Superacid‐Catalyzed Reactions of Cinnamic Acids and the Role of Superelectrophiles.

Abstract

Superacid-Catalyzed Reactions of Cinnamic Acids and the Role of Superelectrophiles. -The chemistry of cinnamic acids and related compounds in the presence of benzene and triflic acid is studied. Two competing reaction mechanisms are proposed which both involve the formation of dicationic intermediates. The cinnamic acid affords indanone derivatives when the aryl group is not deactivated by substituents and give chalcone-type products when the aryl group is deactivated by strong electro-withdrawing groups. -(RENDY, R.; ZHANG, Y.; MCELREA, A.; GOMEZ, A.; KLUMPP*, D. A.; J.

Activation of Electrophilic Sites by Adjacent Cationic Groups

“…The dicationic intermediate (17) exhibits an enhanced electrophilic reactivity compared to the protonated cyclohexanone (16), and consequently there is a reaction with benzene.…”

“…16 As an example of electrophilic activation by a protonated carboxylic acid, β-phenylcinnamic acid (35) is diprotonated in super-acid to give the dication (36, observable by NMR) which then reacts with benzene and gives the indanone (37) in good yield (eq 12). 17 It is known that the 1,1-diphenylethyl protonated amide groups enhance the electrophilic reactivities of the adjacent carboxonium ions. Dication 39 has also been directly observed by low temperature l3 C NMR using Olah's stable ion technique (FS0 3 H-SbF 5 -S0 2 ClF, -50°C).…”

“…The indanone product is formed by a cyclization involving the dicationic species (40). To examine this further, the related amides 41 and 42 were studied in superacid promoted conversions (eqs [16][17]. It was found that amide 42 leads to the indanone product while 41 gives no indanone.…”

Superelectrophiles: Charge–Charge Repulsive Effects

“…Cinnamic acid (112) produces superelectrophile 113 in CF 3 SO 3 H which is capable of reacting with dichlorobenzene, however with greater distance between the carboxonium ion and carbocation centers, the superelectrophile 114 does not react with dichlorobenzene ( Figure 24). [21] Onium sites that are forced into close proximity due to a structural or conformational effect may demonstrate super-A C H T U N G T R E N N U N G electrophilic character ( Figure 25). For example, 1,8-bis(diphenylmethyl)naphthylene dication (115) and 1,1'biphenyl-2,2'-diyl dications (116) are found to have a distance of 3.11 and 3.66 , respectively, separating their carbocationic sites.…”

“…For example, reaction of the cyclopropane carboxylic acid (64) leads to product 66, [21] which arises from In the superacid-catalyzed reactions of oxazolines, diprotonation of the heterocycle gives the 1,3-dication (68) and ring opening leads to the distonic superelectrophile (69, a 1,5-dication). [22] The charge-separated species is shown to be capable of reacting with benzene and dichlorobenzene.…”