Evidence for an important ¹³C NMR shielding effect for carbon atoms bearing a heavy chalcogen substituent

Abstract

An important 13C NMR shielding effect on carbons bearing a heavy chalcogen is demonstrated. This effect is parallel to that induced by iodine, but to a lesser extent. For acyclic compounds and for partially saturated heterocycles, there is an excellent linear correlation between the "C chemical shifts of carbons bearing a chalcogen and carbons bearing a halogen atom in the corresponding compound. The linearity of the relationship is less satisfactory with heteroaromatic compounds.

Carbon-13 nuclear magnetic resonance studies of organotellurium compounds. II. Chemical shift trends

“…However, the available data on tellurium which corresponds to a decrease in the tellurium compounds are limited compared with polarity of the Te-C bond. ~a g n e t i c anisotropic those for sulphur and selenium (5)(6)(7)(8)(9)(10)(11)(12)(13)(14). effects originating in the tellurium substituents are we have recently reported our studies on 1 3~ nmr not expected to vary much in this closely related spectra of phenyl and p-methoxyphenyl tellurium series; moreover, they are expected to be small (8).…”

2,3,4,9-Tetrahydrothiopyrano[2,3-b]indoles: synthesis via an intramolecular indole Grignard reaction and conformational study by 1H and 13C NMR spectroscopy

“…The key intermediates were the diols erythro-( 21) and thre0- (22). Each diol was converted into the epoxides cis- (24) and trans- (25), respectively, by the method of Newman et These diols could also be converted into epoxides by monomesylation, demesylation sequences. In the latter method, inversion of the configuration occurred during epoxidation, and (21) afforded trans- (25), and vice uersa.15 Subsequently, the cisepoxide (24) was converted into the pure threo-iodohydrin (26) by the method of Cornforth et all6 In a similar manner, erythro- (27) was obtained from trans- (25).…”

“…The reaction of the cis-epoxide (24) with EtMgBr in ether was completed smoothly at O"C, and exclusively afforded (28) in 92% yield. However, the reaction of the trans-epoxide (25) was sluggish, giving a 36% yield of (29) and a 32% yield of a dimeric product (34) after 6 h at 25 "C. Cyclization of iodohydrins was conducted using commercially available methylmagnesium iodide (MeMgI) instead of EtMgBr in order to avoid complication by halogen exchange.…”

“…The resonances of aromatic carbons of tetrahydrothiopyrano [2,3-b]indoles are closely related to those of 2,3-dimethylindole and indole alkaloids, especially 1,2,3,4-tetrahydr0-9& carbazole20*21 and yohimbine alkaloids,20*22 except for the C-9a positions which are shielded by a sulphur atom. 24 Assignments of aliphatic carbons were determined by the single-frequency off-resonance decoupling method (SFORD), comparison of 'J(C,H), and SCS. C-2 and C-3 could be differentiated owing to their different 'J(C,H) values.…”

“…Analytical HPLC (p-Porasil; CHC1,-hexane, 1 : 5,3 ml/min) indicated that the unpurified material contained 2% of the cis-epoxide (24) (tR 5.9 min). The desired compound (25) (24).-In a manner similar to that described for the preparation of ( 25), the erythro-diol…”