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Non‐Resonance‐Assisted Hydrogen Bonding in Hydroxymethylene and Aminomethylene Cyclobutanones and Cyclobutenones and Their Nitrogen Counterparts.
Author(s) -
Sanz Pablo,
Mó Otilia,
Yáñez Manuel,
Elguero José
Publication year - 2007
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200700266
Subject(s) - tautomer , cyclobutene , chemistry , ring (chemistry) , intramolecular force , hydrogen bond , cyclobutadiene , acceptor , stereochemistry , crystallography , molecule , organic chemistry , physics , condensed matter physics
Abstract The characteristics of intramolecular hydrogen bonds (IMHB) have been systematically analyzed for a series of 32 different enols of derivatives of cyclobutane, cyclobutene, and cyclobutadiene bearing oxygen and nitrogen functionalities, at the B3LYP/6‐311+G(3df,2p)//B3LYP/6‐311+G(d,p) level of theory. In those cases where two tautomers (interconnected by a hydrogen shift through the IMHB) exist, tautomer a , in which the HB‐donor group (YH) is attached to the four‐membered ring, is less stable than tautomer b , in which is the HB‐acceptor (X) is the one attached to the four‐membered ring. As expected the OH group behaves as a better HB‐donor than the NH 2 group and the CNH group as a better HB‐acceptor than the CO group, although the first effect clearly dominates. Accordingly, the expected IMHB strength follows the [donor, acceptor] trend: [OH,CNH]>[OH,CO]>[NH 2 ,CNH]>[NH 2 ,CO]. Quite unexpectedly, in all those compounds in which the functionality exhibiting the IMHB is unsaturated, the IMHB is weaker than in their saturated counterparts, verifying that the primary effect on the strength of the IMHB is the structure of the σ‐skeleton of the system. In the conjugated systems investigated here, the severe constraints imposed by the four‐membered ring force the HB donor and acceptor to be far apart and the IMHB is rather weak. These geometrical constraints are less severe for the saturated analogues and the IMHB becomes stronger, confirming that the characteristics of the σ‐skeleton, and not the resonance‐assisted hydrogen bond (RAHB) phenomenon, are the primary contributors to the strength of the IMHB in conjugated compounds.