Eand Z-(m-methoxymesityl)-l,2-dimesitylethenols (3a and 4a), their isopropyl ethers 3c and 4c, and l-(m-methoxymesityl)-2,2-dimesitylvinyl isopropyl ether (2c) were prepared. X-ray structures of 3a and 3c were determined. The rotational barriers for the 3-ring flips are 18.8 (3a) and 18.3 (4a) kcal mol-1. Each of 2c-4c display two MeO singlets and four i-Pr doublets and exist as a mixture of two pairs of enantiomers differing in helicity and in the "up" and "down" disposition of the OMe in relation to the C=C bond. The threshold process for correlated rotation around the Ar-C bonds of 2c-4c is a three-ring flip which leads to diastereomerization with helicity reversal, with barriers of 15.8-16.1 kcal mol-1 resembling that for the three-ring flip enantiomerization of trimesitylvinyl isopropyl ether. The DNMR at the i-Pr region showed two coalescence processes. The lower barrier is for a three-ring flip diastereomerization, and the higher barrier is for the two-ring flip enantiomerization of the residual isomers where the MeO-labeled ring passes via the C=C plane. The barriers for the ß,ß'-, ,ß-, and a,/S' two-ring flips are 25.2 (2c), 23.1 (4c), and 21.1 (3c) kcal mol-1. MM calculations on the ground-state conformation are in reasonable agreement with the X-ray data, but in the calculated transition states the rings are distorted and the barriers differ from the observed ones. When ring planarity and o-Me group constraints are imposed, an agreement of the calculated and observed barriers is achieved. The conformation of the i-Pr group significantly affects the calculated barrier. The order of the barriers for the isomeric two-ring flip processes is ascribed mainly to different steric interactions between neighboring rings in the isomeric transition states.
(E)-2-(m-methoxymesityl)-1,2-dimesitylethenol (3a) isomerizes in the absence of a catalyst in solution to a 1.0:0.9 E/Z (3a/3b) equilibrium mixture. In CDCl3 the isomerization is first order in 3a within a run, but the plot of the rate constant k(obs) vs the changing [3a]0 in different runs is a half-parabola, indicating self-catalysis by more than one enol molecule. At 0.09 M enol, the isotope effect k(3a)/k(3a)-OD = 2.1. In the presence of 0.025-0.25 M pyridine-d5, the k(obs) vs [pyridine-d5] plot displays a bell-shaped profile. The change in the shape of the OH signals of the 3a/3b mixture at 295-430 K in C6D5NO2 was followed by DNMR. The four signals of the diastereomeric 3a/3b mixture observed at 330 K coalesce at 350 K with barriers of 18.3 and 18.4 kcal x mol(-1) due to the diastereomerization of the vinyl propellers. The resulting two signals observed at >360 K further coalesce at 425 K with a barrier of 22.9 kcal x mol(-1) due either to oxygen-to-oxygen proton exchange or to E/Z isomerization. The estimated upper limit for the rate of proton exchange of k(ex) < or = (2-4) x 10(3) M(-1) x s(-1) at 425 K between the enol molecules is sufficiently slow to be a rate-controlling step in the isomerization. A process in which several enol molecules catalyze the isomerization is suggested, and several mechanistic routes are analyzed.
9-Xanthenecarboxaldehyde (7a) and methyl (7b), mesityl (7c), 2,4,6-triisopropylpheny 1 (Tip) (7d), and terf-butyl (7e) 9-xanthenyl ketones were prepared and found to be ketonic in CDC13. In DMSO-¿6, 7a, 7c, 7d, and diphenylacetaldehyde 16 are in rapid equilibrium with the enols 15a, 15c, 15d, and 17, respectively. Keto ^enol equilibrium constants, Ktnol, were measured in DMSO-í/6 at various temperatures. The values at 294 K are 7a (101) > 16 (5.06) > 7d (1.1) > 7c (0.48). The AH0 values are -2.0 to -4.4 kcal mol"1, and the AS0 values are -9.1 to -11.8 cal mol'1 K"1. From the decrease of o(OH) values on increasing the temperature, the association constants, Kassoc, of the enols with DMSO-d6 and the derived AHmsx and ASass0C values were obtained. The qualitative rates of formation of the enol acetates in DMSO-pyridine/Ac2Q follow the Xcnol values. X-ray diffraction data for 7b, 7c, and 18c-the acetate of 15c-gave the Ar-C=C and Mes-CO dihedral angles and showed that the xanthenyl moiety had a butterfly conformation. Several triarylethanones PhCH
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