Pyridinium salts photolysis

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

Photolysis of 3-substituted pyridinium salt 234 in aqueous base provides the highly functionalized bicyclic aziridine 236, albeit in low yield (20%) <2005JOC5618>. Fortunately, the two regioisomers can be separated chromatographically. The reaction presumably proceeds through indiscriminant hydroxide addition onto an intermediate allylic cation 235. Compound 236 can be carried on to the desired acetamidocyclopentene derivative 237 in three steps and 80% yield (Scheme 42). 

Upon photolysis, pyridinium salts undetgo cleavage of the carbon-nitrogen bond to form pyridinium radical cation and alkoxy radical. The nitrogen centered radical cations were spectroscopically detected by laser flash photolysis... 

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

Similar to pyridinium salts, iodonium salts can be reduced by silyl radicals generated by photolysis of polysilanes. However, due to the tail absorption bands of iodonium salts at about 300 nm, polysilanes with relatively longer... 

Hizal et al. [79] have recently shown an interesting variation of pyridinium salt photodecomposition in polymer synthesis. In earlier investigations it became evident that ethanol is formed by the hydrogen abstraction of primary ethoxy radical if the photolysis is carried out in strong hydrogen donor solvents such as THF ... 

UV photolysis of CpMn(CO)3 in toluene leads to loss of CO and formation of CpMn(CO)2( ] -toluene). Kinetic studies suggest that the binding energy of the toluene is ca. 60kJmor. The binding of H2 to CpMn(CO)2 has been studied in supercritical CO2 solvent. It has been proposed that pyrylium and pyridinium salts such as (35) can be used to label proteins and thereby aid in the detection and characterization of receptor sites. Cymantrene bound to lysine residues of bovine serum albumin (BSA) has been used as a redox label. Electrochemical reduction of the label established an impressive BSA detection limit of 2 x 10 M. 

Photolysis of pyridinium salts in water or methanol leads to derivatives of 6-azabicyclo[3.1.0]hexene (e.g. 108 from 1-methylpyridinium chloride). The formation of these products may be due to azoniabenzvalenes (e.g. 109) as precursors ... 

The radical cyclization of 2-bromo-iV-alkenylpyridinium triflates yields precursors to the indolizidine and quinolizidine skeletons after reduction <97TL5383>. Photolysis of the pyridinium salt 29 in water or alcohol gives the interesting bicyclic product 30 <97HCA 121>. The products are useful as precursors to glycosidase inhibitors or bc/a-lactams. 

The photoreactions of quaternary pyridinium salts in water give 6-azabicy-clo[3.1.0]hex-3-en-2-ols. On photolysis of pyridine iV-oxides in alkaline solution, ring opening produces cyano-dienolates. ... 

The pyridinium salt end-functionalized PTHFs can be used as polymeric initiators for photoinduced reactions. Upon photolysis the pyrdinium moiety decomposes and polymeric radicals are formed. Depending on the additives present in the system, hydroxy telechelics (305) or block copolymers (302) are thus formed. 

Tlie existence of the ylide 19, which can formally be interpreted as the deprotonation product from the corresponding salt 7a, has been claimed by trapping chlorocarbene with pyridine during the laser-flash photolysis of e do-7-chlorodibenzo[n,c]bicyclo[4.1.0]heptane (18) (96JPC18426). Bromination of l-vinyl-2-pyridone (20) yields the bicyclic pyridinium bro-... 

Note that the photolysis of various pyridinium borate salts at — 78°C (to prevent thermal reaction) affords the same methyl-transfer products as obtained in thermal reactions (i.e., equation 44). 

There have been further reports on pyridine C-nucleosides. 4-Carbam-oyl-2-p-D-ribofuranosylpyridine 54 and 3-carbamoyl-5-P-D-xylofuranosyl-pyridinel55 have been prepared by Alderweireldt s approach (see Vol. 21, p. 211), and some 2 -deoxy-r,2 -didehydro-pyridine C-nucleosides have been prepared by base-catalyzed elimination. Addition of lithiopyridines to 2,3-C-isopropylidene-5-C-TBDMS-D-ribonolactone gives adducts such as (133), which can be reduced to p-pyridyl C-nucleosides with EtaSiH.l T Coupling of the anomeric free radical, produced by photolysis of an iV-acyl-thiohydroxamate, with pyridinium and quinolinium salts gave 2-pyridyl C-nucleosides and 2-quinolinyl systems such as (135), where P-selectivity was... 

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