Nucleophilicity nitrogen nucleophiles

Hydroxide and other 0-nucleophiles Nitrogen nucleophiles Sulfur and related nucleophiles Halide ions Hydride sources Carbanions Azide ions... 

Nitrogen Nucleophiles. Nitrogen nucleophiles, specifically NH3, amines, azide, and nitrite, resulted in high yields of alkene addition products (eq 7). In the absence of additional nucleophiles, the resulting sulfonium salt reacted with a tertiary amine to yield the demethylated product or the ammonium salt with a retention of configuration (eq 8). ... 

Preparing Aldehydes and Ketones A Review Introduction to Nucleophilic Addition Reactions Oxygen Nucleophiles Nitrogen Nucleophiles Mechanism Strategies Sulfur Nucleophiles Hydrogen Nucleophiles Carbon Nucleophiles Baeyer-Villiger Oxidation of Aldehydes and Ketones Synthesis Strategies... 

In this chapter, we will see many different kinds of nucleophiles that can attack a carbonyl group. In fact, this entire chapter will be organized based on the kinds of nucleophiles that can attack. We will start with hydrogen nucleophiles and continue with oxygen nucleophiles, sulfur nucleophiles, nitrogen nucleophiles, and, finally, carbon nucleophiles. This approach (dividing the chapter based on the kinds of nucleophiles) might be somewhat different than your textbook. But hopefully, the order that we use here will help you appreciate the similarity between the reactions. 

You will have noticed that, throughout this chapter, the heteroatom has always been hie nucleophile. There is one way to use nitrogen as an electrophile however and this provides a good synhion for ammo acid synthesis ... 

Most syntheses of nitrogen heterocycles involve substitution and/or condensation reactions of nitrogen nucleophiles with difunctional halides or carbonyl compounds. Common nitrogen reagents are ... 

Many saturated nitrogen heterocycles are commercially available from industrial processes, which involve, for example, nucleophilic substitution of hydroxyl groum by amino groups under conditions far from laboratory use, e.g. 

Regioselectivity becomes important, if unsymmetric difunctional nitrogen components are used. In such cases two different reactions of the nitrogen nucleophile with the open-chain educt may be possible, one of which must be faster than the other. Hydrazone formation, for example, occurs more readily than hydrazinoLysis of an ester. In the second example, on the other hand, the amide is formed very rapidly from the acyl chloride, and only one cyclization product is observed. 

A completely different, important type of synthesis, which was developed more recently, takes advantage of the electrophilicity of nitrogen-containing 1,3-dipolar compounds rather than the nucleophilicity of amines or enamines. Such compounds add to multiple bonds, e.g. C—C, C C, C—O, in a [2 + 3 -cycioaddition to form five-membered heterocycles. 

The nucleophilicity of the nitrogen atom survives in many different functional groups, although its basicity may be lost. Reactions of non-basic, but nucleophilic urea nitrogens provide, for example, an easy entry to sleeping-pills (barbiturates) as well as to stimulants (caffeine). The nitrogen atoms of imidazoles and indole anions are also nucleophilic and the NH protons can be easily substituted. 

The use of oximes as nucleophiles can be quite perplexing in view of the fact that nitrogen or oxygen may react. Alkylation of hydroxylamines can therefore be a very complex process which is largely dependent on the steric factors associated with the educts. Reproducible and predictable results are obtained in intramolecular reactions between oximes and electrophilic carbon atoms. Amides, halides, nitriles, and ketones have been used as electrophiles, and various heterocycles such as quinazoline N-oxide, benzodiayepines, and isoxazoles have been obtained in excellent yields under appropriate reaction conditions. 

The nucleophilicity of amine nitrogens is also differentiated by their environments. In 2,4,5,6-tetraaminopyrimidine the most basic 3-amino group can be selectively converted to a Schiff base. It is meta to both pyrimidine nitrogens and does not form a tautomeric imine as do the ortho- and /xira-amino groups. This factor is the basis of the commercial synthesis of triamterene. 

Other interesting regioselective reactions are carried out within the synthesis of nitrofurantoin. Benzaidehyde semicarbazone substitutes chlorine in chloroacetic ester with the most nucleophilic hydrazone nitrogen atom. Transamidation of the ester occurs with the di-protic outer nitrogen atom. Only one nucleophilic nitrogen atom remains in the cyclization product and reacts exclusively with carbonyl compounds. 

Asymmetric allylation of carbon nucleophiles has been carried out extensively using Pd catalysts coordinated by various chiral phosphine ligands and even with nitrogen ligands, and ee > 90% has been achieved in several cases. However, in most cases, a high ee has been achieved only with the l,3-diaryl-substitiitcd allylic compounds 217, and the synthetic usefulness of the reaction is limited. Therefore, only references are cited[24,133]. 

