1,3-Diphenylisobenzofuran

1,3-Diphenylisobenzofuran is a highly reactive diene that can scavenge unstable and short-lived dienophiles in a Diels-Alder reaction. It is furthermore used as a standard reagent[6] for the determination of singlet oxygen,[7] even in biological systems.[8] Cycloadditions with 1,3-diphenylisobenzofuran and subsequent oxygen cleavage provide access to a variety of polyaromatics.

1,3-Diphenylisobenzofuran
Names
Preferred IUPAC name
1,3-Diphenyl-2-benzofuran
Identifiers
ChemSpider
ECHA InfoCard 100.024.371
UNII
Properties
C20H14O
Molar mass 270.33 g·mol−1
Appearance pale yellow[1] to dark yellow crystalline powder[2]
Density 1.0717 g·cm−3 bei 25 °C[3]
Melting point * 125–126 °C[4]
  • 128–130 °C[2]
almost insoluble
Solubility in acetonitrile, benzene, dichloromethane, chloroform, dimethylsulfoxide, tetrahydrofuran or toluene soluble[5]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Preparation

The first synthesis of 1,3-diphenylisobenzofuran was reported in 1905 by A. Guyot and J. Catel.[9][10] Phenylmagnesium bromide was reacted with 3-phenylphthalide (the latter accessible from the methyl ester of 3-hydroxyphthalide with phenylboronic acid in 95% yield[11]) to a lactol, which gives with mineral acids upon elimination of water 1,3-diphenylisobenzofuran with 87% yield.[12]

Synthese von 1,3-Diphenylisobenzofuran aus 3-Phenylphthalid
Synthese von 1,3-Diphenylisobenzofuran aus 3-Phenylphthalid

The patent literature describes the preparation of 1,3-diphenylisobenzofuran by [4+2]cycloaddition of 1,3-butadiene and dibenzoylethylene (1,4-diphenyl-2-butene-1,4-dione, accessible from fumaryl chloride and benzene in the presence of aluminium chloride.[13]).[14] Dibenzoylethylene is predominantly present in the trans configuration[15] but it can be converted into the needed cis configuration by simple heating.[16]

Synthese von 1,3-Diphenylisobenzofuran aus Butadien und Dibenzoylethylen
Synthese von 1,3-Diphenylisobenzofuran aus Butadien und Dibenzoylethylen

The 4,5-dibenzoylcyclohexene formed previously is cyclized with acetic anhydride to the dihydroisobenzofuran. By bromine addition and hydrogen bromide elimination, 1,2-dibenzoylbenzene is formed and recyclized with zinc acetic acid to the final product 1,3-diphenylisobenzofuran.[17] A publication from 1940 describes high yields for the individual stages of the extensive reaction sequence.[4]

The (much cheaper) phthaloyl chloride gives also access to 1,2-dibenzoylbenzene via Friedel-Crafts acylation with benzene,[18] which is reduced to 1,3-diphenylisobenzofuran in 78% yield using potassium borohydride.[19]

Synthese von 1,3-Diphenylisobenzofuran aus 1,2-Dibenzoylbenzol
Synthese von 1,3-Diphenylisobenzofuran aus 1,2-Dibenzoylbenzol

The synthesis of 1,3-diarylisobenzofurans from 2-acylbenzaldehydes and boronic acids is less cumbersome and gives better yields,[20]

Synthese von 1,3-Diphenylisobenzofuran aus 2-Formylbenzophenon
Synthese von 1,3-Diphenylisobenzofuran aus 2-Formylbenzophenon

just like the synthesis from salicylaldehydes via phenacylhydrazones, which undergo oxidation with lead(IV) acetate to give ortho-diketones,[21] followed by the reaction with an aryl Grignard reagent.[22]

Synthese von 1,3-Diphenylisobenzofuran aus Salicylaldehyd
Synthese von 1,3-Diphenylisobenzofuran aus Salicylaldehyd

Properties

1,3-Diphenylisobenzofuran is a yellow, light- and air-sensitive, crystalline solid that is soluble in many organic solvents with a maximum absorption around 420 nm (in solution), which generates intense fluorescence.[23] Fluorescence measurements can be performed in DMF and DMSO because of the stability of 1,3-DPBF in those solvents. In chloroform and carbon tetrachloride the dissolved 1,3-diphenylisobenzofuran is rapidly photolyzed by attack of CHCl2 and CCl3 radicals, even in the absence of oxygen.[24] [24 ]

With ethanol, 1,3-diphenylisobenzofuran forms an orange-yellow, fluorescent solution. On irradiation, it forms a colorless photodimer (upon with exclusion of oxygen), upon discolouration of the solution.[25]

Photodimerisierung von 1,3-Diphenylisobenzofuran
Photodimerisierung von 1,3-Diphenylisobenzofuran

The compound's refractive index is 1,6700 at 25 °C and 589 nm.[3]

