Experimental Evidence of Intramolecular C_Ar–H···O=C Hydrogen Bonds in the Structure of (Diaryl)tetrahydrofuranones Using Spectroscopic Tools

Abstract

The occurrence of bifurcate H‐bonds CAr–H···O=C in the structure of (diaryl)‐tetrahydrofuranones was experimentally demonstrated using different methods and techniques. The consistent increasing spin–spin coupling constants 1J(C,H) of the ortho‐H‐atoms and low‐field shift of vC=O in IR spectra of 2,2‐(diaryl)tetrahydrofuran‐3(2H)‐ones relative to their 5,5‐diaryl counterparts, as well as pronounced dependence of the ortho‐C–H H‐atoms chemical shifts on the temperature and solvent polarity along with X‐ray diffraction analysis data unambiguously point to the existence of weak CAr–H···O=C H‐bonds in these molecules.

An Intramolecular CAr–H•••O=C Hydrogen Bond and the Configuration of Rotenoids

“…2 and 3 and Fig. S12, Supporting Information), which are consistent with those calculated for other H-bonds [42, 43, 47]. …”

“…The existence of non-classical C–H•••O H-bonds was confirmed by analysis of the total charge density method [40], and an intramolecular N–H•••O=C H-bond between the H atom of a cis thioamide and a carbonyl oxygen atom has been supported by single-crystal X-ray diffraction analysis, with a trans – cis geometry of the almost planar thiourea unit found stabilized by this H-bonding [41]. Recently, an intramolecular C Ar –H•••O=C H-bond has been reported, based on X-ray diffraction data [42], and the chemical shift of an intramolecular “non-classical” H-bonding C–H proton was found shifted downfield about 1 ppm [43]. This downfield chemical shift has been proposed as a potential means of evaluating the H-bond, and the “short” C•••O (<3.3 Å) and O•••H (<2.6 Å) interatomic distance and the favored orientation have been evidenced as consequence of such H-bonding [43].…”

“…2), which provided a potential pseudo six-membered ring over the bonds O4–C4–C3–C1'–C6'–H6'•••O4, as indicated by a hydrogen bond angle 107.6° for C6'–H6'•••O4 (>90°) [42]. This conformation and the pseudo six-membered ring would be favored for an intramolecular C Ar –H•••O=C H-bond between H-6' and O-4, as indicated by the “short” C6'•••O4 [2.940 Å (<3.3 Å)] and O4•••H6' [2.492 Å (<2.6 Å)] interatomic distances (Fig.…”

“…Also, a non-classical intramolecular C Ar –H•••O=C H-bond has been demonstrated for some (diaryl)tetrahydrofuranones by their X-ray diffraction data [42]. Inspection of the partial structural units of C 5 -O-H•••O=C 4 of flavonoids, isoflavonoids, or rotenoids and that for the H-bond of (diaryl)tetrahydrofuranones shows that these structural units are closely comparable, indicating a potential intramolecular C 5 –H 5 •••O=C 4 H-bond for rotenoids and another C 6' –H 6' •••O=C 4 H-bonds for trans -rotenoids.…”