Biradicals matrix isolation

The photochemical dissociation of Me2Ge from 7,7-dimethyl-l,4,5,6-tetraphenyl-2,3-benzo-7-germanorbomadiene (14) has been studied by flash photolysis, low-temperature matrix isolation and CIDNP 3H NMR techniques30. The results suggest that a biradical (15) is formed as an intermediate species in the photoreaction. The biradical is initially formed in the singlet state, which undergoes conversion to the triplet state before irreversible decomposition to form Me2Ge and tetraphenylnaphthalene (TPN) (reaction 19). 

The matrix isolation and spectroscopic characterization of m- and p-benzynes and their derivatives have been reported. Fourier transform ion cyclotron resonance mass spectrometry has been employed to investigate the reactivity of w-benzyne biradical with a pyridinium charge site in the 5-position. The chemical properties of w-benzyne in the gas phase differ from those of the monoradical and... 

An intriguing aspect of matrix isolated biradicals is that they can in some cases persist in two different spin states, which may even be interconverted photochemi-cally or thermally. The reader is referred to Chapter 5 in this volume for a more detailed discussion of this phenomenon. 

A new class of localized biradicals, the cyclobutanediyls, have been synthesized from suitable A -pyrazolines and observed by matrix isolation EPR spectroscopy <88JA1356>. These pyrazolines have a methylene bridge between positions 3 and 5 (diazabicyclo[2.1.1]hex-2-enes). The reactivity of bicyclic-A -pyrazolines has been explored by Adam <93JA1257I,94JA7049>. 

TMM was the first biradical to be directly characterized by low temperature, matrix isolation EPR in pioneering work by Dowd (Eq. 11.91). The spectroscopy confirmed the threefold symmetry and the triplet ground state that theory had predicted, and allowed kinetic studies under matrix isolation conditions. 

A prominent strategy in matrix isolation photochemistry has been the expulsion of N2 from stable precursors. Both diazo/diazirine precursors and 1,2-diazene precursors have been useful. The former generate carbenes and related species, while the latter are useful precursors to biradicals. 

Sander, W., Wandel, H., Bucher, G., Grafenstein, J., Kraka, E., and Cremer, D., a,3-Didehydro-5-methyl-6-hydroxytoluene. Matrix isolation of a biradical related to the neocarzinostatin chro-mophore, /. Am. Chem. Soc., 120, 8480,1998. 

The reaction of 33a was shown, by matrix isolation photolysis at 10 K and other methods, to involve acyloxyketene 37 as an intermediate. It was shown in separate experiments that 37, synthesized by conventional treatment of the corresponding acid chloride, reacted with tetraketones to form the same adducts 35 and 36 in the same ratio as had been observed in the photochemical reaction of 33a. The photolysis then involves conversion of excited tetraketone to 37 and CO followed by thermal reaction of 37 with tetraketone presumably via 5-lactones. Limited evidence supports the singlet state as the reactive specie. If this is correct, a cyclic mechanism, as illustrated in Figure 50.9, could be involved a triplet state reaction would require zwitterionic or biradical intermediates, as illustrated. 

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