Nucleophilic addition secondary interaction

The selectivity of RNH2 on M/A1203 and Raney catalysts decreased in the order Co Ni Ru>Rh>Pd>Pt. This order corresponds to the opposite sequence of reducibility of metal-oxides [8] and standard reduction potentials of metalions [9], The difference between Group VIII metals in selectivity to amines can probably been explained by the difference in the electronic properties of d-bands of metals [3], It is interacting to note that the formation of secondary amine, i.e. the nucleophilic addition of primary amine on the intermediate imine can also take place on the Group VIII metal itself. Therefore, the properties of the metal d-band could affect the reactivity of the imine and its interaction with the amine. One could expect that an electron enrichment of the metal d-band will decrease the electron donation from the unsaturated -C=NH system, and the nucleophilic attack at the C atom by the amine [3], Correlation between selectivity of metals in nitrile hydrogenation and their electronic properties will be published elsewhere. 

Cyclohexanones in which the chair inversion is constrained by substitution undergo diastereoselective nucleophilic addition, the nature of which (i.e., preferentially axial or preferentially equatorial) depends on the nature of the substituents. The explanation of this effect has been extensively explored [95, 189, 193-197]. The simplest explanation, shown in Figure 8.5, involves a distortion of the carbonyl group from planarity in such a way as to improve 71-type donation from the ring C— bond or the axial bond (often a C—H bond) in the a position, whichever is the better donor. A secondary effect is the improved interaction between the distorted n orbital and the HOMO of the... 

The slow nucleophilic addition of dialkylzinc reagents to aldehydes can be accelerated by chiral amino alcohols, producing secondary alcohols of high enantiomeric purity. The catalysis and stereochemistry can be interpreted satisfactorily in terms of a six-membered cyclic transition state assembly [46,47], In the absence of amino alcohol, dialkylzincs and benzaldehyde have weak donor-acceptor-type interactions. When amino alcohol and dialkylzinc are mixed, the zinc atom acts as a Lewis acid and activates the carbonyl of the aldehyde. Zinc in this amino alcohol-zinc complex is regarded as a kind of chirally modified Lewis acid. Various kinds of polymer-supported chiral amino alcohol have recently been prepared and used as ligands in dialkylzinc alkylation of aldehydes. 

Fullerenes are excellent electron acceptors. The early examples for the high electron affinity of fullerenes include efficient nucleophilic addition reactions of fullerenes with electron donors such as primary and secondary amines. Since then, there have been many studies of electron transfer interactions and reactions involving fullerene molecules. It is now well established that both ground and excited state fullerene molecules can form charge transfer complexes with electron donors. The photochemically generated fullerene radical anions as a result of excited state electron transfers serve as precursors for a wide range of functionalizations and other reactions. 

Notably, proline was unique for this transformation, as all the other chiral secondary amines tested failed to promote the reaction. Another well-estabhshed organo-catalyst (4), invented by MacMillan [27], and unable to form secondary interactions with electrophiles like proUne, was used in the addition of aldehydes to indolyl and other carbocations derived from alcohols. The formation of stable carbenium ions from alcohols and their compatibility with water, generated by the organocatalytic cycle (formation of enamines from the corresponding carbonyl derivatives), was estabUshed by Cozzi in a SnI nucleophilic substitution of alcohols in the presence of water [28]. The enamine formed in situ by the MacMUlan catalyst approaches the carbocation from the less hindered side and the hindrance of the incipient carboca-tion controls the stereoselectivity of the reaction (Scheme 26.2) [29]. 

The group proposed that the hydrocyanate underwent a formal Brpnsted base interaction with the guanidine catalyst, thus activating the nucleophile for addition (Fig. 9). In contrast to the bifunctional catalysts, the guanidines are basic enough to activate the substrates without the need for secondary moieties. 

Cationic polymerization of alkenes involves the formation of a reactive carbo-cationic species capable of inducing chain growth (propagation). The idea of the involvement of carbocations as intermediates in cationic polymerization was developed by Whitmore.5 Mechanistically, acid-catalyzed polymerization of alkenes can be considered in the context of electrophilic addition to the carbon-carbon double bond. Sufficient nucleophilicity and polarity of the alkene is necessary in its interaction with the initiating cationic species. The reactivity of alkenes in acid-catalyzed polymerization corresponds to the relative stability of the intermediate carbocations (tertiary > secondary > primary). Ethylene and propylene, consequently, are difficult to polymerize under acidic conditions. 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

Ammonia, a soft nucleophile for being neutral, reacts with methyl acrylate 100 in methanol in conjugate manner to give the primary amine 101. The reaction continues in the same sense and the secondary amine 102 and the tertiary amine 103 are formed successively [39]. It is to be noted that ammonia, and other primary and secondary amines, do not react with simple esters to form amides. Combine this with the known observation that attack at the carbonyl group is irreversible and also rate determining [40], the above conjugate addition must necessarily be a product of kinetic control, supported by HOMO-LUMO interaction. 

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