Formation of oxonium ylide evidenced by the laser flash photolysis of (biphenyl-4-yl)chlorodiazirine in ethers

Abstract

Laser flash photolyses of (biphenyl-4-yl)chlorodiazirine (1) in 2,2,4-trimethylpentane were carried out in the presence of ethers in order to study the reaction intermediates formed from singlet carbenes and ethers. The transient species generated in the absence of ether showed an absorption maximum at 360 nm with a long life time (τ= 24.7 µs), which was assigned to singlet (biphenyl-4-yl)chlorocarbene (5). In the presence of various kinds of ethers, additional transient absorption could not be detected but the spectrum of 5(360 nm) shifted by about 15 nm to longer wavelength. The effect of ethers added to the reaction mixture was studied by means of typical singlet carbene reactions, i.e. trapping by 2,3-dimethyl-2-butene or pyridine. It was found that the rate constants of these reactions decreased by the addition of ethers and, hence, it is concluded that the reaction of 5 with ethers proceeds via oxonium glide intermediate 7. By changing the concentration of tetrahydrofuran (THF), the equilibrium constant of the reaction (5 + THF ⇌ oxonium glide) was determined to be 0.5 dm³ mol^–1 at 293 K in 2,2,4-trimethylpentane (i-Oc).