl acetate [110-19-0] is employed mainly as a solvent, particularly for nitrocellulose coatings. 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

The application to a mixture of two different esters, each with a-hydrogens, is seldom of preparative value, since a mixture of the four possible condensation products will be obtained. If however only one of the starting esters contains a-hydrogens, the crossed Claisen condensation often proceeds in moderate to good yields. 

In conclusion, the self-condensation of 2-furaldehyde promoted by heat occurs with the formation of di- and trifurylic intermediates. The functionality of the growing chain increases after each oligomerization step until gelation and precipitation of the resin occurs. Thus, the process is non-linear from the onset since the condensation product 4 possesses three sites for further attack, namely the free C-5 position and the two formyl groups. It is interestering to note that while the polycondensation of 2-furfuryl alcohol is essentially linear and cross-linking is due to side reactions, the thermal resinification of 2-furaldehyde is intrinsically non-linear and gel formation occurs at earlier conversions. 

Formation of intra- and interchain cross-links via Schiff bases and aldol condensation products... 

Reactions involving a crossed anodic condensation are of practical interest when they combine organic radicals of different type. In solutions containing anions RCOO and R COO, condensation products of the type RR are formed together with the standard products RR and R R by the reactions described. 

Other unsymmetrical aminoporphyrazines have been prepared accordingly. The base-catalyzed cross-condensation of bis(dimethylamino)maleonitrile (99) with 1,2-dicyanobenzene (132) produced a statistical mixture of six possible products that represent hybrids between a heterosubstituted aminoporphyrazine and pc (9). The individual yields of all six products depend on the molar ratio of both starting dinitriles. For instance, a 25 1 ratio of dinitriles 99 and 132 produced mainly pz 133 (35%) along with 100 (Scheme 23). 

Pentaerythritol, used in the production of alkyds, is produced by a crossed Cannizzaro reaction of the aldol condensation product of formaldehyde and acetaldehyde. The by-product formate salt is a major source of formic acid. 

Several cross-aldol condensations have been performed with alkaline earth metal oxides, including MgO, as a base catalyst. A general limitation of the cross-aldol condensation reactions is the formation of byproducts via the self-condensation of the carbonyl compounds, resulting in low selectivities for the cross-aldol condensation product. For example, the cross-condensation of heptanal with benzalde-hyde, which leads to jasminaldehyde (a-K-amylcinnamaldehyde), with a violet scent... 

The photo-cross-linkability of a polymer depends not only on its chemical structure, but also on its molecular weight and the ordering of the polymer segments. Vinyl polymers, such as PE, PP, polystyrene, polyacrylates, and PVC, predominantly cross-link, whereas vinylidene polymers (polyisobutylene, poly-2-methylstyrene, polymethacrylates, and poly vinylidene chloride) tend to degrade. Likewise, polymers formed from diene monomers and linear condensation products, such as polyesters and polyamides, cross-link easily, whereas cellulose and cellulose derivatives degrade easily. ... 

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