Absolute reactivity of halo(pyridyl)carbenes.

A comprehensive series of halo(pyridyl)carbenes was generated by laser flash photolysis of the appropriate diazirines. Only the chloro- and bromo(2-pyridyl)carbenes and the chloro- and bromo(3-pyridyl)carbenes could be directly observed, but the reactivity of all nine halo(pyridyl)carbenes could be directly studied using the standard and a modified pyridine-ylide approach. The carbenes were all ambiphilic, being highly reactive toward both electron-rich and election-deficient alkenes. Second-order rate constants for these reactions ranged from 2.9 x 10(6) to 3.5 x 10(9) M(-1) s(-1) and depended on both the position of the nitrogen atom within the pyridine ring and the nature of the halogen group, as well as the electrophilicity or nucleophilicity of the alkene. A reactivity trend with respect to the location of the nitrogen within the pyridine ring was observed, with the 4-pyridyl carbenes being the most reactive followed by the 2-pyridylcarbenes and then the 3-pyridylcarbenes being the least reactive. This observed reactivity trend is consistent with the pyridyl ring acting as an overall electron-withdrawing group. The results also show that resonance delocalization of electron density into the nitrogen atom of the 4-pyridyl- and 2-pyridylcarbenes in the transition state significantly reduces the effect of the adjacent halogen (F, Cl, or Br) on the reactivity of the pyridyl carbenes with a series of alkenes.