Conspectus Asymmetric synthesis has posed a significant challenge to organic chemists for over a century. Several strategies have been developed to synthesize enantiomerically enriched compounds, which are ubiquitous in the pharmaceutical and agrochemical industries. While many organometallic and organic catalysts have been found to mediate thermal enantioselective reactions, the field of photochemistry lacks similar depth. Recently, chiral 1,3,2-oxazaborolidines have made the transition from Lewis acids that were exclusively applied to thermal reactions to catalysts for enantioselective photochemical reactions. Due to their modular structure, various 1,3,2-oxazaborolidines are readily available and can be easily fitted to a given chemical transformation. Their use holds great promise for future developments in photochemistry. This Account gives an overview of the substrate classes that are known to undergo enantioselective photochemical transformations in the presence of chiral 1,3,2-oxazaborolidines and touches on the catalytic mode of action, on the proposed enantiodifferentiation mechanism, as well as on recent computational studies. Based on the discovery that the presence of Lewis acids enhances the efficiency of coumarin [2 + 2] photocycloadditions, chiral 1,3,2-oxazaborolidines were applied in 2010 for the first time to prepare enantiomerically enriched photoproducts. These Lewis acids were then successfully used in intramolecular [2 + 2] photocycloaddition reactions of 1-alkenoyl-5,6-dihydro-4-pyridones and 3-alkenyloxy-2-cycloalkenones. In the course of this work, it became evident that the chiral 1,3,2-oxazaborolidine must be tailored to the specific reaction; it was shown that both inter- and intramolecular [2 + 2] photocycloadditions of cyclic enones can be conducted enantioselectively, but the aryl rings of the chiral Lewis acids require different substitution patterns. In all [2 + 2] photocycloaddition reactions in which chiral 1,3,2-oxazaborolidines were used as catalysts, the catalyst loading could not be decreased below 50 mol % without sacrificing enantioselectivity due to competitive racemic background reactions. To overcome this constraint, substrates that reacted exclusively when bound to an oxazaborolidine were tested, notably phenanthrene-9-carboxaldehydes and cyclohexa-2,4-dienones. The former substrate class underwent an ortho photocycloaddition, the latter an oxadi-π-methane rearrangement. Several new 1,3,2-oxazaborolidines were designed, and the products were obtained in high enantioselectivity with only 10 mol % of catalyst. Recently, an iridium-based triplet sensitizer was employed to facilitate enantioselective [2 + 2] photocycloadditions of cinnamates with 25 mol % of chiral 1,3,2-oxazaborolidine. In this case, the relatively low catalyst loading was possible because the oxazaborolidine–substrate complex exhibits a lower triplet energy and an improved electronic coupling compared to the uncomplexed substrate, allowing for a sele...

Upon irradiation at λ = 350 nm, cyclohept-2-enone undergoes an isomerization to the strained (E)-isomer. The process was studied by XMS-CASPT2 calculations and found to proceed by two competitive reaction channels on either the singlet or the triplet hypersurface. (E)-Cyclohept-2-enone is a reactive dienophile in thermal [4 + 2] cycloaddition reactions with various dienes. Ten different dienes were probed, most of whichexcept for 1,3-cyclohexadieneunderwent a clean Diels−Alder reaction and gave the respective trans-fused six-membered rings in good yields (68− 98%). The reactions with furan were studied in detail, both experimentally and by DLPNO−CCSD(T) calculations. Two diastereoisomers were formed in a ratio of 63/35 with the exoproduct prevailing, and the configuration of both diastereoisomers was corroborated by single crystal X-ray crystallography. The outcome of the photoinduced Diels−Alder reaction matched both qualitatively and quantitatively the calculated reaction pathway. Apart from cyclohept-2-enone, five additional cyclic hept-2-enones and cyclooct-2-enone were employed in their (E)-form as dienophiles in the Diels−Alder reaction with 1,3-cyclopentadiene (80−98% yield). The method was eventually applied to a concise total synthesis of racemic trans-α-himachalene (four steps, 14% overall yield).

Lewis acids have recently been recognized as catalysts enabling enantioselective photochemical transformations.M echanistic studies on these systems are however rare, either due to their absorption at wavelengths shorter than 260 nm, or due to the limitations of theoretical dynamic studies for larger complexes.I nt his work, we overcome these challenges and employs ub-30-fs transient absorption in the UV,incombination with ahighly accurate theoretical treatment on the XMS-CASPT2 level. We investigate 2-cyclohexenone and its complex to boron trifluoride and analyze the observed dynamics based on trajectory calculations including nonadiabatic coupling and intersystem crossing. This approach explains all ultrafast decaypathways observed in the complex. We show that the Lewis acid remains attached to the substrate in the triplet state,w hich in turn explains why chiral boronbased Lewis acids induce ah igh enantioselectivity in photocycloaddition reactions.

In intramolecular [2+2] photocycloaddition reactions, the two tethered olefins can approach each other in a straight or in a crossed fashion. Despite the fact that the latter reaction mode leads...

Am ethodf or the nucleophilicf luorination of heptamethyl aryl trisiloxanest of orm fluoroarenesi sr eported. The reaction proceeds in the presence of Cu(OTf) 2 and KHF 2 as the fluorides ource under mild conditions for ab road range of heptamethyltrisiloxyarenes with high functional group tolerance. The combination of this method with the silylationo fa ryl CÀHb onds enables the regioselective fluorination of non-activated arenes controlled by steric effects following at wo-stepp rotocol.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.