N-METHYL-4-PICOLINIUM ESTERS AS PHOTOREMOVABLE PROTECTING GROUPS BASED ON PHOTOINDUCED ELECTRON TRANSFER
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The goal of this project is to develop an improved photoremovable protecting group (PRPG) for the carboxylate functionality that can be released using high wavelength light through a photoinduced electron transfer (PET) process.
The N-methyl-4-picolinium group has been demonstrated to be an efficient PRPG. Various carboxylic acids and amino acids as well as diethyl phosphate were released from their corresponding N-methyl-4-picolinium esters in high yields upon sensitized irradiation. The quantum yields for photorelease were also found to be satisfactory.
Some advantages of the picolinium esters over previous PRPGs include their aqueous solubility, ease of protection and removal, clean and fast release, harmless byproducts, and high reduction potential. This latter property is especially desirable as it allows for protection of carboxylic acids that can be easily reduced and also widens the choice of photosensitizers and wavelengths of light used for deprotection. Several photosensitizers were shown to effect PET and the subsequent fragmentation of the picolinium esters.
High wavelength visible light sensitizers were also employed and successfully released the picolinium group. To our knowledge, this is the first report of high wavelength (450-525 nm) release of PRPGs using a PET process, and proceeding with high chemical and effective quantum yields.
In order to further improve the efficiency of this system by suppressing back electron transfer, a mediated relay electron transfer strategy was used to release the picolinium group. Two triplet sensitizers, benzophenone and xanthone, were found to effectively carry out a relay electron transfer to the picolinium esters and release the free substrates in high yields. The quantum yields of release were higher than in the direct sensitized case.
The successful release of picolinium esters by both direct and mediated electron transfer prompted us to study the corresponding covalently linked systems. Several carbazole linked picolinium esters failed to release the free acids upon photolysis. In the mediated case however, the benzophenone linked picolinium esters released the free acids efficiently.