Substituents nucleophilic

Line No. Pyridine substituents Nucleophile (solvent) Rate constant" (temp. °C) 10 fc liter mole- seo-i Activation energy kcal mole-1 Entropy of activation cal mole i deg-i Frequency factor logioA Ref. 

Line No. Pyrimidine substituents Nucleophile in 99.8% EtOH Rate constant (temp. °C) 10 k liter mole i sec Activation energy kcal mole i Entropy of activation cal mole deg-i Frequency factor logic A Ref. 

Line No. Triazine substituents Nucleophile + catalyst Solvent Rate constant (temp. °0) 106 liter mole i sec i Kin param Ex etic eters > JSt Ref. 

Line No. Benzene substituents Nucleophile Solvent Rate constant (temp. °C) 106 jc liter mole-1 see-i Energy of activation keal mole-1 Frequency factor logioA Ref. 

No. substituents Nucleophile (solvent) liter mole-1 sec-i kcal mole-1 calmole-ideg" -1 Ref. W 9... 

The chemistry of diazines remains an area of intense interest, both academic and industrial, with applications in many areas, from biomedical to materials science and electronics. They are versatile, having very varied reactivity, giving many opportunities for manipulation of substituents. Nucleophilic substitutions, electrophilic substitution in oxy and amino derivatives, organometallic and transition metal-catalysed coupling reactions are all subjects of substantial research effort. There are obvious similarities in reactivity of the three diazine systems but also many interesting and practically important, often subtle, differences. 

To sum up, the rate retardation attributed to steric effects of bulky alkyl groups can arise from substituent-electrophile, substituent-substituent and substituent-solvent interactions in the first ionization step of the reaction and also from substituent-nucleophile interactions in the product-forming step. It is therefore not surprising that the usual structure-reactivity correlations or even simpler log/log relationships cannot satisfactorily describe the kinetic effects of alkyl groups in the electrophilic bromination of alkenes. 

The reaction occurs because a favourable tertiary carbocation is generated. Since the carbocation also has three different substituents, nucleophilic attack of water forms a chiral centre, and thus enantiomeric products. 

Conjugation to the free electron pair of the thiazine nitrogen makes these 3-substituents nucleophilic. The first example is reaction of 42 <1968CHE322> and its 2-ethoxycarbonyl derivatives <1983JME559> with triethyl-oxonium tetrafluoroborate (Scheme 42). Another is the reaction of 27 with hexamethyldisiloxane to give 112 (Equation 31) <1988JME1575>. 

When the electron density of a carbon-carbon bond is reduced by strongly electron-withdrawing substituents, nucleophilic attack at one of the vinylic or acetylenic carbons may occur. Electron withdrawal may be either by induction or by resonance. Examples of nucleophilic addition are shown in Equations 7.49-7.53. 

INFLUENCE OF NITRO GROUPS ON REACTIVITY OF HYDROGEN ATOMS AND SUBSTITUENTS. NUCLEOPHILIC REACTIONS... 

So far little information is available on electrophilic substitution reactions these are mainly expected to occur in the azine ring when activated by electron-releasing substituents. Nucleophilic substitution reactions, however, occur readily in either ring. The N—S bond may be cleaved by nucleophilic attack at sulfur and this may be the preferential reaction path in some cases. The N—S bond may also be cleaved as a result of proton abstraction from the azole ring. 

The alkoxy groups in alkoxyazoles undergo easy dealkylation to the corresponding hydroxyazoles (azolinones) when several nitrogen atoms are present or when they are additionally activated by another substituent. Nucleophilic displacement of alkoxy groups on cationic rings occurs readily. 

Line No. Triuine substituents Nucleophile + catalyst Solvent Rate constcuit (temp. °C) 106A liter mole i S60 l Kin paraxn E.1 etic Bters -" JSt Bef. 

Line No. Quinoline substituents Nucleophile (solvent) Bate constant (temp. C) 10 lfc liter mole-1 seo i Activation Entropy of energy activation koal mole-1 oalmole-ideg -1 Ref. 

Line No. Naphthalene substituents Nucleophile (solvent) Rate constant/ (temp. C) 10 ifc liter mole-t sec Activation energy kcal mole-t Entropy of activation cal mole-1 deg Frequency factor 1 logioA Ref. ... 

There is an opportunity for multiple substitution, via regeneration of the cyclohexadienyl cationic complex, after the initial nucleophile addition. Unfortunately, the required exo hydride abstraction is severely retarded by the exo substituent (nucleophile unit) (equation 88). 

When the six-membered ring is strongly activated, e.g. by a 6-nitro substituent, nucleophilic attack by alcoholic alkali may lead to rearrangement as in Scheme 15 (81T3423). These reactions are analogous to those occurring in the corresponding 1,2,4-triazolo-pyridine and -pyrimidine systems. 

In simple carbonyl compounds containing an a-amino substituent, nucleophilic additions generally occur via the cyclic chelate model. However, just as in the a-hydroxycaibonyl series, selectivities can be greatly influenced by substitution of the amino group. Examples of moderate to high stereoselection in both cyclic chelation and nonchelation controlled additions have been reported. 

Substituents at the 2- and 3-positions of the cyclopropane ring decrease the reactivity of the double bond (Table 2) but, if only one face of the ring system bears substituents, nucleophiles almost exclusively undergo trans addition relative to these substituents. Even the monomethyl derivative methyl 2-chloro-2-(2-methylcyclopropylidene)acetate (7 a) upon hydride addition with sodium borohydride afforded the cis-product cw-8a with high diastereoselectivity. Larger... 

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