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Racemization and resolution in the organic solid state Wilson, Keith Rainier

Abstract

Two examples of the simplest type of organic solid-state reaction - the thermal interconversion of optical isomers - have been extensively studied by means of polarimetry, differential scanning calorimetry, and X-ray powder diffraction. The first reaction investigated was the reverse Diels-Alder reaction and recombination of the cyclopentadiene-fumaric acid adduct. Polycrystalline samples of (+)-enantiomer (m.p. 176°) racemize completely in the solid state from 130° to 165°. First-order kinetics (ΔH = 40.0 kcal mole⁻¹, ΔS = 14 cal deg⁻¹mole⁻¹) are strictly obeyed; the racemization rate is insensitive to variations in crystal size and optical purity. The reaction, which is only five times slower than the melt rate extrapolated to these temperatures (for the melt from 176° to 194°, ΔH = 29.7 kcal mole⁻¹, ΔS = -6.9 cal deg⁻¹mole⁻¹) occurs throughout the polycrystalline sample rather than at crystallite boundaries, dislocations, or other preferred sites. Phase studies show that the product separates as a racemic compound (m.p. 186°) which forms a eutectic (at 165°) with the resolved enantiomers. From 165° to 176°, the racemization shows autoacceleration and sigmoid-shaped kinetic curves characteristic of concurrent reactions in the solid and melt phases. The second system studied was that formed between R-(-)- and S-(+)-1,1'-binaphthyl, and surprisingly, resolution, rather than racemization, was observed to occur from 76° to 158°. This unprecedented solid-state resolution is made possible by a solid-solid phase change from a racemic compound (m.p. 145°) to a. eutectic mixture (m.p. 158°) of crystals of pure enantiomers, at temperatures where interconversion occurs in the reactant-product interface. Polycrystalline 1,1’-binaphthyl samples of very low optical activity having the correct phase content (racemate plus crystals of only one enantiomer) for a controlled resolution can be easily and reproducibly prepared. These samples resolve from [α][sub D] = 2° to [α][sub D] = ca. 210° (in either (+) or (-) directions) in less than one hour at 150°. The limit of resolution ([α][sub D] = ±245°) is attained simply by recrystallization of the resolved sample from acetone. The resolution therefore involves the conversion of all of a racemic material to only one enantiomer. Kinetic studies of the solid-state resolution show a smooth development of optical activity with time. A Prout-Tompkins analysis indicates that crystallites of growing enantiomer spread throughout the racemic sample, requiring 62 kcal mole⁻¹ activation energy. Crystallization of completely racemic 1,1'-binaphthyl melt in a closed system gives rise to optical activity. The probability distribution of 200 individual crystallizations is symmetric about [α][sub D] = 0° and proves that optically active samples can be created under absolutely spontaneous conditions (i.e., in the complete absence of external dissymmetric influences).

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