Substituent chemical shifts in NMR. 2—Bromine SCS in rigid molecules

Abstract

The complete analysis and assignment of the 'H NMR spectra of 2-exo-bromonorbornane (1) and 2-bromoadamantane is reported, using high-field NMR and COSY and proton-carbon correlation experiments to assign the spectra. These data, together with those for the parent hydrocarbons, when combined with previous analyses of 1-bromoadamantane, 1-bromooctadeuterocyclohexane and the simple alkyl bromides, provide a data set of 38 proton SCS values for bromine in molecules of accurately known geometry. The effect of solvent on the SCS was investigated for 1 in five common solvents of very different polarity (C,D,, to DMSO). Only small ( < 0.1 ppm) solvent shifts were observed, apart from the 2-endo and 7s protons. A noteworthy feature of the SCS is the downfield shift of the CHX proton in all the series investigated, as X changes from C1 to Br to I. This is the reverse of the 'normal' trend and cannot be explained by present theories of SCS.

Structural Ambiguities Revisited in Two Bridged Ring Systems Exhibiting Enantiotopic Elements, Using Natural Abundance Deuterium NMR in Chiral Liquid Crystals

“…Analysis of Norbornene. To illustrate our approach, norbornene, 1 , is an interesting bridged ring system because, during several years with earlier NMR, the assignment of the syn and anti protons with regard to the double bond was unclear. − Thus, it was claimed that the syn proton is more shielded than the anti 15 before the opposite conclusion was drawn by other authors using the lastest NMR techniques. , Although the modern, isotropic NMR techniques are powerful, there remain some situations where no definitive assignments are possible. Consequently, it is pertinent to propose new methodologies to go beyond such analytical limitations.…”

Structural elucidation of dioxa‐cage compounds from tetrahydroisobenzofuran‐1(3H)‐one: analysis of NMR data and GIAO chemical shifts calculations

“…According to all correlations observed in the COSY contour map, the bridgehead proton H1 should exhibit a complex splitting pattern; however, for compounds 2 and 3, only the J vic H1‐H2 was possible to determine, being the H1 multiplicity assumed as a broad doublet located at δ 2.90 (4.5 Hz) and δ 2.83 (4.6 Hz), respectively. For compound 4 , the H1 was assigned to the doublet of doublets at δ 2.63 in which was possible to identify the J 1,2 (4.8 Hz) and also a small constant of 1.3 Hz, compatible with the vicinal coupling H1‐H10 (or H1‐H10') and also with the long‐range W coupling H1‐H7 …”

“…For compound 4, the H1 was assigned to the doublet of doublets at δ 2.63 in which was possible to identify the J 1,2 (4.8 Hz) and also a small constant of 1.3 Hz, compatible with the vicinal coupling H1-H10 (or H1-H10') and also with the long-range W coupling H1-H7. [37][38][39][40] As can be observed, the effect of the C-9 substituents in the chemical shifts of protons H8 and H9 is noticeable. Curiously, the β-proton H8 (δ 4.27-4.77) is more deshielded than the α-proton wileyonlinelibrary.com/journal/mrc H9 (δ 3.82-3.94), being this effect more pronounced according to the following substitution order: Br > Cl > O.…”

Synthesis of spiro[bicyclo[2.2.1]heptane-2,2′-furan]-3-amines via stereoselective cycloadditions of trimethylenemethane to (1S,3EZ,4R)-3-arylimino-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones

“…[23][24][25][29][30][31]41,42 The dihedral angles between H-C(3) and H-C(4) in the secondary exo-amines 8a-d, 9d, and 10d are close to 90°and, following the Karplus equation, 43 no appreciable coupling between these protons would be expected. Accordingly, negligible coupling constants, 3 J H(3)-H(4) $ 0 Hz, were observed in 1 H NMR spectra of the secondary exo-amines 8a-d, 9d, and 10d.…”