The effects of H-bonding and sterics on the photoreactivity of a trimethyl butyrophenone derivative

Abstract

The irradiation of ester 1 in methanol and chloroform does not yield any photoproducts, whereas the photolysis of 1 in dry argon-saturated benzene produces cyclobutanol 4, which is converted to lactone 5 by the addition of HCl. Laser-flash photolysis of ester 1 demonstrates that 1 undergoes intramolecular H-atom abstraction to form the biradical 2 (λ(max)∼ 310 nm, τ = 200 ns, benzene), which intersystem crosses to photoenols, Z-3 (λ(max)∼ 380 nm, τ = 30-60 μs, benzene) and E-3 (λ(max)∼ 380 nm, τ = 11 ms, benzene). Density functional theory calculations were performed to support the proposed mechanism for forming cyclobutanol 4 and to explain how steric demand facilitates photoenol E-3 to form cyclobutanol 4 rather than lactone 5.

Steric Demand and Rate‐determining Step for Photoenolization of Di‐ortho‐substituted Acetophenone Derivatives

“…These transition states were located 19 and 20 kcal mol −1 above photoenols E ‐ 1P and E ‐ 2P , respectively. Thus, the ortho substituents do not strongly affect the electrocyclic ring closure reactions of E ‐ 1P and E ‐ 2P .…”

Photoenolization of o‐Methylvalerophenone Ester Derivative

“…In comparison, the transition state barrier for lactone 7 to undergo electrocyclic ring closure to yield lactone 8 is 25 kcal mol −1 , whereas the transition state barrier for cyclobutanol 6 to lactonize is 39 kcal mol −1 . However, it is difficult to compare these transition state barriers because solvation plays an important role in lactonization and we cannot accurately model the solvation effects .…”

“…Esters 1 , 9 and 10 all form long‐lived E‐photoenols, but each reacts differently, and therefore yields insight into the factors that govern their reactivity. Photoenol E‐ 11 releases its methanol moiety and forms lactone 13 , whereas E‐ 12 does not release its alcohol moiety spontaneously and only undergoes electrocyclic ring closure to form cyclobutanol 14 . The additional ortho ‐methyl substituents on photoenol E‐ 12 make it non‐planar, and this alignment is better for electrocyclic ring closure than that of more planar photoenols .…”

“…Photoenol E‐ 11 releases its methanol moiety and forms lactone 13 , whereas E‐ 12 does not release its alcohol moiety spontaneously and only undergoes electrocyclic ring closure to form cyclobutanol 14 . The additional ortho ‐methyl substituents on photoenol E‐ 12 make it non‐planar, and this alignment is better for electrocyclic ring closure than that of more planar photoenols . Thus, we previously theorized that E‐ 12 undergoes ring closure to form cyclobutanol 14 more efficiently than intramolecular lactonization.…”

“…In addition, alcohols can also be released through photoenolization of ester derivatives, as long as the photoenolization is followed by spontaneous intramolecular lactonization between the ester and the enol alcohol (Scheme 3) ( [14][15][16][17][18].…”