C−O Bond Fragmentation of 4-Picolyl- and N-Methyl-4-picolinium Esters Triggered by Photochemical Electron Transfer

Abstract

Photochemical reduction of several 4-picolyl- and N-methyl-4-picolinium esters was examined using product analysis, laser flash photolysis, and fluorescence quenching. It is demonstrated that the radical (anions) formed in these reactions readily fragment to yield a carboxylic acid and a 4-pyridylmethyl radical intermediate. The high chemical and quantum yields observed for these photoreactions suggests that these esters can be used as photolabile protecting groups.

Photoinduced Electron Transfer (PET)‐Mediated Fragmentation of Picolinium‐Derived Redox Probes

“…Cyclic voltammograms (see Supporting Information) showed irreversible redox processes for all six substrates, 7 – 12 . The first reduction potential was observed between E red =−1.09 V and E red =−1.37 V, with E red =−1.11 V for 7 which is very close to the reported E red =−1.08 V value measured for the corresponding perchlorate salt . Compounds 7 , 8 and 11 , 12 (i.e.…”

“…Although sensitized probes, in which the light‐harvesting and working elements are functionally disconnected, allow considerably larger flexibility in the optimization process, somewhat surprisingly only modest interest has been paid for the development of such probes . Redox‐gated systems are among those, developed essentially around the 4‐picolyl family . The reduction potentials of the parent phenacyl ( E red =−2.20 V vs. SCE) and 4‐picolyl esters ( E red =−2.62 V vs. SCE) were reduced by N ‐alkylation of the heterocycle ( N ‐methyl‐4‐picolinium: E red =−1.10 V vs. SCE), and also by adding electron‐withdrawing groups to the pyridinium ring (2‐cyano‐picolinium esters E red ≈−0.50 V vs. SCE) .…”

“…[2] Redox-gated systems are among those, developed essentially aroundt he 4-picolyl family. [3] The reduction potentials of the parent phenacyl (E red = À2.20 Vv s. SCE) and 4-picolyl esters (E red = À2.62 Vv s. SCE) were reduced by N-alkylation of the heterocycle (Nmethyl-4-picolinium: E red = À1.10 Vv s. SCE), and also by adding electron-withdrawing groups to the pyridinium ring (2cyano-picolinium esters E red %À0.50 Vv s. SCE). [4] In parallel, Nalkylpicolinium (NAP) + mediators ystemsw ith ketocoumarin dyes, [5] gold nanoparticles [6] and tris(bipyridyl)ruthenium(II) (Ru(bpy)) dyes [4] weredeveloped for the liberation of carboxylic acids, amino acids, phosphates, primary aliphatica nd aromatic amines,u nder redox conditions.…”

Model dyads for 2PA uncaging of a protecting group via photoinduced electron transfer

“…The corresponding reduced radicals of the acceptor units, Pyr À and NB À , are not detected in the spectrum since their absorption bands (for Pyr À at 410 nm, 25 for NB À at 437 and 431 nm (ref. 26)) are out of the spectral range of detection of the pump and probe apparatus used.…”

A Mild Method for the Cleavage of the 4-Picolyloxy Group with Magnesium under Neutral Conditions

“…It was also reported that the 4-picolyl groups were deprotected by a light-absorbing photosensitizer. 7 An excellent yield for the free carboxylate has been obtained using visible-lightabsorbing citrate-stabilized metal complexes to mediate electron transfer between dithiothreitol and an N-methylpicolinium ester in aqueous solution by Falvey's group. 8 Our continued interest in peptide synthesis recently prompted us to explore the efficient deprotection of 4-picolyl esters.…”