Aldehyde oligomerization

Some aldehydes oligomerize in the presence of acids. The polymerization of formaldehyde (formation of H see Figure 9.10) as well as that of aldehydes with strong electronegative sub-... 

The cationic polymerization of cardanol under acidic conditions has been referred to earlier [170,171], NMR studies [16] indicated a carbonium ion initiated mechanism for oligomerization. PCP was found to be highly reactive with aldehydes, amines, and isocyates. Highly insoluble and infusible thermoset products could be obtained. Hexamine-cured PCP showed much superior thermal stability (Fig. 12) at temperatures above 500°C to that of the unmodified cardanol-formaldehyde resins. However, it was definitely inferior to phenolic resins at all temperatures. The difference in thermal stability between phenolic and PCP resins could be understood from the presence of the libile hydrocarbon segment in PCP. 

A recent example where Co2(CO)8 serves as a precatalyst is in the preparation of linear and branched aldehydes via propylene hydroformylation in supercritical C02 (93-186 bar 66-108 °C). Cyclohexane carbaldehyde is produced from cyclohexene using Co2(CO)8 and an acid RCOOH, or else is successful with another established Co catalyst, Co(OOCR)2, assumed to form in situ in the former case. Oligomerization of aldehydes such as n-butanal is achieved with Co2(CO)6L2 as catalyst (L = CO, PR3).1364... 

In another approach, a polymer-bound TEMPO-species, referred to as PIPO (polyamine immobilized piperidinyl oxyl), was derived by oxidation of commercially available Chimasorb 944, an oligomeric sterically hindered amine. Together with bleach as co-oxidant PIPO (12) afforded aldehydes and ketones in short reaction times and with yields from 80 to > 99% (Scheme 4.6) [70]. 

The simplest possible aldehyde donor, acetaldehyde, can also be used as the donor Very recently, Hayashi and coworkers discovered how to use acetaldehyde in crossed-aldol reactions - the trick is to use diarylprohnol as the catalyst and to optimize the reaction conditions carefully to prevent oligomerization of acetaldehyde. However, so far the acetaldehyde aldol reactions appear to be limited to aromatic aldehyde acceptors [205],... 

Burcham, P. C. and Pyke, S. M. (2006). Hydralazine inhibits rapid acrolein-induced protein oligomerization Role of aldehyde scavengers and adduct trapping in cross-link blocking and cytoprotection. Mol. Pharm. 69,1056-1065. 

Although the ozonolysis product exists in oligomeric form, the amount of acid used was calculated by assuming a theoretical yield of the corresponding monomeric aldehyde—methoxy hydroperoxide. p-Toluenesulfonic acid monohydrate, purchased from Aldrich Chemical Company, Inc., was not further purified. 

A number of mechanistic studies of the reaction have been made and a cyclic transition state in which hydrogen transfer occurs from the / -C—H bond of an alkoxide to a coordinated ketone or aldehyde has been postulated.245 However, kinetic measurements are complicated by the oligomeric nature of the aluminum reagent (see 3).24[Pg.354]

Chiral binaphthol(BINAP)-titanates (59 X = OR) have been used as asymmetric catalysts of additions to aldehydes, and show evidence of oligomeric Ti—O—Ti... 

The additions of H20 or alcohols to the C=0 double bond of carbonyl compounds as well as the oligomerizations or polymerizations of aldehydes are reversible reactions. Therefore, the extent of product formation is subject to thermodynamic control. The equilibrium constant of the formation of the respective addition product is influenced by steric and electronic effects. 

For a given nucleophile the equilibrium lies farther on the product side the smaller the substituents R1 and R2 of the carbonyl compound are (Figure 9.1). Large substituents R1 and R2 inhibit the formation of addition products. This is because they come closer to each other in the addition product, where the bonds to R1 and R2 are closer than in the carbonyl compound, where these substituents are separated by a bond angle of about 120°. Formaldehyde is the sterically least hindered carbonyl compound. In H20 this aldehyde is present completely as dihydroxymethane, and anhydrous formaldehyde is exists completely as polymer. In contrast, acetone is so sterically hindered that it does not hydrate, oligomerize, or polymerize at all. 

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... 

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