Previous assignment of the signals in the spectra of carboxymethylcellulose, based on an incremental calculation and hence on low-molar-mass model substances, made determination of the partial degree of substitution fundamentally impossible because overlapping of the relevant signals was predicted. However, the methods of acid and enzymatic hydrolysis employed generate the monomer or a mixture of polymers, oligomers and monomers and hence a large number of end groups, which also influence the nuclear magnetic resonance (NMR) spectra. By using ultrasound, it was possible to degrade the molar mass (lower limit of molar mass approx. 100000 g/mol), without oligomers being generated and cleavage of side groups occurring. The viscosity of 10 wt.-Vo polymer solutions was so low that the partial degree of substitution could be determined quantitatively for the first time. An additional I3C NMR spectroscopic examination of the acid hydrolysate enabled the composition of the eight monomers of sodium carboxymethylcellulose (NaCMC) to be determined in a degree of substitution (DS) range of 0,8-3,0. Knowledge of the monomer compositions is then used to reassign the signals of the polymer spectra, which are then evaluated. Comparison with titrimetric methods (polyelectrolyte titration and ASTM method) showed some wide discrepancies with the absolute method (NMR) in the case of samples with a DS greater than 1. The results of the 13C NMR spectroscopic examination of the acid hydrolysate were also used to determine the relative rate constants of the etherification reaction ( k , : k , : k , = 3,O : 1,O: 2,1), and these were then compared with published data.
On irradiation (j, > 305 nm) in alcohols, 5.5-dimethyl-2(5H)-thiophenone (1 b) is converted to (E)-4-mercapto-4-methyl-2-pentenoates 8. These esters undergo a consecutive light-induced reaction affording thiolanes when irradiated in the presence of alkenes, and either 2,3-dihydrothiophenes or 3-thiabicyclo[3.1.0]hexanes with alkynes.We have recently presented results [l] [2] on the photochemical behaviour of 2(5H)-thiophenone (la) which, in contrast to the corresponding furan derivative
On irradiation (A = 300 nm) in MeOH 5-(2-propenyl)-2(5H)-thiophenone (lc) affords methyl (E)-Cmercapto-2,6-heptadienoate (Zc) which undergoes light-induced homolysis of the S-H bond to give the alkylthio radical 3c. This species can be trapped by 2,3-dimethyl-2-butene or 2-butyne to afford octahydrobenzo-[blthiophene 9 or 3a,4,5,6,7,7a-hexahydrobenzo[b]thiophene 13, respectively, as main products via consecutive radical ring closure. In the absence of unsaturated hydrocarbons, 3c cyclizes to give methyl (E)-3(thiolan-2-y1)-2-propenoate (8).We have recently presented results on the photochemical behaviour of the 2(5H)-thiophenones l a [l] and l b [2] which react with alcohols to give 4-mercaptocrotonates 2a and (E)-4-methyl-2-pentenoates 2b (Scheme 1). These esters undergo a subsequent light-induced homolysis of the S-H bond giving alkylthio radicals 3 which add to alkenes or alkynes to afford thiolanes 4 or 2,3-dihydrothiophenes 5 via radical ring closure of 6 or 7, respectively. We have now synthesized 5-(2-propenyl)-2(5H)-thiophenone (lc) in order to investigate the effect of the additional C=C bond on the photochemistry of unsaturated thiolactones 1 and on the reactivity of radicals 3,6, and 7. Compound l c has, up to now, only been observed by 'H-NMR in a product mixture [3]. In contrast to the corresponding furan derivative [4], l c does not undergo intramolecular [2 + 21 photocycloaddition,
Dedicated to Prof. Dr. 0. E. Polunsky on the occasion of his 70th birthday (4.XI. 88)Mechanistic evidence for the light-induced ring opening of thiophen-2(5H)-ones 1 in alcohols affording c@-unsaturated mercapto esters 2 is presented. Regio-and stereochemical aspects of the ring closure of alkenylthio (type 3) radicals 15 and 17 to S-heterocycles 16 and 18, of 3-thiahex-5-enyl radicals 4 to (tetrahydrothien-3-yl)methyl radicals 6 and of (2,3-dihydrothien-3-yl)methyl radicals 30 (type 7, but-3-enyl radicals) to cyclopropanemethyl radicals 29 are discussed. Irradiation (A. 350 nm) of 1 in cyclohexane in the presence of 2,3-dimethylbut-2-ene affords [2 + 21 cycloadducts 14 albeit in very low yields.We have recently reported [ 1-71 that on irradiation thiophen-2(5H)-ones 1 behave different from the corresponding 0-heterocycles. While furan-2(5H)-ones exhibit a typical enone-like behaviour yielding cyclodimers, [2 + 21 cycloadducts with alkenes or photoreduction products in alcohols from their triplet state [S] [9], the unsaturated thiolactones 1 undergo ring opening in alcohols to give a$-unsaturated mercapto esters 2 from the excited singlet state. Mercapto esters 2 undergo consecutive light-induced S-H homolysis to afford alkenylthio radicals 3 which in reacting with alkenes or alkynes proved to be useful synthetic intermediates for one-pot syntheses of thiolanes, 2,3-dihydrothiophenes as well as bi-and tricyclic thiolane derivatives via intermediates 4-7 (Scheme 1). Such
-Scheme 1
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.