Electron-Deficient Free Radicals

SCHEME 3.2 Radiation will cause breaking of a C—H bond in CH4 and will form two free radicals. 

SCHEME 3.3 Two molecules made from Si and O atoms (a) S102, which is an analogue of CO2, and (b) 8104. The electrons participating in connectivity are highlighted in red. 

ELECTRON-DEFICIENT MOLECULES, GIANT MOLECULES, AND CONNECTIVITY 

2 Definitions of Terms That Follow from the Si02 Story Stoichiometry and Polymers 

---

Neutral free radicals are electron-deficient, so radicals centered on less electronegative elements are lower in energy than radicals centered on more electronegative elements. As a result, the order of stability for first-row radicals is alkyl ( CR3) > aminyl ( NR2) > alkoxyl (RO), and for halogens it is 1- > Br- > Cl > F. 

A stepwise process is used to convert a molecular formula into a Lewis structure, a two-dimensional representation of a molecule (or ion) that shows the relative placement of atoms and distribution of valence electrons among bonding and lone pairs. When two or more Lewis structures can be drawn for the same relative placement of atoms, the actual structure is a hybrid of those resonance forms. Formal charges are often useful for determining the most important contributor to the hybrid. Electron-deficient molecules (central Be or B) and odd-electron species (free radicals) have less than an octet around the central atom but often attain an octet in reactions. In a molecule (or ion) with a central atom from Period 3 or higher, the atom can hold more than eight electrons by using d orbitals to expand its valence shell. 

The double bond of alkenes consists of a strong sigma bond and a weak pi bond. The loosely held tt (pi) electrons are readily available to a reagent that seeks electrons. Hence, the double bond acts as a source of electrons, that is, it acts as a base. Compounds deficient in electrons (acids), such as electrophilic reagents (which seek a pair of electrons) and free radicals (which seek an electron), react with the bond. 

Molecules with an electron-deficient atom (central Be or B) or an odd-electron atom (free radicals) have less than an octet around the central atom but often attain an octet in reactions. 

The alkylation of pyridine [110-86-1] takes place through nucleophiUc or homolytic substitution because the TT-electron-deficient pyridine nucleus does not allow electrophiUc substitution, eg, Friedel-Crafts alkylation. NucleophiUc substitution, which occurs with alkah or alkaline metal compounds, and free-radical processes are not attractive for commercial appHcations. Commercially, catalytic alkylation processes via homolytic substitution of pyridine rings are important. The catalysts effective for this reaction include boron phosphate, alumina, siHca—alurnina, and Raney nickel (122). 

The electron-rich carbon—carbon double bond reacts with reagents that are deficient in electrons, eg, with electrophilic reagents in electrophilic addition (6,7), free radicals in free-radical addition (8,9), and under acidic conditions with another butylene (cation) in dimerization. 

Reactions with Free Radicals and Other Electron Deficient Species... 

The first example of a cyclization of fluorine-containing 5-hexenyl radicals was the study of the radical-iniOated cyclodimenzation reaction of 3,3,4,4-tetra-fluoro-4-iodo-1-butene. In this reaction, the intermediate free radical adds either to more of the butene or to an added unsaturated species [54, 55] (equation 56). Electron-deficient alkenes are not as effective trapping agents as electron-nch alkenes and alkynes [55]. 

In another nonelectrolytic process, arylacetic acids are converted to vi c-diaryl compounds 2A1CR2COOH —> ArCR2CR2Ar by treatment with sodium persulfate (Na2S20g) and a catalytic amount of AgNOs." Both of these reactions involve dimerization of free radicals. In still another process, electron-deficient aromatic acyl chlorides are dimerized to biaryls (2 ArCOCl —> ArAr) by treatment with a disilane RsSiSiRs and a palladium catalyst." " ... 

Chemiluminescence has been used to demonstrate increased free-radical activity after induction of caerulein pancreatitis, with levels peaking at about 20 min and decreasing rapidly to control values thereafter (Gough et al., 1990). Electron spin resonance has been used to demonstrate increased hydroxyl radical activity in choline-deficient diet pancreatitis in the mouse (Nonaka etal., 1989a). 

Most reactive metabolites are electrophiles or free radicals. An electrophile is a molecule that is electron deficient and reacts with nucleophiles, which usually have a negative charge or a lone pair of electrons that can form a bond to the electrophile. Although there may be cases in which reactive metabolites are strong nucleophiles rather than electrophiles, there are no clear examples. 

Boron also appears to be involved in redox metabolism in cell membranes. Boron deficiency was shown to inhibit membrane H -ATPase isolated from plant roots, and H -ATPase-associated proton secretion is decreased in boron-deficient cell cultures [71]. Other studies show an effect of boron on membrane electron transport reactions and the stimulation of plasma reduced nicotinamide adenine dinucleotide (NADH) oxidase upon addition of boron to cell cultures [72, 73]. NADH oxidase in plasma membrane is believed to play a role in the reduction of ascorbate free radical to ascorbate [74]. One theory proposes that, by stimulating NADH oxidase to keep ascorbate reduced at the cell wall-membrane interface, the presence of boron is important in... 

The simple insertion was favored by Walling and Buckler (25) since they could find no evidence for a chain process. However, Lamb (27) has recently reported that during autoxidation 5-hexenylmagnesium bromide undergoes cyclization to give eventually cyclopentylmethanol—a cyclization which is typical of the 5-hexenyl free radical. If this is true, either Reactions 21 and 22 or 21, 22, and 23 are indicated. In view of the close parallel between the autoxidations of all these electron-deficient molecules, some of which are essentially covalent in structure, the displacement scheme (Reactions 21 to 23) may have wider application than has been realized. 

---