Nuclear magnetic resonance, force field, and MNDO studies on 1-methoxycycloalkenes

Abstract

The 1H, 13C, and 17O NMR spectra of 4- to 9-membered 1-methoxycycloalkenes have been recorded in CDCl 3, solution. The 17O NMR chemical shifts reveal unexpectedly large and irregular variations with ring size. The other NMR shift data suggest that these variations are due to differences in the relative amounts of planar and nonplanar conformers about the OC( sp 2) bond. The same NMR data also suggest that whereas the MeO groups of the 5- to 9-membered ethers adopt mainly the expected planar s- cis conformation, the 4-membered ether exists predominantly in the exceptional s- trans conformation. The geometries of and charge densities in these ethers have been studied by force field (COSMIC, MM2) and MNDO calculations. In summary, the evidence given by the 17O, 13C, and 1H NMR chemical shifts shows that extensive p-Π conjugation exists, and planar conformers dominate, in each of the present compounds. The highest contributions of planar forms are found in the 4- and 7-membered 1-methoxycycloalkenes; moreover, in the 5- to 9-membered ethers, the s- cis conformers expectedly prevail. However, both the 13C and 1H NMR data strongly point to an exceptional predominance of the planar s- trans conformer in 1-methoxycyclobutene ( 1). If this interpretation is correct, it appears that this compound is the only known derivative of methyl vinyl ether preferring the s- trans conformation though there is no obvious steric hindrance for the attainment of the s- cis form. This observation has additional significance concerning attempts to explain the origin of the favor of the s- cis conformation of methyl vinyl ether: this conformation has no inherent preference, it is favored only because the other planar form, s- trans, is destabilized by steric interactions between the Me group and the α hydrogen of the vinyl group. If this steric hindrance is relieved, which seems to be the case in 1, the s- trans form becomes the more favored one.