Stable Simple Enols. 8.¹ Synthesis and Keto ⇌ Enol Equilibria of the Elusive 2,2-Dimesitylethanal and 1,2,2-Trimesitylethanone. Conformations of 1,2,2-Trimesitylethanone and 1,2,2-Trimesitylethanol

2,2-Dimesitylethanal (2) and 1,2,2-trimesitylethanone (4) were prepared by pyridinium dichromate oxidation of 2,2-dimesitylethanol (5) and 1,2,2-trimesitylethanol (7), respectively. For 4, e = 11 000 in hexane at 250 nm, δ(¹³C=0) = 204,65 ppm, and v_c0-1688, 1695 cm^-1, indicating a substantial conjugation between the a-mesityl and the C=O moieties. The calculated dihedral angle Mes-CO θis 39° and the observed value in the solid is 48°. Other triarylethanones show similar effects. Consequently, bulky 0 substituents on crowded a-aryl ketones can reduce, rather than increase, the Mes-CO angle. The conformation of 4 is “bisected” with a H-C-C-O angle of ca. 180°, and the three rings adopt a propeller conformation. At 210 K the two 0-mesityl groups of 4 are magnetically equivalent, but the a ring o-Me and m-H protons and the corresponding carbons are anisochronous. A DNMR study suggests that the rapid rotational process which equalizes the β-mesityl groups is a two-ring flip with AG_c <9.5 kcal mol^-1. The coalescence of diastereotopic groups on the a-ring gives △G_CC = 13.8 kcal mol^-1for rotation around the Mes-CO bond. Equilibration of 2 and 4 with their enol isomers was followed from both sides in hexane at 353.6 K. The enols consist of >95% in the mixtures and k_enol= [enol]/[carbonyl compound] values are 20 ± 1 for 2 and 79 ±1 for 4, which are the highest values known for simple enols. They are 9 and 11 orders of magnitude higher than the values for acetaldehyde and acetone. Conformational studies showed that 7 exists in CDC1₃ at 223 K as a 1:1.4 mixture of the ap and the sc conformers, whereas X-ray diffraction shows that the solid has the ap conformation. 7 undergoes a dynamic process at room temperature which was briefly studied.