Pyridine-DABCO

Chlorination and oxidation. This reagent is stable and easy to handle. It can be used to introduce chlorine atoms to C-2 of 2-substituted 1,3-dioxolanes, the a-position of aldehydes besides alkenes and alkynes. Oxidation of alcohols such as benzyl alcohol and cyclooctanol in MeCN requires pyridine-DABCO (4 1) as acid scavenger. 

H2NOH-HC1, DABCO, MeOH, rt, 87% for a camphor derivative. This method was reported to be better than when pyridine was used as the solvent and base. 

TsCl, DABCO, CH2CI2, MTBE or AcOEt, 45-97% yield. In many cases, these conditions were found to be superior to the use of pyridine as a base. DABCO is also less toxic than pyridine, which may prove useful in a commercial setting. 

The high selectivity that the system shows to pyrazine 20 compared to the stronger base pyridine, indicates that the diamine is chelated between the carboxylic acid functions as in 21. Spectroscopic evidence in the form of upfield shifts in the NMR spectra of the complexes supports such structures. Not only aromatic diamines are accommodated but also aliphatics such as l,4-diazabicyclo[2.2.2]octane (DABCO) in complex 22. Typically, exchange rates into and out of these complexes are such that they appear fast on the NMR time scale at ambient temperature, but exchange can be frozen out at low temperatures20. For DABCO, an activation barrier of 10.5 kcal M 1 was observed at Tc = 208 °K. 

A commercially available 5% palladium on activated carbon catalyst from Degussa was used for the investigation. Commercially supplied N-(Carbo-benzyloxy)-L-phenylalanine (99%) was purchased from Aldrich. Modifiers such as pyridine, triethylamine, ethylenediamine and DABCO (Diazabicyclooctane) with a purity >99 % are also available commercially and were used as received. 

Recently, polymers in which [0s04] is covalently bound to either a pyridine group, a dabco group in pvp, or a dabco-derivatized cross-linked styrene-divinylbenzene copolymer have been prepared and used as selective catalysts for the oxidation of alkenes (570, 571). 

Tertiary amines catalyze the homopolymerization of epoxy resins in the presence of hydroxyl groups, a condition which generally exists since most commercial resins contain varying amounts of hydroxyl functionality (B-68MI11501). The efficiency of the catalyst depends on its basicity and steric requirements (B-67MI11501) in the way already discussed for amine-catalyzed isocyanate reactions. A number of heterocyclic amines have been used as catalytic curatives pyridine, pyrazine, iV,A-dimethylpiperazine, (V-methylmorpholine and DABCO. Mild heat is usually required to achieve optimum performance which, however, is limited due to the low molecular weight polymers obtained by this type of cure. 

Six-Membered Heterocycle Ring Formation. Heterocycle formation involving diketene usually involves acetoacetylation of a substrate, followed by intramolecular condensation. Diketene itself readily dimerizes through self-condensation forming mainly dehydro acetic acid [771-03-9] (DHA) (13). Dehydroacetic acid and sodium dehydro acetate [4418-26-2] are used as preservatives for foods and cosmetics. DHA is found as an unwanted by-product in many diketene reactions, but can be obtained intentionally by dimerizing diketene in the presence of pyridine [110-86-1] in benzene, diazabicyclo[2.2.2]octane [280-57-9] (DABCO), and other basic catalysts. 

Selective silylation of ribonucleosides.2 Only the 5 -hydroxyl group of ribonucleo-sides is silylated by reaction with the reagent in THF in the presence of silver nitrate. On addition of pyridine to the reaction, 2, 5 -disilyl derivatives are formed in 80-90% yield. The actual reagent may be r-butyldimethylsilyl nitrate. Highly selective 3, 5 -disilylation can be achieved in the presence of several silver salts (AgN03, AgC104, and AgOAc) in the presence of either DABCO or 4-nitropyridine N-oxide. 

