Donor-Acceptor-Complexes

This reaction is based on a stoichiometric reaction of multifunctional olefins (enes) with thiols. The addition reaction can be initiated thermally, pho-tochemically, and by electron beam and radical or ionic mechanism. Thiyl radicals can be generated by the reaction of an excited carbonyl compound (usually in its triplet state) with a thiol or via radicals, such as benzoyl radicals from a type I photoinitiator, reacting with the thiol. The thiyl radicals add to olefins, and this is the basis of the polymerization process. The addition of a dithiol to a diolefin yields linear polymer, higher-functionality thiols and alkenes form cross-linked systems. 

The most important attribute of the thiol-ene system is its insensitivity to oxygen thus, it is not inhibited by it. Another attractive feature is its very high cure speed. The disadvantage of the thiol-ene system is an unpleasant odor of the volatiles emitted from some polythiol compounds.  

Methoxy styrene (donor)-maleic anhydride (acceptor) 

Triethylene glycol divinyl ether (donor)-maleimide (acceptor) 

The anion HOHOH is isoelectronic with HjO and HF2, and a suitable valence-bond stmcture for it is similar to stmcture (25), with OH replacing F, i.e. 

The bridging O-H bonds of H5O2 and HOHOH have been estimated to have similar lengths and strengths.  

Firestone has also nsed Liimett stmctures to describe the electronic stmctures of synunetrical hydrogen-bonded molecnles. 

Two molecules of acetone or dioxan can interact with one molecule of Br to form the intermolecular complexes Me2C0.Br2.0CMe2 and C5H, O.Br2.0C5H (,. The reported Br-Br lengths of 2.28 A and 2.31 A are not sufficiently different from the length of 2.28 A for free Brj to indicate much interaction of Br2 with these solvents. The increased-valence stmcture (31), which we may generate from the Lewis stmcture (30) 

For each of the structures (31) and (32), there is an increased-valence representation for the 6-electron 4-centre bonding unit, and we remind the reader that its wave-function corresponds to the covalent component of the delocalized molecular orbital configuration for the six electrons (Section 10-2). The relevant atomic orbitals for Br are displayed in Fig. 2-6. 

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Wynne K, Galli C and Hochstrasser R M 1994 Ultrafast charge transfer in an electron donor-acceptor complex J. Cham. Phys. 100 4796-810... 

Morokuma K 1977. Why Do Molecules Interact The Origin of Electron Donor-Acceptor Complexes, Hydrogen Bonding, and Proton Affinity. Accounts of Chemical Research 10 294-300. 

The radical-catalyzed polymerization of furan and maleic anhydride has been reported to yield a 1 1 furan-maleic anhydride copolymer (89,91). The stmcture of the equimolar product, as shown by nmr analyses, is that of an unsaturated alternating copolymer (18) arising through homopolymerization of the intermediate excited donor—acceptor complex (91,92). 

However, strong protic acid catalysts are needed when 7T- or CJ-donor alkylating agents are used to produce carbocationic or highly polarized donor-acceptor-complexes as the reactive alkylating iatermediates ... 

Alternating copolymers of chloroprene have been prepared from a number of donor acceptor complexes in the presence of metal haUdes. 

The strength of the complexation is a function of both the donor atom and the metal ion. The solvent medium is also an important factor because solvent molecules that are potential electron donors can compete for the Lewis acid. Qualitative predictions about the strength of donor-acceptor complexation can be made on the basis of the hard-soft-acid-base concept (see Section 1.2.3). The better matched the donor and acceptor, the stronger is the complexation. Scheme 4.3 gives an ordering of hardness and softness for some neutral and ionic Lewis acids and bases. 

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

Factors Affecting the Stability of Donor-Acceptor Complexes ... 

Eullerene-based donor-acceptor complexes and ion-radical salts with tetrathia-fulvalenes, metalloporphyrins, and cyclic amines as donors 99UK23. 

The reaction is initiated by formation of a donor-acceptor complex 4 from acyl chloride 2, which is thereby activated, and the Lewis acid, e.g. aluminum trichloride. Complex 4 can dissociate into the acylium ion 5 and the aluminum tetrachloride anion 4 as well as 5 can act as an electrophile in a reaction with the aromatic substrate ... 

We have also used poly(propynoic acid) in our studies of the photochemical interaction of PCSs with dienophiles, such as maleic anhydride, tetracyanoethylene, and styrene. This photochemical reaction of Diels-Alder type is accompanied by the breakdown of the conjugation system and the formation of slightly colored adducts266. Together with the cycloaddition reaction, photodegradation of PPA and its adducts takes place. A cycloaddition reaction is always preceded by the formation of a donor-acceptor complex of a PCS with a dienophile. 

Catalysis by Electron Donor-Acceptor Complexes Kenzi TAMARU... 

However, these observations are not proof of the role of a donor-acceptor complex in the copolymcrization mechanism. Even with the availability of sequence information it is often not possible to discriminate between the complex model, the penultimate model (Section 7.3.1.2) and other, higher order, models.28 A further problem in analyzing the kinetics of these copolyincrizations is that many donor-acceptor systems also give spontaneous initiation (Section 3.3.6.3). 

Since intermediates usually cannot be observed directly, the exact nature of the donor-acceptor complex and the mechanisms for their interaction with radicals are speculative. At least three ways may be envisaged whereby complex formation may affect the course of polymerization ... 

The UV spectra suggest that the equilibrium between the diazonium ion and the solvent, on the one hand, and an electron donor-acceptor complex (8.58) on the other, lies on the side of the complex. The latter may possibly exist also as a radical pair (8.60) or a covalent compound (8.59). Dissociation of this complex within a cage to form an aryl radical, a nitrogen molecule, and the radical cation of DMSO is slow and rate-determining. Fast subsequent steps lead to the products observed. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

Bagal et al. (1975) investigated in more detail the role of donor-acceptor complexes in the azo coupling reaction of the 4-nitrobenzenediazonium ion with 2-naphthylamine-3,6-disulfonic acid and that of the 4-chlorobenzenediazonium ion with 2-naphthol-6-sulfonic acid. Their kinetic results are, as would be expected, compatible with the mechanisms shown in Schemes 12-74 or 12-75. 

Guryanova EN, Goldstein P, Romm IP (1975) In Donor-Acceptor Complexes. Wiley, New York... 

When the reaction of two compounds results in a product that contains all the mass of the two compounds, the product is called an addition compound. There are several kinds. In the rest of this chapter, we will discuss addition compounds in which the molecules of the starting materials remain more or less intact and weak bonds hold two or more molecules together. We can divide them into four broad classes electron donor-acceptor complexes, complexes formed by crown ethers and similar compounds, inclusion compounds, and catenanes. 

The analogous PEt2Ph derivative may also be used, but (AlMe3)2 and McjGa are unable to form donor-acceptor complexes with iridium. 

Schoeller WW (2003) Donor-Acceptor Complexes of Low-Coordinated Cationic p-Bonded Phosphorus Systems. 229 75-94... 

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