Electron-withdrawing ability

However, the equilibrium amount of the imino tautomers 88b and 91b is significantly increased with the increase in the electron-withdrawing ability... 

The reaction of Ab-acetyl-1 -hydroxytryptamine (39) with mesyl chloride (MsCl) in THF in the presence of EtsN provides 1-acetyl-1,2,3,8-tetrahydropyrrolo[2,3-(j] indole (49a, 35%) (70JA343), Ab-acetyl-6-mesyloxytryptamine (50a, 4%), Ab-acetyl-2,3-dihydro-2-oxotryptamine (51a, 5%), l-acetyl-3a-(4-chlorobutoxy)-l,2,3,3a,8,8a-hexahydropyrrolo[2,3-(j]indole (52a, 7%), and Ab-acetyltryptamine (53a, 2%) as shown in Scheme 6 (2000H483). In the same reaction with MsCl, l-hydroxy-Ab-methoxycarbonyltryptamine (34) produces 50b (7%), 51b (34%), and 52b (9%), while the formation of 49b is not observed at all. In the case of Ab-trifluoroacetyl-l-hydroxytryptamine (48), 49c (45%), 50c (8%), 51c (4%), and 52c (6%) are produced. These data suggest that the yield of 49 increases, whereas the yield of 51 decreases in the order of electron-withdrawing ability of Ab substituents (COOMe < COMe < COCF3). Stability of 49 seems to govern the quantity of 51, which is probably formed by hydrolysis of 49. 

The last remark deserves a more quantitative explanation. Horowitz et al. [64] have shown that useful correlations are obtained between the oxidation voltages of the anion and the averaged Hammett <7 -function or Ingold and Taft s cr-function of the substituents around a boron atom. Both empirical functions reflect the electron-withdrawing ability of the substituent group. Hammett s averaged cr -values are... 

A number of radical anions of sulfur-containing aromatic compounds have been studied essentially by means of ESR spectroscopy and sometimes by electronic spectroscopy. The studied compounds include aromatic rings separated by the oxidized sulfur functionality. The effects caused by the latter depend on the geometry and topology of the aromatic systems as well as on the electron-withdrawing ability of the other substituents. 

Dirhodhun (II) carboxylate catalyzed cyclization of a series of y-alkoxy-a-diazoesters has been shown to proceed with substantial diastereoselectrvity, producing the 2,3,5-trisubstituted tetrahydrofiirans. Hie diastereo selectivity of the cyclization improved as the electron-withdrawing ability of the substituent R increased (Scheme 22, <96JOC6706>). 

On interaction with a variety of XB acceptors, 1,4-DITFB forms shorter and stronger contacts than 1,4-DIB. Clearly, the electron withdrawing ability of fluorine boosts the donor ability of the iodine atoms... 

The EFISHG technique has also been applied to some pyridyl carbonyls of Rh1 or Ir1 by Bruce et al72,13 These complexes (e.g., (9) and (10)) contain stilbazole ligands bearing resolved 2-methylbutyloxy substituents, incorporated with the aim of encouraging noncentrosymmetric crystal packing structures. The observed 1.5- to 3-fold enhancements of (3 of the free stilbazole on complexation are attributable to the er-electron-withdrawing abilities of the metal centers.72,73... 

Variation of the X-substituent in (190) may be used to tune the pH at which efficient extraction occurs. For example, for the case where X = N02, the electron-withdrawing ability of this group results in more facile deprotonation of the amino proton. Thus the extraction may be performed at a lower pH than for the system with X = H. Similarly, variation of n in (190) may be used to tune the system for metal ions of different radii. 

Due to the strong electron-withdrawing ability of the carbonyl group, a 1,2-allenyl ketone is a very good Michael acceptor. Hence it can undergo 1,4-addition with all kinds of nucleophiles. 

As seen, fullerenes behave as unsaturated ligands. Hence it is expected that coordination to metal fragments might saturate (at least partially) their electron-affinity. Hence it is expected that the electron-withdrawing ability of fullerenes is lowered after complexation with respect to that in their free state. 

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