Detection of Carboxyl Ylide Evidenced by Laser Flash Photolysis of (Biphenyl-4-yl)chlorodiazirine in the Presence of Carbon Dioxide

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Ylide formation of a singlet carbene with carbon dioxide (CO₂ was studied using laser flash photolysis (LFP) of (biphenyl-4-yl)chlorodiazirine (BCD) in CO₂ saturated acetonitrile (MeCN) and in CO₂ dissolved in 2,2,4-trimethylpentanone (i-Oc) at high pressure (ca. 2 X 10⁶ Pa). (Biphenyl-4-yl)chlorocarbene (BCC), produced by LFP, adducts with an oxygen atom of CO₂ to form a carboxyl ylide (CY, &lamda;_max = ca. 510 nm). The apparent rate constant for the addition of BCC to CO₂ was 5.2 x 10⁷ dm³ mol^-1 s^-1 (by measuring the decay rate of BCC) and 6.2 X 10⁷ dm³ mol^-1 s^-1 (the rate of CY formation) in MeCN. CY decays via unimolecular and bimolecular mechanisms. CY is a 1,3-dipole, so in the presence of dimethyl fumarate, it gives a γ-butyrolactone derivative. On adding the carbene scavenger 2,3-dimethyl-2-butene (TME), the decay rate of BCC and the formation and decay rates of CY increased with TME concentration, indicating that CY was in equilibrium with BCC. The equilibrium constant was estimated roughly to be ca. 2 dm³mol^-1.