2-methyl 5-t-butyl-pyrrole(I) reacts with dichlorocarbene, yielding five products (two $\beta$-chloropyridines, IV and VI, two $\beta$-phormyl-pyrroles, III and V, and one 2—dichloromethyl 2H—pyrrole, II). By changing the solvent, these products are formed in relative different amounts, but the ratio "amount of products by attack on the methyl substituted side"/"amount of products by attack on the t-butyl substituted side" is constant. The influence of different protic solvents on the reactions of 2,5 dimethyl-pyrrole(VII) and 2,3 dimethyl indole(X) has been studied: the ratio "expanded products"/"substitution products" increases going from methanol (a more acidic medium) to i-propanol (a less acidic medium). Moreover, it has been shown that the amount of base present in the reaction mixture does not affect the ratio "expanded products/substitution products" unless it is matter of very strong bases, such as K-t-butoxide. This set of data, which are in sharp contrast with the widely accepted mechanism of these reactions, can be rationalized by assuming that in moderate alkaline conditions the neutral molecule of the appropriate substrate reacts with dichlorocarbene, yielding a transient adduct, which subsequently gives all the final (expanded and substitution) products.
Referenze Bibliografiche
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Con il contributo del Ministero per i Beni e le Attività Culturali