The cyclic carbamate (oxazoIidin-2-one) 313 is formed by the reaction of phenyl isocyanate (312) with vinyloxirane[I92]. Nitrogen serves as a nucleophile and attacks the carbon vicinal to the oxygen exclusively. The thermodynamically less stable Z-isomer 315 was obtained as a major product (10 I) by the reaction of 2-methoxy-l-naphthyI isocyanate (314) with a vinyloxir-... 

Carbamates are allylated in the presenee of strong bases in DMSO or HMPA[197], Phthalimide (320) and succimide are allylated with the allyl-isoureu 321 at room temperature or the allylic acetate 322 at 100 C[I98.I99], Di-/-butyl iminodicarbonate is used as a nitrogen nucleophile[200]. 

Addition of Carbon, Oxygen, Nitrogen, and Sulfur Nucleophiles... 

As is broadly true for aromatic compounds, the a- or benzylic position of alkyl substituents exhibits special reactivity. This includes susceptibility to radical reactions, because of the. stabilization provided the radical intermediates. In indole derivatives, the reactivity of a-substituents towards nucleophilic substitution is greatly enhanced by participation of the indole nitrogen. This effect is strongest at C3, but is also present at C2 and to some extent in the carbocyclic ring. The effect is enhanced by N-deprotonation. 

Piperazinothiazoies (2) were obtained by such a replacement reaction, Cu powder being used as catalyst (25. 26). 2-Piperidinothiazoles are obtained in a similar way (Scheme 2) (27). This catalytic reaction has been postulated in the case of benzene derivatives as a nucleophilic substitution on the copper-complexed halide in which the halogen possesses a positive character by coordination (29). For heterocyclic compounds the coordination probably occurs on the ring nitrogen. 

As in the pyridine series, acid catalysis facilitates this reaction because the 2-position of the ring is far more sensitive to the nucleophilic reagents when the nitrogen is quaternized (30). 

This section is organized according to the electrophilic center presented to the nucleophilic nitrogen of the active species. This organization allow s a consistent treatment of the reactivity. However, a small drawback arises when ambident electrophilic centers are considered, and these cases are treated as if the more reactive center were known, which is not always the case. 

Small amounts of salt-like addition products (85) formed by reaction on the ring nitrogen may be present in the medium. (Scheme 60) but. as the equilibrium is shifted by further reaction on the exocyclic nitrogen, the only observed products are exocyclic acylation products (87) (130. 243. 244). Challis (245) reviewed the general features of acylation reactions these are intervention of tetrahedral intermediates, general base catalysis, nucleophilic catalysis. Each of these features should operate in aminothiazoles reactivity. 

The reaction of propiolic acid or its esters with 2-aminothiazole yields 7H-thiazolo[3.2o]pyTimidine 7-one (109) (Scheme 74) (273), The reaction probably proceeds by initial nucleophilic attack of 2-aminothiazole on the sp C followed by intramolecular addition of ring nitrogen to spC. 

Nevertheless, the puzzling fact to be explained is that the harder ring nitrogen prefers the softer electrophilic center and that this preference is more pronounced than the one observed for the amino nitrogen. Much remains to be done to explain ambident heterocyclic reactivity it was shown recently by comparison between Photoelectrons Spectroscopy and kinetic data that not only the frontier densities but also the relative symmetries of nucleophilic occupied orbitals and electrophilic unoccupied orbitals must be taken into consideration (308). 

The principal reactions of this class of compounds are summarized in Scheme 172. In most of these reactions the reactive nucleophilic center is the terminal NHj group, although the other exocyclic nitrogen may also be involved, as shown by acetylation, which yields 284 and 285. However, the structure of compound 281 is not the one proposed in a recent report (1582) that attributes the attack to the other exocyclic nitrogen. The formation of osazones (287) from sugars, 2-hydrazinothiazoles, and hydrazine has been reported (525, 531). 

Reactivity of A-4-thiazoline-2-thiones and derivatives involves four main possibilities nucleophilic reactivity of exocyclic sulfur atom or ring nitrogen, electrophilic reactivity of carbon 2 and electrophilic substitution on carbon 5. 

The kinetics of the reaction between 2-methylthiothiazoles and methyl iodide show that the nucleophilic center is the ring nitrogen. The 2-methylthio group decreases the nucleophilicity of the this atom (269). 

A-2-Thiazoline-4-one possesses three nucleophilic centers (the C-5 atom, the oxygen, and the nitrogen) and two electrophilic centers (the C-4 and C-2 atOT.rs). In the literature all these reactive centers have been involved in autocondensation reactions. 

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