Electron-donating reactant

Electrophilic fluorination is the process by which fluorine is delivered to an electron-donating reactant, such as an alkene, aromatic ring or carbanion, by a formal positive-fluorine reagent to form a carbon-fluorine covalent bond. These reactions are fast and have proven extremely valuable for some important fluorine-18-labelled radiopharmaceuticals. Over the years several reviews on electrophilic fluorination were written. The reader is encouraged to seek out these works for greater detail on the subject [7,68-70]. 

Example The EC mass spectrum of benzo[a]pyrene, C20H12, shows the negative molecular ion exclusively at m/z 252 (Fig. 7.14). This spectrum is representative for EC spectra of polycyclic aromatic hydrocarbons (PAHs) [87,88]. One particular PAH, fluoranthene, has recently received much attention as its ion serves as electron donating reactant ion in electron transfer dissociation (ETD, Chap. 9.15). 

A cyclic conjugate molecule composed of n bonds, A, B,..., X, and Y interacts at A with a reactant Z (Scheme 12). When the molecule is an electron donor (Scheme 12a), electrons delocalize from a to z. The resulting electron hole in a is supplied with an electron by the neighboring b. Similar delocalization sequentially follows from c to b, from d to c and so on. This is also the case with the opposite side Y, X,. It follows that the cyclic orbital interaction of a, b,.x, andy is important in the conjugated molecule. The orbitals are all electron-donating orbitals. When each neighboring pair of orbitals is combined out of phase, the interaction of the cyclic... 

Scheme 36 Cyclic transition states prefer electron-donating geminal a bonds at the Z-position in the reactants...

A majority of chemical reactions are liable to take place at the position and in the direction where the overlapping of HO and LU of the respective reactants is maximum in an electron-donating species, HO predominates in the overlapping interaction, whereas LU does so in an electron-accepting reactant in the reacting species which have SO MO s, these play the part of HO or LU, or both"... 

Table 22.5. Electron donating and accepting redox couples and limiting reactant species for various anaerobic and aerobic microbial metabolisms favored in fluid from a subsea hydrothermal vent, as it mixes with seawater...

Micellar rate enhancements of bimolecular, non-solvolytic reactions are due largely to increased reactant concentrations at the micellar surface, and micelles should favor third- over second-order reactions. The benzidine rearrangement typically proceeds through a two-proton transition state (Shine, 1967 Banthorpe, 1979). The first step is a reversible pre-equilibrium and in the second step proton transfer may be concerted with N—N bond breaking (17) (Bunton and Rubin, 1976 Shine et al., 1982). Electron-donating substituents permit incursion of a one-proton mechanism, probably involving a pre-equilibrium step. 

It was found that the rate constant of the forward decomposition of the surface bidentate formate (DCOO ) to produce D2 and C02 increased from 0.34X10 4 sec-1 under vacuum to 5.3 X10-4 sec-1 under ambient water. Electron donors such as NH3, CH3OH, pyridine, and THF also increased the decomposition rate the rate constants of the forward decomposition of the surface formates at 553 K were determined to be 28.0X10 4, 7.7X10 4, 8.1X10-4, and 6.0X10 4 sec-1 under NH3, methanol, pyridine, and THF vapors (0.4 kPa), respectively. It is likely that the driving force for the forward decomposition of the formate is electron donation of the adsorbed molecule to the Zn ion on which the bidentate formate adsorbs. The reactant-promoted mechanism for the catalytic WGS reaction on ZnO is illustrated in Scheme 8.2. 

These results led to a separation of the observed Diels-Alder reactivities into three categories (a) increase of the rate constants on increasing the Lewis acid character of the solvent as quantified by the AN parameter this behaviour reflects the interactions between the LUMO of the solvent and the HOMO of the reactants and is similar to Lewis acid catalysis (vide supra) (b) reaction retardation by electron donation, as quantified by the D-ji parameter the HOMOsoivent-LUMOreactant interactions are held responsible for this effect, representing an anti-Lewis acid interaction which increases the HOMO-LUMO gap and hence hampers the reaction (c) the Diels-Alder reactions show very small solvent effects and are relatively insensitive to specific reactant-solvent interactions, and... 

In most, if not all, of the transition metal-catalyzed organometallic reactions including the Pd- or Ni-catalyzed cross-coupling, solvents are not used merely to dissolve and dilute reactants and reagents. They often serve as cocatalysts or promoters and even reactants in a limited number of cases. More than 20 solvents have been used for the Pd- or Ni-catalyzed cross-coupling (Table 2). In the absence of any specific information, it is not unreasonable to choose first THF. Frequently, it is desirable to use one or more cosolvents typically for an increased level of solvent polarity and/or electron-donating ability. One of the most frequently used solvents for this purpose is DMF, but some others, such as NMP, pyridine and NMI (A-methylimidazole), have also been used frequently. In some extreme cases,... 

This may be the formation of the ion-pair (281) which in non-polar solvents forms the second cr-bond to give the cycloadduct (280), but in polar solvents forms (282) as shown in Scheme 76. However, the possibility cannot be ruled out that in non-polar solvents the cycloaddition is concerted although formally forbidden, the activation energy for this may be lowered because the reacting 77-bonds have electron-donating and electron-withdrawing substituents attached to them. When the reactant is dicyanoacetylene, the thienopyrrolizine is formed even in dichloromethane. 

There is also the possibility that one combination of reactants will give more isomeric products than the other. For example, this may occur when electron-donating substituents are present in the olefinic reactant. This effect is seen in the reciprocal reactions of methyl /3-bromomethacrylate with 1-hexene and of Z-l-bromo-l-hexene with... 

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