Neckers studies

Template Polymers. Template effects in chelating polymers constitute an interesting development in the field of metal containing polymers. The Template effects are interpreted by the fact that the small molecule is templating a pattern in the macromolecule which can be recognized by the same molecule in a subsequent process. The idea is to prepare a polymer from the metal-chelated monomer, to remove the metal ion, and then to measure the selectivity of the prepared polymer for the metal ion of the template [36]. Typical examples of template systems are 4-vinyl-4 -methylbipyridine (Neckers [36]) and 1-vinyl-imidazole (Tsuchida [37]). These are polymerized in presence of divinylbenzene [36] and appropriate metal salts (Co2+, Cu2+, Ni2+, Zn2+). The template metal ions are removed by acid leaching and the polymer subsequently used for metal ion absorption studies (Fig. 16). 

The photochemistry of Eosin under both reductive and oxidative conditions has been studied by several groups [145-151], Photoreduction by amines such as tribenzylamine (R = CH2, R" = ) produces two leuco analogues, the dihydro derivative, and the cross-coupled product formed from the amine radical and the dye radical anion (2) [152], In addition, debromination of Eosin is reported during photobleaching with amines and phenols. The reader however is referred to the extensive studies of Rose Bengal dehalogenation by Paczkowski and Neckers [153]. Radiolysis of Eosin in methanol shows that debromination is a consequence of the photochemical decomposition of semireduced Eosin [154],... 

Saffran M, Bedra C, Kumar GS, Neckers DC (1988) Vasopressin a model for the study of effects of additives on the oral and rectal administration of peptide drugs. J Pharm Sci 77 33-38... 

These have been extensively studied by Neckers and coworkers [21]. Those which undergo y-hydrogen abstraction cleave cleanly to aldehyde or ketone, and thus provide a possible methodology for environmentally friendly oxidation of alcohols. The secondary photolysis of the hydroxyketene forms benzaldehyde and carbon monoxide, both of which could be considered nuisances, although it might be possible to trap the carbene intermediate in order to make a 1-phenylcyclopropanol. 

PET cyclizations of donors-substituted phenylgloxylates have been intensively studied by the groups of Neckers and Hasegawa. For the thioether derivatives 1 the efficiency of the cyclization depended on the linker chain length (Sch. 5) [18]. With increasing carbon tether (>7 = 2-10) the yield dropped steadily and secondary reductive dimerizations or Norrish Type II processes became competitive. Consequently, the Cn-linked substrate solely underwent dimerization and cleavage [18b]. 

Aluminum chloride and its derivatives are the most familiar Lewis acids and are routinely employed in many Lewis acid-promoted synthetic transformations. The first polymer-supported metal Lewis acids to be studied were polymers attached by weak chemical or physical interactions to a Lewis acid. In the 1970s Neckers and coworkers reported the use of styrene-divinylbenzene copolymer-supported AlCl,- or BF3 as catalyst in condensations, esterifications, and acetalization of alcohols [11,12]. This type of polymer-supported AICI3 (1) is readily prepared by impregnation of a polystyrene resin with AICI3 in a suitable solvent. Subsequent removal of the solvent leaves a tightly bound complex of the resin and AICI3. The hydrophobic nature of polystyrene protects the moisture-sensitive Lewis acid from hydrolysis, and in this form the Lewis acid is considerably less sensitive to deactivation by hydrolysis. This polymer complex could be used as a mild Lewis acid catalyst for condensation of relatively acid-sensitive dicyclopropylcarbinol to an ether (Eq. 1) [13],... 

Covalently bound templates have been extensively studied from 1972 [27, 28] up to the present day by Wulff et al. [62-64] who were joined by Damen and Neckers (1980) [65,66], by Sarhan et al. (1982) [67-69] and by Shea and Thompson (1978) [70]. A variety of parameters have been investigated and their influence on the selectivity of binding and the application of these polymers as chiral stationary phases have been quantified. The parameters investigated are the degree of crosslinking [65,66,71-79] the structure of the crosslinker [71,72,... 

Neckers and his co-workers have investigated the photochemical reactivity of ammonium borate salts both in solution and tethered to a polymer substrate as a means of producing free tertiary amines. The reactions are carried out in acetonitrile solution on the benzoylbenzyl molecules (201) using wavelengths > 300 nm. Irradiation of (202) results in the formation of the products shown in Scheme 6. More detailed studies examined the influence of different borate salts as the counterions to the ammonium salts. Some of these results are shown for the decomposition of the salt (201). From these data, it can be seen that the most efficient quantum yield for the release of tri- -butyIamine is obtained with the triphenylbutylborate counterion. 

Valdes Aguilera, O., Pathak, C. P., Shi, J., Watson, D., Neckers, D. C., Photopolymerization Studies Using Visible light Photoinitiators, Macromolecules 1992, 25, 541 547. 

Cycloaddition reactions of the indole 2,3-double bond are not limited to alkenes as partners. Acetylenic compounds have also been used in photochemical cycloaddition reactions with indoles to produce cyclobutenone derivatives. There have been extensive studies on the reaction of indoles with dimethyl acetylenedi-carboxylate (61, DMAD), which produce a number of structurally distinct products [31]. By devising a photosensitized cycloaddition reaction of DMAD to activated indoles 60 in the presence of acetophenone, Neckers and Davis were able to produce the cyclobutene derivatives 62 in good yields (Scheme 14) [32]. The resulting cyclobutenes can then be converted to the corresponding benzazepines 63 via thermal ring-opening reactions. 

The water content in the medium plays a key role to control the colloid formation. The influence of the water content can be studied by using dynamic light scattering (DLS) and UV-vis spectroscopy. DLS indicates that when the water content increases above CWC, the hydrodynamic radius (i b) gradually increases as the water content increases. When the water content is above a certain value, i h starts to decrease as the water content further increases and then stabilizes at the final value. The structure evolution in the process can be better understood from the UV-vis spectroscopic investigation. It is well known that the photoisomerization rate and isomerization degree at the photostationary state are related to the free volume surrounding the azo chromophores (Kumar and Neckers, 1989). For azobenzene-type molecules, the isomerization behavior can be monitored by UV-vis spectroscopy and used as a molecular probe to detect the environmental variation in the systems. 

Mondal R, Okhrimenko AN, Shah BK, Neckers DC (2008) Photodecarbonylation of a-diketones a mechanistic study of reactions leading to acenes. J Phys Chem 112 11-15... 

Based on the studies of Gottschalk and Neckers in 1985, the group of Lei developed a radical Cu-catalyzed cross-dehydrogenative olefination reaction that gives direct access to a,p-unsaturated ketones (Scheme 1.45). ... 

Lee, S.-K., Valdes-Aguilera, O., and Neckers, D.C., A low temperature luminescence study of the colored merocyanine form of substituted 6-nitrobenzospiropyrans, /. Photochem. Photobiol. A Chem., 67, 319,1992. 

The opposite behavior is evident in the Norrish type II photochemistry of arylglyoxylates that Neckers and co-workers have studied (Scheme The 1,4-biradicaI intermediate resulting from triplet-state... 

Bernier, M., Kusama, Y, Borgers, M., VerDonck, L., Valdes-Aguilera, O., Neckers, D.C., and Hearse, D.J., Pharmacological studies of arrhythmias induced by rose bengal photoactivation. Free Radic. Biol. Med., 10, 287, 1991. 

---