2,4-Dimethylene-1,3-Cyclobutanediyl and 2,4-Dimethylenebicyclobutane. Synthesis, Spectroscopy, and Reactivity of a Triplet Biradical and Its Covalent Isomer

Citation

Snyder, ‌Gary James (1988) 2,4-Dimethylene-1,3-Cyclobutanediyl and 2,4-Dimethylenebicyclobutane. Synthesis, Spectroscopy, and Reactivity of a Triplet Biradical and Its Covalent Isomer. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4ftp-kr64. https://resolver.caltech.edu/CaltechTHESIS:03202013-082024971

Abstract

The preparation and direct observation of triplet 2,4-dimethylene-1,3-cyclobutanediyl (1), the non-Kekulé isomer of benzene, is described. The biradical was generated by photolysis of 5,6-dimethylene-2,3-diazabicyclo[2.1.1]hex-2-ene (2) (which was synthesized in several steps from benzvalene) under cryogenic, matrix-isolation conditions. Biradical 1 was characterized by EPR spectroscopy (‌‌‌‌‌│D/hc│ = 0.0204 cm⁻¹, │E/hc│ = 0.0028 cm⁻¹) and found to have a triplet ground state. The Δm_s = 2 transition displays hyperfine splitting attributed to a 7.3-G coupling to the ring methine and a 5.9-G coupling to the exocyclic methylene protons. Several experiments, including application of the magnetophotoselection (mps) technique in the generation of biradical 1, have allowed a determination of the zero-field triplet sublevels as x = -0.0040, y = +0.0136, and z = -0.0096 cm⁻¹), where x and y are respectively the long and short in-plane axes and z the out-of-plane axis of 1.

Triplet 1 is yellow-orange and displays highly structured absorption (λ_max = 506 nm) and fluorescence (λ_max = 510 nm) spectra, with vibronic spacings of 1520 and 620 cm⁻¹ for absorption and 1570 and 620 cm⁻¹ for emission. The spectra were unequivocally assigned to triplet 1 by the use of a novel technique that takes advantage of the biradical's photolability. The absorption є = 7200 M⁻¹ cm⁻¹ and f = 0.022, establishing that the transition is spin-allowed. Further use of the mps technique has demonstrated that the transition is x-polarized, and the excited state 1s therefore of B_1g symmetry, in accord with theoretical predictions.

Thermolysis or direct photolysis of diazene 2 in fluid solution produces 2,4-dimethylenebicyclo[l.l.0]butane (3), whose ¹H NMR spectrum (-80°C, CD₂Cl₂) consists of singlets at δ 4.22 and 3.18 in a 2:1 ratio. Compound 3 is thermally unstable and dimerizes with second-order kinetics between -80 and -25°C (∆H^‡ = 6.8 kcal mol⁻¹), (∆S^‡ = -28 eu) by a mechanism involving direct combination of two molecules of 3 in the rate-determining step. This singlet-manifold reaction ultimately produces a mixture of two dimers, 3,8,9-trimethylenetricyclo[5.1.1.0^2,5]non-4-ene (75) and trans-3,10-dimethylenetricyclo[6.2.0.0^2,5]deca-4,8-diene (76t), with the former predominating. In contrast, triplet-sensitized photolysis of 2, which leads to triplet 1, provides, in addition to 75 and 76t, a substantial amount of trans-5,10-dimethylenetricyclo[6.2.0.0^3,6]deca-3,8-diene (77t) and small amounts of two unidentified dimers.

In addition, triplet biradical 1 ring-closes to 3 in rigid media both thermally (77-140 K) and photochemically. In solution 3 forms triplet 1 upon energy transfer from sensitizers having relatively low triplet energies. The implications of the thermal chemistry for the energy surfaces of the system are discussed.

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