Rotations Methoxy groups

Barriers to Rotation and Equilibrium Constants of Arylfluorenes Carrying a Hydroxy or a Methoxy Group in the 2-Position of the Aryl Group at 56.3°C in Chloroform ... 

The protected diol side-chain of 456 is introduced by asymmetric dihydroxylation and directs diastereoselectivity in the formation of 457 and 458 by lithiation. The most acidic position of 456, between the two methoxy groups, is first protected by silylation. Suzuki coupling of 459 with the boronic acid 460 gives the kinetic product 461—the more severe hindrance to bond rotation in this compound does not allow equilibration to the more stable atropisomer of the biaryl under the conditions of the reaction. 

As in other l-(p-anisyl)-substituted carbocations, in the l-(p-anisyl)vinyl cations 401 and 403 there exists a torsional barrier for a methoxy group rotation around the... 

Quantum chemical ab initio calculations were performed to reveal additional details of the structures and charge stabilization of l-(p-anisyl)vinyl cations. The l-(p-anisyl)vinyl cation 403 and the model cation structures 410 and 411 with >S-SiH3 and /l-Cfh groups, respectively, were calculated along with the corresponding transition state structures 403-TS, 410-TS and 411-TS for the rotation of the p-methoxy group around the C4-oxygen bond. 

Due to a partial 77-character, aromatic carbonyl compounds have an activation energy barrier for rotation around the phenyl-carbonyl bond, the value of which is substantially increased upon protonation.44 In para-anisaldehyde a second protonation of the methoxy group will drastically decrease their barrier. The temperature-dependent NMR spectrum will reflect both exchange processes, intra- and intermolecular, as shown in Scheme 1.1. 

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