Donor-acceptor concept

The impact of the donor-acceptor concept. V. Gutmann and R. Resch, Comments Inorg. Chem., 1982,1, 265-278 (44). 

Much of the current research into halogen bonding involves organohalogen or inorganic halide acceptors. However, the dihalogens (X2) and inter halogens (XY) continue to attract attention. This review will focus specifically on structural and theoretical studies of such systems over the past decade. While polyhalide anions are closely related to the neutral donor-acceptor concepts that are the focus here, they will be discussed only when necessary to elaborate on the central theme of the review. 

The cu-bonding model provides a more complete and fundamental description of hypervalent molecules that are often interpreted in terms of the VSEPR model.144 In the present section we examine some MX species that are commonly used to illustrate VSEPR principles, comparing and contrasting the VSEPR mnemonic with general Bent s rule, hybridization, and donor-acceptor concepts for rationalizing molecular geometry. Tables 3.32 and 3.33 summarize geometrical and NBO/NRT descriptors for a variety of normal-valent and hypervalent second-row fluorides to be discussed below, and Fig. 3.87 shows optimized structures of the hypervalent MF species (M = P, S, Cl n = 3-6). 

Miiller and co-workers have developed an enantioselective enzymatic crossbenzoin reaction (Table 2) [43, 44], This is the first example of an enantioselective cross-benzoin reaction and takes advantage of the donor-acceptor concept. This transformation is catalyzed by thiamin diphosphate (ThDP) 23 in the presence of benzaldehyde lyase (BAL) or benzoylformate decarboxylase (BFD). Under these enzymatic reaction conditions the donor aldehyde 24 is the one that forms the acyl anion equivalent and subsequently attacks the acceptor aldehyde 25 to provide a variety of a-hydroxyketones 26 in good yield and excellent enantiomeric excesses without contamination of the other cross-benzoin products 27. The authors chose 2-chlorobenzaldehyde 25 as the acceptor because of its inability to form a homodimer under enzymatic reaction conditions. 

P. Dtinkelmann, D. Kolter-Jung, A. Nitsche, A. S. Demir, P. Siegert, B. Lingen, M. Baumann, M. Pohl, M. Muller, Development of a donor-acceptor concept for enzymatic cross-coupling reactions of aldehydes, the first asymmetric cross-benzoin condensation. f Am. Chem. Soc. 2002, 124, 12084-12085. 

The selective donor-acceptor concept can be transferred to other ThDP-dependent enzymes. For example, enantiopure mixed benzoins were obtained when 2-chlorobenzaldehyde reacted with a variety of selective donor aldehydes in the presence of BAL [67]. By performing various cross-benzoin condensation reactions with this enzyme, not only new selective donors but also additional aldehydes reacting selectively as acceptors, such as 2-iodobenzaldehyde or 2,6-difluorobenzaldehyde, could be identified. Again all the mixed benzoins generated exhibited an R-configuration and were obtained with high to excellent enantiomeric excesses [69]. 

The scope of donor-acceptor concept is extremely broad and includes interaction such as electrostatic, solvation and complexation of metal ions, etc. 

Donor-acceptor concept and reactivity of heteroaromatic compounds 92H(33)1129. 

The cyclopropylimine ynoline rearrangement (equation 68) has been exploited by Stevens in alkaloid syntheses. - Wenkert s cyclopropylcarbinyl rearrangement (equation 69) served extremely well in the design of 1,4-dicarbonyl synthons or 3,-y-unsaturated carbonyl compounds which then were expressed in numerous syntheses of terpenoid and alkaloid natural products. Donor-acceptor concepts continue to be express in the applicability of these rearrangements to organic synthesis. 

Duenkelmann, P., Kolter-Jung, D., Nitsche, A., Demir, A. S., Siegert, P., Lingen, B., Baumann, M., Pohl, M., Mueller, M. Development of a Donor-Acceptor Concept for Enzymatic Cross-Coupling Reactions of Aldehydes The First Asymmetric Cross-Benzoin Condensation. J. 

Bilayer devices [27,65] apply the donor-acceptor concept introduced above here the exciton is dissociated at their interface, leading to holes on the donor and electrons on the acceptor. Thus, the different types of charge carriers may travel independently within separate materials and bimolecu-lar recombination is largely suppressed. Therefore fight intensity-dependent... 

CHEC-I <84CHEC-I(5)167> described the known situation until the early 1990s when Begtrup put some order in the reactivity of heteroaromatic compounds <92H(33)l I29>. His model is based on the donor-acceptor concept and contains many references and useful discussions concerning pyrazoles and pyrazole-A-oxides. The period between 1983 and 1989 has been covered by Katritzky and Taylor <90AHC(47)139>. 

