Aldehyde cyclic oligomer

Aldehydes can add to themselves only occasionally (Section 7.2.3). If so, cyclic oligomers or acyclic polymers such as paraformaldehyde are formed. 

The mechanism of the reaction has been studied in some detail by Hogberg i2,7i,72) jn contrast to the base-catalyzed oligomerization, the acid catalyzed process involves electrophilic aromatic substitutions by cations, as outlined in Fig. 8. Although formaldehyde does not react with resorcinol to produce cyclic oligomers, other aldehydes such as acetaldehyde and benzaldehyde give excellent yields of... 

In bulk copolymerizations ([PO] = 1.9 mol -l-1, [THF], = 10.7 mol 1 1), the concentration of cyclic PO tetramer did not exceed 4%. The proportion of cyclic oligomers may be further decreased i.e., to about 1 %, by addition of propionic aldehyde in a concentration comparable to that of [PO]q. Addition of propionic aldehyde also influences the molecular-weight distribution in the presence and absence of aldehyde, Mw/Mn 2 and 4, respectively. 

Resorcinarene A cyclic oligomer (usually a tetramer) arising from the condensation of resorcinol and an aldehyde under acidic conditions. Other synonyms are resorcarene, calixresorcarene, calixresorcinarene, as well as resorcin-[n]-arene or calix-[n]-resorcinarene, where [n] specifies the number of monomers in the oligomer. 

In the 1970s Cram began to develop cavitands [7]. Cavitands are structurally preorganized, which enhance the van der Waals contacts between the enforced cavities and the guest molecules. In 1983, Cram introduced the word carce-plex to define molecular systems that can only release the entrapped guest by rupture of a covalent bond [88]. Calixarenes are macrocycles or cyclic oligomers based on the hydroxyalkylation product of a phenol and an aldehyde, and are a particular case of cavitands, which have been developed in the early 1980s (Fig. 13.8) [89]. 

Resorcinarenes are typically synthesized from the acid-catalyzed condensation of resorcinol and an aldehyde (Scheme 9.1). Br0nsted acids, such as HCl, are most common [6,7], but Cabaleiro showed that a range of Lewis acids also work [8], and allow acid-sensitive aldehydes to be used in this macrocyclization [9]. Unlike their calixarene cousins, resorcinarenes are primarily available as cyclic tetramers. This macrocycle is the thermodynamic product, and their relative insolubility means that they can be precipitated as the sole product of a condensation process producing a multitude of acyclic and cyclic oligomers. By inducing precipitation early on in the reaction, higher resorcinarenes as kinetic products can be isolated, albeit in low yields [10]. 

Smaller aldehydes form cyclic acetal-type oligomers readily in aqueous conditions.60 Diols and polyols also form cyclic acetals with various aldehydes readily in water, which has been applied in the extraction of polyhydroxy compounds from dilute aqueous solutions.61 E in water was found to be an efficient catalyst for chemoselective protection of aliphatic and aromatic aldehydes with HSCH2CH2OH to give 1,3-oxathiolane acetals under mild conditions (Eq. 5.7).62... 

Reactions and Uses. Ihe common reactions that o hydroxy acids undergo such as self- or bimolecular estcrihcation to oligomers or cyclic esters, hydrogenation, oxidation, etc. have been discussed in connection with lactic and hydroxyacetic acid A reaction that is of value lor the synthesis of higher aldehydes is tleearbonylation under boiling sulfuric-acid with loss of waler. 

Fig. 8 (a) Reversible formation of cyclic and linear acetal oligomers from triethylene glycol and 4-nitrobenzaldehyde. (b) Reversible formation of hemithioacetal (HTA) from reaction between a thiol and an aldehyde derivative... 

Most unsaturated substances such as alkenes, alkynes, aldehydes, acrylonitrile, epoxides, isocyanates, etc., can be converted into polymeric materials of some sort—either very high polymers, or low-molecular-weight polymers, or oligomers such as linear or cyclic dimers, trimers, etc. In addition, copolymerization of several components, e.g., styrene-butadiene-dicyclo-pentadiene, is very important in the synthesis of rubbers. Not all such polymerizations, of course, require transition-metal catalysts and we consider here only a few examples that do. The most important is Ziegler-Natta polymerization of ethylene and propene. 

The main chains of polyacetals consist of strictly alternating carbon-oxygen bonds of the type —CHR—O—. They are produced by polymerization of aldehydes or their cyclic trimers. The cyclic trimers and tetramers of formaldehyde are, respectively, known as trioxane and tetroxane. The formaldehyde oligomer with 6-10 monomeric units produced spontaneously in aqueous formaldehyde solutions is known as paraformaldehyde. The cyclic trimer of acetaldehyde is called paraldehyde and the cyclic tetramer is called metaldehyde. 

Formaldehyde f6r- mal-d9- hld, fer- [ISV form- + aldehyde] (1872) (formic aldehyde, methanal, oxymethylene) n. HCHO. A colorless gas with a pungent, suffocating odor, obtained most commonly by the oxidation of methanol or low-boiling petroleum gases such as methane, ethane, etc. The gas is difficult to handle, so it is sold commercially in the form of aqueous solutions (formalin), solvent solutions, as its oligomer, paraformaldehyde, and as the cyclic trimer, 1,3,5-trioxane (a-trioxym-ethyl-ene). High-molecular-weight, commercial polymers of formaldehyde are called poly-oxymethylene or acetal resin. Formaldehyde is also used in the production of other resins such as phenolic resin (phenol-formaldehyde) and amino resin (urea formaldehyde). Syn methylene oxide, methanal. See formalin. 

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