US EPA

1627808 

Journal Article 

2-benzoylbenzoic acid: A photolabile mask for alcohols and thiols 

Jones, PB; Pollastri, MP; Porter, NA 

1996 

Yes 

Journal of Organic Chemistry
ISSN: 0022-3263
EISSN: 1520-6904 

61 

26 

9455-9461 

WOS:A1996WC12000057 

Photolysis of 2-benzoylbenzoate esters of primary and
secondary alcohols 1 in the presence of a hydrogen donor (2-isopropanol) or an electron donor
(primary amines) produces the corresponding alcohol in high yield. The fate of the benzoate is
dependent on the conditions used for the photoreaction. In 2-propanol, the ketyl radical that
derives from photoreduction dimerizes, to afford the benzpinacol product 3,3'-
diphenylbiphthalidyl, 5. In the presence of amines the product is 3-phenylphthalide, 6, a
benzhydrol derivative which is the result of simple reduction of the ketone followed by
lactonization. While the photoproduct of the benzoate-2-propanol reaction results from
anticipated free radical chemistry, the amine-promoted reaction appears to result from a second,
''dark'', electron transfer process. We conclude that 2-benzoylbenzoic acid is an effective
photolabile protecting group for primary and secondary alcohols, and preliminary studies indicate
that thiols can be protected in an analogous way. Studies on the effect of benzophenone
substituents and reaction solvent on the benzhydrol:benzpinacol product ratio provide mechanistic
insight into the process.