The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4, whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.

In the presence of a chiral thioxanthone catalyst (10 mol %) the title compounds underwent a clean intermolecular [2 + 2] photocycloaddition with electron-deficient olefins at λ = 419 nm. The reactions not only proceeded with excellent regio- and diastereoselectivity but also delivered the respective cyclobutane products with significant enantiomeric excess (up to 95% ee). Key to the success of the reactions is a two-point hydrogen bonding between quinolone and catalyst enabling efficient energy transfer and high enantioface differentiation. Preliminary work indicated that solar irradiation can be used for this process and that the substrate scope can be further expanded to isoquinolones.

3-Allyl-substituted quinolones undergo at ripletsensitized di-p-methane rearrangement reaction to the corresponding 3-cyclopropylquinolones upon irradiation with visible light (l = 420 nm). Ac hiral hydrogen-bonding sensitizer (10 mol %) was shown to promote the reaction enantioselectively (88-96 %y ield, 32-55 %e e). Surprisingly,i tw as found that the enantiodifferentiation does not occur at the state of initial product formation but that it is the result of aderacemization event. The individual parameters that control the distribution of enantiomers in the photostationary state have been identified.Triplet sensitization represents an efficient and elegant way to promote amolecule from its ground state to atriplet state, in most cases to the lowest-lying triplet state T 1 .T he mechanism of the process has been elucidated, [1] and apart from an energetic driving force,itiscrucial that the sensitizer and the substrate are in close spatial proximity to allow for efficient sensitization. Thei dea to employ triplet energy transfer by ac hiral sensitizer and to perform photochemical reactions enantioselectively was first disclosed in the 1960s. Pioneering studies were directed to the formation of chiral trans-1,2-diphenylcyclopropane (1)from its racemate (rac-1). After initial work by Hammond with as ensitizer that was later shown to operate on the singlet hypersurface, [2] Ouanns and co-workers found in 1973 that chiral ketone 2 induced an otable but small enantioselectivity (3 % ee)i nt he formation of 1 (Scheme 1). [3] Thee fficiency of the reaction suffered from the simultaneous formation of the achiral cyclopropane 3.In the 1980s and 1990s,l ittle attention was paid to the topic of enantioselective triplet-sensitized reactions. [4,5] The few reported studies were discouraging,a nd enantioselectivities did not exceed 20 % ee. [6] With the advent of efficient hydrogen-bonding templates and catalysts, [7] it became evident, however,t hat the issue of insufficient enantioface differentiation could be overcome,a nd the first highly enantioselective triplet-sensitized reaction was disclosed in 2009. [8] Am odified version of the xanthone catalyst used in this and related studies [9] was presented in 2014 and was based on at hioxanthone as the sensitizing unit. [10] Thec ompound, which is available in both enantiomeric forms 4 ( Figure 1) [11] and ent-4,has the advantage to operate with visible light and holds promise to be av ersatile catalyst for several photochemical reactions. [12,13] In the current study,weattempted to use catalyst 4 in an enantioselective [14] di-p-methane rearrangement reaction [15] of 3-allyl-substituted quinolones.A lthough the enantioselectivities of the reaction remained moderate,the mode of action turned out to be remarkable.I tw as found that the enantioenriched cyclopropanes are formed in ad eracemization process,w hich resembles the seminal work on chiral cyclopropanes [2,3] (see Scheme 1), and which likely proceeds via a1 ,3-diradical intermediate.T he results of our preliminary exper...

3‐Allyl‐substituted quinolones undergo a triplet‐sensitized di‐π‐methane rearrangement reaction to the corresponding 3‐cyclopropylquinolones upon irradiation with visible light (λ=420 nm). A chiral hydrogen‐bonding sensitizer (10 mol %) was shown to promote the reaction enantioselectively (88–96 % yield, 32–55 % ee). Surprisingly, it was found that the enantiodifferentiation does not occur at the state of initial product formation but that it is the result of a deracemization event. The individual parameters that control the distribution of enantiomers in the photostationary state have been identified.

Vinylogous Nazarov-type cyclizations yield seven-membered rings from butadienyl vinyl ketones via a photochemical [6π] photocyclization followed by subsequent isomerization steps. The mechanism of this recently developed method was investigated using unrestricted DFT, SF-TDDFT, and CASSCF/NEVPT2 calculations, suggesting three different pathways that lead either to pure trans, pure cis, or mixed cis/trans configured products. Singlet biradicals or zwitterions occur as intermediates. The computational results are supported by deuterium-labeling experiments.