John R. Scheffer

Control of solid-state photodimerization of trans-cinnamic acid by double salt formation with diamines

By double salt formation, diamines can steer the solid-state [2+2] photodimerization of trans-cinnamic acid (1). Thus, the yields for the photodimerization were significant only in three double salts, i.e., the ones with tn and t- and c-chxn, which are assumed to have an overlap structure. The resultant photodimer is generally β-truxinic acid, although in one case, e-truxillic acid was formed. α-Truxillic acid was not produced from any of the double salts studied. The X-ray crystal structures of three double salts of low photoreactivity (1·en, 1·pen, 1·hen) were consistent with Schmidts rule.

The Norrish type II reaction in the crystalline state: Toward a better understanding of the geometric requirements for γ-hydrogen atom abstraction

By correlating the success or failure of the Norrish type II reaction for a series of compounds in the crystalline state with geometric data derived from X-ray crystallography (the so-called Crystal Structure-Solid State Reactivity Method), the distance and angular requirements for photochemical γ-hydrogen atom abstraction have been established. These studies indicate that abstractions are preferred when the CO…Hγ distance is close to the sum of the van der Waals radii of hydrogen and oxygen (2.72 A), and that the γ-hydrogen atom can deviate by as much as 50–60° from the plane containing the oxygen n-orbital responsible for abstraction. Because molecular conformations and interatomic distances and angles can be calculated readily by using empirical force field methods, such data permit predictions to be made concerning the outcome of hypothetical Norrish type II processes. By correlating the success or failure of the Norrish type II reaction for a series of compounds in the crystalline state with geometric data derived from X-ray crystallography, the distance and angular requirements for photochemical γ-hydrogen atom abstraction have been established.

CONTROL OF THE SOLID-STATE PHOTODIMERIZATION OF SOME DERIVATIVES AND ANALOGS OF TRANS-CINNAMIC ACID BY ETHYLENEDIAMINE

Some of double salts derived from ethylenediamine (en) and a variety of trans-cinnamic acids and their analogs underwent photodimerization in the solid state, giving predominantly β-truxinic dimers. X-Ray studies demonstrate that (a) the conformation of en is gauche for the highly photoreactive double salts (o-1b·en and m-1e·en), whereas it is anti for lessphotoreactive o-1a·en or photoinert 1d·en and (b) for highly reactive o-1b·en and m-1e·en, the monomer acid molecules are arranged in an overlap configuration and a reactive monomer pair is hydrogendashbonded to the same en molecule.

Norrish type II reaction in the solid state: Involvement of a boatlike reactant conformation

Untersucht wird das Verhaltnis der Produkte (II)-(IV), das bei Bestrahlung der Acetophenone (I) einmal in kristallinem Zustand und zum anderen in verschiedenen Losungsmitteln, insbesondere MeCN, entsteht.

Crystal to molecular chirality transfer: Supramolecular photochemistry of crystalline carboxylate salts

Abstract Solid state irradiation of a salt formed between a prochiral, photochemically reactive carboxylic acid and a non-absorbing, optically active amine is shown to lead to an optically active Norrish type II photoproduct whose optical purity depends on the dimorphic crystalline modification photolyzed.

Monitoring structural transformations in crystals. 8. Monitoring molecules and a reaction center during a solid-state Yang photocyclization

Structural changes taking place in a crystal during an intramolecular photochemical reaction [the Yang photocyclization of the alpha-methylbenzylamine salt with 1-(4-carboxybenzoyl)-1-methyladamantane] were monitored step-by-step using X-ray structure analysis. This is the first example of such a study carried out for an intramolecular photochemical reaction. During the photoreaction, both the reactant and product molecules change their orientation, but the reactant changes more rapidly after the reaction is about 80% complete. The distance between directly reacting atoms in the reactant molecule is almost constant until about 80% reaction progress and afterwards decreases. The torsion angle defined by the reactant atoms that form the cyclobutane ring also changes in the final stages of the photoreaction. These phenomena are explained in terms of the influence of many product molecules upon a small number of reacting molecules. The adamantane portion shifts more than the remaining part of the anionic reactant species during the reaction, which is explained in terms of hydrogen bonding. The structural changes are accompanied by changes in the cell constants. The results obtained in the present study are compared with analogous results published for intermolecular reactions.

ABSOLUTE CONFIGURATION CORRELATION STUDIES IN SOLID STATE ORGANIC PHOTOCHEMISTRY

An ever-increasing number of examples in the literatun? attest to the fact that photochemical reactions carried out in optically active crystalline media can lead to enantioineridly enriched products in high optical yields. Taken from our own work and that of others, this article examines the relatively few examples of such reactions in which the absolute stereochemical arrangements of the molecules in the reactant crystals are correlated with the absolute configurations of the optically active products formed by irradiation of the same crystals. Studies of this type provide penetrating insights into the mechanisms of the reactions in question, and in addition, help to pinpoint the crystal latticederived stereoelectronic factors that arc responsible for tbe observed enantioselectivities.

ASYMMETRIC INDUCTION IN THE SOLID STATE PHOTOCHEMISTRY OF SALTS OF CARBOXYLIC ACIDS WITH OPTICALLY ACTIVE AMINES

Abstract A general approach to using crystal chirality in asymmetric synthesis is described, It consists of the preparation of crystalline salts of prochiral carboxylic acids with optically active amines followed by photolysis of the salts in the solid state. Conversion of the photoproducts to the corresponding methyl esters by using diazomethane followed by chiral shift reagent NMR analysis revealed enanttomeric excesses ranging from 14–80% depending on the optically active amine employed. In contrast to the solid state results, photolysis of the salts in solution gave only racemic products.

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

Abstract Irradiation of crystals of salts formed between 4-(2,4,6-triisopropylbenzoyl)benzoic acid ( 1a ) and various optically active amines is shown to lead to optically active Norrish type II photoproducts in low to moderate enantiomeric excesses, a process in which the ammonium ion has acted as an “ionic chiral handle.” X-ray crystallography reveals that the salts crystallize with two independent, mirror image-related molecules of the benzophenone moiety in the asymmetric unit, and this is suggested to be the source of the low ee s observed.

Reaction selectivity in solid state photochemistry

Abstract Steric compression, arising from intermolecular or intermolecular H...H non-bonded repulsive interactions generated along the reaction pathway, is used to explain abnomal solid state photoreactivity. Computer simulations of the motions involved in each case, with calculations of the resulting non-bonded steric compression energies were performed. Three systems were studied: ( A ) Enone photorearrangements, where the observed changes in the solid state photoreactivity of eight closely related α,β-unsaturated cyclohexenones are correlated with their crystal and molecular structures as determined by X-ray diffraction methods. ( B ) Failure of [2+2] photocycloadditilon of an α,β-unsaturated ketone when irradiated in the solid state despite an almost perfect crystal lattice alignment of the potentially reactive double bonds. ( C ) An unsymmetrically substituted ene-dione for which solution photoreactlon results in four products and solid state photoreaction yields only one.