Topochemical, Single‐Crystal‐to‐Single‐Crystal [2+2] Photocycloadditions Driven by Chalcogen‐Bonding Interactions

Abstract The face‐to‐face association of (E)‐1,2‐di(4‐pyridyl)ethylene (bpen) molecules into rectangular motifs stabilized for the first time by chalcogen bonding (ChB) interactions is shown to provide photoreactive systems leading to cyclobutane formation through single‐crystal‐to‐single‐crystal [2+2] photodimerizations. The chelating chalcogen bond donors are based on original aromatic, ortho‐substituted bis(selenocyanato)benzene derivatives 1–3, prepared from ortho‐diboronic acid bis(pinacol) ester precursors and SeO2 and malononitrile in 75–90 % yield. The very short intramolecular Se⋅⋅⋅Se distance in 1–3 (3.22–3.24 Å), a consequence of a strong intramolecular ChB interaction, expands to 3.52–3.54 Å in the chalcogen‐bonded adducts with bpen, a distance (<4 Å) well adapted to the face‐to‐face association of the bpen molecules into the reactive position toward photochemical dimerization.


I) General considerations
Oxygen-and moisture-sensitive experiments were carried out under a dry oxygen-free argon atmosphere using standard Schlenk techniques. Solvents were dried by standard methods. The NMR spectra were recorded on Bruker spectrometers (300 MHz) referenced to residual solvent signals as internal standards. Elemental analyses were performed at Centre Regional de Mesures Physiques de l'Ouest (CRMPO) with Elementar Vario/Perkin Elmer 2400 series. Single crystal XRD were performed at Centre de Diffraction de Rennes (CDIFX) with a Bruker AXS D8 Venture diffractometer. (E)-1,2-di(4-pyridyl)ethylene (bpen) was obtained from Aldrich and used as received.

II) General procedure for the Miyaura borylation reactions
To bis(pinacolato)diborane (2.5 mmol), anhydrous potassium acetate (6 mmol) and a dibromo derivative (1 mmol) 3.5 ml of anhydrous DMF was added and the mixture was degassed with argon. [1,1'-Bis(diphenylphosphino)ferrocene]palladium(II) dichloride (73 mg, 0.1 mmol) was added and the mixture was stirred in argon atmosphere at 80°C overnight. After cooling to room temperature, the mixture was poured to a separatory funnel, diluted with water and extracted with AcOEt (50 ml). The aqueous layer was discarded and the organic layer was washed with water (twice) and brine and then dried over MgSO4. Volatiles were removed on a rotavap and the crude product was purified by flash chromatography (petroleum ether:AcOEt = 9:1) to give pure diboronic acid pinacol diesters 4-6.

III) General procedure for the synthesis of the bis(selenocyanato)arenes
To SeO2 (6 mmol) and malononitrile (2 mmol) 2 ml of anhydrous DMSO was added in argon atmosphere and the mixture was stirred for 30 min at room temperature. To the resulting orange solution diboronic acid pinacol diester 4-6 (1 mmol) was added and the mixture was stirred at 60°C overnight. After cooling to room temperature water (10 ml) was added and the mixture was extracted with DCM (3x 20 ml). The organic layers were combined, dried with MgSO4 and volatiles were removed on a rotavap. The crude product was purified by column chromatography (petroleum ether:DCM = 1:1 or petroleum ether:AcOEt = 9:1) to give pure bis(selenocyanato)arenes 1-3.

1:
The compound was dissolved in small amount of CHCl3 and left for slow evaporation of the solvent. After one day yellow rods appeared (mp 111-113°C).

2:
The compound was dissolved in small amount of hot AcOEt and petroleum ether was added in portions. Resulting solution was left to cool down yielding white needles (mp 112-113°C).

3:
The compound was dissolved in small amount of hot AcOEt and petroleum ether was added in portions. Resulting solution was left to cool down yielding dark yellow rods (mp 122-123°C).

V) Photo reactions of (1)4(bpen)5 and (3)(bpen)2
Crystals of (1)4(bpen)5 were placed in a drop of Paraton® oil on a glass plate and exposed to 254 nm UV radiation under a 6W laboratory lamp for two hours. A small piece of the resulting crystals was cut and used for single crystal X-ray diffraction experiment.
Crystals of (3)(bpen)2 were placed in a drop of Paraton® oil on a glass plate and exposed for 4 days to 253.7 nm (35W) UV radiation in a Rayonet RPR-100 reactor. A small piece of the resulting crystals was cut and used for single crystal X-ray diffraction experiment.

VII) ESP calculations
The geometry of molecules 1-3 was optimized starting from their "syn" conformation using the Gaussian 16 program using the B3LYP method with D3 Grimme dispersion correction and the Def2TZVPP basis set (as reported in A. Dhaka, O. Jeannin, E. Aubert, E. Espinosa M. Fourmigué, Chem. Commun. 2021, 57, 4560). For compound 3, preliminary optimizations with HF/6-311G were performed to shorten the time during the second calculation with more complex method and basis set. Frequency calculations were also performed in order to check that true energy minima were obtained. ESP maps generated with isovalue = 0.002 are presented below. The values for both 0.001 and 0.002 isovalue are summarized in Table S1.

Figure S3
Network of ChB interactions in 2. See Table S2 for structural characteristics. Hydrogen atoms were omitted for clarity.

Figure S6
Illustration of the solid-state organization within the alternating layers in (1)4(bpen)5

Figure S7
Projection view along a of the unit cell of (3)(bpen)2 before UV irradiation, showing the two crystallographically independent bpen molecules, one involved in ChB with 3 and the other interspersed between the dimethoxybenzene moieties. Hydrogen atoms were omitted for clarity.

Figure S8
Projection view along a of the unit cell of (3)(bpen)2 after UV irradiation, showing the photo-cyclized bpen molecules involved in ChB with 3 and the other unmodified bpen molecule. Hydrogen atoms were omitted for clarity.