Product Selection through Photon Flux: Laser‐Specific Lactone Synthesis

Abstract

In ground-state chemistry, such as metal-catalyzed processes, exquisite levels of reaction control can often be achieved by subtle changes to the catalyst or reaction conditions. In photochemistry the situation is much more complex, as fates of excited states are governed by competing pathways for reaction, coupling to other electronic states, possible further photoexcitation, and relaxation to the ground state; thus, the outcomes of such processes are difficult to control once initiated. Some useful selectivity through singlet or triplet states can be achieved by a number of means, including use of triplet quenchers or sensitizers, [1][2][3] irradiation at user-selected wavelengths, [4] through higher exited states from multiphoton absorption, [5, 6] and feedback-based optical control using evolutionary algorithms and pulse shapers. [7] Herein, we describe how photon flux [8] can be used to switch cleanly between Norrish type I and type II pathways [1, 5, 6f-l,9] in the photochemical rearrangement of 1,3-indandiones.[10] In particular, the study demonstrates that photon flux, not average power or wavelength, is the controlling factor in product selection.As part of an ongoing study of the [5+2] photocycloaddition of maleimides, [11,12] we investigated the photochemistry of 2-ethyl-1,3-indandione 1 (Scheme 1). Unfocused [13] Nd:YAG laser irradiation of 1 at 266 nm [14] gave rise to the volatile g-alkylidinephthalide lactones 2 as an E/Z mixture (18 %). Using the higher power output of the laser at 355 nm (350 mW), a yield of 60 % of isolated 2 was obtained after 18 h. Addition of isoprene as a triplet quencher had no effect, suggesting a singlet mechanism. Quantum-yield (F) measurements using tunable lasers at five specific wavelengths between 222 and 355 nm indicated an inefficient photochemical reaction (F = 0.5-3.3 %).At this point we attempted to scale up the formation of 2 by irradiating 1 with higher (average) power UV lamps. To our surprise, however, irradiation of 1 with a low-pressure phosphor-coated mercury lamp (312 nm) gave only the diones 3 and 4 (43 %, 2:1). Similar results were obtained using medium-pressure lamps (125-600 W). Irradiation of 1 in CD 3 CN and direct analysis of the photosylate by 1 H NMR spectroscopy showed greater than 99 % of 3 and 4 and a trace of 2 (less than 1 %). These results indicate that the pathway leading to 2 is effectively closed for lamp irradiation. Significantly, addition of isoprene (one equivalent) resulted in quenching of the reaction and recovery of 1, thus indicating that 3 and 4 are formed from a different (triplet) pathway on lamp irradiation.This stark, source-dependent difference in behavior can be explained by the mechanisms in Scheme 2. Initial excitation of 1 with either source leads to the singlet state (S 1 ), from which two separate pathways can evolve. With the laser, S 1 undergoes a cleavage to the diradical 5 and subsequent recombination to the lactones 2 through a Norrish type I pathway. A singlet manifold is supported by the lack of quenchin...

Anti-influenza Virus Polyketides from the Acid-Tolerant Fungus Penicillium purpurogenum JS03-21

“…Analysis of the 1 H and 13 C NMR data (Table ) for 5 revealed a structure similar to 4 , except that the C-3–C-8 fragment was replaced by an olefin. The relatively upfield shift of H-8/C-8 (δ H/C 5.59/109.9) indicated the Z -configuration of the double bond at C-3 and C-8 . Thus, the structure of 5 was established as ( Z )-5,6-dihydroxy-7-methyl-3-propylideneisobenzofuran-1(3 H )-one.…”

“…The relatively upfield shift of H-8/C-8 (δ H/C 5.59/109.9) indicated the Z-configuration of the double bond at C-3 and C-8. 17 Thus, the structure of 5 was established as (Z)-5, 6-dihydroxy-7-methyl-3-propylideneisobenzofuran-1(3H)-one.…”

Neuroprotective and Cytotoxic Phthalides from Angelicae Sinensis Radix

“…Among these compounds, five known compounds were identified as ligustilide (1) [16], E-butylidenephthalide (2) [17], Z-butylidenephthalide (3) [17], riligustilide (4) [18], cis-Z,Z'-3a. 7a',7a.…”

The Reactivity of the 1,4-Biradical Formed by Norrish Type Reactions of Aqueous Valerophenone: A QM/MM-Based FEP Study

“…Some advanced approaches such as replica exchange , and weighted histogram analysis method − have been developed for the QM/MM free energy calculation, but most of them need to sample different QM conformations on the QM/MM potential energy surface, which requires the enormous computational cost. In comparison, the FEP method is more suitable for computing free energy of photochemical systems in solution, which has been discussed in previous studies. , For a QM/MM system, the partition function can be represented as Z 0 ( r QM , r MM ) = ∫ e − normalβ E false( r QM , r MM false) .25em d r QM .25em d r MM where E is the total energy, and r QM and r MM represent coordinates of the QM and MM atoms, respectively. The Helmholtz free energy is related to partition function for an ( N , V , T ) ensemble as F 0 ( r QM , r MM ) = − β − 1 ln Z 0 ( r QM , r MM ) Let us consider the free energy as the potential of mean force (PMF) of r QM .…”

“…However, there is no clear-cut understanding of how the concerted effect of solvent and conformation on partition of the 1,4-biradical between cyclization and cleavage and the behavior of the 1,4-biradical is still an enigma in general. Attempts to understand the factors that influence the reactivity of triplet 1,4-biradicals continue to be of contemporary interest in view of the importance of the Norrish type reactions in many applications. ,,− In the present work, Norrish type reactions of the aqueous valerophenone (VP) have been studied from the viewpoint of theory and more attention was paid to the solvent effect on the conformation of the 1,4-biradical.…”