Ring activation

For substituted benzenes, ring activators (electron-releasing groups) increase sensitivity and ring deactivators (electron-withdrawing groups) decrease sensitivity (exception halogenated benzenes)... 

Substituting deuterium for hydrogen gas in the reduction of BT to DHBT with the catalyst precursor [Rh(NCMe)3(Cp )](BF4)2 has shown that the stereoselective ds-deuteration of the double bond is kinetically controlled by the tj2-C,C coordination of BT. The incorporation of deuterium in the 2- and 3-positions of unreacted substrate and in the 7-position of DHBT has been interpreted in terms of reversible double-bond reduction and arene-ring activation, respectively (Scheme 16.14) [55]. 

In phenols, the reactions that take place on the aromatic ring are electrophilic substitution reactions (Unit 13, Class XI). The -OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become eiectron rich due to the resonance effect caused by -OH group. The resonance structures are shown under acidity of phenols. 

To be really satisfactory, a Friedel-Crafts alkylation requires one relatively stable secondary or tertiary carbocation to be formed from the alkyl halide by interaction with the Lewis acid, i.e. cases where there is not going to be any chance of rearrangement. Note also that we are unable to generate carboca-tions from an aryl halide - aryl cations (also vinyl cations, see Section 8.1.3) are unfavourable - so that we cannot nse the Friedel-Crafts reaction to join aromatic gronps. There is also one further difficulty, as we shall see below. This is the fact that introduction of an alkyl substitnent on to an aromatic ring activates the ring towards fnrther electrophilic substitution. The result is that the initial product from Friedel-Crafts alkylations is more reactive than the... 

Any substituent whose atom attached to the benzene contains a lone pair of electrons is ortho-para directing (but not necessarily a ring activator). Substituents without a lone pair on the atom attached to the ring are likely meta directors (with the exception of alkyl groups and aromatic rings, which turn out to be ortho-para directors). 

In this connection three oases may be considered 1) the strength of aprotic centres is sufficient for the activation of olefin and the following alkylation by interaction with aromatic hydrocarbon frem gaseous phase, but insufficient for aromatic ring activation. Para-dialkylbenzene formed in this case does not undergo further isomerization on these centres. The group of centres... 

The double bonds of the oxadiazole ring activate certain functional groups in the 5-position. For instance, when a keto group is in /S position, alkaline hydrolysis is facilitated as in the case of p diketones [50, 83, 95, 107,117), e.g. ... 

Electrophilic aromatic substitutions The chemistry of pyrimidine is similar to that of pyridine with the notable exception that the second nitrogen in the aromatic ring makes it less reactive towards electrophilic substitutions. For example, nitration can only be carried out when there are two ring-activating substituents present on the pyrimidine ring (e.g. 2,4-dihydroxypyrimidine or uracil). The most activated position towards electrophilic substitution is C-5. 

The synthetic utility of anodic reactions that couple aromatic rings activated by alkoxy groups has been explored for the formation of natural products. Thus laudanosine (78 R1 = R2 = R3 = R4 = CH3) has been oxidized... 

The color yield of acacetin is low in spite of a 4 -methoxyl group, and this is attributed to suppression of the A-ring activity owing to 5-hydroxyl to 4-keto hydrogen bonding, carbonyl attraction of electrons, and perhaps low solubility. 

This chapter covers reactions in which coordination of a transition metal to the ir-system of an arene ring activates the ring toward addition of nucleophiles, to give V-cyclohexadienyl-metal complexes (1 Scheme 1). If an electronegative atom is present in the ipso position, elimination of that atom (X in 1) leads to nucleophilic aromatic substitution (path a). Reaction of the intermediate with an electrophile (E+) can give disubstituted 1,3-cyclohexadiene derivatives (path b). If a hydrogen occupies the ipso posi-... 

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