Nucleophilic capture conjugation

Many of these reactions support a measure of thermodynamic control in nucleophilic capture Conjugated radicals or products formed with release of ring strain are favored. For example, the addition of ethanol to radical cation 110 + is regiospecific, forming the more stable (benzylic) intermediate 111 + the capture of 112 + likewise forms a benzylic radical (113 ). Radical cation 48 + generates a... 

The vinylcyclopropane radical cation, is another radical cation of structure type B, which is stabilized by conjugation. Its proposed structure was based exclusively on ab initio calculations (B3LYP/6-31G ) because the electron-transfer photochemistry of this species failed to provide clear-cut CIDNP effects [128], In this context it is worth noting that product studies cannot, in principle, establish the cyclopropane radical cation structure type. Irrespective of the structure, nucleophilic capture is expected to result in the cleavage of the strained ring. 

Conjugate Addition with Intramolecular Nucleophilic Capture... 

As with other electrophilic substitutions, conjugate addition can be used in conjunction with nucleophilic capture of an indolenine intermediate. Marko and coworkers used such a reaction to synthesize a versatile intermediate in alkaloid synthesis [298]. The amide 106-A cyclized to 106-B, when exposed to Si02 in CH2CI2. The nucleophilic addition was then conducted in a separate step. The... 

Section 10 10 Protonation at the terminal carbon of a conjugated diene system gives an allylic carbocation that can be captured by the halide nucleophile at either of the two sites that share the positive charge Nucleophilic attack at the carbon adjacent to the one that is protonated gives the product of direct addition (1 2 addition) Capture at the other site gives the product of conjugate addition (1 4 addition)... 

As in the case of addition reactions of carbon nucleophiles to activated dienes (Section HA), organocopper compounds are the reagents of choice for regio- and stereoselective Michael additions to acceptor-substituted enynes. Substrates bearing an acceptor-substituted triple bond besides one or more conjugated double bonds react with organocuprates under 1,4-addition exclusively (equation 51)138-140 1,6-addition reactions which would provide allenes after electrophilic capture were not observed (cf. Section IV). 

Besides direct nucleophilic attack onto the acceptor group, an activated diene may also undergo 1,4- or 1,6-addition in the latter case, capture of the ambident enolate with a soft electrophile can take place at two different positions. Hence, the nucleophilic addition can result in the formation of three regioisomeric alkenes, which may in addition be formed as E/Z isomers. Moreover, depending on the nature of nucleophile and electrophile, the addition products may contain one or two stereogenic centers, and, as a further complication, basic conditions may give rise to the isomerization of the initially formed 8,y-unsaturated carbonyl compounds (and other acceptor-substituted alkenes of this type) to the thermodynamically more stable conjugated isomer (Eq. 4.1). 

The reaction is conjugate addition of the thiolate anion 2 to the enone 7 making an enolate intermediate that captures a proton from PhSH 8 to give the target molecule 4 and regenerate the nucleophile 2. 

Similar to the addition reactions of acceptor-substituted dienes (Scheme 16), the outcome of the transformation depends on the regioselectivity of the nucleophilic attack of the organocopper reagent (1,4- vs. 1,6-addition) and of the electrophilic capture of the enolate formed. The allenyl enolate obtained by 1,6-addition can afford either a conjugated diene or an allene upon reaction with a soft electrophile, and thus opens up the possibility to create axial chirality. The first copper-mediated addition reactions to Michael acceptors of this type, for example, 3-alkynyl-2-cyclopentenone 75,... 

Alternatively, interconversion between the stereoisomeric allyl cations can take place by capture of a nucleophile at either end, followed by rotation about the more or less normal single bond, and then regeneration of the cation by ionisation. Interconversion between the corresponding anions can take place similarly by cr coordination (771) to a metal at one end or the other. Because of the availability of these pathways, experimental measurements of the barrier to rotation have confirmed that it is less than the very approximate theoretical value of 116 kJ mol-1 (28 kcal mol ). Furthermore, measurements have generally been made on significantly more substituted systems. Such substitution can stabilise the filled, half-filled or empty p orbital, or the double bond, even when these components are no longer conjugated, and so appropriate substituents lower the barrier to rotation. 

Addition of nucleophiles such as ammonia or alcohols or their conjugate bases to benzynes takes place very rapidly. These nucleophilic additions are believed to involve capture of the nucleophile, followed by protonation to give the substituted benzene. ... 

In reactions with organic molecules e q reacts as nucleophilic reagent it attacks molecules with low-lying molecular orbital, like aromatic hydrocarbons, conjugated olefinic molecules, carboxyl compounds, and halogenated hydrocarbons (Swallow 1982 Buxton 1982, 1987 Buxton et al. 1988). In the latter case, addition is usually followed by halide ion elimination, so the reaction can be considered as a dissociative electron capture. For instance, the reaction with chlorobenzene yields phenyl radical and chloride ion... 

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