Effects of Metal Ions on the Equilibria of Methanol and Methoxide Ion Addition to Benzaldehydes. Effect of a Poly(oxyethylene) Side Arm

The effect of added alkali (Na, K) and alkaline-earth (Sr, Ba) metal bromides on the equilibria for the addition of methanol and methoxide ion to 2-(1,4,7,10,13-pentaoxatetradecyl)benzaldehyde (2) has been studied in methanol at 25-degrees-C. Whilst alkali cations do not significantly perturb the above equilibria, alkaline-earth metal ions cause a dramatic increase of the equilibrium constant for the addition of methoxide ion (ca. 420 and 150 times in the presence of 0.1 M SrBr2 and BaBr2, respectively). A detailed study of the system afforded equilibrium constants for associations of metal ions with all of the species involved in the process. An analogous set of equilibrium measurements was carried out for comparison purposes with benzaldehyde (1). All of the added salts did not affect both the addition of methanol and methoxide ion to 1, thus indicating that a substantial contribution to the ligand ability of the hemiacetal anion of 2 arises from interaction of metal ions with at least some of the donor sites of the polyether side arm.