Cycloaddition of singlet carbene to 2-vinylpyridine studied by laser flashphotolysis of 3-(biphenyl-4-yl)-3-chlorodiazirine

The laser flash photolyses (LFPs) of 3-(biphenyl-4-yl)-3-chlorodiazirine (B CD) were carried out in the presence of 2-vinylpyridine (VP) in 2,2,4-trime thylpentane. Immediately after the flash, the absorption spectrum of (biphe nyl-4-yl)chlorocarbene (BCC) was recorded (lambda (max) = 360 nm). Correspo nding to the decrease in the BCC band, a new band formed by first-order kin etics in the wavelength range from 440 to 620 nm (lambda (max) = 540 nm). T his band is assigned to a pyridinium ylide (PY) produced by the addition of BCC with VP. The PY formation rate was equal to the BCC decay rate. Becaus e PY has a 1,5-zwitter ion (ZI) structure as a contributing resonance form, this species decayed by the intramolecular cyclization of the ZI (tau = 43 mus). Also the rates of the BCC decay, the PY formation and the PY decay i ncreased with increasing 2,3-dimethyl-2-butene (TME) concentration, suggest s that the BCC was in equilibrium with PY. The equilibrium constant was rou ghly estimated to be 80 dm(3) mol(-1). The photoreaction of BCD in the pres ence of VP produced the indolizine (IND) derivative (56%) and cyclopropane compounds (total yield, 12%). (C) 2001 Elsevier Science B.V. All rights res erved.