Bonding electron donation

The best known of metal carbene reactions, cydopropanation reactions, have been used since the earliest days of diazo chemistry for addition reactions to the carbon-carbon double bond. Electron-donating groups (EDG) on the carbon-carbon double bond facilitate this catalytic reaction [37], whereas electron-withdrawing groups (EWG) inhibit addition while facilitating noncatalytic dipolar cycloaddition of the diazo compound [39] (Scheme 5). There are several reviews that describe the earlier synthetic approaches [1, 2,4, 5,40-43], and these will not be duplicated here. Focus will be given in this review to control of stereoselectivity. 

Ni(niox)2] Quadrupole splitting of a square-planar nickel complex, the sign of which demonstrates that the EFG of the nonbonding electrons outweighs that of the bonding electrons donated by the ligands... 

The frontier orbital interaction between the olefin HOMO 7t(C-C) and the orbitals with c (0-0) character in the LUMO group of the metal peroxo moiety controls the activation of 0-0 bond. Electron donating alkyl substituents at the olefin double bond raise the energy of the HOMO, with the epoxidation barrier dropping concomitantly. On the other hand, a base coordinated at the metal center pushes the a (0-0) LUMO to higher energies and thus entails a higher barrier for epoxidation. 

Although bissulfonyl peroxides can decompose by homolysis of the 0—0 bond, in the presence of electron donor reagent (jr, n and a donors) they react as electrophiles. Conversely, in the case of an electron-deficient peroxide bond, electron-donating reagents can be regarded as attacking nucleophilicaUy the 0—0 bond. 

What structural features are responsible for the reactivity order of carboxylic acid derivatives Like the other carbonyl containing compounds that we ve studied they all have a planar arrangement of bonds to the carbonyl group Thus all are about the same in offering relatively unhindered access to the approach of a nucleophile They differ m the degree to which the atom attached to the carbonyl group can stabilize the carbonyl group by electron donation... 

Infrared IR spectroscopy is quite useful in identifying carboxylic acid derivatives The, carbonyl stretching vibration is very strong and its position is sensitive to the nature of IKT the carbonyl group In general electron donation from the substituent decreases the double bond character of the bond between carbon and oxygen and decreases the stretch mg frequency Two distinct absorptions are observed for the symmetric and antisym metric stretching vibrations of the anhydride function... 

The importance of steric effects in determining the oxidation state of the product can be illustrated by a thioether linkage, eg (57). If a methyl group is forced to be adjacent to the sulfur bond, the planarity required for efficient electron donation by unshared electrons is prevented and oxidation is not observed (48). Similar chemistry is observed in the addition of organic nitrogen and oxygen nucleophiles as well as inorganic anions. 

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