Pyridine amine based catalysts

Pyridine, 3-(dimethylamino)-amination, 2, 236 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342 Pyridine, 4-(dimethylamino)-in acylation, 2, 180 alkyl derivatives pK, 2, 171 amination, 2, 234 Arrhenius parameters, 2, 172 as base catalysts, 1, 475 hydrogen-deuterium exchange, 2, 286 ionization constants, 2, 172 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342... 

Suitable reagents for derivatizing specific functional groups are summarized in Table 8.21. Many of the reactions and reagents are the familiar ones used in qualitative analysis for the characterization of organic compounds by physical means. Alcohols are converted to esters by reaction with an acid chloride in the presence of a base catalyst (e.g., pyridine, tertiary amine, etc). If the alcohol is to be recovered after the separation, then a derivative which is fairly easy to hydrolyze, such as p-nltrophenylcarbonate, is convenient. If the sample contains labile groups, phenylurethane derivatives can be prepared under very mild reaction conditions. Alcohols in aqueous solution can be derivatized with 3,5-dinitrobenzoyl chloride. 

As has already been mentioned, boron halides are electron-deficient molecules. As a result, they tend to act as strong Lewis acids by accepting electron pairs from many types of Lewis bases to form stable acid-base adducts. Electron donors such as ammonia, pyridine, amines, ethers, and many other types of compounds form stable adducts. In behaving as strong Lewis acids, the boron halides act as acid catalysts for several important types of organic reactions (see Chapter 9). 

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... 

The various alkylaminations appear in Table IV. All were carried out by refluxing sodium sand in toluene containing the primary amine and catalyst until all or most of the sodium was consumed. The pyridine base was added in the second step with evolution of hydrogen (80UP1). 

The polymer-supported 4-(A -methylamino)pyridine (Figure 3.4), prepared by reaction of 4-chloromethylstyrene and 4-(methylamino)pyridine sodium salt and successive copolymerization with styrene and divinyl benzene,was utilized as reusable catalyst for promoting the Baylis-Hillman reaction usually this kind of reaction uses stoichiometric or overstoichiometric amounts of an organic amine as a base catalyst. 

The coupling reaction between an electron-deficient alkene and an aldehyde (Baylis-Hillman reaction) usually requires a cata-lyst/catalytic system (typically, a tertiary amine and a Lewis acid) to be successful. The base catalyst is not necessary when pyridine-2-carboxaldehyde is employed as electrophile it is enough with the activation effected by stoichiometric amounts of TMSOTf for the reaction to proceed to give indolizidine derivatives (eq 71). ... 

Mukhopadhyay and Ray (2011) described the synthesis of 2-amino-5-alkyl-idenethiazol-4-ones from ketones, rhodanine, and secondary amines promoted by a silica-pyridine-based catalyst. Compounds containing a 2-amino-5-arylidene-l,3-thiazol-4(5H)-one moiety display a wide range of interesting pharmaceutical activities such as antiviral (Abdel-Ghani, 1999), antimicrobial (Soltero-Higgin et al., 2004), cardiotonic (Andreani et al., 1996), and anti-inflammatory (Nasr and Said, 2003) effects. The one-pot synthesis of thiazol-4-ones was optimized on a model... 

Mukhopadhyay, C., Ray, S. 2011. Synthesis of 2-amino-5-alkylidenethiazol-4-ones from ketones, rhodanine, and amines with the aid of re-usable heterogeneous sihca-pyridine based catalyst. Tetrahedron 67(41 ) 7936-7945. 

In the formation of the first synthetic intermediate in Sequence D, the very effective Verley-Doebner modification of the fundamental Knoevenagel condensation is used. This modification uses malonic acid in place of the conventional ester to promote enoUzation. In addition, the heterocyclic amine, pyridine, functions as both the base catalyst and the solvent. A cocatalyst, P-alanine (an amino acid), is also introduced. Mechanistically, the reaction closely resembles the aldol condensation in that in both cases a carbanion is generated by abstraction, by base, of a proton alpha to a carbonyl group. The resulting carbanion is stabilized as an enolate anion (see below). 

The polyesters were typically prepared by mixing the diol and the diacid chloride in equivalent amounts (0.2-0.3 M cone, of monomer) in 1,2-dichloroethane and adding an organic tertiary amine (pyridine or triethylamine) as a base catalyst. The mixture is heated under nitrogen on a constant temperature oil bath and after the appropriate time the polymer was recovered by precipitating into methanol, and purifying by reprecipitation from halogenated solvents into methanol or ether. 

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