Effect of a-Conjugated Substituents

Both aromatic and cyclopropyl groups conjugate strongly with an electron-deficient center. Styrene, the simplest olefin with phenyl substituent, is much less reactive than the simple 1-alkene. 

On the other hand, cyclopropyl enhances the reactivity of the double bond. 

The electron-donating substituent on aromatic ring enhances the rate, whereas the effect is opposite for electron-withdrawing substituent. 

The relative rates of reaction ofp-CHjO, p-CH, unsubstituted, p-F, p-Cl, m-Cl, and m-NOj systems with 9-BBN are determined as shown in Chart 5.2 [9]. 

A Hammett plot, using a, is reasonably linear with p = -0.49, which corresponds to a small buildup of positive charge at the a-carbon of the styrene and reaction with bond. In all these cases the (3-phenylethyl alcohol is 

---

The effect of a conjugating substituent in the monomer may be summarized by observing that its influence is much greater in the product radical than in the monomer. In the activated complex, which is intermediate in character between reactants and product, resonance stabilization is substantially greater than in the monomer reactant, though less than in the product radical. The substituent therefore lowers the activation energy for the process, and enhances thereby the reactivity of the monomer. 

The effect of a chiral substituent at the alkene terminus on the course of the 1,7-electrocyclization of diene-conjugated diazo compounds has also been examined [88TL6361 94JCS(P1)3149]. Cyclization of diazo com-... 

The effect of a remote substituent on the facial selectivities in a nucleophilic conjugate addition has been investigated for the reaction of EtSH with a series of dibenzobicyclo[2.2.2]octatrienes (84). Syn-addition proved to be favoured for nitro substituent and polar solvents increased the selectivity (Table 5).19... 

Claisen rearrangements introduce the complication of oxygen lone pairs within the rearranging system, rather than as substituents on the perimeter. They may be ignored and the transition structure treated as benzene-like, or they may interrupt the conjugation, and the transition structure is then like a heptatrienyl anion.915 Predictions based on the simple theory above, whichever of these models is taken, match most of the substituent effects, and more elaborate treatments with calculations account for the anomalous accelerating effect of a donor substituent at C-6.916... 

A recent study applied the PSP equation to good effect in discussing the gas-phase and aqueous solution basicities of about fifty 2-, 3- or 4-substituted pyridines and some 2,6-disubstituted compounds ". The substituents studied included 2-, 3- and 4-CN, and these conformed fairly well to various relations and graphical plots. CN also features extensively among the +R substituents in a recent paper on the inherent dependence of resonance effects of strongly conjugated substituents on electron demand . 

Figure 4. Effect of acetaminophen-conjugate substituents on antibody recognition in the acetaminophen-adduct immunoassay. The decrease in inhibition due to particular substituents is shown. These values were calculated by comparing the ability of the following acetaminophen-conjugate analogs to competitively inhibit antibody binding to solid-phase acetaminophen-bound metallothionein. a N-acetyl-L-cysteine compared to 3-(N-acetyl-L-cystein-S-yl)acetaminophen. b acetaminophen compared to 3-(N-acetyl-L-cystein-S-yl)acetaminophen. c 2-(N-acetyl-L-cystein-S-yl)hydroquinone compared to 3-(N-acetyl-L-cystein-S-yD-acetaminophen. d 3-(methylthio)acetanilide compared to 3-(methyl-thiolacetaminophen. e 2-(L-cystein-S-yl)-2-aminophenol compared to 3-(L-cystein-S-ylacetaminophen. f 3-(L-cystein-S-yl)acetaminophen compared to 3-(N-acetyl-L-cystein-S-yl)acetaminophen. g 3-(methyl-thiolacetaminophen compared to 3-(N-acetyl-L-cystein-S-yD-acetaminophen (Reproduced fimm Ref. 14).

The same effect of a hydrazide substituent has also been observed in 1,4-endo-peroxidation of conjugated dienes related to norcaradienes. The results have been discussed in terms of frontier molecular orbital theory. 

The chemical properties of BFL are very similar to those of FL. The greatest difference is that under similar conditions there is more singlet reaction from BFL than from FL. This observation is reflected in the estimate of AGsr obtained, exactly as it was for FL, from the observed rate of reaction with methyl alcohol in acetonitrile. For BFL, use of (28) gives A(7st = 1.0 kcal mol 1. This value implies that there is a significant amount of BFL in the equilibrium mixture, and that the effect of the conjugating benzo-substituent is to stabilize the singlet carbene more than the triplet. 

It is clear that any kind of addition polymerization of the norbornenyl double bond will benefit from the electronic stabilization provided by a conjugating substituent. A simple radical addition process such as is known for both styrene and acrylate monomers may be a reasonable analogy to our system. Whether this effect alone is enough to account for our observations is not clear. A possible additional effect, at least in the case of the phenyl substituted monomers, is suggested below as part of our work on polymer structure. 

---