Here also, for a general theory of nucleophilic reactions of pyrazole-A-oxides in the framework of the donor-acceptor concept see <92H(33)U29>. The most relevant results in this field are those... 

Thiazole (28) may be considered to be derived from benzene by replacing a CH group with a nitrogen atom and a CH=CH group with a sulfur atom. Hence, the chemistry of thiazole shows similarities to those of five- and six-membered aromatic rings. The closest system is thiophene. The donor-acceptor concept which has been successfully applied to 1,3-diazoles <92H(33)ll29> can be extended to thiazole (28) in order to predict both reactivity and regioselectivity. 

An analysis, based on the donor-acceptor concept concerning the general reactivity of the thiazole ring, shows that the 2-position is electron-deficient while the 5-position is electron-rich and the 4-position is neutral <92ACS372,92H(33)l 129>. In consequence, the reactivity of the substituents linked to carbon atoms of the thiazole ring depends on their position in the heterocyclic nucleus. As an example, 2-alkylthiazoles are more easily deprotonated in a-position than 5-alkyl derivatives 2-halothiazoles are more reactive towards nucleophilic displacement than 4- or 5-halothiazoles. In general, the different i-electron delocalization exhibited by the thiazole ring makes the 2-position the most reactive. 

The scope of the donor-acceptor concept is extremely broad and encompasses many types of chemical interaction, including the solvation and complexation of metal ions and the formation of coordination compounds by transition metals (see Topics El, E3, H2 and H9). Many chemical reactions also depend on donor-acceptor interactions. For example, the hydrolysis of SiCl4 to give Si... 

Notably, the spectrochemical series introduced in Section 3.3.1 is also rationalized partly by the ir-donor/acceptor concept, since ligands on the left-hand side tend to be ir-donors, whose interaction with the octahedral Ug set tends to raise the energy of this set and decrease A o, whereas ligands on the right-hand side tend to be ir-acceptors, whose interaction lowers the Ug set and increases A Q. Ligands which are pure lone pair after the bonding pair, or vacant orbitals) do not participate in ir overlap at all, and thus tend to lie in the middle of the spectrochemical series. 

Throughout this paper we seek to illustrate the many similarities between ground-state and excited-state NBO descriptors, as well as their characteristic differences. We believe these illustrations justify considerable optimism that key features of excited-state potential-energy surfaces can be fruitfully described in terms of NBO donor-acceptor concepts that strongly parallel those for ground states. In this manner we hope to expand awareness of available NBO-based techniques for excited states, encouraging intrepid spectroscopic investigators to explore and extend applications of NBO donor-acceptor concepts to frontier areas of molecular and supramolecular photochemistry. 

The statements concerning the reactivity and selectivity of the reactions of azoles with electrophiles and nucleophiles can be further defined on the basis of the donor-acceptor concept [142]. 

Although there have been several approaches to porphyrinic catenanes and rotaxanes [46-48], these have been based on assembly through charged [49] or neutral [50,51] 7r-donor/acceptor concepts, or by using coordination chemistry [47]. Nevertheless, there is a series of non-porphyrinic amide-based catenanes and rotaxanes that have received considerable attention [52,53]. The primary recognition factor in these systems is H-bonding. Gunter [54] has extended these systems to include porphyrinic supramolecu-les that use similar H-bonded amide-based motifs for the self-assembly of multiporphyrin catenanes and rotaxanes. 

The proton donor-acceptor concept of acids and bases... 

These two sets of scales agree in their general trend, but are often at variance when values for any two particular solvents are taken. Some intercorrelations have been presented by Taft et al., e.g., the parameters E, AN and Z can be written as linear functions of both a and 7t. Originally, the values of E,- and Ji were conceived as microscopic polarity scales reflecting the local polarity of the solvent in the neighborhood of solutes ( effective dielectric constant in contrast to the macroscopic one). In the framework of the donor-acceptor concept, however, they obtained an alternative meaning, based on the interrelationships found between various scales. Along these lines, flie common solvents may be separated into six classes as follows. 

U. Salzner, Does the donor-acceptor concept work for designing synthetic metals 1. Theoretical investigation of poly(3-cyano-3 -hydroxybithiophene), /. Phys. Chem. B, 106, 9214-9220 (2002). 

We will start this chapter with a brief review of the origin of the donor-acceptor concept, followed by a short summary of the key chemistries that are used for the synthesis of D-A copolymers e.g., Suzuki coupling and Stille coupling), with a focus on their application towards the polymer synthesis. The classical step-growth polymerization will be next discussed within the context of Pd-catalyzed polycondensation. Then we will briefly discuss two recent, interesting advances in the synthesis of D-A copolymers Pd-free polycondensation and chain-growth Suzuki polymerization. Since the synthesis of D-A... 

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