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Conformational preferences of 2‐methoxy‐acetophenone
Author(s) -
Schaefer T.,
Peeling James,
Wildman Timothy A.
Publication year - 1984
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270220802
Subject(s) - substituent , chemistry , ring (chemistry) , methyl group , acetophenone , side chain , stereochemistry , group (periodic table) , proton , benzene , stereospecificity , crystallography , physics , polymer , organic chemistry , quantum mechanics , catalysis
On the basis of the presence or absence of long‐range spin–spin coupling constants between side–chain and ring nuclei in 2‐methoxyacetophenone, some literature ambiguities about the conformational preferences of the side‐chains in this compound can be resolved. The long‐range coupling between the methoxy protons and the ring proton ortho to the methoxy group, 5 J(H, CH 3 )o, is (−)0.28 ± 0.02 Hz, as expected for a conformation in which the methoxy group lies in the benzene plane and cis to H‐3. The methyl protons of the acetyl group do not couple to H‐6, implying that this methyl group does not approach H‐6 closely. However, the 13 C nucleus of this methyl group couples by +0.4 Hz to H‐5 and not to H‐3. This stereospecific five‐bond coupling implies that the acetyl group predominantly prefers an arrangement in which the carbonyl group lies trans to the other substituent, as would be expected electrostatically. Large twists out of the ring plane are not consistent with the observed couplings.