Ionic chiral handle-induced asymmetric synthesis in a solid state Norrish type II photoreaction

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.

Two-step photomechanical motion of a dibenzobarrelene crystal

Upon light irradiation, a dibenzobarrelene crystal quickly bent, and then slowly bent towards the opposite direction.

The crystal and molecular structure of sodium 4-(2,4,6-triisopropylbenzoyl)benzoate in terms of the photochemical behaviour of the anion

Contrary to the known 4-(2,4,6-triisopropylbenzoyl)benzoate salts, di-μ-aqua-bis[tetraaquasodium(I)] bis[4-(2,4,6-triisopropylbenzoyl)benzoate] dihydrate, [Na2(H2O)10](C23H27O3)2·2H2O, (1), does not undergo a photochemical Norrish-Yang reaction in the crystalline state. In order to explain this photochemical inactivity, the intermolecular interactions were analyzed by means of the Hirshfeld surface and intramolecular geometrical parameters describing the possibility of a Norrish-Yang reaction were calculated. The reasons for the behaviour of the title salt are similar crystalline environments for both the o-isopropyl groups in the anion, resulting in similar geometrical parameters and orientations, and that these interaction distances differ significantly from those found in salts where the photochemical reaction occurs.

Diastereospecific Photocyclization of a Isopropylbenzophenone Derivative in Crystals and the Morphological Changes

Reaction of crystals of 2,4,6-triisopropylbenzophenone derivative with the (S)-phenylethylamide group caused diastereospecific Norrish type II photocyclization by UV irradiation to give (R,S)-cyclobutenol as a sole product. In contrast, the solution photolysis gave an almost 1:1 mixture of (R,S)- and (S,S)-cyclobutenol. The specific diastereodifferentiation in the crystalline state is attributed to the smooth transformation with minimum molecular motion due to the very similar molecular shapes as well as the 2-fold helical arrangements between the reactant crystal and the product (R,S)-cyclobutenol crystal. UV irradiation of the bulk crystals led to cracking and breaking into small fragments. In contrast, the microcrystals maintained the single-crystalline morphology in the course of photocyclization, suggesting the single-crystal-to-single-crystal transformation.

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.

2000 Alfred Bader Award LectureIn the footsteps of Pasteur: asymmetric induction in the photochemistry of crystalline ammonium carboxylate salts

This review describes the development of a new, synthetically useful method of asymmetric synthesis in organic photochemistry. Similar in many ways to the Pasteur procedure for resolving racemic carboxylic acids and organic amines, the method relies on the use of crystalline organic salts in which the enantioselectivity of a photochemical reaction of an achiral organic ion (for example, a carboxylate anion) is governed in the solid state by the presence of an optically pure counterion (for example, an optically active ammonium ion). Such optically pure counterions are termed ionic chiral auxiliaries. Salts containing ionic chiral auxiliaries are required to crystallize in chiral space groups, which provide the asymmetric environment necessary for chiral induction. Using this methodology, we have obtained near-quantitative optical yields in a wide variety of photochemical reactions.Key words: photochemistry, solid state, chiral auxiliaries, asymmetric synthesis, crystal structure–reactivity relationships.