DABCO catalysis

The three-component Baylis-Hillman reaction was also performed on 2-chlorotrityl chloride resin by treating polymer-bound acrylic acid with aldehydes and sulfonamides in dioxane at 70 °C for 16 h under DABCO catalysis (Fig. 6.4). Both scaffolds, 3-hydroxy-2-methylidene propionic acids as well as 2-methylidene-3-aminoarylsulfonyl-propionic acids, are precursors for the synthesis of MCSLs. 

High pressure was used to improve reactivity. Crotonaldehyde could be reacted with aldehydes in this way under DABCO catalysis [47]. A recent report examined the phosphine-catalyzed dimerization of activated alkenes under pressure (Scheme 10.17) [48]. 

Other aliphatic aldehydes with ehains of about six earbons or less appear to be only slightly less reaetive than formaldehyde. Longer chain and especially a-branched aldehydes react rather slowly (Scheme 1.44). With DABCO catalysis, pivalaldehyde (2,2-dimethylpropanal) does not reaet with methyl acrylate and only very slowly with phenyl vinyl sulfone. Electron-withdrawing groups on the a carbon enhance reactivity (Scheme 1.44). ... 

For polynuclear aromatic aldehydes, the reactivity decreases in the order benzaldehyde> l-naphthaldehyde>9-anthraldehyde. The last does not add to methyl acrylate with DABCO catalysis at room temperature/ ... 

Recently, the MBH reaction of p-substituted a,p-unsaturated CF3 ketones with acrylonitrile was also found to proceed in aqueous THF solution, with DABCO catalysis, to give the corresponding adducts in good yields. In the case of ketones containing EtO and Mc2N groups, only polymerization was observed and no target product was isolated (Chapter 2.2.1). ... 

Figure 10.12. Photograph of a polyurethane aerogel obtained by supercritical CO2 drying of a gel synthesized by cross-linking of pentaerythritol with 4,4-diphenyhnethane diisocyanate under DABCO catalysis.

Catalysis by DABCO in the reactions of FDNB with piperidine, r-butylamine, aniline, p-anisidine and m-anisidine (usually interpreted as base catalysis as in Section B) was also assumed to occur by the formation of a complex between DABCO and the substrate14913. The high (negative) p-value of —4.88 was deemed inappropriate for the usually accepted mechanism of the base-catalysed step (reaction 1). For the reactions with p-chloroaniline, m- and p-anisidines and toluidines in benzene in the presence of DABCO a p-value of —2.86 was found for the observed catalysis by DABCO (fc3DABC0). The results were taken to imply that the transition state of the step catalysed by DABCO and that of the step catalysed by the nucleophile have similar requirements, and in both the nucleophilic (or basicity) power of the nucleophile is involved. This conclusion is in disagreement with the usual interpretation of the base-catalysed step. 

Vj/Vj/V j/V -Tctramethylclhylcncdiaminc (TMEDA) has been compared with DABCO in its catalysis of the reaction in aqueous medium.170... 

Curing agents and cyclotrimerization catalysis should be properly chosen for the epoxide-BPA/DC systems. As an example, a mixture consisting of Zn octoate, 2-ethylimidazole and triethylenediamine (DABCO) can be mentioned [79]. 

B0C2O (65) is prepared from potassium tcrt-butoxide and carbon dioxide to form the terf-butyl potassium carbonate, which is converted with phosgene into the di-tert-butyl tricarbonate. The tricarbonate is smoothly converted by basic catalysis with 1,4-diazabicy-clo[2.2.2]octane (DABCO) into the dicarbonate.f ... 

This study compares the effect of catalysts on aliphatic and aromatic isocyanates. With the exception of di-n-butyltin dithiocarbonate, all the di-n-butyltin catalysts perform similarly. The DABCO catalyst shows excellent catalysis for aromatic isocyanates and is less effective for aliphatic isocyanates. Combining this amine catalyst with DBTDL gives excellent catalytic activity for both aliphatic and aromatic isocyanates. Stannous, zirconium, and zinc octanoate show reduced activity in comparison to organotin. 

In the past years, only a few reports have dealt with a chiral auxiliary induced diastereoselective aza MBH reaction. In 1994, Kiindig et al. explored the reaction of methyl acrylate and acrylonitrile with enantiopure planar chiral o substituted Cr(CO)3. Under the catalysis of DABCO, the corresponding aza MBH adducts were obtained in good yields. Removal of the metal provided chiral amines in high yields and enantiomeric excesses [12]. Later on, Aggarwal ef al. used enantiopure Nsulfinimines in the aza MBH reaction with methyl acrylate in the presence of 3 hydroxyquinuclidine (3 HQD) and Lewis add. The desired adducts, functionalized P sulfonated amino acid derivatives, were obtained with good diastereoselectivities (Scheme 13.4) [13]. 

Ethylene diisocyanate reacts with active-hydrogen compounds to form 1-substituted 2-imidazolidones in high yield. The order of reactivity is R2NH > RNH2 > ROH > RSH in the last case, catalysis by DABCO ( triethylenediamine ) is required. 

In a very useful modification, simple ketones with CH2 adjacent to the carbonyl (cyclic ketones work much better than acyclic ketones) and ortfto-iodo-arylamines react under palladium catalysis to give indoles directly. The use of dimethylformamide as solvent and DABCO as the base are crucial to the success of the route. Mechanistically, the sequence certainly proceeds through the enamine. As well as being conceptually and practically simple, this method tolerates functional groups that would be sensitive to the acid of the traditional Fischer sequence. This method can also be applied to aldehydes, thus providing a direct route to 2-unsubstituted indoles, including side-chain-protected tryptophans. ... 

