Nucleophilic reagents, definition

Whereas carbenoid character is definitely present in metalated alkyl vinyl ethers, lithiated alkyl and aryl vinyl sulfides and thioesters, which are easily available by hydrogen-lithium exchange, do not display carbenoid-typical reactions . They rather behave like nucleophilic reagents, so that their discussion is beyond the scope of this overview despite their utility in synthesis The same appiies to various derivatives of enamines, deprotonated in the vinyiic a-nitrogen position - . 

The syntheses of iron isonitrile complexes and the reactions of these complexes are reviewed. Nucleophilic reagents polymerize iron isonitrile complexes, displace the isonitrile ligand from the complex, or are alkylated by the complexes. Nitration, sulfonation, alkylation, and bromina-tion of the aromatic rings in a benzyl isonitrile complex are very rapid and the substituent is introduced mainly in the para position. The cyano group in cyanopentakis(benzyl isonitrile)-iron(ll) bromide exhibits a weak "trans" effect-With formaldehyde in sulfuric acid, benzyl isonitrile complexes yield polymeric compositions. One such composition contains an ethane linkage, suggesting dimerization of the transitory benzyl radicals. Measurements of the conductivities of benzyl isonitrile iron complexes indicate a wide range of A f (1.26 e.v.) and o-o (1023 ohm-1 cm.—1) but no definite relationship between the reactivities of these complexes and their conductivities. 

Hydrated-electron reactions are, by definition, electron-transfer processes, which are not very common in classical organic chemistry. The kinetic studies have shown, however, that the eleotron behaves analogously to a classical nucleophilic reagent and, although this analogy... 

In this case, however, 1-isopropyl-4,4-difluoro-2-hexafluoroethyl-3-tri-fluoromethyl-li/-azetine is formed, which points to a direct reaction of olefin 1 with isopropylamine. This suggests that steric factors play a definite role in this reaction. Indeed, the interaction between olefin 1 and ethylamine is the second route of this reaction. If, however, no isomerization of perfluorolefin takes place under the action of the starting alkylamine, or if a symmetrically substituted structure is formed, then only one heterocyclic compound is obtained. This is realized for perfluoro-5-azanon-4-ene. Irrespective of the character of the nucleophilic reagent, diazete derivative 20 is produced. 

The Lewis definitions of acid-base interactions are now over a half a century old. Nevertheless they are always useful and have broadened their meaning and applications, covering concepts such as bond-formation, central atom-ligand interactions, electrophilic-nucleophilic reagents, cationic-anionic reagents, charge transfer complex formation, donor-acceptor reactions, etc. In 1923 Lewis reviewed and extensively elaborated the theory of the electron-pair bond, which he had first proposed in 1916. In this small volume which had since become a classic, Lewis independently proposed both the proton and generalized solvent-system definitions of acids and bases. He wrote ... 

This might be an appropriate time to remind the reader of important definitions in chemistry—electrophilic and nucleophilic compounds. An electrophilic reagent is a compound that in the course of its reaction with another compound acquires electrons or seeks to share electrons belonging to some other atom or ion. The nucleophilic reagent is one that transfers electrons or shares its electrons with another atom. Hydroxyl groups are thus nucleophilic.)... 

Although the final products of the amination of 5-bromo-4-R-pyrimidines are the 6-amino-4-R-pyrimidines,106 107 two mechanisms, by which these same products are formed, have been recognized. The substrate is converted in part to the product via C-6 adduct 41, whereas the remaining part follows an ANRORC mechanism requiring attack of the reagent at C-2.105 Evidence for the existence of C-2 adducts as intermediates in nucleophilic substitutions is hard to obtain. However, the formation of an adduct of this kind (34) from 4,6-diphenyIpyrimidine was definitely proved by H-NMR measurements (Table XI).98 C-6 adducts such as 42-44 have also been described (see also Table XI) and shown to be the sole intermediates in the amination of 5-chIoro-4-te/T-butyIpyrimidine and 5-X-2,4-di-tert-butyIpyrimidines (X = Cl, Br), the ANRORC mechanism remaining excluded with the latter substrates.98... 

Thus, by definition, electrophiles are electron-pair acceptors and nucleophiles are electron-pair donors. These definitions correspond closely to definitions used in the generalized theory of acids and bases proposed by G. N. Lewis (1923). According to Lewis, an acid is any substance that can accept an electron pair, and a base is any substance that can donate an electron pair to form a covalent bond. Therefore acids must be electrophiles and bases must be nucleophiles. For example, the methyl cation may be regarded as a Lewis acid, or an electrophile, because it accepts electrons from reagents such as chloride ion or methanol. In turn, because chloride ion and methanol donate electrons to the methyl cation they are classified as Lewis bases, or nucleophiles ... 

Although no definitive study of scavengers in photoaffinity labeling experiments has yet been made, the present indications are that thiols are the most suitable reagents. It is expected that as strong nucleophiles they should react with most photogenerated species which are electrophiles as well as with free radicals which can abstract hydrogen atoms from the... 

Markovnikov orientation the 1,2-addition of an unsymmet-rical electrophilic reagent to an alkene with attachment of the more positive moiety of the reagent (H in the original definition) to the less substituted carbon Michael addition conjugate addition addition of a nucleophile to the /3-position of a carbon-carbon double or triple bond that is conjugated to a carbonyl or other electron-withdrawing group... 

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