Trimerization, of aldehydes

Trimerization of aldehydes affords trialkyl-1,3,5-thioxanes 11. Compounds of this type were obtained from acetaldehyde, propionaldehyde (29,33), isobutyraldehyde (47), and isovaleralde-hyde (35). The two isomers normally formed can be separated by GC. 

Trioxanes 726 (Z = 0) are trimers of aldehydes or ketones formed by acid-catalyzed condensations of the monomers. 1,3,5-Trithiane 726 (Z = S) is prepared by passing H2S through formaldehyde and hydrochloric acid <1943OSC(2)610>. The reaction of formaldehyde and H2S with amines affords the corresponding thiadiazines 727 (Scheme 311) <2003RCB1817, CHEC-III(9.09.9.3)511>. 

MTO also catalyzes the trimerization of aldehydes - one of the three oxygen atoms in the product is derived fromMTO (eq 42). ... 

Primary aromatic amines react with aldehydes to form Schiff bases. Schiff bases formed from the reaction of lower aUphatic aldehydes, such as formaldehyde and acetaldehyde, with primary aromatic amines are often unstable and polymerize readily. Aniline reacts with formaldehyde in aqueous acid solutions to yield mixtures of a crystalline trimer of the Schiff base, methylenedianilines, and polymers. Reaction of aniline hydrochloride and formaldehyde also yields polymeric products and under certain conditions, the predominant product is 4,4 -methylenedianiline [101 -77-9] (26), an important intermediate for 4,4 -methylenebis(phenyhsocyanate) [101-68-8], or MDI (see Amines, aromatic amines, l thylenedianiline). 

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. 

Dihydro-2,5-dimethoxyfuran 824 reacts neat at 24°C with TCS 14 via the intermediate 825, the 2-trimethylsilyloxyfuran 826 (which can also be readily prepared from 5H-furan-2-one), and 827-829 to give the crystalline trimer 830 in 20% yield [21]. In the presence of aldehydes such as thiophen-2-aldehyde 831a or benzaldehyde 831b, however, 824 reacts via 832 to give the condensation products 833 a and 833 b, which are obtained in 30 and 62% yield, respectively [22, 23] (Scheme 6.11). Because it is postulated 2-trimethylsilyloxyfuran 826 is a intermedi-... 

The volatility of nonanal is sufficiently low to require unpractically high levels of recycle gas. It would not be possible to remove the aldehyde dimers and trimers of nonanal by gas recycle. Practically speaking, pentanal is at the border of aldehydes that may be isolated this way. Removal of the dimers and trimers of pentanal would be problematic. 

Nitrogen-containing heterocyclic compounds, including 1,2,3,4-tetrahydroqui-noline, piperidine, pyrrolidine and indoline, are also popular hydrogen donors for the reduction of aldehydes, alkenes, and alkynes [75, 76]. With piperidine as hydrogen donor, the highly reactive 1-piperidene intermediate undergoes trimer-ization or, in the presence of amines, an addition reaction [77]. Pyridine was not observed as a reaction product. 

By organic chemistry formalism, polyacetals are reaction products of aldehydes with polyhydric alcohols. Polymers generated from aldehydes, however, either via cationic or anionic polymerization are generally known as polyacetals because of repeating acetal linkages. Formaldehyde polymers, which are commercially known as acetal resins, are produced by the cationic ring opening polymerization of the cyclic trimer of formaldehyde, viz., trioxane [29-30] (Fig. 1.5). 

Aluminum methoxide Al(OMe)3 is a solid which sublimes at 240 °C in vacuum. Aluminum isopropoxide melts in the range 120-140 °C to a viscous liquid which readily supercools. When first prepared, spectroscopic and X-ray evidence indicates a trimeric structure which slowly transforms to a tetramer in which the central Al is octahedrally coordinated and the three peripheral units are tetrahedral.162,153 Intramolecular exchange of terminal and bridging groups, which is rapid in the trimeric form, becomes very slow in the tetramer. There is MS and other evidence that the tetramer maintains its identity in the vapour phase.164 Al[OCH(CF3)2]3 is more volatile than Al[OCH(Me)2]3 and the vapour consists of monomers.165 Aluminum alkoxides, particularly Al(OPr )3, have useful catalytic applications in the synthetic chemistry of aldehydes, ketones and acetals, e.g. in the Tishchenko reaction of aldehydes, in Meerwein-Pondorf-Verley reduction and in Oppenauer oxidation. The mechanism is believed to involve hydride transfer between RjHCO ligands and coordinated R2C=0— A1 groups on the same Al atom.1... 

