Nucleophilic addition reaction variations

In nucleophilic addition reactions, the imbalance results in exceeding jSnuc i nuc e normalized structure-reactivity coefficients that are the analogs of a and /3, respectively, in proton transfers.Note that since is usually obtained from the variation of a phenyl substituent, it is equal to the normalized Hammett p value. A representative example is shown in equation (59) where = 0-67 and i nuc — 037 Again, the exalted coefficient is the result of the negative charge at the transition state 76 being closer to the Z substituent than it is in the adduct. 

In summary, there now exists a body of data for the reactions of carbocations where the values of kjkp span a range of > 106-fold (Table 1). This requires that variations in the substituents at a cationic center result in a >8 kcal mol-1 differential stabilization of the transition states for nucleophile addition and proton transfer which have not yet been fully rationalized. We discuss in this review the explanations for the large changes in the rate constant ratio for partitioning of carbocations between reaction with Bronsted and Lewis bases that sometimes result from apparently small changes in carbocation structure. 

To what extent are the variations in the rate constant ratio /cs//cpobserved for changing structure of aliphatic and benzylic carbocations the result of changes in the Marcus intrinsic barriers Ap and As for the deprotonation and solvent addition reactions It is not generally known whether there are significant differences in the intrinsic barriers for the nucleophile addition and proton transfer reactions of carbocations. 

Nucleophilic additions to carbonyl groups lead to alcohols which on dehydration, furnish alkenes70,71. This two-step protocol has been extremely useful for diene and polyene synthesis with wide variation in the carbonyl substrate and the nucleophilic addendum. Diene synthesis using aldol-type condensation as well as phenyl sulphonyl carbanion (the Julia reaction) are also discussed in this section. 

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

A variation of GTP, referred to as aldol GTP, involves polymerization of a silyl vinyl ether initiated by an aldehyde [Sogah and Webster, 1986 Webster, 1987]. Both initiation and propagation involve nucleophilic addition of the vinyl ether to the aldehyde carbonyl group with transfer of the trialkyl silyl group from vinyl ether to the carbonyl oxygen (Eq. 5-82). The reaction has similar characteristics as GTP. The product is a silated poly(vinyl alcohol) (PVA), which can be hydrolyzed by acid to PVA (Eq. 5-83). 

Aromatic compounds undergo many reactions, but relatively few reactions that affect the bonds to the aromatic ring itself. Most of these reactions are unique to aromatic compounds. A large part of this chapter is devoted to electrophilic aromatic substitution, the most important mechanism involved in the reactions of aromatic compounds. Many reactions of benzene and its derivatives are explained by minor variations of electrophilic aromatic substitution. We will study several of these reactions and then consider how substituents on the ring influence its reactivity toward electrophilic aromatic substitution and the regiochemistry seen in the products. We will also study other reactions of aromatic compounds, including nucleophilic aromatic substitution, addition reactions, reactions of side chains, and special reactions of phenols. 

This chapter presents computational studies of organic reactions that involve anions. These reactions are usually not grouped together in textbooks. However, these reactions are fundamentally variations on a theme. Anions, acting as nucleophiles, can attack sp carbon atoms we call these as nucleophilic substitution reactions that follow either the S l or 8 2 mechanism. Reactions where the nucleophile attacks sp or sp carbon atoms are addition reactions. The 1,2- and... 

Alternatively, the SrnI mechanism can be coupled to nucleophilic additions as was the case during the formation of substituted indole 212 [182], an interesting variation on a reaction first reported by Beugelmans and Roussi [183]. 

The fruitfulness of the idea of a stepwise addition with an independent variation of the addends was brilliantly illustrated by Normant s studies, which resulted in the elaboration of a general method of alkene synthesis based on the reaction of alkyne carbometallation. Basically this reaction represents a case of the well-known nucleophilic addition to a carbon-carbon triple bond. In the Normant reaction, however, the initial addition of a nucleophile (an organome-tallic reagent) across the triple bond results in the formation of a stabilized carbanion-like intermediate equivalent to a vinyl carbanion. This intermediate can similarly be further reacted with an external electrophile. Most typically, copper-modified Mg or Li reagents, which are unable to react with acidic acetylenic hydrogens, are used in this sequence. 

Most reactions of carbonyl groups occur by one of four general mechanisms nucleophilic addition, nucleophilic acyl substitution, alpha substitution, am carbonyl condensation. These mechanisms have many variations, just a alkene electrophilic addition reactions and 8 2 reactions do, but the varia tions are much easier to learn when the fundamental features of the mechanisms are understood. Let s see what the four mechanisms are and what kinds of chemistry carbonyl groups undergo. 

An intramolecular variation of the Heck reaction was used in the construction of the monoterpene alkaloid ( )-oxerime 395 [138]. Nucleophilic addition to 392 of an allyl anion equivalent produced 393, a precursor to an intramolecular Heck cyclization. Palladium-catalyzed ring closure afforded 394, an intermediate in the synthesis of the natural product 395. 

In contrast to these transformations, the introduction of longer alkyl chains by copper-promoted 1,6-addition reactions to -steroids normally proceeds with unsatisfactory a p ratios . In some cases, however, the diastereoselectivity could be improved by fine tuning of the reaction conditions for example, the ratio of a- and /1-epimeric products in the copper-catalyzed 1,6-addition of 4-pentenylmagnesium bromide to 17/1-acetoxy-4,6-androstadien-3-one rose from 58 42 to 82 18 upon variation of the number of equivalents of the nucleophile and the solvent composition (equation 25). ... 

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