Title : Mechanisms of forward and reverse [2+2] photocycloaddition of ethylene derivatives: Concerted addition according to the Woodward-Hoffman rules and stepwise ring opening via predissociation
Abstract:
The [2+2] photocycloaddition (PCA) reaction of ethylene derivatives results in the formation of cyclobutane derivatives and is one of the most fundamental reactions widely used in organic synthesis for the construction of strained carbocyclic systems, natural product synthesis, and in materials chemistry. According to the symmetry of the molecular orbitals, the ground state of the two ethylene molecules participating in the PCA reaction correlates with the excited state of cyclobutane, and vice versa. Therefore, both the forward ring-closing reaction and the reverse ring-opening reaction (retro-PCA) are considered to obey the Woodward–Hoffmann rules for concerted pericyclic reactions: both reactions are thermally forbidden in the ground state, but photochemically allowed in the excited state.
However, experimental and theoretical data indicate that for substituted ethylenes having an unsaturated substituent conjugated with the π-bond of ethylene, the mechanisms of the forward and reverse reactions are fundamentally different.
If the ethylene double bond is incorporated into the common π-conjugated chain, the long-wavelength absorption band (LWAB) of the substituted ethylene is determined by the entire conjugated π-system. Upon irradiation with relatively soft light in the LWAB region, the desired “ethylene” ππ*-excited state is populated, and the forward PCA reaction proceeds in a concerted manner according to the Woodward—Hoffman rules.
To obey the Woodward–Hoffmann rules, the reverse retro-PCA reaction requires excitation of cyclobutane into the σσ*-excited state, which can only be populated upon irradiation with hard UV light. The LWAB of substituted cyclobutane is determined by the absorption of the substituents. In this case, upon irradiation with light in the LWAB region, the binding ππ*-excited state, in which the excitation energy is localized on the substituent, is first populated. The energy must then be transferred to the cyclobutane core and localized on the cyclobutane σ-bond to initiate bond cleavage and ring opening. The transition from the binding state to the dissociative state corresponds to the predissociation mechanism. The rupture or the first σ-bond of cyclobutane leads to the formation of a tetramethylene biradical intermediate, which then either completely cleaves into two molecules of substituted ethylene or is converted back to cyclobutane.
It is obvious that the predissociation mechanism is characteristic of the majority of practically important cyclobutanes, in which, upon irradiation with light, the π-system of the substituent, rather than the σ-system of the cyclobutane ring, is excited. The study was performed in accordance with the State task No. 124013000686-3
