Site-selective radical reactions of kinetically stable open-shell singlet diradicaloid difluorenoheteroles with tributyltin hydride and azo-based radical initiators

We have demonstrated site-selective radical reactions of the kinetically stable open-shell singlet diradicaloids difluoreno[3,4-b:4′,3′-d]thiophene (DFTh) and difluoreno[3,4-b:4′,3′-d]furan (DFFu) with tributyltin hydride (HSn(n-Bu)3) and azo-based radical initiators. Treatment of these diradicaloids with HSn(n-Bu)3 induces hydrogenation at the ipso-carbon in the five-membered rings, while treatment with 2,2′-azobis(isobutyronitrile) (AIBN) induces substitution at the carbon atoms in the peripheral six-membered rings. We have also developed one-pot substitution/hydrogenation reactions of DFTh/DFFu with various azo-based radical initiators and HSn(n-Bu)3. The resulting products can be converted into substituted DFTh/DFFu derivatives via dehydrogenation. Theoretical calculations unveiled a detailed mechanism of the radical reactions of DFTh/DFFu with HSn(n-Bu)3 and with AIBN, and that the site-selectivity of these radical reactions is controlled by the balance of the spin density and the steric hindrance in DFTh/DFFu.

to give 1-6. An analytical sample was obtained by recycling gel-permeation chromatography.

Preparation of 1
Dihydro compound 1H (30 mg, 41 µmol) was allowed to react with DDQ (19 mg, 82 µmol, 2 equiv) in dry toluene (10 mL) according to the general procedure for the synthesis of 1-6 to give 1 (30 mg, 42 µmol) in almost quantitatively as a deep blue solid. . 1  Scheme S1. Cyclo-dimerization of in-situ formed trimethylsilylethynyl-substituted indeno [1,2-b]fluorene derivative reported by Zhao and co-workers. [3] Competitive reaction experiment: A mixture of DFTh (10.2 mg, 17.3 µmol) and DFFu (10.0 mg, 17.3 µmol) in toluene-d8 (5 mL) was heated at 120 °C under a nitrogen atmosphere. Tributyltin hydride (0.10 mL, 0.35 mmol, 20 equiv) was added, and the resulting mixture was stirred for 0.5 h. The solution was directly transferred to the NMR tube, and the 1 H NMR spectrum was measured ( Figure S1). The radio of DFThH to DFFuH to DFFu was determined to be 1.0 to 0.81 to 0.19; DFTh was almost completely consumed. Figure S1. 1 H NMR spectrum of the products in toluene-d8, which was obtained by the competitive reaction experiment using DFTh, DFFu, and tributyltin hydride.

Reaction of DFTh with AIBN:
To a solution of DFTh (40 mg, 67 µmol) in dry toluene (10 mL) was added AIBN (33 mg, 202 µmol, 3.0 equiv) at room temperature. The resulting mixture was heated at 110 °C for ca. 10 min under a nitrogen atmosphere; the color of the solution rapidly changed from deep blue to pale blue. After the reaction solvent was removed under reduced pressure, the residue was subjected to recycling gel-permeation chromatography to give the mixture of 1 and 1H (0.36:1, 21 mg) as a blue solid in a total yield of 42%; the ratio of 1 and 1H was determined by NMR spectroscopy ( Figure S2).  Low-temperature X-ray diffraction data for 1H were collected on a Rigaku AFC10 diffractometer coupled to a Rigaku AFC HyPix-6000 detector with Mo Kα radiation (λ = 0.71073 Å) from an FR-E+ X-ray source. The diffraction images were processed and spaced using the CrysAlisPro software. [4] Using Olex2, [5] the structures were solved through intrinsic phasing using SHELXT [6] and refined against F 2 on all data by full-matrix least squares with SHELXL [7] following established refinement strategies. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms bound to carbon were included in the model at geometrically calculated positions and refined using a riding model.

S15
The open-shell singlet diradical character index y was calculated based on the natural orbital occupation number (NOON) of the LUMO in spin-unrestricted LC-BLYP calculation using 6-311G* basis set. According to the Yamaguchi scheme, 8 y value is expressed as where T is the orbital overlap between the corresponding orbital pairs and it can be presented using the NOON of HOMO and LUMO.
[c] Open-shell singlet diradical character index.    Figure S8. Electronic absorption spectra of DFTh and its derivatives 1-5 in CH2Cl2. Figure S9. Electronic absorption spectra of DFFu and its derivatives 6 in CH2Cl2.

Theoretical Calculations
All optimized structures for the analysis of reaction mechanisms were obtained using B3LYP functional [9,10] with 6-31G** basis set [11] as implemented in the Gaussian 16 program package. [11] Using the vibrational frequency analysis, we confirmed that the obtained local minima have no imaginary frequency.