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

Abstract

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, lmax = 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.