Donor-acceptor interactions Michael acceptors

Besides the Lifshitz-van der Waals interactions, there are short-range (<0.2 nm) forces due to the donor-acceptor interaction(67-69) or the acid-base interaction.( o) xhe role of the acid-base interaction in polymer adhesion has been studied by Fowkes(7i 72) and Bolger and Michaels.( 73) Recently Good et have examined the role of the interfacial hydrogen bond... 

The mechanism of the former reactions, at least in principle, is fairly easy to understand. The anionic donor (enolate) interacts with chiral cationic species such as ammonium salts or metal cations, which differentiate the enantiofaces of the donor. In contrast, the latter reaction appears to be more compHcated since the chiral enolate complex recognizes the enantiofaces of the Michael acceptor. The following discussions maybe helpful in some cases for the better understanding of the prochiral acceptor reaction [2,3]. 

These results show that not only are novel radical-anions formed by interaction of Michael acceptors with I and related organometallics, but that the epr technique is an excellent method of ascertaining the basicity of the latter, as well as shedding light on the subtle aspects of the donor-acceptor interactions involved. The chemistry in Equation (1) is exactly analogous to that which occurs in initiation of Atom Transfer Radical Polymerization (ATRP) catalysis, Equation (2), where the same basic metal complexes and others are active (X = halide). 

To interpret the stereochemical outcome of this tandem reaction, the authors suggested a transition-state model, in which the bifunctional catalyst 121 interacted with the Michael donor and acceptor in a mode that the enolic Michael donor approached the acceptor from its Si face providing the observed adduct (Figure 6.42). 

Mechanistically, this catalytic reaction proceeds via enantioselective Michael addition and the subsequent protonation of the transient enol intermediate in a stereoselective manner (Scheme 9.27). Thus, the authors proposed that the catalysts serve as a dual-function catalyst for this tandem reaction namely, the stereochemical outcome of this tandem reaction resulted from a network of hydrogen-bonding interactions between the catalyst with the reacting donor and acceptor in the addition step and, subsequently, with the putative enol intermediate (78) in the protonation step (Scheme 9.28). 

In certain instances, however, Lewis-acid-mediated Michael additions show a slight dependence on the geometry of both the donor and acceptor (vide supra). Hence, the first-order analysis must be modified to include the differential effects induced by the double-bond substitution patterns. By consideration of these effects and by minimization of the adverse gauche-type interactions, trends in Lewis-acid-mediated additions where the conjugate addition is likely to be the actual product-determining step can be rationalized. 

It has to be pointed out that simple enolizable aldehydes and ketones, which are not acidic enough compounds to be directly used as pro-nucleophiles in this context, can nevertheless be employed as Michael donors in the reaction with enals or enones, which have been previously activated as the corresponding iminium ion, but their use requires prior activation via enamine activation. In these cases, it is usually proposed that the amine catalyst is involved in a dual activation profile interacting with both the Michael donor and the acceptor, although the enamine activation of the pro-nucleophile is mandatory for the reaction to occur, the activation of the acceptor being of less relevance in most cases. For these reasons, this chemistry has been covered in Chapter 2. 

Figure 10.2 The catechol side chain of DOPA is capable of forming reversible interactions and irreversible covalent bonds. The benzene ring of the catechol is capable of n-n interactions (A). Catechol -OH groups can function both as a hydrogen bond donor and acceptor (B). Catechol forms strong coordination complexes with metal ions (C). When catechol is oxidized to form highly reactive quinone (D), it can undergo dimer formation (E) and subsequently polymerize into oligomers. Quinone can form intermolecular crosslinking with nucleophile such as -NH2 through Schiff base substitution (F) and Michael-type addition (G).

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