Acetylenic groups electronic effects

Another illustrative example is that of phenylacetylene. Table 6-7 summarizes the H NMR chemical shifts of its alkyne H-atom in a variety of solvents [273], Most solvents (except aromatic solvents) decrease the shielding of the acetylenic hydrogen nuclei. The corresponding low-field shifts have been interpreted in terms of weak specific association between the alkyne as hydrogen-bond-donor and electron pair-donor groupings of the solvent [273], The high-field shifts in aromatic solvents arise from the magnetic anisotropy of the solvent molecules (see below). The order of effectiveness of the solvent... 

Acetylenic groups are used for their electronic effects, as equivalents of aromatic rings and to impose structural constraints. 

Reaction of Ta-alkyne complexes with R R2C=0. The Ta complexes formed from TaCls/Zn with unsymmetrical alkynes react with carbonyl compounds to form two rcgioisomcric allylic alcohols with a ratio depending on the substituents on the alkync (both steric and electronic effects) as well as the size of the substituents in the carbonyl group. The complexes from acetylenic esters react with carbonyl compounds mainly at the position or to the ester group, whereas complexes from acetylenic amides react mainly at the position fi to the amide. 

Intermolecular atom transfer reactions of simple alkyl iodides to acetylenes bearing electron-withdrawing substituents have been observed [15] and are exemplified in Scheme 4. Yields were relatively poor when primary iodides are employed, but improved when secondary or tertiary iodides were added. Electron-withdrawing activating groups are needed on the acetylene in order to accentuate the polar effects and hence increase the initial rate of radical attack. This is done at the cost, however, of diminishing the rate of subsequent iodine transfer to the better-stabilized radical. 

Electronic effects of cyano, isocyano, acetylenic and diazonio groups... 

In a short section the electronic effect of the isocyano group is discussed. The possibility of including unipolar substituents in Hammett and similar treatments has long been a matter of controversy and this problem is discussed with special reference to the N2 substituent. The effects of acetylenic groups are examined in some detail and various unsatisfactory aspects are uncovered. Multiparameter treatments such as the Yukawa-Tsuno equation and the dual substituent-parameter equation have long been important and further treatments have been devised in recent years. A final section is devoted to some of these, with an indication of the place of CN, and to a lesser extent of the other groups of interest, in these treatments. 

X. THE ELECTRONIC EFFECTS OF ACETYLENIC GROUPS A. The Characteristics of the Ethynyl Group... 

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