Carbocations from thiophene

As concerns the factors leading to the observed reactivity scale, it is our opinion that the relative ground-state energies play a more important role than the relative stabilities of the intermediate carbocations. From a qualitative point of view, it is observed that the more aromatic is the starting molecule, the smaller is the rate of substitution. The order of reactivity is in fact the reverse of the order of the ground-state aromaticities, as determined by several different approaches thiophene > selenophene > tellurophene > furan (see Section II,C). A quantitative confirmation of this hypothesis has been obtained by analysis of the activation parameters for the formylation reaction.57... 

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

First examples of stable carbocations were reported from several classes of thia-PAHs with four fused rings, namely, benzo[Z ]naphtho[2,l-J thiophene (40) and its 3-methoxy derivative (41), phenanthro[4,3-Z ]thiophene (42) and its 7-methoxy (43), 10-methoxy (44), and 9-methoxy (45) derivatives, phenanthro[3,4-Z ]thiophene (46) and its 7-methoxy (47) and 9-methoxy (48) derivatives, and 3-methoxybenzo[Z ]-naphtha[l,2-J thiophene (50) (Fig. 25). Regioselectivity issues and charge... 

A report has described a route to multiply alkylated thiophene derivatives from electrochemically derived diarylcarbenium ions (Scheme 46). Using the cation-pool technique - in which the cationic intermediate is generated in high concentration and pooled - the carbocation (228) alkylates thiophene, providing up to 58% of the trialkylated product (229) and 24% of the dialkylated product (230). 

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