Chromophore Activation of α,β‐Unsaturated Carbonyl Compounds and Its Application to Enantioselective Photochemical Reactions

Abstract

The first law of photochemistry, as described by Theodor von Grotthuß and John W. Draper, states that only the light absorbed by the irradiated matter can effect photochemical change. Consequently, the photochemical behavior of a molecule can be controlled by bringing its absorbance properties in line with the emission of the light source. A compound with a chromophore that only absorbs light at short wavelengths will not be excited by light of longer wavelengths. If one can reversibly modify the photophysical properties of a compound with a chemical activator, then it is possible to photoexcite only the activated species. For α,β‐unsaturated carbonyl compounds, the use of Lewis acids, Brønsted acids, or the formation of the respective iminium ions can bring about the desired chromophore activation to catalyze a photochemical reaction at a given wavelength. In this Minireview, the concept of chromophore activation will be illustrated, and examples of its implementation in enantioselective catalysis will be discussed.

Enantioselective Visible‐Light‐Mediated Formation of 3‐Cyclopropylquinolones by Triplet‐Sensitized Deracemization

“…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). [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 experiments are summarized in this Communication.Initial experiments were performed with quinolone 5a, which was readily obtained from the respective methyl (2-chloro-7-benzyloxyquinolin-3-yl)acetate by double amethylation, reduction to the aldehyde,m ethylenation, and final hydrolysis of the 2-chloroquinolone (see the Supporting Information for further details). [4,5] The few reported studies were discouraging,a nd enantioselectivities did not exceed 20 % ee.…”

“…[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. [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 experiments are summarized in this Communication. [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 experiments are summarized in this Communication.…”

Intramolecular [2+2] Photocycloaddition of Cyclic Enones: Selectivity Control by Lewis Acids and Mechanistic Implications

“…As mentioned in the introduction, Lewis acids display a profound influence on the absorption properties of enones . Figure shows the absorption spectrum of enone 5 a in dichloromethane solution ( c= 0.5 m m ).…”

“…In both cases, there is a significant absorption at longer wavelength that exceeds in terms of quantitative absorbance the n–π* absorption of the uncomplexed substrate 5 a . In the presence of a Lewis acid there is no n–π* transition …”

“…The chiral Lewis acid acts by coordination to the carbonyl carbon atom and lowers the energy difference between the ground state (S 0 ) and the first excited state (S 1 ). The chromophore is activated and the allowed π–π* absorption is red‐shifted to absorb at λ ≥360 nm. Although there is a weak n–π* absorption of uncomplexed dihydropyridone at a similar wavelength, the Lewis acid complex has a much higher absorption coefficient and the reaction thus proceeds enantioselectively.…”