Photochemical Asymmetric Synthesis. Irradiation of Ring and Open-chain Derivatives of L-Erythritol 1,4-Dicinnamate

Stereoselective [2 + 2] photodimerization: a viable strategy for the synthesis of enantiopure cyclobutane derivatives

The [2 + 2] photodimerization of cinnamic acid derivatives to afford enantiopure cyclobutanes has been investigated. The use of a chiral auxiliary represents a convenient and straightforward method to exert enantiocontrol on the reaction. By exploiting Evans oxazolidinones, the stereoselective light-driven cyclisation affords a functionalised cyclobutane ring with up to 99% enantiocontrol after removing the chiral auxiliary. In-flow experiments allowed us to improve further the efficiency of the methodology, leading to high conversion and excellent enantioselectivity.

Photochemical Stereocontrol Using Tandem Photoredox-Chiral Lewis Acid Catalysis

The physical, biological, and materials properties of organic compounds are determined by their three-dimensional molecular shape. The development of methods to dictate the stereochemistry of organic reactions has consequently emerged as one of the central themes of contemporary synthetic chemistry. Over the past several decades, chiral catalysts have been developed to control the enantioselectivity of almost every class of synthetically useful transformation. Photochemical reactions, however, are a conspicuous exception. Relatively few examples of highly enantioselective catalytic photoreactions have been reported to date, despite almost a century of research in this field. The development of robust strategies for photochemical enantiocontrol has thus proven to be a long-standing and surprisingly difficult challenge. For the past decade, our laboratory has been studying the application of transition metal photocatalysts to a variety of problems in synthetic organic chemistry. These efforts have recently culminated in the discovery of an effective system in which the activity of a visible light absorbing transition metal photoredox catalyst is combined with a second stereocontrolling chiral Lewis acid catalyst. This dual catalyst strategy has been applied to a diverse range of photochemical reactions; these have included highly enantioselective photocatalytic [2 + 2] cycloadditions, [3 + 2] cycloadditions, and radical conjugate addition reactions. This Account describes the development of the tandem Lewis acid photoredox catalysis strategy utilized in our laboratory. It provides an analysis of the factors that we believe to be particularly important to the success of this seemingly robust approach to photocatalytic stereocontrol. (1) The photocatalysts utilized in our systems are activated by wavelengths of visible light where the organic substrates are transparent, which minimizes the possibility of competitive racemic background photoreactions. (2) The high degree of tolerance that Ru(bpy)₃²⁺ and similar octahedral metal polypyridine complexes exhibit toward Lewis acids affords great flexibility in tuning the structure of the stereocontrolling chiral catalyst without perturbing the photoredox properties of the photocatalyst. (3) Synthetic chemists have amassed a substantial understanding of the features that are common in highly successful chiral Lewis acid catalyzed reactions, and these deep, well-validated insights are readily applied to the reactions of a variety of photogenerated intermediates. We hope that the recent success of this and similar dual catalytic systems will provide a useful foundation for the further development of powerful, stereocontrolled photochemical reactions.

Investigations of the Asymmetric Intramolecular [2 + 2] Photocycloaddition and Its Application as a Simple Access to Novel C(2)-Symmetric Chelating Bisphosphanes Bearing a Cyclobutane Backbone

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.