Benzophenone sensitizer

The Diels-Alder reaction of isopyrazoles 365 with MTAD gives azoalkanes 366. Direct as well as triplet-sensitized (benzophenone) photolysis of these compounds leads to the corresponding housanes (bicyclo[2.1.0]pentanes) 367. Under acidic conditions, the housanes rearrange to the corresponding bicyclic products 368 <1995JOC308,... 

Schemer s observations (1964) on spin inversion and bond rotation in nitrogen-containing diradicals lends support to Skell s view. Direct photolysis of the triazolines 42 and 43 gives the related aziridines, 44 and 45 with predominant retention of the geometric arrangement of the methyl and adjacent phenyl substituents. Photolysis in the presence of a triplet sensitizer, benzophenone, results in a product distribution showing much lower stereoselectivity. The inference is that...

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

Among the investigated compounds, diphenyl sulphoxide, the structural analogue of classical sensitizer - benzophenone - was found to be the most effective. 

Reaction of methyl coumalate 327a (Scheme 65) with an electron-poor dienophile such as fumaronitrile at high temperature (120°C, no solvent) gives the [4 -I- 2]-adduct 332 in 28% yield (89NKK1765). However, the reaction of 327a with acrylonitrile takes place only under irradiation in the presence of a sensitizer (benzophenone) to give two types of [4 + 2]-cycloadducts 333 endo 17% exo 12%) along with [2 -1- 2]-cycloadduct 334 (9%). 

Quenchers (naphthyl moiety see 212, and 213) and sensitizers (benzophenone moiety, see 214) of the SP triplet state exerted similar effects when linked to the SP molecule. The half-life cycles of compounds 211-214 were more than doubled (Figure 17). 

Photosensitized crosslinking of polymers has been the subject of numerous publications [l - 30], concerned mainly with poly(ethylene), poly(vinyl alcohol), various vinyl copolymers, copolymers of maleic anhydride and/or phtalic anhydride with styrene and some polymers derivated from cinnamic acid. The following compounds were used as sensitizers benzophenone, 4-chloro- and 4,4-dimethylbenzophenone [l, 3-6, 8, 9l, oC -and -derivatives of anthraquinone [3, 23] acetophenone, hydroquinone, triphenylmethane and pyridine li.] chlorobenzene and no less than trichlorinated n-paraffins [6], a complex of zink chloride with o-dia-nizidine fill potassium bichromate [l2j, anthracene fl3, 14] 2,5-methoxy-4-amino-trans-stilbene [l5], benzyl ideneacetophenone fl6-l8] -thiophenylacetophenone,... 

The concept of intramolecular alkylation of AT-substituted amino acid derivatives via 1,5-diradicals also turned out to be an excellent system for studying the different stereochemical course of spinisomers as discussed in Section 6.2.2. Thus, the a-ketoester 9, which contains an alanine moiety, was prepared. In contrast to aryl ketones, a-ketoesters are not completely converted into the triplet state after photochemical excitation. Upon addition of either a triplet quencher (naphthalene) or a triplet sensitizer (benzophenone), each of the two spin states may be forced (Scheme 3, Table 1). The chiral center at the d-position with respect to the keto carbonyl group raises the question whether a memory effect of chirality may be observed during the cyclization. The results summarized in Table 1 amply demonstrate the specific properties of spinisomeric biradicals. In the presence of naphthalene, which probably acts not only as a triplet quencher but also as a singlet sensitizer, the chiral information of the reactant 9 is almost entirely conserved in the helical diradical 10 because of its very short lifetime. In contrast, the addition of benzophenone results in almost complete racemization, and also the cis/trans selectivity is... 

An attempt to prepare triplet benzyne by the triplet-sensitized (benzophenone) decomposition of the peroxide (254) has shown that a singlet benzyne is produced.150 This conclusion was reached even though a kinetic study showed that the decomposition occurred from the triplet state of the peroxide. The two products (255), obtained in the ratio 82 18 by trapping the benzyne with trans-cyclo-octene, could be produced in identical yield using benzyne formed by a variety of paths. The absence of other products which might have arisen from a biradical such as (256) is thought to be conclusive evidence for the absence of triplet benzyne in these experiments. 

More informative is a process described by Roth (1972). The direct and the sensitized (benzophenone) photolysis of diazomethane in toluene yields ethylbenzene, but a CIDNP effect can only be observed from the sensitized reaction. This result leads to the conclusion that the direct photolysis is a concerted CH insertion of a highly reactive species, whereas the sensitized reaction takes place by a hydrogen abstraction, followed by a radical addition (8-24). It is, therefore, likely that it is the singlet that reacts in the first reaction, but the triplet in the second process. 

UV absorber, acrylic adhesives pressure-sensitive Benzophenone-1 UV absorber, acryiic sheets Benzophenone-1 UV absorber, adhesives Drometrizoie UV absorber, alkyds... 

The ratio of the products, divinyl cyclobutanes to cyclohexene depends on the triplet energy (Et) of the sensitizer used. With sensitizer Et > 60 kcal/mol, both s-trans and s-cis are excited having a preference to s-trans. With sensitizer Et 55 kcal/mol, the s-cis is preferentially excited. Excited s-trans gives only cyclobutanes, while the excited s-cis conformer gives both cyclobutane and cyclohexene. For instance, sensitizer benzophenone of Et 68.5 kcal/mol gives divinylcy-clobutanes and cyclohexene in a ratio of 92 8, whereas sensitizer benzil of Et 53.7 kcal/mol, gives divinylcyclobutanes and cyclohexene in a ratio of 55 45 [22]. 

Incorporation of a triplet sensitizer benzophenone into the macrocycle 33c has little influence on the self-assembly nor the dimension of the resulting nanotube, but the reaction environment of the cavity is totally changed. Photodimerization of the above enones could not happen within 33c-tube. Instead, the nanotube was able to act as a triplet sensitizer for the cis-trans photoisomerization of trans- >-mQ hy -styrene and selective oxidation of 2-methyl-2-butene to a primary allylic alcohol (Scheme 3.14) [70, 109]. As well, an enlarged nanotube 33d-tube was shown to facilitate the selective photodimerization of coumarin to the awr/-head-to-head product [71]. 

---