Use

Reagent for determination of singlet oxygen
1,3-Diphenylisobenzofuran Reaktion mit Singulett-Sauerstoff
1,3-Diphenylisobenzofuran Reaktion mit Singulett-Sauerstoff
Graph showing change in absorbance of DPBF vs. irradiation time in the presence of photosensitizer (porphyrin)[26]
Electron absorption spectrum showing photodegradation of DPBF in the presence of a porphyrin photosensitzier, during intermittent periods of irradiation with ~630 nm light

In the presence of methylene blue irradiated with red laser light, 1,3-diphenylisobenzofuran reacts with intermediate singlet oxygen 1O2, forming an unstable peroxide that decomposes into (colorless) 1,2-dibenzoylbenzene.[27] The detection of singlet oxygen by 1,3-diphenylisobenzofuran is based on this reaction, even in biological systems. For biological systems, water-soluble derivatives of 1,3-diphenylisobenzofuran were developed.[28] The singlet oxygen generation of photosensitizers were monitored by photolysis of 1,3-diphenylisobenzofuran (DPBF). 1,2-Dibenzoylbenzene absorbs at <300 nm, therefore making DPBF an optimal chemical trap for detecting singlet oxygen, as most photosensitizers absorb <400-600 nm. This allows for an accurate determination of the photodegradation of the molecule.

Dienophile in Diels-Alder reactions

Isobenzofurans like 1,3-diphenylisobenzofuran are among the most reactive Diels-Alder dienes known to date,[29] and are useful for scavenging short-lived and unstable olefins and alkynes. The group led by Georg Wittig made important contributions to this topic.

With the unstable cyclohexyne, 1,3-diphenylisobenzofuran reacts to a tricyclic compound that gives a 9,10-diphenylcyclohexenonaphthalene after hydrogenation and hydrogen abstraction.[30]

Reaktion von 1,3-Diphenylisobenzofuran mit Cyclohexin
Reaktion von 1,3-Diphenylisobenzofuran mit Cyclohexin

1,3-Diphenylisobenzofuran gives similarly with benzyne (dehydrobenzene) an oxygen-bridged anthracene (in 85% yield), which can be reduced with zinc to 9,10-diphenylanthracene (88% yield).[31]

Reaktion von 1,3-Diphenylisobenzofuran mit Dehydrobenzol
Reaktion von 1,3-Diphenylisobenzofuran mit Dehydrobenzol

Cyclopropenone (which is unstable above its melting point of -29 °C) reacts quantitatively at room temperature with 1,3-diphenylisobenzofuran to form a Diels-Alder adduct,[32] which is exclusively an exo isomer.[33]

Reaktion von 1,3-Diphenylisobenzofuran mit Cyclopropenon
Reaktion von 1,3-Diphenylisobenzofuran mit Cyclopropenon

Dimethyl acetylenedicarboxylate reacts with 1,3-diphenylisobenzofuran as dienophile in 84% yield to yield the corresponding adduct.[34]

Reaktion von 1,3-Diphenylisobenzofuran mit Acetylendicarbonsäureester
Reaktion von 1,3-Diphenylisobenzofuran mit Acetylendicarbonsäureester

1,3-Diphenylisobenzofuran reacts also with heterocyclic dienophiles such as 3-sulfolene to the corresponding Diels-Alder adduct.[35]

Molecular building block for polyaromatics

Polyaromatic hydrocarbons (PAHs) are of interest as precursors to graphite but also raise concern as ingredients of pollution. They have persistence and carcinogenicity. 1,3-diphenylisobenzofuran reacts quantitatively with acenaphthylene when heated to 160 °C to give 7,12-diphenylbenzo[k]fluoranthene.[36]

Reaktion von 1,3-Diphenylisobenzofuran mit Acenaphthylen
Reaktion von 1,3-Diphenylisobenzofuran mit Acenaphthylen

The twice occurring Diels-Alder reaction of 1,3-diphenylisobenzofuran with p-benzoquinone yields almost quantitatively a product that can be reacted further with p-toluenesulfonic acid to give a pentacene derivative in 49% yield.[37]

Reaktion von 1,3-Diphenylisobenzofuran zum Pentacen
Reaktion von 1,3-Diphenylisobenzofuran zum Pentacen

Literature
  • W. Friedrichsen (1980), "Benzo[c]furans", Adv. Heterocycl. Chem., Advances in Heterocyclic Chemistry, vol. 26, pp. 135–234, doi:10.1016/S0065-2725(08)60141-5, ISBN 9780120206261
  • W. Friedrichsen (1999), "Recent advances in the chemistry of benzo[c]furans and related compounds", Adv. Heterocycl. Chem., Advances in Heterocyclic Chemistry, vol. 73, pp. 1–96, doi:10.1016/S0065-2725(08)60940-X, ISBN 9780120207732
  • R. Rodrigo (1988), "Progress in the chemistry of isobenzofurans: Applications to the synthesis of natural products and polyaromatic hydrocarbons", Tetrahedron, vol. 44, no. 8, pp. 2093–2135, doi:10.1016/S0040-4020(01)81720-8

References
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