Chiral crystalline salts from achiral biphenylcarboxylic acids and tryptamine

Physicochemical Properties and Photochemical Reactions in Organic Crystals

Background:

Molecular organic photochemistry is concerned with the description of physical and chemical processes generated upon the absorption of photons by organic molecules. Recently, it has become an important part of many areas of science: chemistry, biology, biochemistry, medicine, biophysics, material science, analytical chemistry, among others. Many synthetic chemists are using photochemical reactions in crystals to generate different types of organic compounds since this methodology represents a green chemistry approach.

Objective & Method:

Chemical reactions in crystals are quite different from reactions in solution. The range of organic solid state reactions and the degree of control which could be achieved under these conditions are quite wider and subtle. Therefore, for a large number of molecular crystals, the photochemical outcome is not the expected product based on topochemical principles. To explain these experimental results, several physicochemical factors in crystal structure have been proposed such as defects, reaction cavity, dynamic preformation or photoinduced lattice instability and steric compression control. In addition, several crystal engineering strategies have been developed to bring molecules into adequate orientations with reactive groups in good proximity to synthesize complex molecules that in many cases are not available by conventional methods. Some strategies involve structural modifications like intramolecular substitution with different functional groups to modify intermolecular interactions. Other strategies involve chemical techniques such as mixed crystal formation, charge transfer complexes, ionic and organometallic interactions. Furthermore, some examples of the single crystal to single crystal transformations have also been developed showing an elegant method to achieve regio and stereoselectivity in a photochemical reaction.

Conclusion:

The several examples given in this review paper have shown the wide scope of photochemical reactions in organic molecular crystals. There are several advantages of carrying photochemical reaction in the solid state. Production of materials unobtainable by the traditional solution phase reactions, improved specificity, reduction of impurities, and enhancement in the yields by the reduction of side reactions. These advantages and the multidisciplinary nature of solid-state photochemistry make this discipline quite likely to develop a lot in the future.

2-(Biphenyl-4-yl)acetic acid (felbinac)

The structure of the title compound, C₁₄H₁₂O₂, displays the expected inter­molecular hydrogen bonding of the carb­oxy­lic acid groups, forming dimers. The dihedral angle between the two aromatic rings is 27.01 (7)°.

Experimental aspects of solid state circular dichroism

The interest of circular dichroism in the solid state is stimulated by several needs, such as the desire to get solvent free spectra, the insolubility of the sample or the intrinsic process in which the sample itself is prepared or manipulated. We approach the argument on the basis of the sampling technique, since each different case calls for specific care in getting proper results.

Absolute Asymmetric Photocyclization of Isopropylbenzophenone Derivatives Using a Cocrystal Approach Involving Single-Crystal-to-Single-Crystal Transformation

Absolute asymmetric photocyclization of isopropylbenzophenone derivatives was achieved by means of a cocrystal approach. Three chiral salt crystals formed by carboxylic acid derivatives with achiral amines could be prepared by spontaneous crystallization. In the M-crystal of 4-(2,5-diisopropylbenzoyl)benzoic acid with 2,4-dichlorobenzylamine, a twofold helical arrangement occurs in a counterclockwise direction to generate the crystal chirality. Conversely, the clockwise helix exists alone in the P-crystal. Irradiation of the M-crystal at >290 nm caused highly enantioselective Norrish type II cyclization to give the (R,R)-cyclopentenol, (R)-cyclobutenol, and (R)-hydrol in a 6:3:1 molar ratio, resulting in successful absolute asymmetric synthesis, while irradiation at around 350 nm afforded the (R,R)-cyclopentenol as the sole product. The reaction proceeded via single-crystal-to-single-crystal transformation, and therefore the reaction path producing the (R,R)-cyclopentenol could be traced by X-ray crystallographic analysis before and after irradiation.

Optical activity induced by helical arrangements of tryptamine and 4-chlorobenzoic acid in their cocrystal

Optical rotatory powers of chiral cocrystals formed from the achiral molecules tryptamine and 4-chlorobenzoic acid were determined by the HAUP (high accuracy universal polarimeter) method. These cocrystals belonged to space group P2(1)2(1)2(1), and their absolute configuration was confirmed by the Flack parameter. In the M-crystal, 2-fold helical arrangements are formed in a counterclockwise direction between the two components through the quaternary ammonium salt bridge, hydrogen bond, and the aromatic pi-piinteraction along the c axis, while clockwise helices alone exist in the P-crystal. Large rotatory powers rho(3)(M) = -355 and rho(3)(P) = +352 deg mm(-)(1) were obtained along the c axis in the M- and P-crystal, respectively, at 632.8 nm and 303 K. The magnitude was 10 to 100 times larger than those for ordinary organic crystals. Further, it was confirmed that the negative sign was induced by the counterclockwise helical structures and the positive sign by the clockwise helices. In contrast, the rotations along the a and b axis which are in perpendicular directions to the screw axis were rho(1)(M) = +138, rho(1)(P) = -140 deg mm(-)(1), and rho(2)(M) = -56, rho(2)(P) = +58 deg mm(-)(1), much smaller than rho(3)(M) and rho(3)(P) . The results revealed that the helically arranged aromatic pi electrons as well as the helical ionic and hydrogen bond networks in the crystal contributed to the enhancement of the magnitude of these rotations.

Chiral Bimolecular Crystallization of Tryptamine and Achiral Carboxylic Acids

Although tryptamine (1), 2-thiophenecarboxylic acid (2), and 3-indoleacetic acid (4) are achiral compounds, chiral crystalline salts 1.2 and 1.4 were prepared by recrystallization from solutions of the two components. The crystal chirality is generated by the formation of a 2-fold helix in only one direction between the two molecules through the salt interaction and hydrogen bonding in the lattice. Two enantiomorphous crystals (P and M) of 1.2 and 1.4 were obtained by spontaneous crystallization and were easily discriminated by the measurement of solid-state CD spectra. The absolute configuration of P-1.2 was determined with high certainty by the Bijvoet method based on the anomalous dispersion of the sulfur atom during X-ray analysis.