Conformational analysis of 1-substituted 2,3-epoxypropanes study. A carbon-13 nuclear magnetic resonance

Abstract

The 13C NMR shift data for nine 1-substituted .2,3-epoxypropanes were obtained and analyzed. It was found that the 1 shift was useful in evaluating rotational preferences of the CHlX moiety. Comparison with 1-substituted propane data is also presented. Solvent effects on rotational populations were found to be unimportant.The 1-substituted 2,3-epoxypropanes, of general formula I, pose an interesting problem in conformational analysis CH H2X-CH-'H because the barrier to rotation of the CHzX moiety is expected to be small. The problem involving rotational populations can be more simply analyzed if all eclipsed conformations are ignored, leaving only rotamers A, B, and C to be considered. *IThe stereochemistry of the epoxypropanes, particularly the halogenated derivatives, has been previously investigated using electron diffraction spectroscopy,' ab initio calculations,2 dipole moments," and IH NMR s p e c t r o~c o p y .~.~ However, the conclusions concerning the relative importance of conformations A, 13, and C are not in complete agreement. For instance, Thomas4b concluded that rotamer C was the least populated state for X = fluorine, while Reynolds3 reported that B is the minor conformer. The gas-phase diffraction data apparently indicate that for X = chlorine and bromine A and B are of equal importance,l while Reynolds states that A is the major conformation." In view of this controversy, it was felt that 13C NMR shift data would provide useful information such that the above situation could be clarified.The application of '3C NMR spectroscopy to the assessment of molecular struct.ure and stereochemical assignment is well documented.,j These applications are facilitated by empirical correlations of substituent effects generated from systematic studies of closely related compounds.6 Of particular utility for obtaining stereochemical assignments is the induced uplfield shift observed for the resonance of a carbon which is in a gauche disposition to another carbon or heteroatom at .the y position.' The "y effect" is ascribed to induced polarization of the charge along .the 13C-H bond caused by nonbonded interactions, through space, such that the carbon in question becomes more electron rich.* Some caution must be exercised in this type of evaluation when the compound contains N, 0, or F, since these heteroatoms also induce upfield shifts when y-antiperiplanar to a particular carbon.9 The proposed mechanism for this effect is amply described elsewl~ere.~ Owing to the sensitivity of the carbon-13 chemical shift to this stereochemical probe, the importance of the rotamer populations for the 1-substituted 2,3-epoxypropanes can be evaluated.Experimental Section All of the compounds, except where X = I, were commercially available and of high purity. These compounds were used as received.The iodo derivative was prepared by literature procedure^.^ The carbon-13 magnetic resonance spectra were obtained in the Fourier transform mode on a JEOL FX-60 spectrometer system equipped with a Texas Instruments computer with a 24K m...

Conformational Stability of (+)-Epichlorohydrin

“…Epichlorohydrin, which is used in the synthesis of the optical isomers of antihypertensive and antianginal agents and in preparation of glycidyl ethers, has been studied using infrared (IR), Raman, nuclear magnetic resonance (NMR), microwave, electron diffraction, and dipole moment measurements. Three different conformations (See Figure ) have been considered for epichlorohydrin.…”

Epihalohydrins in Organic Synthesis

“…The determination of the conformational stability of epifluorohydrin has been the subject of several spectroscopic studies, including microwave, , infrared, and/or Raman − and NMR studies. − In all these studies, there is agreement that the most stable conformer in the solid, liquid, and gas phases is the conformer 1 (Figure ), but there is disagreement as to the second most stable conformer. In some of these studies, the conformer 2 has been reported as the second most abundant conformer, ,,, whereas others have reported the conformer 3 as the second most abundant conformer (Figure ) ,, or that it was impossible to determine the relative stability of the two high energy conformers. ,, …”