Abstract
The irradiation of (Z)-2-phenyl-4-aryliden-5(4H)-oxazolones 1 in deoxygenated CH2Cl2 at 25 °C with blue light (465 nm) in the presence of [Ru(bpy)3](BF4)2 (5% mole ratio) as a triplet photocatalyst promotes the [2+2] photocycloaddition of the C=C bonds of the 4-arylidene moiety, thus allowing the completely regio- and stereoselective formation of cyclobutane-bis(oxazolone)s 2 as single stereoisomers. Cyclobutanes 2 have been unambiguously characterized as the μ-isomers and contain two E-oxazolones coupled in an anti-head-to-head form. The use of continuous-flow techniques in microreactors allows the synthesis of cyclobutanes 2 in only 60 min, compared with the 24–48 h required in batch mode. Ring opening of the oxazolone heterocycle in 2 with a base affords the corresponding 1,2-diaminotruxinic bis-amino esters 3, which are also obtained selectively as μ-isomers. The ruthenium complex behaves as a triplet photocatalyst, generating the reactive excited state of the oxazolone via an energy-transfer process. This reactive excited state has been characterized as a triplet diradical 3(E/Z)-1* by laser flash photolysis (transient absorption spectroscopy). This technique also shows that this excited state is the same when starting from either (Z)- or (E)-oxazolones. Density functional theory calculations show that the first step of the [2+2] cycloaddition between 3(E/Z)-1* and (Z)-1 is formation of the C(H)–C(H) bond and that (Z) to (E) isomerization takes place at the 1,4-diradical thus formed.
Highlights
The synthesis of organic compounds by photocatalysis is becoming increasingly popular due to the intrinsic advantages of this methodology.[1−8] the use of visible light as a renewable source of energy to reach high-energy excited states that cannot be achieved using conventional thermal methods, the different reactivity of the excited states with respect to the ground state, the virtual absence of waste material, and the high atom economy and efficiency of these reactions make photochemical processes attractive from the point of view of a sustainable synthetic methodology.[1−8] The synthesis of cyclobutanes by [2+2] photocycloaddition of alkenes is a paradigm of photochemical reactions due to the versatility, high efficiency, and atom economy of the process, and the mild reaction conditions required.[9]
We have recently reported the synthesis of 1,3-diaminotruxillics, expanding the scope, reducing the reaction times, and achieving full conversions with remarkable selectivity considering that the reactions occur in solution.[18]
Due to the excellent stereoselectivity shown by the Ru species, we focused our research on this photocatalyst
Summary
The synthesis of organic compounds by photocatalysis is becoming increasingly popular due to the intrinsic advantages of this methodology.[1−8] the use of visible light as a renewable source of energy to reach high-energy excited states that cannot be achieved using conventional thermal methods, the different reactivity of the excited states with respect to the ground state, the virtual absence of waste material, and the high atom economy and efficiency of these reactions make photochemical processes attractive from the point of view of a sustainable synthetic methodology.[1−8] The synthesis of cyclobutanes by [2+2] photocycloaddition of alkenes is a paradigm of photochemical reactions due to the versatility, high efficiency, and atom economy of the process, and the mild reaction conditions required.[9]. 1,3-Diaminotruxillic and 1,2-diaminotruxinic species make up a very interesting group of bis-amino acids with a cyclobutane core (Figure 1). Both derivatives, among other structurally related compounds, are found in coca leaves (Erythroxylum coca and Erythroxylum truxillense) in very low concentrations[13] and have been known since the late 19th.
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