Photoenolization of o‐Methylvalerophenone Ester Derivative

Abstract

Photolysis of ester 1 in argon-saturated methanol and acetonitrile does not produce any product, whereas irradiation of 1 in oxygen-saturated methanol yields peroxide 2. Laser flash photolysis studies demonstrate that 1 undergoes intramolecular H atom abstraction to form biradical 3 (λmax ~340 nm), which intersystem crosses to form photoenols Z-4 and E-4 (λmax ~380 nm). Photoenols 4 decay by regenerating ester 1. With the aid of density functional theory calculations, it was concluded the photoenol E-4 does not undergo spontaneous lactonization or electrocyclic ring closure because the transition state barriers for these reactions are too large to compete with reketonization of E-4 to form 1.

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

“…Thus, our results mirror those of Haag et al for the photoenolization of o-methylacetophenone. In comparison, we have shown that Z-photoenols from o-methyl substituted valerophenone and butyrophenone ester derivatives reketonize slower than their biradical precursors (18,19). Similarly, 2,5-dimethylphenacyl ester derivatives also form Z-photoenols that are longer lived than their triplet biradical precursors (46).…”

“…Efficient photoenol reactivity has been captured in applications such as synthesizing complex natural products and pharmaceutical drugs, and initiating release from photoremovable protecting groups (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). The mechanism of photoenolization, which has been determined using transient spectroscopy, can be outlined as follows: Upon excitation ortho-substituted aryl ketones form the singlet excited state of the ketone chromophore, which undergoes efficient intersystem crossing to form its triplet configuration (Scheme 1) (15)(16)(17)(18)(19). The triplet ketone decays by intramolecular H-atom abstraction to form a triplet 1,4-biradical.…”

Light-driven catalyst-free access to phthalazines: Merging two metal-free domino reactions in one-pot

“…The photochemical enolization 36 of o-alkyl benzophenones towards transient substituted o-quinodimethanes or o-xylylenes is a remarkable and extensively investigated transformation. 37 mechanism of this reaction, which was discovered by Cava and Napier in 1957 38 and later extended by Yang and Rivas, 39 has been comprehensively studied by flash photolysis, 40 ultrafast laser 41,42 and supersonic jet spectroscopies, 43 paramagnetic resonance 44,45 and time-resolved thermal lensing 46 techniques, as well as by isotope exchange and trapping studies. [47][48][49][50] Moreover, theoretical investigations have been carried out.…”

Light-Induced Step-Growth Polymerization of AB-Type Photo-Monomers at Ambient Temperature

“…21 The exceptionally diverse mechanistic nature of SGP chemistry enables the tailoring of distinct property profiles of the resulting polymers with highly specific features including degradability, 8,21,22 solubility, 11 fluorescence, 23,24 and residual functional groups for postsynthetic modification. 10,25 A remarkable reaction, the photoenolization of, for example, α-methyl benzaldehydes, 26 an example of a photoinduced Diels−Alder (DA) reaction harking back to the pioneering work of Tchir and Porter, 27,28 describes a light-induced tautomerization process 29 and has led to a wide ranging and potent toolbox for synthetic organic chemistry, 30,31 surface functionalization, 32−34 and ligation chemistry on synthetic macromolecules. 35−38 The so-called photoenols (i.e., photocaged dienes, see Figure 1a) are hydroxy-o-quinodimethanes formed in situ by the tautomerization of o-methylphenyl ketones or aldehydes.…”

“…A remarkable reaction, the photoenolization of, for example, α-methyl benzaldehydes, an example of a photoinduced Diels–Alder (DA) reaction harking back to the pioneering work of Tchir and Porter, , describes a light-induced tautomerization process and has led to a wide ranging and potent toolbox for synthetic organic chemistry, , surface functionalization, − and ligation chemistry on synthetic macromolecules. − The so-called photoenols (i.e., photocaged dienes, see Figure a) are hydroxy- o -quinodimethanes formed in situ by the tautomerization of o -methylphenyl ketones or aldehydes . The photocaged diene can subsequently undergo a DA cycloaddition with various dienophiles.…”