Effect of Z Substituents

Figure 9.4 Effect of Z substituent on effectiveness of RAFT agents 164 in various polymerizations. Dashed line implies limited effectiveness with a particular monomer (broad molecular weight distribution).401...

Figure 12.7. Orbital interactions in the Cope rearrangement (a) symmetry-allowed interactions of the g and orbitals (b) effect of X (or C ) and Z (or C ) substituents in the 3-position (c) effect of Z substituents at the 1-position.

Mutagenic activities across series 28 (entries 22-32) showed a distinct dependence upon the electronic effect of the substituent activity is greatest with electron donor and weakest with electron-withdrawing substituents at Z. 

The A-benzoylhydrazones of acylpinacolines 75 (R = alkyl R = H R = r-Bu R" = Ph) in solution form equilibria involving the participation of two hydrazone (75A- and Z) tautomers, one or two enhydrazine (75A -E and Z) tautomers, and one cyclic (75B) tautomers (84ZOR1371). Here, clear evidence is presented characterizing the influence of the steric effect of the substituent on the C = N bond an increase in the steric demands in the series R = H < Me < t-Pr < t-Bu shifts the equilibrium toward the cyclic tautomer 75B. When R = H, this tautomer is absent but when R = r-Bu, the equilibrium is shifted entirely in favor of 75B. 

The ElcB mechanism has the same features as the E2 mechanism except that proton abstraction by the base proceeds essentially to completion prior to departure of the leaving group. A variant of this mechanism may intervene whenever the leaving group is a poor leaving group or an exceptionally stable carbanion may be formed (i.e., due to the presence of Z substituents in addition to the polar a bond and/or a hybridization effect). The factors which lead to stabilization of carbanions have been discussed in Chapter 7. 

Studies of vinyl ether hydrolysis have demonstrated a strong retardation effect of jS-carboxy and /J-carbomethoxy groups (2000- to 25,000-fold). The rate profile for (Z)-/5-met boxy methacry lie acid indicates that ionization of the carboxylate raises the rate of hydrolysis by a factor of 240. It has been proposed that this difference in reactivity of ionized and non-ionized forms of the substrate is due to the conjugative and inductive effect of the substituents, rather than /5-lactone formation65,66. 

The 1H, 13C, and 15N NMR chemical shifts of the (Z)-phenylhydrazones of 5-alkyl-and 5-aryl-3-benzoyl-l,2,4-oxadiazoles have provided information on the ground-state electronic distribution.27 The effects of the substituents on the chemical shifts of N(4) and C(5) correlate with kinetic results of the rearrangement of 1,2,4-oxadiazoles to 1,2,3-triazoles (Boulton-Katritzky reaction) support the importance of the leaving group. 

The electron-withdrawing effect of the substituent Z responsible for stability and the concentration of carbanion 9 is the main factor determining the cyclization rate. The tautomeric equilibrium 10 11 is completely shifted to the amino form. It was empirically established that the activity of the electron-withdrawing substituents Z increases in the following series (1989MI2, 1992SR1) ... 

With respect to a-substituents bearing p- or 7r-electrons which are directly attached to the C—Cl bond (Table 6, Z = CH2=CH to CH3CH20), these may delocalize their electrons through resonance or mesomeric effects with the positively charged carbon atom in the transition state. Because of this, they were not plotted in the Taft figure for a-substituted ethyl chlorides. Furthermore, the rates for these substituents also could not be correlated with the electrophilic substituent constants a+. The o+ parameters have been defined for substituents on the benzene ring which are far from the reaction site. Even though steric effects may interfere with the coplanarity and hence with delocalization, the effect of these substituents was believed to be polar in nature. 

Kinetic data for the HBr elimination of secondary alkyl bromide, i.e., a-substituted ethyl bromide, in the gas phase are shown in Table 11121. Contrary to primary -substituted ethyl bromides, the rate constants for these secondary halides could not be correlated by the use of the Taft equation. This limitation arised because the corresponding olefin products underwent rapid isomerization with HBr gas acting as a catalyst. The lack of a kinetic control prevented evaluation of the factor by which the Z substituent in ZCH(Br)CH3 affected the direction of elimination. However, as the electron-releasing effect of Z increases (Table 11), a small but significant increase in the overall rate constant was obtained. In view of the catalysis by HBr in the isomerization process of the olefin products, a general mechanism for this process was suggested (equation 40). 

In the hydrogenation of 1-alkyl-substituted naphthalenes over Pd-C, the proportion of hydrogenation of the substituted ring increases with increasing bulkiness of the 1-alkyl groups, as shown in eq. 11.67.238 The effect of the substituents is most pronounced in 1-Z-butylnaphthalene, where the ring with f-butyl was hydrogenated... 

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