Alkenylbenzenes conjugation

The reaction of diolefins with aromatics is similar to that of styrenes with aromatics. Hoffman and Michael (44) reported that toluene and various conjugated diolefins could be reacted at 60-90 in the presence of sodium to yield alkenylbenzenes [Reaction (25)]. 

A carbon atom that is directly attached to a benzene ring is called a benzylic carbon (analogous to the allylic carbon of C=C—C). A phenyl group (C H —) is an even better conjugating substituent than a vinyl group (H2C=CH—), and benzylic carbocations and radicals are more highly stabilized than their allylic counterparts. The double bond of an alkenylbenzene is stabilized to about the same extent as that of a conjugated diene. 

Alkenylbenzenes. With Co2(CO)8, whatever the structure of the alkenylbenzene, when the double bond is conjugated with the aromatic... 

With Rh, the phenomena are strikingly different since the aromatic ring does not influence the selectivity which is always very high, no matter what olefinic structure (except for 1,1-diphenylpropene). Isomer selectivity depends on several factors, mainly the structure and the stereochemistry of the alkenylbenzene. With conjugated alkenylbenzenes, addition of the CHO group occurs preferentially on the a carbon when the a and / carbons are monosubstituted. When the ft carbon is disubsti-tuted, because of steric requirements, hydroformylation is less selective and occurs on both the a and y carbon ... 

In a similar way, alkenyl benzenes undergo the other addition reactions characteristic of the carbon-carbon double bond. Let us look further at the reactions of conjugated alkenylbenzenes, and the way in which the ring affects orientation and reactivity,... 

Addition to conjugated alkenylbenzenes orientation. Stability of the benzyl cation... 

On the other hand, we have seen (Sec. 12.17) that conjugated alkenylbenzenes are more stable than simple alkenes. On this basis alone, we might expect addition to conjugated alkenylbenzenes to occur more slowly than to simple alkenes. 

The fact is that conjugated alkenylbenzenes are much more reactive than simple alkenes toward both ionic and free-radical addition. Here again—as in most cases of this sort—resonance stabilization of the transition state leading to a carbonium ion or free radical is more important than resonance stabilization of the reactant. We must realize, however, that this is not always true. 

Alkenylbenzenes are not directly hepatotoxic or hepatocar-cinogenic. Cytochrome P450 enzymes in the liver oxidize the double bond of alkenylbenzenes to an epoxide, which is mostly conjugated by glutathione for excretion, but at levels exceeding the detoxification capacity, the overflow can... 

Alkenylbenzenes that have their side-chain double bond conjugated with the benzene ring are more stable than those that do not ... 

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