Optically Active Amines. 40.¹ Application of the Benzene Sector Rule to the Circular Dichroism of Chiral Benzylcarbinamines and Benzylcarbinols

Abstract

The sign of the 1 L b Cotton effects (CEs) from about 240 to 270 nm in the circular dichroism (CD) of enantiomers of chiral benzylcarbinamines and benzylcarbinols is correlated with their absolute configurations using the benzene sector rule and a consideration of the equilibrium between their two conformers of lowest energy and of oppositely signed rotatory powers. For chiral benzylalkylcarbinamines, carbinamine salts, and carbinols, which show a single sign for their 1 L b CEs, a shift in the conformational equilibrium can explain a sign reversal of the CEs with a change in solvent, (R)-2-amino-1-phenylpropane showing negative 1 L b CEs in methanol but positive ones in cyclohexane. L-Phenylalanine in water shows positive 1 L b CEs, but in methanol it shows both negative and positive CD maxima. In methanol, the two longest wavelength maxima comprise a double CE associated with the band origin absorption maximum at 267 nm, the conformational equilibrium shifted from the positive conformer of L-phenylalanine to its negative one and the greater amount of the latter is now detected at slightly longer wavelength in the CD spectrum.

Chiral, Polyionic Dendrimers with Complementary Charges − Synthesis and Chiroptical Properties

“…They are similar to those of phenylalanine derivatives, [26,27] but with larger molar ellipticities and less fine structure, and the CEs are bathochromically shifted approximately 7Ϫ8 nm. On the basis of this similarity, and the sensitivity of the CD spectra of phenylal- anines as a measure of their conformation, [26,27] we assume that significant perturbations of the conformational equilibrium of our monomers will show as changes in the CD spectra. Since the dendrimers presented here are of low generation number, we did not expect to see steric packing effects, but hydrogen-bonding networks could potentially distort the dendritic structures, causing them to fold on themselves, and this would be expected to alter the CD spectra.…”

“…If there is an additive relationship between the chiroptical parameters for the individual monomers and the whole dendritic structure, it can be inferred that the conformational equilibrium of the monomers is unperturbed in the dendrimer. [27] From this investigation, it is known that phenylalanine derivatives exist mainly as two conformers, 35a and 35b (see Figure 2), with Cotton effects (CEs) of opposite sign in the 240Ϫ270 nm region. Conversely, if there are deviations from this additivity, packing effects may be operative, and the monomer subunits of the dendrimer may exist in different conformations than the free monomers.…”

Optically Active Amines. 41.1 Application of the Benzene Chirality Rule to Chiral Ring-Substituted Benzylcarbinamines and Benzylcarbinamine Salts

“…In an earlier report, an outline was given for the correlation of the sign of the 1 L b Cotton effects (CEs) from about 240 to 270 nm in the circular dichroism (CD) of enantiomers of chiral benzylcarbinamines and benzylcarbinols with their absolute configurations using the benzene sector rule. , This rule was initially formulated for the correlation of the sign of the 1 L b CEs of phenylcarbinamines and phenylcarbinols, such as ( R )-α-phenylethylamine [( R )- 1a , Table ] and ( R )-α-phenylethyl alcohol [( R )- 1b ], with their absolute configurations. The CEs are associated with transitions from the lowest energy vibrational mode in the ground state to totally symmetric vibrational modes in the 1 L b electronically excited state of the benzene chromophore , and are the result of vibronic borrowing , from allowed transitions at shorter wavelength.…”

The role of chlorine position in the electronic circular dichroism of chlorophenyl‐ethanol investigated by vibronic calculations

“…The ECD of the 1 L b band of benzene derivatives is usually interpreted on the grounds of qualitative rules like the sector and benzene chirality ones 2,3,40 . They successfully predict the ECD sign and therefore the absolute configuration of a number of related molecules, 21,22,41–47 but some exceptions have been documented like ( R )‐methylmandelate, 48 ( R )‐1‐indancarboxylate, 49 and ( R )‐α‐isopropylphenylethane 20 …”

“…The ECD of the 1 L b band of benzene derivatives is usually interpreted on the grounds of qualitative rules like the sector and benzene chirality ones. 2,3,40 They successfully predict the ECD sign and therefore the absolute configuration of a number of related molecules, 21,22,[41][42][43][44][45][46][47] but some exceptions have been documented like (R)methylmandelate, 48 (R)-1-indancarboxylate, 49 and (R)α-isopropylphenylethane. 20 Sector rule predicts that the ECD of R enantiomer of PhEtOH is dominated by HT effects and is negative, according to the preferred position of the benzylic hydrogen and the sectors where the OH and CH 3 groups are located.…”