A theoretical study on the molecular mechanism for the normal Reimer–Tiemann reaction

Normal Reimer–Tiemann rearrangement (reaction of pyrrole ring and dichlorocarbene in basic media to yield pyrrole-2-carboxaldehyde) has been theoretically characterized at HF/6-311++G∗∗, MP2//6-31++G∗∗ and B3LYP//6-31++G∗∗ computing levels. Analysis of the results shows that the molecular mechanism comprises six steps; the first is a barrierless process associated with the addition of dichlorocarbene on the α position of pyrrole anion; the second and rate-limiting step corresponds with an intramolecular hydrogen transfer from the α carbon of the pyrrole ring to the carbon atom of the incoming dichlorocarbene fragment; the subsequent pathways are a nucleophilic substitution of Cl− by OH− on this carbon atom along an uncharged intermediate, followed by a Cl− leaving process and finally the product, pyrrole-2-carboxaldehyde, is obtained along an intramolecular hydrogen transfer from the hydroxyl group to the nitrogen atom of the ring. Solvent effects of the ether medium on the rate-limiting step are analyzed by using a polarizable continuum model.