Indoles dimer

Mainly based on the failure of 2-methylindole to form a dimer, Schmitz-DuMont proposed structure (24) for indole dimer, the argument being that a methyl group in position 2 would effectively hinder the formation of a skeleton of type (24), whereas one in position 3 would not. ... 

Dimerization is markedly subject to steric hindrance, thus, whereas 3-n-propylindole dimerizes readily, neither 3-isopropyl- nor Z-tert-butyl-indole dimerizes. This failure is most probably the result of steric hindrance of approach of the electrophilic reagent to position 2 by the bulky 3-substituent in the unprotonated molecule. On the other hand, models show that approach of a nucleophilic reagent to position 2 of a 3-protonated molecule is quite open, it should, there-... 

The trimerization step can then be seen as formation of the reactive cation (52) by ring-opening of protonated indole dimer (53), followed by addition of indole as shown to give the trimer (27). ... 

Nevertheless, the further mechanistic steps leading to indole dimerization is not defined and a computational investigation could suggest feasible reaction pathways, providing important anticipation about IR and UV absorption spectra, which could be very useful for the assignment of the intermediates involved. It has been experimentally proposed that the semiquinone 1-SQ may decay via disproportionation to... 

The formation of 2-(indolin-2-yl)indole dimers from indole-3-acetic acid and its propyl ester in trifluoroacetic acid and phosphoric acid has been studied." The reaction involves electrophilic attack of the protonated species (24) on the free substituted indole to give the trans stereochemistry at the C(2)-C(3) bond. 

Although most common carboxylic acids can be used successfully in this context (equation 57), formic acid gives the indole dimer derived side product (21 equation 58) and trifluoroacetic acid gives the condensation product (22 equation 59), ° in addition to the expected products. The particular prob-... 

Cation (40, X = H), since it contains the system —NH—CH—, will be a good electrophilic reagent, and will thus attack the exposition of a neutral indole molecule leading to indole dimer (25) by way of the cation (43). The reversal of the dimerization then involves... 

When indole dimer is subjected to acid treatment in the presence of indole, indole trimer , C24H21N3, is produced. Suggest a structure for the trimer . (Hint consider which of the two reactants would be most easily protonated, and at which atom.)... 

The C2-C3 bond of an acylindole can act as the alkene component and add to the triplet excited state of an acylindole to produce indole dimers. This was first reported by Hino and co-workers in 1977, who found that UV light irradiation of 1,3-diacetylindole yields a single dimer [55]. The structure was determined by X-ray crystallography to be the syn head-to-tail dimer 62 (Scheme 23). More recent work has shown that this stereochemistry of addition is exceptional and that other N-acylindoles give mixtures of the anti head-to-head and head-to-tail dimers 63 and 64 only (Scheme 24) [56]. 

The novel indole dimers peronatin A (404) and B and two related oxygenated compounds are... 

By extenuating the reaction temperatures, Gaunt et al. was able to selectively phenylate C3-position of indole nucleus using aryliodonium salts [215]. At temperatures below 60°C, Cu(OTf)2-catalyzed C3-arylation took place selectively between N-acylindoles with aryliodonium salts using dtbpy as the agent to prevent indole dimerization. 

In the indole series, the A -pyridylsulfonyl indole 3.189 gave the product of C2 vinylation 3.190, rather than the typical C3 vinylation, showing that the coordination effect of the pyridine can overwhelm the normal indole reactivity (Scheme 3.74). The reaction presumably proceeds via a chelated palladium intermediate 3.192. Interestingly, an indole dimer 3.191 is produced in the absence of an added alkene, showing that the palladium(II) intermediate is capable of attacking a second molecule of the substrate. 

Karthikeyan S, Nagase S (2012) Origins of the stability of imidazole imidazole, benzene imidazole, and benzene indole dimers CCSD(T)/CBS and SAPT calculations. J Phys Chem A 116 1694-1700... 

Other monomer and the angle formed by the planes of the two monomers is 63° [71]. The calculated interaction energy was - 5.11 kcal/mol. The indole dimer also prefers the T-shaped orientation [72]. MP2/6-31G -level calculations of the interaction in the pyridine-ammonia complex have also been reported [93]. 

---