engleski

Spirobinaphthopyrans: Synthesis, X-ray Crystal Structure, Separation of Enantiomers and Barriers to Thermal Racemization

Chiral spirobinaphtopyrans 1-9 were synthesised using the acid catalyzed aldol condensation of 2-hydroxy-1-naphthaldehyde or 1-hydroxy-2- naphthaldehyde with the appropriate ketone and subsequent reaction of the isolated pyrylium salt with base. Separation or enrichment of enantiomers was accomplished by low-pressure liquid chromatography (LPLC) on triacetylcellulose, tribenzoylcellulose or (+)- poly(triphenylmethyl methacrylate) on SiO2. The barriers for the interconversion of enantiomers were determined by dynamic 1H NMR measurements and by on line- or off line-measurements of the enriched enantiomers after LPLC. Gibbs energies of activation G for reversible cleavage of the Cspiro-O bond in 1 - 9 were found in the range 85-105 kJ/mol and are rationalized by steric and electronic effects. The G values decrease with decreasing length of the bridge or decreasing size of substituents in the 3- and 3'-positions. Therefore, G for 7 is higher than 9 higher than 6, 3 higher than 2 and 4 higher than 1. Geometrical data from X-ray crystal sructure analysis show that the mean value of the Cspiro-O bond length in the spirobibenzopyran 14 is shorter, i.e. the bond is stronger, than that in the spirobinaphtho derivatives 4, 3 and 6. This bond length shortening is explained by the weaker conjugation of the oxygen -lone electron pair with the -system of the benzene ring in 14 compared to the naphthalene moiety in 3, 4, and 6.

Chiral spirobinaphthopyrans ; enantioselective liquid chromatography ; triacetylcellulose ; barriers to racemization ; X-ray crystallography

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