Minisci reaction

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The Minisci reaction is a named reaction in organic chemistry. It is a radical substitution to an aromatic compound, in particular to a heteroaromatic base, that introduces an alkyl group. The reaction was published about in 1971 by F. Minisci.[1] The aromatic compound is generally electron-deficient and with N-aromatic compounds the nitrogen atom is protonated.[2] A typical reaction is that between pyridine and pivalic acid to 2-tert-butylpyridine with silver nitrate, sulfuric acid and ammonium persulfate. The reaction resembles Friedel-Crafts alkylation but with opposite reactivity and selectivity.[3]

The Minisci reaction proceeds regioselectively and enables the introduction of a wide range of alkyl groups.[4] A side-reaction is acylation.[5] The ratio between alkylation and acylation depends on the substrate and the reaction conditions. Due to the simple raw materials and the simple reaction conditions the reaction has many applications in heterocyclic chemistry.[6][7]

Reaction between pyridine and pivalic acid to 2-tert-butylpyridine

Mechanism[edit]

A free radical is formed from the carboxylic acid in an oxidative decarboxylation with silver salts and an oxidizing agent. The oxidizing agent reoxidizes the silver salt. The radical then reacts with the aromatic compound. The ultimate product is formed by rearomatisation. The acylated product is formed from the acyl radical.[4][5]

Mechanism of the Minisci-Reaction

References[edit]

  1. ^ F. Minisci, R. Bernardi, F. Bertini, R. Galli, M. Perchinummo: Nucleophilic character of alkyl radicals—VI : A new convenient selective alkylation of heteroaromatic bases, in: Tetrahedron 1971, 27, 3575–3579.
  2. ^ Minisci reaction Jie Jack Li in Name Reactions 2009, 361-362, doi:10.1007/978-3-642-01053-8_163
  3. ^ Strategic applications of named reactions in organic synthesis: background and detailed mechanisms László Kürti, Barbara Czakó 2005
  4. ^ a b F. Fontana, F. Minisci, M. C. N. Barbosa, E. Vismara: Homolytic acylation of protonated pyridines and pyrazines with α-keto acids: the problem of monoacylation, in: J. Org. Chem. 1991, 56, 2866–2869; doi:10.1021/jo00008a050.
  5. ^ a b M.-L. Bennasar, T. Roca, R. Griera, J. Bosch: Generation and Intermolecular Reactions of 2-Indolylacyl Radicals, in: Org. Lett. 2001, 3, 1697–1700; doi:10.1021/ol0100576.
  6. ^ P. B. Palde, B. R. McNaughton, N. T. Ross, P. C. Gareiss, C. R. Mace, R. C. Spitale, B. L. Miller: Single-Step Synthesis of Functional Organic Receptors via a Tridirectional Minisci Reaction, in: Synthesis 2007, 15, 2287–2290; doi:10.1055/s-2007-983792.
  7. ^ J. A. Joules, K. Mills: Heterocyclic Chemistry, 5. Auflage, S. 125–141, Blackwell Publishing, Chichester, 2010, ISBN 978-1-4051-9365-8.