Photochemistry of 2,2,4,4-Tetramethyl-3-thietan-1-ylidene: A Heterocyclic Carbene with an Unusually Short Lifetime and Evidence for a Nonclassical Structure

Abstract

Laser flash (308 nm) and steady-state photolysis (300 nm) of Δ3-1,3,4-oxadiazoline (3) produces a transient that can be trapped with pyridine to give an adduct with λmax = 350 nm, attributed to ylide 10. Laser flash photolysis (LFP) studies of the ylide indicate that the value of kpyrτ (where kpyr is the absolute rate constant of carbene reaction with pyridine and τ is the carbene lifetime) of the reactive intermediate is 25 times smaller than that deduced for cyclobutylidene. The sulfur-containing transient may have a much shorter lifetime than cyclobutylidene or a much smaller value of kpyr. These experimental results, along with ab initio calculations, suggest that the structure of 2,2,4,4-tetramethyl-3-thietan-1-ylidene is best represented as nonclassical, bisected structure 2 and not classical structure 1.