The planar conformation of 2-methylthiobenzaldehyde in solution

Abstract

. Chem. 61. 26 (1983). The 'H nmr spectral parameters are extracted for a 4 mol% solution of 2-methylthiobenzaldehyde in CCI, at 305 K. The long-rangc spin-spin coupling constants involving the aldehydic and methyl protons are consistent only with a preferred conformation in which all heavy atoms are coplanar, as are the chemical shifts of the ring and methyl protons. This conclusion contradicts previous interpretations of the dipole moment, the nmr parameters, and of the infrared data for CCI, solutions. The present data show that the 0-sytl and 0-crtlri forms of the conlpound are present in roughly equal proportions.--TED SCHAEFER et RUDY SEBASTIAN. Can. J. Chem. 61, 26 (1983). On a dCduit les paramktres spectraux de la rmn du 'H d'une solution ii 4 mol% de methyl-2 thiohenznldkhyde B 305 K. Les constantes de couplages a longue distance impliquant les protons aldehydiqucs et ceux du mkthyle sont en accord uniquement avec une conformation prCf6rentielle dans laquelle tous les atornes lourds sont coplanaires. 11 en cst dc m&me pour les deplacements chimiques des protons du cycle et du mkthyle. Cctte conclusion contredit les interpretations anterieures du moment dipolaire, des paramhtres de la rnmn et dcs donnkes infrarouges dcs solutions dans le CCI,. Les donnkes actuelles rnontrent que les forrnes 0-sytl et 0-anti du composd sont prescntes approximativernent en proportions Cgales. Introductionplanar conformation of 1. A discussion (13) of the dipole moThe stereospecificity of the long-range coupling over five ment and infrared Spectrum of 1 came to the same conclusion.

Concerning in 2-methylphenol and its derivatives. A proximate coupling

“…Other examples are 5~~, " C ' ' ' in anisole (2, 3 , 8-10) and thioanisole derivatives (5-7). The magnitude of 5~~~' C " ' drops off rapidly as the OCH, or SCH, moieties move away from the benzene plane (6,(8)(9)(10). Indeed, 'J,!…”

“…Negative proton spin-spin coupling constants over five formal bonds are usually attributable to proximate interactions involving a close approach of the bonds or atoms carrying the protons in question ( I - 10). An example is '~j j ' .~" in 2,5-dichlorostyrene (I), involving an ortlzo ring proton and that P-proton which, in a planar conformation, lies very near the ring C-H bond.…”

“…Indeed, 'J,! :.~"' is a useful conformational indicator (5-10) and a detailed theoretical study for anisole indicates its mechanism and conformational dependence ( 10).…”