Dimer-esters

Miscellaneous Commercial Applications. Dimer acids are components of "downweU" corrosion inhibitors for oil-drilling equipment (see Petroleum Corrosion and corrosion inhibitors). This may account for 10% of current dimer acid use (71). The acids, alkyl esters, and polyoxyalkylene dimer esters are used commercially as components of metal-working lubricants (see Lubrication). Dimer esters have achieved some use in specialty lubricant appHcations such as gear oils and compressor lubricants. The dimer esters, compared to dibasic acid esters, polyol esters and poly(a-olefin)s, are higher in cost and of higher viscosity. The higher viscosity, however, is an advantage in some specialties, and the dimer esters are very stable thermally and can be made quite oxidatively stable by choice of proper additives. 

There are a number of methods for the oxidation of primary alcohols or ethers to dimeric esters, and secondary alcohols to ketones. We recently also found that quaternary ammonium tribromides, especially BTMA Br3, are useful oxidizing agents for the purpose described above (ref. 31). 

That is, the reaction of primary alcohols or ethers with a calculated amount of BTMA Br3 in carbon tetrachloride-water in the presence of Na2HP04 at 60°C gave dimeric esters in good yields. In the case of benzyl alcohol, the only oxidation product was benzaldehyde (Fig. 20). 

The coupling of two molecules of aldehydes into esters (Tishchenko reaction) has been used as an efficient method for the industrial preparation of dimeric esters. Although a number of systems for such reactions using transition-metal catalysts have been reported [73], there is stiU great room for improvement of the synthetic efficiency. 

When the oxidation of a primary alcohol with PCC results in the formation of an aldehyde, activated with an electron withdrawing group at the a-position sometimes, a stable dimeric hemiacetal is formed that is further oxidized to a dimeric ester.331 This reaction, that can also happen with other chromium-based reagents (see page 42), can be minimized by adjusting the reaction conditions. 

The aldehyde reacts with the starting alcohol, yielding a stable hemiacetal that can be further oxidized to a dimeric ester. The formation of the dimeric ester can be minimized by the use of high dilution and the slow addition of the alcohol to the oxidant, resulting in a reaction giving an optimized 5 2 ratio of aldehyde to dimeric ester. 

During the oxidation of primary alcohols with oxoammonium salts, sometimes dimeric esters are formed.20a This can be minimized by increasing the quantity of TEMPO. 

An oxoammonium salt operating as a primary oxidant is generated by oxidation of catalytic TEMPO with Br2, which, in turn, is formed by electrooxidation of bromide anion. The formation of a dimeric ester side-compound is minimized increasing the quantity of TEMPO. 

The most common side reactions during Oppenauer oxidation consist of base-induced condensations of the aldehyde or ketone, generated during the oxidation, with the carbonyl compound used as oxidant.65 This side reaction is particularly prominent during the obtention of aldehydes because they are generally more reactive in aldol condensations than ketones. Furthennore, aldehydes very often suffer Tischtschenko condensations,66 resulting in the formation of dimeric esters during Oppenauer oxidations. That is why, the Oppenauer oxidation is seldom useful for the preparation of aldehydes. 

Figure 19 Diflunsal decarboxylation and dimer ester formation.

Primary alcohols react slowly giving dimeric ester, presumably via hemiacetal intermediates. 

In another classical investigation, Wassermann and coworkers " studied the system cyclopentadiene-trichloroacetic acid for several years and were able to disentangle its intricate behaviour throu a series of brilliant and meticulous experiments. The identification of the dimeric ester and the study of its polymerisation to give polycon-jugated products, a reaction enhanced by the presence of such promoters as mono- and trichloroacetic acids, demonstrate the pcMSibility of propagation by ester-ester interaction assisted by acid. 

