Dedicated to Vladimir Prelog and to the memory of Horst PracejusA large number of successful methods for chirality transfer, using either stoichiometric or catalytic chiral auxiliaries, are in use today. However, there is a lack of practical and dynamic selectivity models, i.e. models which take into account the entire reaction sequence, and which allow simple and reliable assessment, optimization and prediction of selectivity in asymmetric syntheses. The models that are available are either too strongly biased to the steric requirement of the particular molecules reacting, but do not go beyond classical considerations of static facial differentiation, or they take a demanding, theoretical approach, which because of its inherent limitations and the great theoretical effort required has not yet found its way into the practical world of the synthetic chemist. The "Isoinversion Principle", developed on the basis of Eyring's theory, closes this gap. With its aid, the synthetic chemist can determine the characteristic isoinversion temperature for the reaction type of interest from a few temperature-dependent measurements of selectivity parameters. then affords information on interesting questions such as optimization etc. The advantage of this method is that it is useful not only for stereoselectivity, but for any kind of process where selectivity in general (regio-, chemo-, etc) is generated at two or more stages of a reaction sequence, regardless of whether these reactions involve the ground state or a diabatic photoprocess. The present review will demonstrate that this generation of selectivity at two or more stages of a reaction sequence occurs more commonly than is generally thought.

The light of the sun can be used directly for changing chemical structures photochemically. Any industrial application must conform to the limitations imposed by the spectral distribution of the photons from the sun, the interruptions to the radiation due to the day/night rhythm, and the weather. In this review, we describe the photochemical potential of the sun, give a fundamental treatment of the concept of photoreactors driven by sunlight (abbreviated to solar photoreactors), and give an account of the realization of this concept in the first pilot plant on the “Plataforma Solar de Almeria” in southern Spain and in other activities in this field. Based on experimental data from photochemical investigations on the pilot plant scale, possibilities, limitations, and the potential growth of solar photochemistry are described. Solar photochemistry, in our opinion, is a technique which could make a contribution to the chemistry of the future because of its photochemical synthesis potential, the avoidance of waste products, and the direct utilization of the sun, not only as a primary energy source, but also as a reaction partner.

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The utilization of simple photochemical reactions for the storage of solar energy in the form of chemical energy in energy-rich products has often been considered in the further development and improvement of e. g. simple thermosolar techniques. The hitherto proposed criteria for the qualification of an abiotic photochemical system are, however, mostly of a qualitative nature, so a mutal comparison of the systems is not precise enough. In this article it is shown how a useful correlation on the basis of time-independent experimental data can be achieved and how, from the viewpoint of photochemistry, a comparative classification of known reactions is possible. The following reactions are compared: the [2 + 21-photocycloadditions of norbornadiene, dimethyl 2,3-norbornadienedicarboxylate, and dicyclopentadienone, the photoisomerization of trans-to cis-diacetylindigo, the photodissociation of nitrosyl chloride as well as a photocatalytic redox reaction. The quantity of material required and storage efficiency are by far the most favorable in the case of trans-diacetylindigo. The main disadvantage of the latter however, is that the energy-rich cis-form rapidly reverts to the stable frans-form at elevated temperatures

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The asymmetric intramolecular [2 + 2] photocycloaddition of alpha,beta-enoates was evaluated as a simple access to the novel C(2)-symmetric bisphosphanes 22 and 27 possessing a cyclobutane backbone. A source of different chiral auxiliaries for investigations of the photochemical key step was provided by the transacetalization of dialkyl tartrates 3 with the corresponding 3,3-dialkoxybutan-2-ones 4. An insight into the selection mechanism was gained by temperature dependent measurements on the irradiation of the dicinnamates 10a-d, since the corresponding Eyring diagram discloses strictly linear functions as well as an isoselective relationship. Diol 8a turned out to be a structurally optimized auxiliary in terms of chiral induction and product crystallization and was also successfully applied in the first asymmetric photodimerization of 2-indenecarboxylic acid esters. Indeed, in this case excellent diastereoselectivities were achieved, too, but head-to-tail dimers 16a and 16b were formed predominantly. Diesters 11a and 16a were converted by standard procedures into the desired enantiopure 1,4-diphosphane 22 and 1,5-diphosphane 27. Furthermore, the hitherto unknown absolute configuration of delta-truxinic acid was elucidated from a single crystal X-ray structure analysis of 11a.

A series of new rhodium(I) complexes [L*Rh(NBD)Cl] (L* = chiral cyclic phosphonite with a fused 1,4-dioxane or cyclobutane ring in the backbone) was synthesized via the corresponding borane−phosphonite adducts. They turned out to be highly active catalysts in the asymmetric hydrosilylation of ketones in a broad temperature range. Evaluation of the temperature dependent measurements according to the Eyring formalism disclosed a nonlinear relationship between ln P (P = ratio of the enantiomeric product alcohols) and the reciprocal of temperature marked by the occurrence of points of inversion in the middle temperature region between −5 and 30 °C. These findings indicate a complexly composed selection mechanism that is controlled by at least two relevant partial steps. Furthermore, by comparison of different catalysts based on a 1,4-dioxane, a cyclobutane, or a 1,3-dioxolane ring in the backbone of L*, the conformational properties of the ligand backbone were revealed to be a crucial feature determining not only the extent but also the direction of chirality transfer, thus providing a versatile tool for the construction of stereocomplementary ligands. Accordingly, by proper choice of the ligand backbone and the temperature, fairly good enantioselectivities in the hydrosilylation of rather different ketones like acetophenone (82% ee) and pivalophenone (86% ee) can be achieved.