Wang et al. (Chemical Science 2024, 15, 3971) combined reversible amide bonds, pyridine coordination-site regulation, and coordination-polymer self-assembly to preorganize olefins for stereoselective solid-state [2+2] cycloaddition; after photodimerization, destruction of coordination bonds and hydrolysis of the amide bonds release free functionalized cyclobutane derivatives, and embedding the crystals in PVA films converts photostress into macroscopic bending. The paper's conclusions state as a prospect that the amide bonds could also be replaced by other reversible covalent bonds, allowing more types of cyclobutane derivatives to be obtained. Non-amide reversible linkages (imine and dative B-N bonds) were already demonstrated in metal-directed solid-state [2+2] chemistry in 2017, 2019, and 2026, so the generalization problem is to carry at least one reversible covalent linkage class beyond the amide, imine and dative B-N bonds already demonstrated in metal-directed solid-state [2+2] chemistry through the full template cycle of ligand synthesis, topochemical crystallization, photodimerization, and product release.

Background

Solid-state [2+2] photodimerization is a crystal-engineering route in which two neighboring C=C double bonds in a crystal react under ultraviolet light to form a cyclobutane ring (a strained four-membered carbon ring that is a sought-after building block in organic and medicinal synthesis). The reaction proceeds only when the reacting double bonds are preorganized correctly: the classical Schmidt topochemical criterion requires the two C=C bonds to be parallel and separated by roughly 4.2 Angstroms or less. Because the crystal lattice fixes the relative orientation of the reactants, the reaction can deliver stereodefined cyclobutane isomers that are hard to obtain selectively in solution.

Coordination polymers (CPs - crystalline solids built from metal ions bridged by organic ligands) are a popular template for this reaction: an olefin-bearing ligand that also binds a metal is self-assembled into a crystal in which metal-ligand connectivity, rather than weak intermolecular contacts, enforces the topochemical geometry. When the crystal survives the photoreaction as a single crystal (a single-crystal-to-single-crystal, SCSC, transformation), the product stereochemistry can be read directly by single-crystal X-ray diffraction (SCXRD).

A further refinement is the reversible-linkage strategy. A removable directing group (for example a pyridine ring that coordinates the metal) is attached to the olefin through a covalent bond that can later be cleaved - an amide bond (hydrolyzable in acid), an imine bond (also hydrolyzable), or a dative boron-nitrogen (B-N) bond. After the solid-state photodimerization, the template is destroyed: the coordination bonds are broken and the reversible covalent linkage is cleaved, releasing the free functionalized cyclobutane product that carries no ligating groups.

The source paper (Wang et al., Chemical Science 2024, 15, 3971) combined these ideas. Two Cd(II) one-dimensional CPs built from N-pyridinyl olefin ligands ((2E)-N-3-pyridinyl- and N-4-pyridinyl-3-(2-thienyl)-2-propenamide: 3-ptpa, with the pyridine nitrogen in the 3-position, and 4-ptpa, with it in the 4-position, both bearing a thiophene-substituted C=C bond) and 5-methoxyisophthalate linkers place neighboring C=C bonds head-to-tail (3.946 Angstroms) or head-to-head (3.751 Angstroms), respectively. Both undergo SCSC [2+2] cycloaddition under 365 nm light with first-order kinetics; base destroys the coordination bonds and acid hydrolyzes the amides, releasing four stereodefined cyclobutane derivatives (two dicarboxamides and two dicarboxylic acids). The crystals show photosalient behavior (jumping, rolling, fragmentation) driven by anisotropic unit-cell change, and grinding the crystals into poly(vinyl alcohol) (PVA, a water-soluble hydrogen-bonding polymer) films converts the photostress into macroscopic bending (about 90 degrees in 32 s). The paper's conclusions close by stating that the amide bonds could also be replaced by other reversible covalent bonds, allowing more types of cyclobutane derivatives to be obtained.

The strategy has both a lineage and precedents with other linkages. The same group introduced the amide version in 2021 (Zhu et al., Inorganic Chemistry Frontiers 8, 1997). Earlier, MacGillivray and co-workers had demonstrated the reversible-imine variant for the same purpose: Oburn et al. (JACS 2017, 139, 8452) converted trans-cinnamaldehyde to a 4-pyridyl imine, preorganized the olefins at 3.96 Angstroms in a silver(I) 1D coordination polymer, photodimerized them in the solid state to a stereodefined bis-imine cyclobutane, and hydrolyzed the imines to release the aldehyde-functionalized cyclobutane alpha-truxilaldehyde. A dative B-N variant was reported in 2019 (Campillo-Alvarado et al., Angewandte Chemie International Edition 58, 5413), and reversible B-N bond formation and cleavage with catalytic template turnover in a mechanochemical solid-state [2+2] reaction in 2026 (Shaw et al., JACS Au). Cyclobutane products can also escape a CP without any reversible-linkage design at all, as in a sunlight-driven photosalient 1D CP that releases the free cyclobutane ligand during the photoreaction (Dutta et al., Chemical Communications 2019, 55, 11049).

Problem Statement

Wang et al. (Chemical Science 2024, 15, 3971) state as a prospect in their conclusions: can the reversible-linkage template strategy - install a removable directing group on an olefin through a reversible covalent bond, self-assemble a crystalline coordination polymer that enforces the Schmidt topochemical geometry, carry out stereoselective solid-state [2+2] cycloaddition, then destroy the coordination framework and cleave the reversible linkage to release the free functionalized cyclobutane - be extended beyond the amide, imine and dative B-N linkages demonstrated to date, so as to obtain a broader range of functionalized cyclobutane derivatives?

A complete answer would carry at least one reversible covalent linkage class beyond the amide, imine and dative B-N bonds already demonstrated in metal-directed solid-state [2+2] chemistry through the full cycle: ligand synthesis, a crystalline CP with verified topochemical geometry, stereoselective solid-state [2+2] photodimerization, and isolation of a new functionalized cyclobutane derivative after template destruction and linkage cleavage, with the linkage's reversibility demonstrated rather than assumed. A single such linkage carried through the full cycle is a complete answer; linkage classes are counted by cleavage chemistry, so a linkage whose cleavage chemistry differs from amide hydrolysis, from imine hydrolysis and from dative B-N cleavage counts as a class beyond the three already demonstrated.

References

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