Thiophene protonation

The values of bond lengths for neutral thiophene, 2/7-thiophenium ion and 2 H-thiophenium ion were calculated by the RHF/6-31G method and selected values are 

It can be seen that in the 2//-thiophenium imi, the bonds S-C-2, C-2-C-3 and C-4-C-5 are longer than in thiophene, and the bonds S-C-5 and C-3-C-4 are shorter than in thiophene. In the 3//-thiophenium ion, S-C-2 and C-4—C-5 are shorter than in thiophene, and the bonds S-C-5, C-2-C-3 and C-3-C-4 are longer than in thiophene, and thus the calculations are consistent with the structures shown in Fig. 1. 

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Because the amide C-N bond has partial double bond character, there is restricted rotation around this bond. Therefore, in addition to the three signals expected for the thiophene protons in the NMR spectrum, there are two broad singulets for the two amide protons since these are both chemically and magnetically non-equivalent. The amide hydrogen atoms are diastereotopic and therefore can be distinguished by the descriptors pro-Z and pro-E. 

Aluminum chloride-catalyzed acylation may be accompanied by formation of ct complexes (i.e., thiophenes protonated by the very strong acid co-product HA1C14) (75JOU412). This will be more serious under any of the following conditions (1) the thiophenes contain electron-supplying groups, as they will then be better bases (2) solvents that do not contain nitro groups are used, because aluminum chloride is then not coordinated... 

I In isomer 1, there are NOEs from the penicillin methyl resonance G to thiophene protons B and F, showing that the sulphur is down. An NOE at C from A indicates that the thiophene ring faces to the left, so isomer 1 is compound d. 

However, fi adsorption on catalysts is also supported by deuterium exchange studies of thiophene. When thiophene and D2 are passed over Mo-based HDS catalysts at temperatures below which substantial HDS occurs, the thiophene protons exchange with deuterium. This exchange occurs much more rapidly at the 2- and 5-positions than at the 3- and 4-sites. When CpRu(jj -T)+ (equation 9) is dissolved in CD3OD in the presence of OD at 23 °C, the 2,5-hydrogens of thiophene... 

NMR spectroscopy has been the most useful tool in cephalosporin C chemistry. In cephalosporins the carbons are unsaturated or highly substituted with heteroatoms, and the protons are usually widely separated in chemical shift and have simple coupling patterns. Recently, solvent induced chemical shifts, nuclear Overhauser effects, and the anisotropy of the sulfoxide bond have been utilized in chemical studies of cephalosporin C derivatives. Analytical information may be derived from NMR spectra of cephalothin by observing the contribution of the 0-lactam protons, thiophene protons, methylene groups, and methyl protons (from acetate). 

Cyclobutadithiophene 4.1 can formally be regarded as the smallest member of the cyclothiophenes. Also known as cyclo [2]thiophene or cyclodi(3,4-thienylene), it was prepared by Shepherd as the most stable of the four possible isomeric dithia analogues of biphenylene [356]. In the NMR spectrum of 4.1, a paramagnetic shift of the a-thiophene protons has been found, indicating a cyclobutadienoid or [4]annulenoid structure. In the UV spectrum, absorptions are red shifted (334, 318 nm) compared with that of thiophene (230 nm), providing evidence of electronic interaction and conjugation between the two thiophene units. 

Magnetically non-equivalent thiophenic protons have been observed in thienylglycollic acid esters of certain asymmetric alcohols. ... 

Fig. 9-12 Variable-temperature H NMR spectra of the thiophene proton of P30T. After Reference [261], reproduced with permission.

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