Base format form

Ingold makes no differentiation in principle between pseudo bases and those derivatives which are formed by the action of nucleophilic reagents. The latter he denotes by the general term "pseudo salts and considers their formation parallel to the formation of the car-binolamines themselves. If the mesomeric cation combines with a hydroxide ion, a pseudo base is formed if it combines with another nucleophilic ion or molecule, then a pseudo salt is formed. 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

A base has the ability to donate a pair of electrons and an acid the ability to accept a pair of electrons to form a covalent bond. The product of a Lewis acid-base reaction may be called an adduct, a coordination compound or a coordination complex (Vander Werf, 1961). Neither salt nor conjugate acid-base formation is a requirement. 

A similar approach to the one described above was utilized for the formation of quinone methide derivatives of osmium.14 Reaction of OsCl2(PPh3)3 with a phenolic diphosphine ligand in the presence of Et3N resulted in phosphine exchange followed by C—H activation and deprotonation by the base to form the two isomeric QM... 

Aldolases catalyze asymmetric aldol reactions via either Schiff base formation (type I aldolase) or activation by Zn2+ (type II aldolase) (Figure 1.16). The most common natural donors of aldoalses are dihydroxyacetone phosphate (DHAP), pyruvate/phosphoenolpyruvate (PEP), acetaldehyde and glycine (Figure 1.17) [71], When acetaldehyde is used as the donor, 2-deoxyribose-5-phosphate aldolases (DERAs) are able to catalyze a sequential aldol reaction to form 2,4-didexoyhexoses [72,73]. Aldolases have been used to synthesize a variety of carbohydrates and derivatives, such as azasugars, cyclitols and densely functionalized chiral linear or cyclic molecules [74,75]. 

The role of Schiff bases formed between pyridoxal phosphate and amino acid residues as intermediate products in many enzymatic reactions is well known and documented. NMR is an excellent tool for studies of the enzymatic processes involving Schiff bases formation. 

Formaldehyde fixes proteins in tissue by reacting with basic amino acids— such as lysine,5 7—to form methylol adducts. These adducts can form crosslinks through Schiff base formation. Both intra- and intermolecular cross-links are formed,8 which may destroy enzymatic activity and often immunoreactiv-ity. These formaldehyde-induced modifications reduce protein extraction efficiency and may also lead to the misidentification of proteins during proteomic analysis. 

Schiff base interactions between aldehydes and amines typically are not stable enough to form irreversible linkages. These bonds may be reduced with sodium cyanoborohydride or a number of other suitable reductants (Chapter 2, Section 5) to form permanent secondary amine bonds. However, proteins crosslinked by glutaraldehyde without reduction nevertheless show stabilities unexplainable by simple Schiff base formation. The stability of such unreduced glutaraldehyde conjugates has been postulated to be due to the vinyl addition mechanism, which doesn t depend on the creation of Schiff bases. 

Aldehydes and ketones can react with primary and secondary amines to form Schiff bases, a dehydration reaction yielding an imine (Reaction 45). However, Schiff base formation is a relatively labile, reversible interaction that is readily cleaved in aqueous solution by hydrolysis. The formation of Schiff bases is enhanced at alkaline pH values, but they are still not stable enough to use for crosslinking applications unless they are reduced by reductive amination (see below). 

Aldehyde particles are spontaneously reactive with hydrazine or hydrazide derivatives, forming hydrazone linkages upon Schiff base formation. Reactions with amine-containing molecules, such as proteins, can be done through a reductive amination process using sodium cyanoborohydride (Figure 14.21). 

The aqueous solution of methylene blue is not decolorised by sodium hydroxide the blue water-solubie ammonium base is formed. The dye is converted by reducing agents into the easily oxidisable leuco-base. The following experiment, which demonstrates the formation of methylene blue by the introduction of sulphur between the rings of Bind-schedler s green, is very instructive. 

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

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