Pseudotetrahedral Polyhaloadamantanes as Chirality Probes:  Synthesis, Separation, and Absolute Configuration

Abstract

Pseudotetrahedral, conformationally as well as configurationally stable 1-bromo-3-chloro-5-fluoro- (4) and 1-bromo-3-chloro-5-fluoro-7-iodoadamantane (5) (and some related compounds) were prepared by our recently devised phase-transfer catalytic halogenation protocol; the optical antipodes of 4 were separated by HPLC on chiral phase in ee > 99%, and the absolute configurations were assigned by matching observed and computed circular dichroism spectra. Structure 5 is the first chiral aliphatic hydrocarbon containing all stable (nonradioactive) halogens; its structure was proven by NMR spectroscopy and by X-ray crystal data. We emphasize that the combination of experiment and theory is very powerful in assigning absolute configurations even for molecules without typical chromophors, with small values for the optical rotation, and without an atom at the stereogenic center.

Rotationally resolved infrared spectroscopy of adamantane

“…The high density of vibrational states, the presence of several hot bands, and the numerous possible anharmonic interactions prevented us from resolving the rotational structure in this part of the spectrum. Table III lists In the following paragraphs we present the first rotational analysis and fit of 7 fundamental bands of adamantane: ν 23 , ν 24 , ν 25 , ν 26 , ν 27 , ν 28 , and ν 30 . For the spectral analysis, it is possible to use the tools that have been developed previously for smaller tetrahedral molecules, importantly CH 4 .…”

“…As we show below, ν 30 , ν 28 , ν 27 , ν 26 , and ν 23 can be treated as isolated bands. In contrast, ν 25 and ν 24 should be treated as a dyad, with a strong Coriolis interaction between the two bands. Figure 3 shows the high resolution spectra of the ν 30 , ν 28 , ν 27 , ν 26 , ν 23 bands and Fig.…”

“…Figure 3 shows the high resolution spectra of the ν 30 , ν 28 , ν 27 , ν 26 , ν 23 bands and Fig. 8 shows the results for ν 25 and ν 24 . The upper traces (in blue) are the predicted spectra and the lower traces are the experimental spectra (in red).…”

“…Figure 9 shows the weak tetrahedral splittings in P-and R-branches observed for relatively high J values (J ≥ 80), although they are less pronounced than for example the ν 28 mode. As for ν 24 and ν 25 many vibrational levels may be in Coriolis interaction with ν 23 . Despite the presence of neighboring possibly perturbing states, we treated the ν 23 band system as isolated since no large perturbations were noticed.…”

“…23 Furthermore, chiral properties of higher diamondoid molecules, as well as of halogenated adamantane have been studied with optical rotation and vibrational circular dichroism (CD and VCD) spectroscopy. 24,25 Since adamantane is a tetrahedral molecule, it has no permanent dipole moment in its vibronic ground state, so that no pure rotational spectrum exists; moreover, to our knowledge, rotationally resolved IR or UV/vis spectra have not been reported to date for any diamondoid molecule, so that no experimental information on the rotational structure is available whatsoever.…”