Issue in Honor of Prof Henk C. van der Plas ARKIVOC 2009 (vi) 183-190
Table 1. Results of the competition between nitrobenzene A and 2-Z- and 3-Z-substituted nitrobenzenes B for reaction with the carbanion of 2-phenylpropionitrile (as in Scheme 1), and for comparison - the relative activities of substituted nitroarenes in the VNS reaction11
Ratio of Ratio of
2 - Z VNS 3 - Z VNS
PB/PA PB/PA
MeO 0.030 0 MeO 0.44 1.1
I 0.54 0 3 - I 0.64 31
H 1.0 1.0 H 1.0 1.0
Br 12 4.8 Br 1.9 170
Cl 16 6.3 Cl 14 260
COO-iPr 26 9.3 F 21 17
CF3 46 67 CN 1100 (20)c 5500
F 110 (7.0)a 25 CF3 -
CN 8600 (3.9)b 1100 COOMe -
a.Recalculated from results of the competition with 2-chloronitrobenzene given in brackets. bRecalculated from the results of the competition with 3-cyanonitrobenzene given in brackets. cRecalculated from the results of the competition with 2-fluoronitrobenzene given in the brackets.
As was discussed in detail in our preceding papers,11,12 the rate of the nucleophilic addition to substituted nitroarenes can be affected by the substituents in a few ways: primary electronic: inductive and resonance effects on the reaction site, primary steric effect on the reaction site, and also secondary electronic and steric effects on the nitro- group that change its overall electron- withdrawing action activating nitro- aromatic rings. The observed relative rate constants of the addition of 1– to nitroarenes are a superposition of these effects. Substituents Z in 2-Z nitroarenes being in a meta- position to the reaction site, exert primary electronic inductive effect on the addition site, and secondary electronic and steric effects on the nitro group. For example, in the series of 2-halo-nitro-arenes, F>Cl>Br>H>I, it is evident that the secondary steric effect that hinders co-planarity of the nitro group with the ring which is necessary for the addition, dominates over the secondary electron-donating effect of F in 2-F nitrobenzene. Thus, the overall effect is as show in the above sequence. The importance of the secondary steric effect can be seen in the case of 2-CF3- nitrobenzene. Although the CF3 group exerts a very strong electron- withdrawing inductive effect, (much stronger than F) and, unlike F, has no electron- donating conjugative effect, 2-CF3- nitrobenzene is less active than 2-F- nitrobenzene. On the other hand, in 3-Z- nitrobenzenes, the substituents Z act via primary electronic and steric effects on the addition site. Here one can observe the strong influence of primary steric effects. The presence of bulky and moderately electron-withdrawing halogens, I and Br, respectively moderately decelerate, and slightly accelerate, the addition. Of particularly interest is 3-CF3 nitrobenzene. This highly electron-deficient nitroarene adds 1– to form •H-adducts, but due to steric hindrances
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