Photoadditions

Some other simple uniform mechanisms are the basis of photochemical reactions. These are photoadditions, the photoreduction and some sensitised photoreactions which are discussed in the following. 

As in the case of photoisomerisations, the reaction can proceed either via the singlet or the triplet intermediate. The component B is part of the reaction in contrast to the sensitisation mechanism discussed above. In addition both pathways via the singlet or triplet state can be quenched. The photophysical steps are summarised in Table 3.2 and the reaction scheme is derived in Appendix 6.6.1.1. In the following examples this information is used to derive the time laws and the quantum yields for 4 types of photoaddition reactions according to either the normal addition reaction 

The photoaddition via the singlet state results by application of the Bodenstein hypothesis to the photophysical intermediates in the rate law 

In this photoaddition the quantum yield depends on the concentration of the reactant B. Therefore this quantum yield can be used to estimate the minimal concentration which yields a reasonable turnover at optimal conditions. The dependence of the quantum yield on the concentration of compound B turns out to be 

In consequence the concentration of B requires an order of magnitude of 10-2 rnol 1 to yield a reasonable turnover using the assumptions for the rate constants as above. Accordingly the quantum yield is determined to be 

In contrast crystalline (101), with less bulky substituents and with the relative orientation of double bonds practically identical to that in (99), is completely unreactive. For (99) there is a buffer zone i.e. a disordered packing region), which maintains the crystal structure in the monomer state but permits sufficient conformational change of the piperidine rings to occur to avoid steric hindrance as the reaction centres approach at the initiation of dimerisation. For (101), however, there is no such buffer zone available to permit the conformational changes and molecular movements necessary for dimerisation, in addition to the geometrical requirements previously identified by Schmidt. The buffer zone concept has been used to rationalise the different photodimerisation behaviour of 

Diastereomer-differentiating intramolecular [2+2] photocycloaddition has been reported for the diastereomeric 2,6-dienones (105) and (106). Both direct and sensitised irradiation of (105) yields (107) whereas (108) is formed from (106). Reinvestigation, at -78 °C, of the photocycloadditions of 1-cyanonaphthalene (1-CN) and 2-cyanonaphthalene (2-CN) with 1,3-cyclohexadiene has suggested that the primary major singlet-derived products are the corresponding exo-[4+4]adducts, (109) and (110) respectively, and the s /i-[2+2]adducts, (111) and (112) respectively. Consideration of primary and secondary orbital interactions has been used to interpret these observations.  

Allyltrimethylsilanes react with triplet excited 2,3-dicyano-5,6-dimethylpyra-zine to give diazatricyclooctenes (115), possibly via intermediate (114) formed from ring-opening of an initially-formed [2+2]adduct. Intramolecular [2+2]-cycloaddition yields the pentacyclic compound (116) on irradiation of the corresponding tethered enantiopure bis-2,3-dihydro-4-pyridone. Addition of ethylene to the corresponding a,P-unsaturated-y-lactams in acetone yields (117) and (118) as the major photoproducts.  

Direct irradiation of N-acyl-lH-pyrrol-2(5H)-ones (119) results predominantly in cleavage of the exocyclic nitrogen-carbon bond and disproportionation to the 

Irradiation of imides (125)-(129) in solution gives oxetanes (130) in high yield. In the crystalline state the outcome is controlled by the conformations adopted about the imide unit in the crystal lattice. Compound (125) crystallises in a chiral space group and forms the oxetane in high enantiomeric excess. In addition to solid state oxetane formation, (126) and (127) also yield (131) and (132) 

Photoaddition reactions occur at the double bond of unsaturated sugars by  

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Many [2 + 2] photocycloadditions have not been assigned a definitive mechanism, but they serve well as synthetic methods. Thiones add vinyl ethers to give thietanes in very good yields (Section 5.14.4.1.2), and interesting wavelength-stereochemistry relations were found in the photoaddition of 2-adamantanone to dicyanoethylene (Section 5.14.4.1.2). Diheterocyclobutanes can also be prepared by [2 + 2] photocycloadditions (Section 5.13.3.3). 

There are few examples of formation of simple azetidines from [2 + 2] fragments, although this type of approach is important for certain azetidine derivatives, e.g. azetidin-2-ones (see Section 5.09.3). Nitrogen analogues of the Paterno-Buchi route to oxetanes are rare an example involves the photoaddition of 3-ethoxyisoindolone (37) to the enol ether (38) (75JA7288, 72CC1144). 

