Discrimination of Cryptochirality in a Saturated Quaternary Hydrocarbon by Asymmetric Autocatalysis

Discrimination of Cryptochirality in a Saturated Quaternary Hydrocarbon by Asymmetric Autocatalysis 

Significant progress in chiral discrimination has been achieved in recent decades, however, there remains a class of compounds whose chiral discrimination has been very difficult to establish, or has not been possible at all. Chiral saturated hydrocarbons form a class of compounds whose chiral discrimination has often been very difficult [105]. Unlike other functionalized compounds, chiral saturated hydrocarbons do not bear heteroatom(s), -electrons, or chromophores, and therefore, the difference between the four substituents on the asymmetric carbon atom is very small. 

An example of a compound whose chiral discrimination poses the utmost difficulty is a saturated quaternary hydrocarbon bearing similar substituents on the asymmetric carbon atom, with a representative example being 5-ethyl-5-propylundecane,i.e., (n-butyl)ethyl(n-hexyl)(n-propyl)methane (Fig. 8) [ 106]. The enantiomer of this hydrocarbon exhibits practically no optical rotation ( of 0.001) between 280 and 580 nm. The compound is a chiral, but to all intents an optically inactive, compound. To the best of our knowledge, the chirality of this compound has not been discriminated using any current method. Mislow called such hidden chirality cryptochirality [ 14,107]. 

Chiral quaternary alkanes do not have any m-electrons or a heteroatom. Thus, the present chiral discrimination may involve the CH-7T interactions between the CH group of the chiral hydrocarbon and the m-electrons of the pyrimidine-5-carbaldehyde 11. This CH-jt interaction [109] maybe used to discriminate between the Re and Si enantiofaces of aldehyde 11, because the 

Chiral Hydrocarbons Act as Chiral Initiators (S-Indudng Enantiomer) 

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