Chirality and asymmetric transformations of axially chiral 4,5-disubstituted phenanthreneamides
We have investigated the rotation barriers of 4-carbamoylphenanthrene and 4-thiocarbamoylphenanthrene and the asymmetric transformations of the novel 4-carboxy-5-carbamoylphenanthrenes. The similar ArC and CN barriers of 92 kJ/mol of 4-carbamoylphenanthrene indicate a strongly correlated process of both rotations. Only the corresponding thioamide could be separated on microcrystalline triacetylcellulose and a barrier of 115.6 kJ/mol was obtained from thermal racemization. Despite the large steric hindrance of the tightly interlocked substituents of the 4-carboxy-5-carbamoylphenanthrenes, the chirality became visible only at temperatures below −60°C by 1H NMR spectroscopy. Only six of the eight possible stereoisomers, which may form four racemates, have been observed, namely two anti and one syn species. After the asymmetric transformation, one further syn arrangement is less populated or does not exist and the two anti isomers exist in an unequal ratio. Two orientations were found for the amide group: a major form A with the MeE ‘outside’ and the carbonyl group ‘inside’ the bay-area and a minor form B with the reverse arrangement. The low barriers of 4-carboxy-5-carbamoylphenanthrenes indicate that steric hindrance and electrostatic repulsion in the transition state are compensated by correlated ArC and CN rotations and by twisting the phenanthrene plane. In the transition state, the carbonyl group passes the bay area and the pyramidal amide group passes H3 or H6 of the phenanthrene ring.
Christoph Kiefl, Herbert Zinner*, Maria Assunta Cuyegkeng, Alfred Eiglsperger, Chirality and asymmetric transformations of axially chiral 4,5-disubstituted phenanthreneamides, Tetrahedron: Asymmetry, Volume 11, Issue 17, 2000, Pages 3503-3513, ISSN 0957-4166, https://doi.org/10.1016/S0957-4166(00)00298-6.