Azepine, 3,5-bis(t-butyl)-l-ethoxycarbonyl-photoaddition reactions, with oxygen, 7, 523... 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

Paal-Knorr synthesis, 4, 118, 329 Pariser-Parr-Pople approach, 4, 157 PE spectroscopy, 4, 24, 188-189 photoaddition reactions with aliphatic aldehydes and ketones, 4, 232 photochemical reactions, 4, 67, 201-205 with aliphatic carbonyl compounds, 4, 268 with dimethyl acetylenedicarboxylate, 4, 268 Piloty synthesis, 4, 345 Piloty-Robinson synthesis, 4, 110-111 polymers, 273-274, 295, 301, 302 applications, 4, 376 polymethylation, 4, 224 N-protected, 4, 238 palladation, 4, 83 protonation, 4, 46, 47, 206 pyridazine synthesis from, 3, 52 pyridine complexes NMR, 4, 165... 

Photoaddition of acetylene, 349 Photoaddition of allene, 349 Photoaddition of dichloroethylene, 349 Photoaddition of ethylene, 348 Photoaddition of hexafluoroacetone, 345 Photoaddition of maleic anhydride, 348 Photochemical addition of ethylenes and acetylenes to steroidal enones and dienones, 343... 

The Methylene Blue sensitized photoaddition of singlet oxygen to ethyl l//-azepine-l-car-boxylate was reported originally to yield only a C2-C5 adduct 269 however, a reinvestigation with the methyl ester, and using tetraphenylporphine as a sensitizer, revealed that in addition to adduct 46 the [6 + 2] cycloadduct 47 is also produced.270... 

The methanol adduct may arise by photoprotonation of the double bond. A similar photoaddition of methanol has been reported on photolysis of 4973. In a closely related study, King and coworkers74 have shown that photolysis in methanol of the monocyclic unsaturated sulfone (50) gives the methanol adduct (51) and the acyclic sulfonate, CH2= C(Ph)CH—CHCH2S020Me, in 43 and 30% yield, respectively. Formation of the sulfonate ester in this case strongly supports the intermediacy of the sulfene (52) produced on cycloreversion. 

Recent synthetic applications of the photochemical [2 + 2] cycloaddition of unsaturated sulfones have been noted. Musser and Fuchs84 have effected an intramolecular [2 + 2] addition of a 6-membered ring vinyl sulfone and a five-membered ring vinylogous ester in excellent yield, as part of a synthetic approach to the synthesis of the mould metabolite, cytochalasin C. The stereospecificity of the addition was only moderate, however, and later problems with this synthetic approach led to its abandonment. Williams and coworkers85 have used the facile [2 + 2] photoaddition of 73 and... 

Diels-Alder Reaction Facilitated by Physical and Chemical Methods 1,4-Photoaddition of -morphoiinoacryionitriie to 1-acyinaphthaienes [105]... 

A [2 + 2] photoaddition-cycloreversion was applied to the enantioselective synthesis of the natural product byssocMamic add (Figure 6.11). Desymmetrization of a meso-cyclopentene dimethyl ester with PLE in pH 7 buffer-acetone (5 1) provided a monoacid, one of the photopartners. It is noteworthy that both enantiomers of this natural product were synthesized from the same monoacid [58]. 

The photoaddition reaction of 2-phenylbenzimidazole with Michael acceptors was investigated <96JHC1031> as was the preparation and cycloaddition-reactivity of benzimidazole-2-carbonitrile oxide <96AJC199>. The nitration of 1-methylbenzimidazole was found to give only the 5- and 6-nitrated products as a mixture of isomers in 87% yield... 

The photoadditions proceed through 1,4-diradical intermediates. Trapping experiments with hydrogen atom donors indicate that the initial bond formation can take place at either the a- or (3-carbon of the enone. The excited enone has its highest nucleophilic character at the (3-carbon. The initial bond formation occurs at the (3-carbon for electron-poor alkenes but at the a-carbon for electron-rich alkenes.191 Selectivity is low for alkenes without strong donor or acceptor substituents.192 The final product ratio also reflects the rate and efficiency of ring closure relative to fragmentation of the biradical.193... 

Flegel, M. Lukeman, M. Huck, L. Wan, P. Photoaddition of water and alcohols to the anthracene moiety of 9-(2 -hydroxyphenyl)anthracene via formal excited state intramolecular proton transfer. J. Am. Chem. Soc. 2004, 126, 7890-7897. 

Photolysis reactions often are associated with oxidation because the latter category of reactions frequently can be initiated by light. The photooxidation of phenothiazines with the formation of N- and S-oxides is typical. But photolysis reactions are not restricted to oxidation. In the case of sodium nitroprusside, it is believed that degradation results from loss of the nitro-ligand from the molecule, followed by electronic rearrangement and hydration. Photo-induced reactions are common in steroids [36] an example is the formation of 2-benzoylcholestan-3-one following irradiation of cholest-2-en-3-ol benzoate. Photoadditions of water and of alcohols to the electronically excited state of steroids have also been observed [37],... 

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