Dynamic NMR Study of Cyclopropanecarbaldehyde. Comparison of the Conjugating Abilities of the Cyclopropyl and Phenyl Groups

The NMR spectrum of cyclopropanecarbaldehyde (1) shows two chemical shifts for the aldehyde proton at −169.2 °C, separated by 1.71 ppm. The populations of the two conformations at this temperature are 0.012 and 0.988, corresponding to the aldehyde hydrogen cis (1a) or trans (1b) to the methine hydrogen, respectively. From the averaged chemical shift at room temperature, populations of 1a and 1b were estimated to be 0.27 and 0.73. Free-energy barriers of 5.03 kcal/mol (1a → 1b) and 5.95 kcal/mol (1b → 1a) were determined for the compound at −169.2 °C. Comparison of these barriers with the barrier for benzaldehyde indicates that the π-donating ability of the cyclopropyl group is lower than for phenyl.