Solid-state photochemistry of crystalline pyrazolines: reliable generation and reactivity control of 1,3-biradicals and their potential for the green chemistry synthesis of substituted cyclopropanes

Abstract

To expand on the limited number of examples that exist in the literature for the solid-state photodenitrogenation of azoalkanes, a series of crystalline 7-alkyl-2,3,7-triazabicyclo[3.3.0]oct-2-ene-6,8-diones with varying 4,4-substituents were prepared. Their photochemical behavior in solution and in the solid state was dependent on the 4,4-substitution of the 1-pyrazoline ring, with unsubstituted pyrazoline giving a mixture of products both in solution and in the solid state. Diphenyl substituted pyrazolines denitrogenate spontaneously in solution but require light exposure to react quantitatively in the solid state. t-Butyl-phenyl substituted pyrazolines were shown to denitrogenate both chemo- and diastereoselectively in solution and in the solid state to yield a single product in quantitative yield.

New 4-Functionalized Glutamate Analogues Are Selective Agonists at Metabotropic Glutamate Receptor Subtype 2 or Selective Agonists at Metabotropic Glutamate Receptor Group III

“…From here, a 2 + 3 cycloaddition reaction of 9 with an ethereal solution of diazomethane installed the desired syn relationship of the two distal carboxylate groups in pyrazoline 10a/10b . Without further purification, crude pyrazoline 10a/10b was dissolved in acetonitrile in a round-bottomed quartz flash and irradiated with mercury lamps (6 × 24 W) in the presence of 3 equiv of benzophenone to afford cyclopropanes 11a/11b in 86% yield as a (∼1:1) mixture of diastereomers . At this stage, we attempted to separate diastereomers 11a / 11b by HPLC for subsequent stereochemical assignment by NMR.…”

“…Without further purification, crude pyrazoline 10a/ 10b was dissolved in acetonitrile in a round-bottomed quartz flash and irradiated with mercury lamps (6 × 24 W) in the presence of 3 equiv of benzophenone to afford cyclopropanes 11a/11b in 86% yield as a (∼1:1) mixture of diastereomers. 32 At this stage, we attempted to separate diastereomers 11a/11b by HPLC for subsequent stereochemical assignment by NMR. Regrettably, this approach was not successful, and cyclopropanes 11a/11b were therefore fully deprotected to give the final products 2c/2d in 35% yield as a (1:1) mixture of diastereomers.…”

Taming Radical Pairs in Nanocrystalline Ketones: Photochemical Synthesis of Compounds with Vicinal Stereogenic All-Carbon Quaternary Centers

“…In fact, stable molecules that absorb UV or visible light “instantaneously” increase their energy content by 60–90 kcal/mol, which makes the formation of a wide range of reactive intermediates accessible. We have shown that crystals of diazo compounds, diazenes, pyrazolines, and triazolines can be exposed to UV light to generate carbenes, dialkyl radical pairs, 1,3-dialkyl biradicals and 1,3-alkyl-aminyl radical pairs, respectively, which can go on to give clean products in the solid state. In the case of crystalline ketones ( K , Scheme ), excitation to the n,π* excited state followed by intersystem crossing leads to the triplet state 3 K *, which is able to undergo an α-cleavage reaction to form of a triplet acyl-alkyl radical pair ( 3 RP-1 ) with energetics that depend on the radical-stabilizing abilities of the α-substituents (R 1 -R 3 ).…”

Solar Photochemical Synthesis: From the Beginnings of Organic Photochemistry to the Solar Manufacturing of Commodity Chemicals

“…Likewise, continuous-flow operation in microreactors has been shown to significantly reduce solvent needs and photodecomposition losses. − Photochemical syntheses have also been demonstrated in less hazardous media such as water, − micelles, ionic liquids, or supercritical CO 2 . Likewise, higher quantum efficiencies of photochemical transformations have been achieved by use of gaseous CO 2 , or N 2 as leaving groups.…”

“…31−36 Photochemical syntheses have also been demonstrated in less hazardous media such as water, 37−39 micelles, 40 ionic liquids, 41 or supercritical CO 2 . 42 Likewise, higher quantum efficiencies of photochemical transformations have been achieved by use of gaseous CO 2 43,44 or N 2 45 as leaving groups.…”