Skatole dimerization

Again based on the failure of 2-methylindole to form a dimer, Schmitz-DuMont proposed structure (29) for skatole dimer. Structure (30) was, however, shown to be the correct one independently in three... 

The structure (39) proposed for this compound by analogy with indole and skatole dimers was confirmed by an analysis of the NMR spectrum. ... 

Skatole (3-methylindole) readily forms a dimer hydrochloride with HCl in ether or with 15% aqueous HCl, as docs 1,3-dimethyl-indole. 3-n-Propylindole is also readily dimerized in dry ether/HCl/ ... 

Methylindole is, however, incorporated into mixed dimers and trimers. It thus reacts with indole and with skatole to form mixed dimers, and 2 moles react with 1 mole of indole for form a mixed trimcr. Likewise indole forms a dimer with 1,2-dimethylindole, 2-phenylindole, and even with 2,5-dimethylpyrrole, ... 

The dimerization of skatole proceeds in an entirely analogous manner, cation (44) now being the electrophilic reagent. This is sufficiently reactive to effect substitution at the a-position of a neutral skatole molecule. Attack by the less hindered side of cation (44) will be favored, leading to the stereochemistry shown in structure (30). The failure of 2-methylindole to dimerize is paralleled by the failure of 2-methylpyrrole dimer to react with a further molecule of 2-methylpyrrole. The main reason is almost certainly again the reduction in the electrophilic character of the immonium carbon by... 

Skatole is converted into an a,a -linked dimer in acid 2-methylindole, in contrast, is not susceptible to acid-catalysed dimerisation, reflecting the lower electrophilic character of the 3-protonated 2-substituted 377-indolium cation, much as ketones are less reactive than aldehydes. 

---