Document Detail

Unravelling the reaction mechanism of the reductive ring contraction of 1,2-pyridazines.
MedLine Citation:
PMID:  22530929     Owner:  NLM     Status:  Publisher    
Substituted pyrroles may be synthesized from selected 1,2-pyridazines through a reductive ring contraction involving the addition of four electrons and four protons. Our density functional theory computations of this reaction mechanism show that the first reduction event must be preceded by the uptake of one proton by 1,2-pyridazine, and that the reaction proceeds through a 2e-/3H+-bearing intermediate. In the absence of electron-withdrawing groups able to resonate charge away from the ring, this intermediate lies too high in energy, making the reaction sequence thermodynamically inaccessible. After another two-electron reduction and the addition of two more protons, the original 1,2-pyridazine ring opens. Ring contraction and ammonia elimination then proceed with very small barriers, irrespective of the substituents present in the original 1,2-pyridazine. By establishing the need for electron-withdrawing resonant groups in the 3- and 6- positions to stabilize the critical intermediate in the initial stages of the reaction, this work suggests that the scope of the reductive ring contraction of 1,2-pyridazines may be expanded to pyridazines bearing COCH3 groups, amides or aryls in these positions. We also explain the lack of reactivity of unsubstituted 1,2-pyridazine and analyze the feasibility of bypassing the high energy 2e-/3H+-intermediate through disproportionation of earlier 2e-/2H+-bearing intermediates.
Pedro Jorge Silva
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-4-24
Journal Detail:
Title:  The Journal of organic chemistry     Volume:  -     ISSN:  1520-6904     ISO Abbreviation:  -     Publication Date:  2012 Apr 
Date Detail:
Created Date:  2012-4-25     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  2985193R     Medline TA:  J Org Chem     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
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