Furan coupling constants

The furan coupling constants are much smaller in magnitude than the corresponding ortho and meta coupling constants of the benzene derivatives. It is characteristic of the furans and the other heteroaromatic compounds that the "ortho" couplings, J2 3 and J3 4 are not the same. 

As might be anticipated from the behaviour of the parent heterocycles, C-2 of indole, benzo[i]furan and benzo[i]thiophene (Table 13) is shifted to lower field than C-3. However, the shifts for C-2 (O, 144.8 Se, 128.8 S, 126.1 NH, 124.7 Te, 120.8) and C-7a (O, 155.0 Se, 141.3 S, 139.6 NH, 135.7 Te, 133.0) in the benzo[i] heterocycles vary irregularly (80OMR(l3)3l9), and the sequence is different to that observed for C-2 in the parent heterocycles, namely 0>Se>Te>S>NH. Also noteworthy is the upheld position of C-7, especially in indole and benzofuran, relative to the other benzenoid carbons at positions 4, 5 and 6. A similar situation pertains in the dibenzo heterocycles (Table 14), where not only are C-1 and C-8 shifted upheld in carbazole and dibenzofuran relative to the corresponding carbons in dibenzothiophene and fluorene, but similar, though smaller, shifts can be discerned for C-3 and C-6 in the former compounds. These carbon atoms are of course ortho and para to the heteroatom and the shifts reflect its mesomeric properties. Little variation in the carbon-hydrogen coupling constants is observed for these dibenzo compounds with V(qh) = 158-165 and V(c,h) = 6-8 Hz. 

C- F coupling constants, 4, 569 Benzo[b]furan-5-carboxylic acid, 4-hydroxy-2-isopropyl-synthesis, 3, 698... 

Table 6 lists proton chemical shifts for a selection of monosubstituted furans including some with a metal or other less common heteroatom attached to the ring, but perhaps it should be noted that values are not closely comparable unless obtained in closely comparable conditions. Solvents can have a substantial effect upon chemical shifts (66CR(B)(263)1227>, especially benzene (72JA8854). Table 7 lists some ring interproton coupling constants and Table 8 some shift values for disubstituted furans. Polysubstituted furans are covered in the reviews and compilations mentioned above. 

Table 7 11, H Coupling Constants for Ring Protons in Furan Derivatives...

Tables 13 and 14 list chemical shifts for monocyclic furans variously substituted, and Tables 15 and 16 the associated coupling constants. As usual, carbon chemical shifts show...

Table IS 13C, h NMR Coupling Constants in Simple Furan Derivatives1...

Table 17 Chemical Shifts and/or Coupling Constants Directly Attached to a Furan Nucleus...

Table 19a 13C-19F Coupling Constants" in Ethyl 6-Cyano-4,5,7-trifluoro-2-methylbenzo[6]furan-3-carboxylate...

Table 19b 13C- h Coupling Constants for the Furan Nucleus in Furocoumarins and Furochromones (Benzofuran... 

Table 22 H,H Coupling Constants for 2,3-Dihydrofuran and 2,3-Dihydrobenzo[6]furan Derivatives...

NMR data have been recorded for numerous compounds with exocyclic enamine double bonds (79JHC1335). Examples of exocyclic imine double bonds are found in furan-2,3-dione 3-oxime and a related 5-oxime the vie proton coupling constant in the former is 5 Hz and so larger than expected, perhaps because of electron release from the hydroxy oxygen atom (72JCS(P1)2527). 

Although single bond carbon-13 proton coupling constants (Vch) show a dependency on the geometry of the ring, a value of 200 Hz is considered typical for a carbon attached to oxygen in a heteroaromatic system (B-72MI22203). Values for 2- and 4-pyranone and furan are presented in Table 11. 

Table 11 Single Bond l3C- H Coupling Constants for the Pyranones and Furan (74JOC1935)...

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