Electron Donors Nucleophiles

9 monofunctional, open-chain C,H,0-compounds (in the order of rising oxidation number)  

General problems with synthetic organic reactions are discussed together with some practical solutions for specific examples. These problems include 9 regio- and stereoselectivity by exploitation of the substrates stereochemistry (e.g., p. 20ff.) and differentiated nucleophilicity (p. 24f, 44f, 56ff.) 

9 selectivity in competitive cyclisation, polymerieation, and dialkylation reactions (p.23ff.) 

Within the last decade remarkable progress has been made with highly stereoselective addition reactions to C = C and C = 0 double bonds using chiral reagents. These reagents include  

General problems with synthetic organic reactions are discussed together with some practical solutions for specific examples. These problems include 

0 selectivity in competitive cycli2ation, polymerization, and dialkylation reactions (P- 23ff.) 

0 asymmetric boron enolates for aldol additions (p. 61 f) and allylboranes (p. 67ff.) 0 allyltin reagents steered by Lewis acid catalysts bound to chiral groups for Grignard reactions (p. 66f)  

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The reaction of perfluoroalkyl iodides with electron donor nucleophiles such as sodium arene and alkane sulfinates in aprotic solvents results in radical addition to alkenes initiated by an electron-transfer process The additions can be carried out at room temperature, with high yields obtained for strained olefins [4 (equations 3-5)... 

The reactions 33 between tetrachloro-A-n-butylphthalimide (113) and n-butylamine275 in aprotic and apolar media (cyclohexane, benzene, toluene, xylenes) show a third experimental reaction order in the amines explained by the formation of a complex (n-jr-like) between the electron acceptor substrate (the derivative of the phthalimide) and the electron donor nucleophile (the amine). In mixed solvents (such as the mixtures cyclohexane/aromatic solvents) the kinetic investigation reveals the presence of a competition between the electron donor solvent and the amine in complexing the substrate. 

All these observations point to the occurrence of a 8 2 rather than an outer sphere, dissociative electron-transfer mechanism in cases where steric constraints at the carbon or metal reacting centres are not too severe. It is, however, worth examining two other mechanistic possibilities. One of these is an electrocatalytic process of the Sg -type that would involve the following reaction sequence. If, in the reaction of the electron donor (nucleophile), the bonded interactions in the transition state are vanishingly small, the alkyl radical is formed together with the oxidized form of the electron donor, D . Cage coupling (144) may then occur, if their mutual affinity is... 

One important feature of ion-radical organic reactions consists of a possibility to nudge them by the introduction of active reactants. Thus, in the reaction of an electron acceptor with electron donors (nucleophiles), the addition of a tiny amount of a nucleophile, which is more active at initiation of the one-electron transfer allows the less reactive nucleophile to start its own chain propagation. A method called entrainment is widely used in chemical practice as a recent example (see Schmidt et al. 2007). 

These previous examples are reactions where the electron donor (nucleophile) supplies the electronic push to accomplish bond breaking. Many nucleophiles, either neutral or anionic, have lone pairs of electrons that are easily donated. They can be donated to even weak electron acceptors (electrophiles). 

In the previous chapter the formation of carbon-carbon bonds was discussed in terms of polar or two-electron processes. In such reactions one carbon serves as an electron donor (nucleophile) and a second carbon serves as an electron pair acceptor (electrophile). The result of the donor-acceptor interaction of these two species is a new carbon-carbon bond in which the electron pair is shared by die donor and acceptor. 

Excited alkenes and alkynes are highly reactive towards nucleophiles, acids and electron donors. Nucleophilic addition and photoreduction (entries 5 and 8) predominate with alkenes carrying electron-withdrawing substituents. Some electron-rich alkenes or alkynes readily undergo photoprotonation (entries 6 and 7). 

Irradiation of aromatic compounds in the presence of a good electron donor (nucleophile) may promote electron transfer from this species to the excited aromatic substrate in order to form an anion-radical intermediate, which releases the leaving group (Sr-nIAt, where R = radical, = anion Scheme 6.93).836 838 840 852 In contrast, electron transfer from the excited aromatic substrate to good electron acceptors, followed by the reaction of the cation-radical intermediate thereby formed with a nucleophile, is possible when an electron-donating group is present on the aromatic substrate (Sr+n1 Ar, where + = cation not shown).836... 

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