(R)-(+)-[VCD(-)984]-4-Ethyl-4-methyloctane: A Cryptochiral Hydrocarbon with a Quaternary Chiral Center. (2) Vibrational CD Spectra of Both Enantiomers and Absolute Configurational Assignment
To characterize precisely the chiroptical properties of 4-ethyl-4-methyloctane (1), one of the so-called cryptochiral compounds, the enantiomer (S)-(-)-[VCD(+)984]-1 was synthesized in an enantiopure form. The observed specific rotation values at 365 nm of both enantiomers agreed well, but were opposite in sign, confirming their enantiomeric relationship, although their absolute values were very small, reflecting their cryptochirality. The vibrational circular dichroism (VCD) spectra of both enantiomers were measured neat and showed characteristic Cotton effects around 1150-900 cm -1 ; the VCD spectral curves were mirror images of each other. Based on the observed positive VCD band at 984 cm -1 for (S)-(-)-1, its absolute configuration was fully designated as (S)-(-)-[VCD(+)984]-1. The VCD spectrum was quantum chemically calculated by the DFT MO method at the B3PW91/6-31G(d,p) level, and the simulated VCD curve for (S)-1 agreed very well with the observed VCD spectrum of (S)-(-)-1, confirming the absolute configuration.