Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N-arylguanidinium chlorides

Abstract

The ¹³C and ¹⁵N chemical shifts of five N-arylguanidinium chlorides carrying polar substituents, ranging in character from 4-methoxy to 4-nitro groups, have been determined by NMR spectroscopy at the natural-abundance level of ¹³C and ¹⁵N in dimethyl sulfoxide solution. Comparison of the ¹³C shifts of these salts with those of monosubstituted benzenes shows that the guanidinium group induces an average downfield shift of -5.8 ppm of the resonance of the aryl carbon to which it is attached (C1), an average upfield shift of +4.2 ppm for C2 and C6, and a small upfield shift of +1.9 ppm for C4. The shifts of C3 and C5 are small and erratic relative to the corresponding carbons in monosubstituted benzenes. The ¹⁵N resonances of the guanidinium nitrogens are quite sensitive to electric effects resulting from substitution of polar groups at C4. The ¹⁵N shift of the [unk]NAr nitrogen relative to that of the salts suggests that the predominant tautomer for N-arylguanidines is (H₂N)₂C[unk]NAr. The ¹⁵N shifts of the (NH₂)₂ nitrogens correlate rather well with σ_p^- parameters, whereas the shifts of the -NHAr nitrogens seem to correlate only with R values derived from the σ_p^- substituent constants.

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