While there is no doubt that as the degree of conjugation in the polymer increases, stable polyenic carbenium ions are formed in the acidic medium, the nature of the chain carriers involved in the formation of the dimeric ester and in the early sta s of its polymerisation is worth discussing. The authors invoked the formation of carbenium ions from the first step of the process but no proof of their presence, or of the presence of the dimer cation and dication was offered. An alternative mechanism based on activated ester molecules would be more plausible to us in view of the hi reactivity of the cyclopentadienyl cation and the low likelyhood that it would be formed in such mildly acidic conditions as those employed in these experiments. 

Polyols, polyoleates, C36 dimer esters, diesters WGK 0 Phthalates and trimellitates WGK 0-2... 

Two-stroke oils Esters such as C36 dimer esters and polyoleates have several advantages over mineral oils as the lubricant component of two-stroke engine fuel mixtures. First, their clean-burn characteristics result in less engine fouling with considerably reduced ring sticking and lower levels of particulate deposit build-up... 

High performance in the synthesis of hydrolytically resistant polyurethanes was obtained by using in the polyesterification reaction, very hydrophobic fatty dimer acids and fatty dimer alcohols, products obtained from vegetable oils (see Chapter 12.5). The use of fatty dimeric acids and fatty dimeric alcohols (obtained by the hydrogenation of dimeric acids or dimeric esters) to build the polyester structure, creates an extremely high hydrophobic environment alongside a low concentration of labile ester bonds. 

This reaction was shown to be general. In the absence of an added aldehyde, primary alkoxides give dimeric esters, RCHzOCOR, and secondary alkoxides are converted into ketones. If an aldehyde (RCHO) is added to either reaction, esters are formed, R CH20C0R from a primary alkoxide and R R CHOCOR from a secondary alkoxide. 

The synthesis of PLA and its cost-driving bottlenecks are presented in Fig. 4. The first step is the fermentative synthesis of (usually L-) LA with bacteria or yeast. Elegant reviews on this anaerobic fermentation are available elsewhere [55-58]. LA is further converted to its cyclic dimeric ester lactide via a two-stage process consisting of a pre-polymerization and a backbiting reaction (second frame in Fig. 4). This L,L-lactide is the acmal mmiomer for high molecular weight PLA. 

Graca J., Santos S., Linear aliphatic dimeric esters from cork suberin, Biomacromolecules, 7(6), 2006, 2003-2010. 

Along with 3a-tropine derivatives, characteristic for the plants of the family Erythroxylaceae are the esters of 3jS-tropine (pseudotropine) and ecgonine with benzoic, phenylacetic, 3,4,5-trimethoxybenzoic, 3,4,5-trimethoxycinnamic acids, etc. Dimeric esters of methylecgonine (tmxillines) and tropine (mooniines) have been also reported [15]. 

The Tishchenko reaction is more than 100 years old and, with its different variants, has grown over the decades into a niche method for the selective production of dimeric esters from two aldehyde molecules. The reaction also known as the Claisen-Tishchenko reaction, occurs via a Lewis acid-catalysed disproportionation reaction involving a hydride shift (the rate determining step) as depicted in Scheme 18.23. Catalysts with higher Lewis acidity (like pure aluminium(m) alkoxides) afford in most cases the esters. 

Accordingly, deprotection of the TBS in 174 and transesterification of the salicylic acid acetonide with 2-(trimethylsilyl)ethanol gave deactivated monomeric salicylate ester 175. Ester 175 was stable to strong base, whereas 174 (Scheme 6.31) was decomposing under basic conditions therefore, 175 acted as pronucleophile for acylation with 174 to afford 176 as dimeric ester. Protection of dimeric ester 176 followed by desilylation provided the seco-acid, which on macrolactonization using the modified Mukaiyama salt and deprotection furnished 44-membered C2-symmetrical marinomycin A 177. 

General Synthesis.—High yields of esters are obtained when tri-n-butylstannyl derivatives of alcohols are condensed with aldehydes, and the products treated with A-bromosuccinimide (Scheme 27). In the absence of an aldehyde, dimeric esters (109) are produced via oxidation of the alcohol derivative to the corresponding aldehyde (or ketone), which then condenses as shown. 

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