Comparison of photoenolization and alcohol release from alkyl-substituted benzoyl benzoic esters

Abstract

Photolysis of 1B in argon-saturated solutions yields 4B and releases methanol. Laser flash photolysis of 1B shows formation of biradical 2B, which has a lifetime of~50 ns and a l max at 330 nm. Biradical 2B undergoes an intersystem crossing to form photoenols E-3B and Z-3B with a lmax at 390 nm. Laser flash photolysis shows that the lifetimes of E-3B and Z-3B are affected by the solvent. Density functional theory calculations demonstrate that the transition-state barrier for a 1,5-H atom shift from Z-3B to regenerate 1B is affected by the ortho-alkyl substituents, whereas the stereoelectronics of the alkyl substituent affect the transition-state barrier of E-3B as it undergoes electrocyclic ring closure to form 4B. The photoreactivity of 1B was compared with its analogous methyl and isopropyl derivatives 1A and 1C, respectively, to better estimate the effect of the alkyl substituent on reactivity.Résumé : La photolyse de 1B dans des solutions saturées en argon conduit à la formation de 4B et à la libération de mé-thanol. La photolyse éclair au laser de 1B conduit à la formation du biradical 2B dont le temps de vie est d'environ 50 ns et un lmax à 330 nm. Le biradical 2B subit une transformation intersystème avec formation des photoénols E-3B et Z-3B avec un lmax à 390 nm. La photolyse éclair au laser montre que les temps de vie des isomères E-3B et Z-3B sont affectés par le solvant. Des calculs selon la théorie de la fonctionnelle de la densité démontre que la barrière de l'état de transition pour un déplacement 1,5 d'atome d'hydrogène à partir de Z-3B et conduisant à la régénération du produit 1B est affectée par les substituants ortho-alkyles alors que les effets stéréoélectroniques du substituant alkyle affectent la barrière de l'état de transition de l'isomère E-3B lorsqu'il subit une fermeture électrocyclique de cycle conduisant à la formation de 4B. Dans le but de pouvoir mieux évaluer l'effet d'un substituant alkyle sur la réactivité, on a comparé la photoréactivité de 1B à celles de ses analogues, les dérivés méthyle (1A) et isopropyle (1C).Mots-clés : groupes protecteurs photolabiles, photoénols, photolyse éclair au laser, déplacement 1,5 d'un atome d'hydrogène, fermeture électrocyclique de cycle, lactonisation intramoléculaire.[Traduit par la Rédaction]

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

“…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].…”

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

“…In the electrocyclic ring closure, twisting of the butadienyl moiety transfers the reactant along the potential energy surface towards the twisted geometry of the transitions state. 12 Thus, the steric effect of the ortho substituents in photoenol E-3 lowers the barrier for electrocyclic ring closure in comparison to E-10.…”

“…Photorelease mechanisms can be divided into two main categories: those in which the cleavage of the protected molecule takes place directly from an excited state of the PRPGs and those in which the cleavage takes place from the ground state of a high-energy photoproduct that has a low energy barrier for releasing the protected molecule. We have developed several PRPGs based on ortho-alkyl benzophenone [11][12][13] or acetophenone 14 derivatives that, upon exposure to light, form high-energy photoenols 15 that relax to more stable products and concurrently release alcohols (Scheme 1). More precisely, the release from these PRPGs is initiated by intramolecular H-atom abstraction to yield a biradical that intersystem crosses to form Z and E enols.…”