Aldehydes, dimerization

When catalyzed by acids, low molecular weight aldehydes add to each other to give cyclic acetals, the most common product being the trimer. The cyclic trimer of formaldehyde is called trioxane, and that of acetaldehyde is known as paraldehyde. Under certain conditions, it is possible to get tetramers or dimers. Aldehydes can also polymerize to linear polymers, but here a small amount of water is required to form hemiacetal groups at the ends of the chains. The linear polymer formed from formaldehyde is called paraformaldehyde. Since trimers and polymers of aldehydes are acetals, they are stable to bases but can be hydrolyzed by acids. Because formaldehyde and acetaldehyde have low boiling points, it is often convenient to use them in the form of their trimers or polymers. 

Dimeric aldehydes analogous to (II) and (VI) have in fact been isolated from incubates of coniferyl alcohol with laccase (58). There are only 3% such aldehydic groups in lignin 4) [cf. Unit 10 in Fig. 9] but these suffice to give a intense red color with phloroglucinol and concentrated hydrochloric acid, the conventional Wiesner test for lignin. 

A saturated aldehyde was found to absorb hydrogen to yield a dimeric aldehyde thus, propionaldehyde formed a-methylvaleraldehyde. That this product was the result of a base-catalyzed aldol condensation was indicated by the finding that both propionaldol and its dehydration product, a-methylpentenal, yielded a-methylvaleraldehyde when hydrogenated in this catalyst system. 

Polyacetylene aldehydes undergo a curious dimerization with loss of carbon monoxide " - . The reaction occurs spontaneously when concentrated solutions of the aldehydes 230 are allowed to stand at room temperature giving both Z and E isomers of the dimeric aldehydes 231. p-Substituted 5-phenyl-2,4-pcntadiynals (232)... 

A sigruficant use is as a catalyst in a multitude of reactions, such as the formation of acrjdic and methacrjdic acid amides from fatty acid amides (81) of cychc ketones such as 4-phen5icyclopentane-l,2-dione and 2,5-dihydroxy-/)-benzoquinone from benzaldehyde diethji acetal and 2,4-dioxo-5,5-dimethoxy-hexanoic acid methyl ester (82) of dimer aldehydes from propane (83) and for the polymerization of epoxy compounds (84). Mag nesium Kthylate. Magnesium ethoxide [2414-98-4]y Mg(OC2H 2 niol wt 114.4, is an almost white hygroscopic powder density,... 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

A more recent method (31) is to prepare the Aj-thiazoline from the mercaptoacetaldehyde dimer, ammonia, and an aldehyde using Asinger s method (32). 

Only one exception to the clean production of two monomer molecules from the pyrolysis of dimer has been noted. When a-hydroxydi-Zvxyljlene (9) is subjected to the Gorham process, no polymer is formed, and the 16-carbon aldehyde (10) is the principal product in its stead, isolated in greater than 90% yield. This transformation indicates that, at least in this case, the cleavage of dimer proceeds in stepwise fashion rather than by a concerted process in which both methylene—methylene bonds are broken at the same time. This is consistent with the predictions of Woodward and Hoffmann from orbital symmetry considerations for such [6 + 6] cycloreversion reactions in the ground state (5). 

Reactions with Aldehydes and Ketones. The base-catalyzed self-addition of acetaldehyde leads to formation of the dimer, acetaldol [107-89-1/, which can be hydrogenated to form 1,3-butanediol [107-88-0] or dehydrated to form crotonaldehyde [4170-30-3]. Crotonaldehyde can also be made directiy by the vapor-phase condensation of acetaldehyde over a catalyst (53). 

Schiff s bases of etbyleneimine dimer are obtained from the reaction of aromatic aldehydes, eg, ben2aldehyde [100-52-7] or furfural [98-01-00] and ethyleneimine (228). 

Simple olefins do not usually add well to ketenes except to ketoketenes and halogenated ketenes. Mild Lewis acids as well as bases often increase the rate of the cyclo addition. The cycloaddition of ketenes to acetylenes yields cyclobutenones. The cycloaddition of ketenes to aldehydes and ketones yields oxetanones. The reaction can also be base-cataly2ed if the reactant contains electron-poor carbonyl bonds. Optically active bases lead to chiral lactones (41—43). The dimerization of the ketene itself is the main competing reaction. This process precludes the parent compound ketene from many [2 + 2] cyclo additions. Intramolecular cycloaddition reactions of ketenes are known and have been reviewed (7). 

Pyrazolones show a great variety of reactions with carbonyl compounds (B-76MI40402). For instance, antipyrine is 4-hydroxymethylated by formaldehyde and it also undergoes the Mannich reaction. Tautomerizable 2-pyrazolin-5-ones react with aldehydes to yield compound (324) and with acetone to form 4-isopropylidene derivatives or dimers (Scheme 8 Section 4.02.1.4.10). 

With l,3-dimethyl-2,l-benzisoxazolium salts, however, considerable reactivity has been reported. Condensation occurs readily with aldehydes, ketones, orthoesters and diazonium salts to yield styryl, cyanine and azo compounds, respectively (78JOC1233). In the presence of triethylamine, dimerization was observed, and the reactions of the cation were considered to involve the intermediacy of the anhydro base (77JOC3929). 

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. 

Although the catalysis of the dimerization of aldehydes to acyloins by thiazolium ion has been known for some tlrae, the development of procedures using anhydrous solvents which give satisfactory yields of acyloins on a preparative scale was first realized in the submitters laboratories. The mechanism proposed by Breslow - for the thiazolium ion-catalyzed reactions is similar to the Lapworth mechanism for the benzoin condensation with a thiazolium ylide replacing the cyanide ion. Similar mechanisms are involved... 

On the other hand y-pyrones or 1,3-diketones could be obtained from the reactions of ketone derived enamines with diketene 423-426). The addition of dimethyl ketene dimer to aldehyde or ketone derived enamines produced cyclohexanediones 425,426). 

The Baylis-Hillman reaction is usually carried out under mild conditions (0°C or room temperature). The reaction time varies from a few minutes to even days. With the proper catalyst, good yields are possible. In the absence of an aldehyde or ketone as the electrophilic component, a dimerization of the activated alkene can take place under the influence of the catalyst, as also observed as a side reaction under the usual reaction conditions ... 

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