Thiophenes, coupling constants

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for stmeture determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDCl ), 6 = 7.12, 7.34, 7.34, and 7.12 ppm. Coupling constants occur in well-defined ranges J2-3 = 4.9-5.8 ... 

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. 

Typical coupling constants for isothiazoles are given in Table 5. The electronegative nitrogen atom reduces 3,4 and V3.5 from the values of 3.50 Hz and 0.27 Hz, respectively, in thiophene. The V values correlate quite well with rr-bond orders calculated by MO methods (74CJC833). 

Substituting a formyl, carboxy, or carbalkoxy group into a thieno-thiophene or selenophenothiophene molecule has no substantial effect on Ae coupling constants but considerably affects the chemical shifts (see Litvinov and Fraenkel and Michael and Gronowitz ). The chemical shifts of the protons in the selenophene ring are characteristic of different t rpes of condensation of the thiophene and selenophene rings and increase in the order 15 > 14 > 16 irrespective of the solvent used (acetone, CCIJ. A similar sequence is observed in the carbonyl derivatives of the isomeric selenophenothiophenes. 

The formation of a radical-anion with a very short lifetime on the surface of a sodium-potassium alloy during the reduction of thieno[3,2- ]-thiophene (2) at —100° was established by ESR (theoretical and experimental spectra are presented). The formation of the thieno[2,3-61-thiophene (1) radical-anion even under such extreme conditions was not observed. The difference in the stability of radical-anions of thienothiophenes 1 and 2 was accounted for by a greater degree of conjugation in thienothiophene 2 molecule as compared to 1. The spectrum of the thienothiophene 2 radical-anion distinctly exhibits two types of hydrogen atoms with coupling constants 4.87 and 0.52 Gauss. The... 

Proton NMR spectroscopy is an ubiquitous tool for the structural elucidation and study of properties of thiophenes. Both the chemical shifts and the coupling constants provide... 

These values for the ring coupling constants vary only within these small ranges listed above and provide therefore a trustworthy tool for determining substitution patterns. This is perhaps best seen by consideration of a heterogeneous group of thiophene containing structures (116)—(125). 

The JH NMR spectra of 2-substituted tellurophenes have also been systematically studied (76ACS(B)605). As in the selenophene series, a-protons resonate at lower field than /3-protons. Coupling constants are also in the order J45 >J34>J3s- The magnitude of the coupling constants observed for tellurophene derivatives is greater than those of the corresponding selenophenes and thiophenes. 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for structure determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDC13), 6 = H2 7.12, H3 7.34, H4 7.34, and H5 7.12 ppm. Coupling constants occur in well-defined ranges J2 3 = 4.9-5.8 J3 4 = 3.45-4.35 J2 4 = 1.25-1.7 and J2 5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. 13C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. 13C chemical shifts for thiophene, from tetramethylsilane (TMS), are C2 127.6, C3 125.9, C4 125.9, and C5 127.6 ppm. 

---