Thione photochemistry. Dual pathways in aralkyl thione cyclizations

Abstract

It is shown that aralkyl thiones, which possess an oxygen atom at the δ position in the side chain, can therefore not undergo the preferred δ insertion, and insert at the γ and ε positions. Although ε insertion occurs only from the 1(π,π*) state, γ abstraction (followed by cyclization or cleavage) occurs from both 1(π,π*) and 3(n,π*) states. The quantum yields of photocyclization of 2c, 2d, and 2g change little with solvent polarity, which suggests that a thione–ether charge transfer interaction contributes little to the cyclization. Fluorescence and product quenching studies are reported which support this.

Photochemistry of Aliphatic Thioketones in the Gas Phase

“…In general, the quantum efficiencies of ca. 4−9% are comparable to those determined for insertion chemistry in a number of aryl alkyl thioketones. 15b,, …”

“…7D as the major product in eq 13). Most striking is the fact that each of these products may be thought of as resulting from the equivalent of a Norrish type II fragmentation, hitherto unobserved in the alicyclic thioketones and rare among other thiocarbonyl substrates. ,16b …”

The Photoreactivity of Thiocarbonyl Compounds

“…54 Particularly, blocking the &position by oxygen leads to y-and &-insertion resulting in the formation of four-and six-membered cyclic thiols (Scheme 17). The altered reactivity resulting from heteroatom substitution can be either due to activation of the adjacent hydrogens or the consequence of intramolecular charge transfer from oxygen to the excited thioketone.…”