An older paper <1971MI873> reported that ozonolysis of alkenes in the presence of tertiary amines resulted in the formation of aldehydes. A recent reinvestigation <20060L3199> has shown that amine oxides were responsible for this reductive ozonolysis . Indeed, pretreatment of the tertiary amines with ozone, giving rise to amine oxides, accounted for this phenomenon. A preparative method emerged, by treating the alkene (e.g., 1-decene) at 0 °C with a solution of 2% 03/02 in dichloromethane (2 equiv of ozone relative to the alkene) in the presence of an excess (about threefold molar excess) of A-methylmorpholine A-oxide, pyridine A-oxide, or l,4-diazabicyclo[2.2.2]octane A-oxide (DABCO A-oxide). Yields of aldehydes (nonanal in the above example) were 80-96%, and the excess of amine oxide ensured the absence of residual ozonide (Scheme 21). 

Dealkoxycarbonylation.1 Enolizable P-keto esters undergo this reaction in 60-70% yield when refluxed in slightly aqueous toluene (90°) containing 1 equiv. of DMAP and buffered to pH 5-7. DABCO, N,N-dimethylaniline, and pyridine are not effective. 

Pyridylmethylphosphinates can be accessed, albeit in low isolated yield, by palladium-catalyzed cross-coupling of 2- and 3-pyridylmethyl chlorides with anilinium hypophosphites <2005T6315>. For example, 3-chloromethylpyr-idine hydrochloride reacts with anilinium hypophosphite in the presence of Pd(OAc>2, dppf, 1,4-diazabicy-clo[2.2.2]octane (DABCO), and (BuOTSi as esterification agent to give butyl (pyridine-3-ylmethyl)phosphinate 53 in 46% yield (Equation 37). This transformation proceeds in 24% yield when performed on 2-chloromethylpyridine. 

Historically, the most effective N-based organic catalysts were nucleophilic unhindered tertiary amines such as DABCO (diazabicyclo[2.2.2]octane, 1) [23], qui-nuclidine (2), 3-hydroxy quinuclidine (3-HDQ, 3), 3-quinuclidone (4) and indoli-zine (5) (Fig. 5.1) [24]. A direct correlation has been found between pKa and the activity of the quinuclidine-based catalysts the higher the pKa, the faster the rate [25]. More recently, l,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 6), considered as a hindered and non-nucleophilic base, was shown to be a better catalyst than DABCO, or 3-HDQ [26]. The reason for the increased reactivity for this catalyst was attributed to stabilization of the zwitterionic enolate by delocalization of the positive charge. Other N-based catalysts such as N,N-(dimethylamino)pyridine... 

Reagents (i) CH2=CHC02CH3, DABCO (ii) AcCI, pyridine CH2CI2 (iii) CF3C02H, CH2CI2, 0 °C... 

Russell et al. looked at the issue of regioselectivity during a study of the thermal decomposition of organomercurials in pyridine <85JOC3423>. In most cases the ratio of C2 to C4 addition products was -2 1. However, when irradiations were carried out in the presence of DABCO, selectivity was enhanced for additions involving 2°- and 3°-alkyl radicals yet worsened for additions of l°-alkyl radicals (Scheme 9). Yields were determined using a variety of methods (GC, H NMR and isolation) and unusually, no products derived from attack at C3 were observed. 

Electrophilic cyclopropanes 392, which are useful intermediates in organic syntheses, can be prepared by the cyclopropanation of olefins with diethyl dibromomalonate and its derivatives (81MI4). The reaction is carried out in the presence of 1 mol equiv. of copper(II) bromide and 2-4 mol equiv. of DBU. Alternatively, the reaction can be effected with diethyl bromomalonate (83BCJ2687) in the presence of a catalytic amount of copper(II) bromide and a slight excess of DBU in benzene at ambient temperature. When some other base (e.g., triethylamine, DABCO, pyridine, or sodium hydride) was applied instead of DBU, the yield was lower or no reaction occurred. The use of other copper salts led to a decrease in the yield. When cyclopropanation was carried out in dimethyl sulfoxide, dimethylformamide, or acetonitrile, the yield of product 392 was again lower. 

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