Displacement of halides can be catalysed by amines - trimethylamine, pyridine and DABCO have been used. Mechanistically, the catalysis involves formation of an intermediate quaternary ammonium salt that is more reactive towards nucleophiles than the starting halide. The intermediate quaternary salts can be isolated, if required. Trimethylamine gives the most reactive quaternary salt, but DABCO can be more convenient. The relative reactivities for nucleophilic displacement at C-6 are trimethylamine DABCO chlorine = 100 10 1. Cyano and fluorine are amongst the groups that have been introduced in this way. 

TABLE I. SYNERGISTIC EFFECT OF DABCO CATALYST AND DIBUTYLTIN DILAURATE FOR THE CATALYSIS OF PHENYL ISOCYANATE (0.07M) REACTION WITH BUTANOL-2 (0.07H) IN DIOXANE AT 25°C... 

Another supporting evidence for complex formation as a prerequisite to synergism was obtained from the study of the catalysis of phenyl isocyanate-butanol reaction by soluble organic cobalt compounds in presence and absence of DABCO catalyst. The results obtained are presented in Figures 4 and 5. It is evident that the combination of DABCO catalyst with divalent cobalt compounds shows synergistic effects while the trivalent cobalt acetylacetonate shows relatively low activity. The explanation of these observations is the structure of these compounds. 

In pyridine solutions, the statistically corrected relative catalytic coefficients of tertiary amines for 1-methylindene isomerization decreased in the order24 4. quinuclidine, 80 DABCO, 10 triethylamine, 1. The smaller catalytic effectiveness of DABCO than quinuclidine is attributable to its weaker basicity is —30eu for each of these bicyclic bases. On the other hand, triethylamine is about as basic as quinuclidine, but must lose considerable rotational freedom in the rate-limiting proton transfer. This is reflected in the more negative entropy of activation (—39eu) for the triethyl-amine-catalyzed reaction. In pyridine solution, there is a close correlation between pa s of the catalyzing base and A// for 1-methylindene isomerization. Asymmetric catalysis was demonstrated in the quinine-catalyzed isomerization of optically active 1-methylindene in pyridine at 25°C the dextrorotatory indene isomerized nearly twice as fast as its enantiometer247. 

Attempts to add acrolein to aetivated olefins led to polymerization, but methaerolein, crotonaldehyde and cinnamaldehyde reaet normally with acrylates and aerylonitrile (Scheme 1.46). ° Upon treatment of these a,jS-unsaturated aldehydes with methyl vinyl ketone in the presenee of DABCO, the self-eondensation product 3-methylenehepta-2,6-dione was obtained exclusively some MBH adduct was isolated with phosphine catalysis (Scheme 1.46). In addition, crotonaldehyde does not add to phenyl vinyl sulfone. ... 

Compared with chiral nonracemic a-amino carbonyl compounds - which are not suitable substrates for MBH reaction, mainly due to their racemization under normal conditions after prolonged exposure times to catalyst or due to poor diastereoselectivity " a-keto lactams, enantiopure 3-oxo-azetidin-2-ones 168, readily react with various activated vinyl systems promoted by DABCO to afford the corresponding optically pure MBH adducts 169 without detectable epimerization (Scheme 1.69). " However, the Lewis acid-mediated reaction of electron-deficient alkynes with azetidine-2,3-diones 168 as an entry to p-halo MBH adduets was not very sueeessful the coupling product 170 was achieved with concomitant acetonide cleavage as a single ( )-isomer in low yield, in the presence of trimethylsilyl iodide under BF3 OEt2-induced catalysis (Scheme 1.69). 

Various catalysts are used to prepare polyurethane at a relatively low temperature and with a much faster rate of polymerisation than would be the case with an uncatalysed reaction. Catalysts may be classified into two broad categories namely, amine (basic) compounds and organometalhc complex compounds. Tertiary amine is stiU one of the most frequently used urethane catalysts. Commonly used amine catalysts are triethylenedi-amine (TEDA), l,4-diazabicyclo[2.2.2]octane (DABCO), triethylamine (TEA), dimethylethanolamine (DMEA) and dimethylcyclohexylamine (DMCHA). The catalysis mechanism of tertiary amine catalysed urethane reaction involves complexation of the amine with isocyanate groups, followed by reaction of the complex with alcohol to produce polyurethane. A list of catalysts used in polyurethane preparation is given in Table 6.4. 

A phosphine sulfonamide derived from L-threonine promotes aza-Morita-Baylis-Hillman (aza-MBH) reactions of sulfinylimines in up to 96% yield and 97% ee. A review describes the synthesis of chiral amines under mild conditions via catalytic asymmetric aza-MBH reactions. Proline/DABCO (l,4-diazabicyclo[2.2.2]octane) co-catalysis of enantioselective aza-MBH reactions gives good to high yields and up to 99%... 

Two general classes are very well known. The first is composed of tertiary amines, of which DABCO, tertiary alkylamines, substituted morpholines, piperazines, guanidines, and substituted hydroxy amines are representative types. A wide range of activities is represented in this class, but stronger catalysts are sometimes needed to promote the reaction of secondary hydroxyl groups with isocyanates. Stronger catalysis is supplied by catalysts of the second class, organotin compounds like dibutyltin dilaurate... 

The reaction of isocyanates with alcohols and with water can be catalyzed by amines and by organometallic compounds. Tertiary amines, such as l,4-diazo-[2.2.2]-bicyclooctane (DABCO) or triethylamine, are particularly effective in promoting the isocyanate-water reaction, while organometallic complexes, such as dibutyltin dilaurate or stannous octoate, are very useful for catalyzing isocyanate-alcohol reactions. Numerous articles have been written on various aspects of the catalysis of isocyanate reactions and representative examples are cited in refs. 8-10. 

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