Nucleophilic reactions aliphatic carbons

This initial attack of the ozone molecule leads first to the formation of ortho- and para-hydroxylated by-products. These hydroxylated compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions. The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron acceptor groups. In summary, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups. 

There is an ongoing controversy about whether there is any stabilization of the transition state for nucleophilic substitution at tertiary aliphatic carbon from interaction with nucleophilic solvent." ° This controversy has developed with the increasing sophistication of experiments to characterize solvent effects on the rate constants for solvolysis reactions. Grunwald and Winstein determined rate constants for solvolysis of tert-butyl chloride in a wide variety of solvents and used these data to define the solvent ionizing parameter T (Eq. 3). They next found that rate constants for solvolysis of primary and secondary aliphatic carbon show a smaller sensitivity (m) to changes in Y than those for the parent solvolysis reaction of tert-butyl chloride (for which m = 1 by definition). A second term was added ( N) to account for the effect of changes in solvent nucleophilicity on obsd that result from transition state stabilization by a nucleophilic interaction between solvent and substrate. It was first assumed that there is no significant stabilization of the transition state for solvolysis of tert-butyl chloride from such a nucleophilic interaction. However, a close examination of extensive rate data revealed, in some cases, a correlation between rate constants for solvolysis of fert-butyl derivatives and solvent nucleophicity. " ... 

Nucleophilic reaction — The nucleophilic reaction is found locally on molecular sites showing electronic deficits and, more frequently, on carbons carrying electron-withdrawing groups. The molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds, as well as to specific functional groups. 

The Sn2 reaction involves the attack of a nucleophile from the side opposite the leaving group and proceeds with exclusive inversion of configuration in a concerted manner. In contrast to the popular bimolecular nucleophilic substitution at the aliphatic carbon atom, the SN2 reaction at the vinylic carbon atom has been considered to be a high-energy pathway. Textbooks of organic chemistry reject this mechanism on steric grounds [175]. 

The following gas-phase reactions of anions have been studied and will be briefly reviewed in the next sections proton transfer reactions, nucleophilic displacement reactions at both aliphatic and aromatic carbon centers, elimination reactions, electron transfer (ET) reactions, reactions with carbon-carbon double bonds and carbonyl functions, and association or complex (cluster)-forming reactions of various types. 

Several classes of nucleophilic substitution reactions can be distinguished (in addition to similar processes, such as acyl transfer to a nucleophile) substitution on aliphatic carbons, on aromatic carbons, and on elements other than carbon, such as metals in metal-ligand coordination complexes. Of these, substitution on aliphatic carbon is most frequently encountered. 

II. NUCLEOPHILIC REACTIONS AT ALIPHATIC CARBONS A. Primary Carbon Centers... 

In aprotic media, O2-- exhibits exceptional reactivity as a nucleophile towards aliphatic halogenated hydrocarbons and carbonyl carbons with adequate leading groups. Chapter 7 discusses the reaction chemistry of O2-- with electrophiles, reductants, transition metals, and radical species, and the chemistry of its conjugate acid (HOO) is outlined in Chapter 5. 

Nucleophilic substitution at aliphatic carbon is a reaction that plays an important role in organic synthesis. The use of the reaction in construction of carbon-carbon bonds will be discussed in Part B, but the role of the reaction in interconverting certain functional groups has such a basic place in organic chemistry that it is appropriate to illustrate some examples of the synthetic utility of nucleophilic substitution. The conversion of alcohols to alkyl halides and the conversion of alcohols to ethers are among the most fundamental organic transformations. 

Thus, this chapter focuses on the most investigated classes of metal-catalyzed coupling reactions—namely, reactions of aromatic and aliphatic electrophiles with main group carbon or nitrogen nucleophiles, reactions of aromatic halides with olefins, including enanti-oselective versions of these reactions, and direct coupling processes. The mechanisms of these reactions are presented with reference to the chapters on the stoichiometric steps of these catalytic processes. 

The reactions presented thus far are examples of nucleophilic aliphatic substitution reactions, in which a nucleophile substitutes for a leaving group at an aliphatic carbon. Nucleophilic aliphatic substitution proceeds with backside attack and inversion of configuration in a bimolecular process. In other words, the nucleophile collides with the electrophilic carbon atom to initiate the reaction. A shorthand symbol is used to describe this reaction Sjfl, where S means substitution, N means nucleophile, and 2 means bimolecular, or nucleophilic bimolecular substitution. Once a reaction is identified as Sn2, back-side... 

Chapter 22 returns to carbonyl chemistry and a discussion of the acid-base properties of carbonyl compounds. The proton on the a-carbon (directly attached to the carbonyl) is slightly acidic and removal with a suitable base leads to an enolate anion. Enolate anions react as nucleophiles in aliphatic substitution, acyl addition, and acyl substitution reactions. 

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