Trimerization of imidates is a valuable route to 1,3,5-triazines. Imidates can be considered as activated nitriles and cyclotrimerize more readily. Most symmetrical 2,4,6-trialkyl-1,3,5-triazines are easily formed, although large alkyl substituents may give rise to steric hindrance (61JOC2778). Symmetrical isocyanurates (525) are readily available from isocyanates, RNCO catalysts include tertiary amines, phosphines and sodium methoxide. Aldehydes RCHO and ammonia give hexahydro-1,3,5-triazines (526), known as aldehyde ammonias (73JOC3288). 

The so-called trimerization of propynal in the presence of piperidine acetate provides a synthesis of 4-ethynyI-4//-pyran-3,5-dicarbaldehyde (149) (50LA(568)34> it should be noted that the structure proposed for the product in the original work has been corrected (64CB1959). In the absence of moisture, the reaction fails and it seems likely that the synthesis involves hydration of the alkyne to the divinyl ether. Finally, condensation with the third molecule of the aldehyde results in cyclization to the product (Scheme 20). 

Tetrahydro-l,3-oxazines are normally assumed to adopt a chair conformation in which the NH bond has an axial orientation. This view is substantiated by NMR spectroscopy and also by dipole moment analysis <73JCS(P2)325). The ring system is not stable and, for example, when the parent molecule is allowed to stand, it slowly ring opens and then forms a trimer (Scheme 33) (78AF937). In acidic media ring fission is accelerated and the open-chair imines may then hydrolyze. This property has been utilized in the synthesis of aldehydes as previously noted (see Section 2.27.2.2.4(0). 

Ethanal (acetaldehyde) polymerizes under the influence of acids to the cyclic trimer, paraldehyde, and a cyclic tetramer, metaldehyde. Paraldehyde has been used as a relatively nontoxic sleep-producing drug (hypnotic). Metaldehyde is used as a poison for snails and slugs, Snarol. Ketones do not appear to form stable polymers like those of aldehydes. 

The aldehyde-ammonia adducts usually are not very stable. They readily undergo dehydration and polymerization. 1-Aminoethanol, for example, gives a cyclic trimer of composition C6Hl5N3-3H20, mp 97°, with structure 4 ... 

The enantiomeric purity of the 3-pinanecarbaldehyde corresponds to the a-pinane utilized (70-85%). Enantiomerically pure aldehyde can be obtained by the acid-catalyzed trimerization of the aldehyde, with only one enantiomer being preferentially cyclotrimerized to a crystalline compound.2311 Cleavage of the trimer results in enantiomerically pure aldehyde. If cobalt catalysts are employed in the cyclization, rearrangement to the bomane structure takes place (equation 9).25... 

A most convenient procedure for the conversion of carboxylic acids to aldehydes results from their initial treatment with borane-dimethyl sulphide to give first, the triacyloxyborane (9), which is then reduced further to the intermediate (10) the trimer of (10) is the trialkyloxyboroxine (11). Oxidation of (11) with PCC then yields the aldehyde.110 The method is illustrated for the preparation of octanal (Expt 5.81), but it has been applied to cyclohexanecarboxylic acid and to various aromatic carboxylic acids (X-C6H4C02H, X = p-Cl, NOz, OMe, ni-CN). 

The aqueous solution (about 180 cc.) of non-volatile products from the steam distillation is cooled to room temperatime, filtered to remove a little insoluble material, and then cooled in ice water. The somewhat impure azelaic half aldehyde (3.33 g., or 76%) which separates is collected, dried, and extracted with 400 cc. of boiling light petroleum (b.p. 40-60°) in which all but 0.6 g., identified as the trimer of the aldehyde acid, dissolves. When the petroleum solution is cooled in ice-salt mixture, the semialdehyde separates as plates which, after several re-crystallizations from 50 parts of warm water, yields 1.6 g. of pure azelaic aldehyde acid as colorless rhombic plates melting at 38°. 

As you can see from Figures 9.10 and 9.11, all steps of the oligomerization or polymerization of aldehydes in the presence of protons are reversible. The trimerizations of the mentioned aldehydes acetaldehyde and isobutyraldehyde are therefore thermodynamically controlled. This is the reason why they take place stereoselectively and the trimers C (R = Me... 

Cross aldol reaction between two different aldehydes and/or ketones without prior activation or protection should provide a straightforward methodology for the synthesis of aldols, Mahrwald recently reported that treatment of aldehydes with TiCU and NEta (or TMEDA) gives rise to syn- do reaction in good yields (Eqs 38 and 39) [141], This method was extended to the aldehyde-ketone cross aldol reaction catalyzed by TiCU [142], an advantage of which is that reaction occurs at the more encumbered a-position of unsymmetrical ketones, as illustrated in Eqs (40) and (41) [143], The use of aliphatic aldehydes instead of PhCHO usually reduced stereoselectivity. When TiCU was replaced by a catalytic amount of BuTi(0-/-Pr)4Li, the aldol reaction was followed by the Tischenko reaction [144], Methyl vinyl ketone trimerized to give a chlorinated cyclic product with TiCU [145],... 

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