Induced optical activity rotation

Another example of a geometrical photoisomerization process influenced by CD is the cis-trans photoconversion of cyclooctene in solid P-CD complexes. After prolonged irradiation of the cis-cyclooctene complex, an apparent photostationary state with a trans/cis ratio of 0.47 was detected, which was considerably smaller than that obtained in homogeneous fluid solution (0.96). This result is explained by the reduced rotational mobility of the guest in the inclusion complex. The asymmetric CD environment however, is not able to induce optical activity in the photoproduct, which shows an enantiomeric excess of only 0.24%. In this study, light of 185 nm, also absorbed by the CD, was used [302]. 

Introduction of the allene structure into cycloalkanes such as in 1,2-cyclononadiene (727) provides another approach to chiral cycloalkenes of sufficient enantiomeric stability. Although 127 has to be classified as an axial chiral compound like other C2-allenes it is included in this survey because of its obvious relation to ( )-cyclooctene as also can be seen from chemical correlations vide infra). Racemic 127 was resolved either through diastereomeric platinum complexes 143) or by ring enlargement via the dibromocarbene adduct 128 of optically active (J3)-cyclooctene (see 4.2) with methyllithium 143) — a method already used for the preparation of racemic 127. The first method afforded a product of 44 % enantiomeric purity whereas 127 obtained from ( )-cyclooctene had a rotation [a]D of 170-175°. The chirality of 127 was established by correlation with (+)(S)-( )-cyclooctene which in a stereoselective reaction with dibromocarbene afforded (—)-dibromo-trans-bicyclo[6.1 0]nonane 128) 144). Its absolute stereochemistry was determined by the Thyvoet-method as (1R, 87 ) and served as a key intermediate for the correlation with 727 ring expansion induced... 

For the optical activity of achiral chromophores with a dissymmetric environment, two types of theoretical treatments have been proposed coupled oscillator treatment and one-electron treatment. The charge distribution of the magnetic dipole transition correlates Coulombically with an electric dipole induced in the substituents, and the colinear component of the induced dipole provides, with the zero-th order magnetic moment, a non-vanishing rotational strength. 

Carbodiimide Polymers An optically active carbodiimide, (/ )-152 ([a]365 +7.6°), gives a polymer by polymerization using a titanium (IV) isopropoxide catalyst (Scheme 11.9) [203], The polymer showed optical activity essentially identical to the monomer however, on heating, the polymer indicated mutarotation and specific rotation reached a plateau value of [a]363 -157.5°, which is considered to be based on excess helical sense of the main chain. The mutarotation has been ascribed to a conformational transition from a kinetically controlled one to a thermodynamically controlled one. Excess single-handed helical conformation can be induced for polyfdi-/ -hexyl carbodiimide) by protonating the polymer with chiral camphorsul-fonic acid. 

Photochemical experiments with BN in cholesteric and optically active isotropic phases. Photoinduced interconversions of BN atropisomers are in competition with thermally induced racemization at the temperatures of our experiments. Thus, the observed rotations reflect lower limits to the actual atropisomeric pho-... 

So far, optical rotations of compounds have been discussed where the optical activity is associated with the particular structure of the cyclopropane ring. This means that the rotations are generated by a chiral arrangement of (achiral) ligands attached to the (achiral) molecular skeleton. For certain substituent patterns of I, in particular, the rotations are induced by atomic asymmetry. This is true for III and IV. The effect of the (achiral) cyclopropane moiety (viewed as a ligand) on open-chain molecules with an asymmetric carbon atom can be seen from the rotations of (S)-( —)-l-methyl-1-(1-ethoxyethyl) cyclopropane (80) and its counterpart 81 with only acyclic substituents. ... 

If one analyzes the rotation of D-a-(methylenecyclopropyl)glycine (82) the optical activity must come from (at least) four sources. One rotation contribution is associated with the atomic asymmetry of the open-chain moiety (methylenecyclopropane being viewed as a ligand). On the other hand, optical activity will also be induced by the asymmetric carbon atom of the ring and the asymmetry in the electron density distribution of the exocyclic double bond system (with diastereotopic faces). Finally also helix optical activity may be operative. The example of 82 demonstrates the complexity of the optical rotation of an apparently simple cyclopropane derivative. Further discussions of optical rotations of similar compounds, therefore, will cling to only the qualitative level. 

In an external magnetic field all matter becomes optically active. This observation was first made by Faraday in 1845 the magnetically induced rotation of the plane of polarized light is therefore referred to as the Faraday effect. In recent years, the common mode of study of this phenomenon has been the measurement of magnetic circular dichroism (MCD). Similarly to natural circular dichroism, magnetic circular dichroism is defined as the difference Af = fi, - of the extinction coefficients for left-handed and right-... 

In 1815 the French physicist Biot made the important observation that when a beam of plane polarized light is passed through a solution of certain naturally occurring organic compounds, the plane of polarized light is rotated either to the left or to the right. Molecules that induce this rotation are said to be optically active. 

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