1,3,5-Trineopentylbenzene. VIII. Restricted Rotation of Alkyl and Acyl Substituents.

Artificial Molecular Rotors

“…A number of groups have investigated rotations in alkylbenzenes, − benzyl alkali salts, − benzyl halides, − substituted aromatic amine derivatives, dialkyl disulfides, methylated pyridinium salts, and variously alkylated phenyl rings. , A more detailed analysis of rotations of alkyl groups can be found in a number of reviews. ,,, However, several investigators , have cautioned that gearing in alkyl systems is quite complicated and cogwheeling may not be the only mechanism for the rotation of the groups.…”

“…A number of groups have investigated rotations in alkylbenzenes, [253][254][255][256][257][258][259][260] benzyl alkali salts, [261][262][263][264] benzyl halides, [265][266][267][268] substituted aromatic amine derivatives, 269 dialkyl disulfides, 270 methylated pyridinium salts, 252 and variously alkylated phenyl rings. 271,272 A more detailed analysis of rotations of alkyl groups can be found in a number of reviews.…”

Conformational Studies by Dynamic NMR. 62.1 Stereomutations of Rotamers and of Conformational Enantiomers in 1,2-Diacylbenzenes

“…Whereas benzaldehydes, even when ortho -substituted, maintain an essentially planar conformation, ortho -substituted diaryl or alkyl aryl ketones adopt twisted conformations, with the Ar−CO dihedral angle in the range 45−90° . The latter situation can be directly detected in solution by monitoring, usually at low temperature, the anisochronous NMR signals due to diastereotopic groups of appropriate prochiral substituents. − The free energy of activation (Δ G ⧧ ) for the Ar−CO rotational process can be determined by computer simulation of the temperature dependent line shape of these NMR signals. If the ortho substituents are such as to eliminate the plane of symmetry, two conformational enantiomers are created at low temperature, by virtue of the Ar−CHO chirality axis.…”

Conformational Studies by Dynamic NMR. 85.1 Stereomutation of Conformational Atropisomers of o-tert-Butylphenyl Alkyl Ketones

“…2,3 Conversely, hindered aromatic ketones (for instance, those bearing ortho or ortho-like substituents) have the plane of the carbonyl group significantly twisted with respect to that of the aromatic ring, as shown by the X-ray diffraction structures determined in the crystalline state. [4][5][6] In these cases, the rotation barriers (steric barriers) can be measured by dynamic NMR only if prochiral probes are also available, [7][8][9][10][11][12] or if pairs of diastereoisomers are present at the equilibrium. [13][14][15] In favorable circumstances the barriers become NMR detectable when the hindered ketone adopts a propellerlike chiral conformation, 16 or if the enantiotopic groups in the molecule can be made diastereotopic by the presence of an appropriate chiral agent.…”

“…18 When dealing with non-coplanar phenyl ketones, the rotation barriers are sufficiently high for NMR detection only when substituents are present in both the ortho positions. 7,[9][10][11] So far, therefore, restricted rotation has never been measured by NMR in phenyl ketones containing solely one ortho substituent, 19 the only exception being the peculiar situation encountered in the racemic conformer of a phenyl bearing two i-PrCO groups in an ortho relationship. 12a The latter finding thus suggests that it should be possible to render NMR visible the Ar-CO bond rotation if a sufficiently large group is purposely introduced in one of the two ortho positions of a phenyl ketone.…”