Photo-induced addition

The photo-induced addition reaction of 1,4-dibromopiperazine-2,5-dione with several alkenes has been studied (80BCJ219 83BCJ1705 86BCJ479). In acetonitrile, the reaction with cyclohexene occurs by a radical chain mechanism as shown in Scheme 15 (in a sequence of reactions). 

This methodology was extended to the addition of functionalized carbon-based radicals generated by photo-irradiation of alcohols in the presence of benzophe-none [58], The general reaction mechanism is shown in Eq. (13.46). High diaste-reoselectivities of >98 2 and yields of 97-99% are obtained. This photo-induced addition of hydroxyalkyl radicals also proceeded with high selectivity and yield with acyclic substrates, contrary to what was observed with simple alkyl radicals. 

Block s building block 22 is also amenable to prepare eneynes. Photo-induced addition of 22 to alkyne 93 gave the adduct 94, which was converted to eneyne 95 with the aid of potassium f-butoxide.23 The same sequence was applied to terminal alkyne 96 to afford eneyne 97. 

A novel route to secodine derivatives (Scheme 29) involves the photo-induced addition of indoline-2-thiones, e.g. (189), to methyl acrylate. The product... 

A new and elegant solvent-free approach to main chain LCEs was found by Yang et al. [39] who, based on the work on linear LC polymers by Lub et al. [40 2], made use of the photo-induced addition of thiols and olefins (click-chemistry) to synthesize nematic polymer networks. Starting from a mixture of the mesogen, a tetrafunctional crosslinker and a photo-initiator networks with a around 170 °C were obtained by UV crossUnking (Scheme 8). 

Photo-induced addition reaction of aromatic carbonyl compounds with alkynes gives unstable oxetanes. For example, the reaction of benzaldehyde with 2-butyne 138 gives a,(3-unsaturated carbonyl compound 139 by cleavage of intermediate oxetane [80]. 

Photo-induced additions of ferrocene to electron-deficient alkynes, giving 1-ferrocenylalkenes, photo-ethoxycarbonylation of ferrocene derivatives ( v-CCU-EtOH), and electrochemically-initiated additions of alkyl radicals (from RCOg ) to ferricenium cation have been reported. Electrochemical reductive cleavage of the ring-metal bonds in carbonyl-substituted ferrocenes has been achieved using polarographic methods. ... 

Noyori, R. and Kato, M., Photo-induced addition of protic solvents to cycloalkenones. Evidence for the ground-state trans isomers as chemically reactive intermediates. Bull. Chem. Soc. Jpn., 47, 1460-1466, 1974. 

Fig. 28 Raman spectra of polymeric sulfur (S ) prepared by various methods [109,173], of large disordered rings (S ) [182], and of photo-induced amorphous sulfur (a-S) [119], respectively. The spectrum of a-S has been smoothed for clarity. The position of the stretching vibration of a-S is pressure-shifted to higher wavenumbers. The very weak signals in the spectra of Sj, at ca. 150 and 220 cm are probably caused by the presence of Sg. In addition, the weak shoulder at ca. 470 cm observed in spectra of Sj, may originate from Sg, too...

Properly substituted phosphonates, which serve as key intermediates for the synthesis of biologically active compounds [39,54], have been prepared via the photo-induced 1,4-addition of phosphites onto Michael ketones (Scheme 19). 

There is always interest in the photochemistry of the pyrimidine nucleic acid bases and related simple pyrimidinones, due to its importance in genetic mutation. In addition to damaging DNA, photo-induced reactions may also repair the damage, as in the reduction, by FADH, of the thymine glycol 64 back to thymine <06JACS10934>. Another report related to repair of DNA involved a model study, by means of the linked dimer 65, of the involvement of tryptophan in the electron-transfer leading to reversion of thymine oxetane adducts <06OBC291>. 

Photo-induced conversions are also important synthetically. The photocycloaddition of 5-substituted uracils 69 with ethylene has been used for the synthesis of 2-aminocyclobutanecarboxylic acids. The addition reaction worked well with carbon and fluorine substituents and also with a Cbz-protected amino. However, uracils with other 5-nitrogen substituents (NH2, NHBn and NO2) failed, only starting material being recovered. The sequence also worked for the 6-isomers but somewhat less consistently <06SL1394>. 

Dual Photo/Thermal Initiation Studies. A series of studies were performed using reactive formulations containing both a photoinitiator and a thermal initiator dissolved in the Derakane resin. The objective of these studies was to investigate a dual cure strategy in which the heat liberated by the photo-induced polymerization leads to the production of additional active centers by the dissociation of a thermal initiator. In this way, the dual cure strategy could offer both the temporal control of the start of the reaction afforded by the photopolymerization, as well as enhanced reaction rate and completeness of cure provided by the thermal initiation. 

Monte Carlo calculations have been carried out to simulate the spin transition behaviour in both mono- and dinuclear systems [197]. The stepwise transition in [Fe(2-pic)3]Cl2-EtOH as well as its modification by metal dilution and application of pressure have been similarly modelled by considering short- and long-range interactions [52, 198, 199]. An additional study of the effect of metal dilution was successfully simulated with the Monte Carlo treatment considering direct and indirect inter-molecular interactions [200]. A very recent report deals with the application of the Monte Carlo method to mimic short- and long-range interactions in cooperative photo-induced LS—>HS conversion phenomena in two- and three-dimensional systems [201],... 

The light-induced addition of water to the 9,10-double bond of bromocriptine yielding the so-called lumi-products, is of high probability (12,18). They have, however, not yet been isolated or characterized. The corresponding lOa-methoxy-lumi-derivative could be prepared by the photo-catalyzed addition of methanol (19) under slightly acidic conditions. 

The reactivity and jr-facial selectivity of CH3OH and H2O additions to the radical cations of 7-benzhydrylidenenorbornene derivatives (68), generated by photo-induced... 

The stereoselectivity of anti-Markovnikov adducts (161) and (162) produced through photo-induced electron-transfer reaction of (160) with MeOH in MeCN depends on the optimum structures and stabilities of the corresponding radical and carbanion intermediates (163) and (164). In PhH, steric hindrance in an exciplex, comprising an excited singlet sensitizer and (160), forced cis addition of MeOH to (160) to give trans-isomer (161) as the major addition product. 

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