Triplet sensitized reactions

Upon low conversion direct photolysis the cis isomer (10) gave only the cis isomer (12) and the trans isomer (11) gave only the trans isomer (13). The triplet sensitized reaction of (10) and (11) gave rise only to cis-trans isomerization. Thus the di-ir-methane photorearrangement from the triplet state cannot compete with triplet state deactivation via cis-trans isomerization (Zimmerman has termed this the free rotor effect). Several other examples of regio-specilicity and stereospecificity in di-w-methane photoreactions are as followsa8 a3) ... 

Direct irradiation of BDAF in benzene or acetonitrile containing methyl alcohol gives the expected ether in high yield. The triplet-sensitized reactions give the same result. Similarly, BFL reacts with styrene and with a-methyl-styrene to give cyclopropanes. In the latter case, 80% retention of configuration is observed for both the direct and triplet-sensitized irradiations. 

I. Anthracene and Its Derivatives. Liu234 has convincingly demonstrated that several reactions sensitized by anthracene and its derivatives proceed through the second excited triplet state, T2. The first case studied was the photosensitized rearrangement of 54 to yield the products shown below. A break in the efficiency of triplet sensitized reaction was found between... 

These rules also predict the nature of photoproducts expected in a metal-sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation. 

It has been assumed so far that the sensitizer acts by an energy-transfer mechanism, but in some cases other modes of interaction may occur. It is possible that electron transfer takes place to give the radical anion or the radical cation of the alkene, which is the species that subsequently isomerizes. This is likely to be the case in the chlorophyll-sensitized isomerization of vitamin A acetate, which is used commercially to obtain the required all-trans isomer 12.8) from the mixture of Isomers resulting from the synthesis. Unlike triplet-sensitized reactions, electron-transfer isomerizations frequently lead to a predominance of the most thermodynamically stable isomer. 

Stern-Volmer constant from product quantum yield data and limiting quantum yield for triplet sensitized reactions. 

The observation of triplet sensitized cycloaddition reactions of 10 but not t-1 with vinyl ethers reflects the requirement of a planar stilbene excited state for effective interaction with ground state electron-rich or electron-poor alkenes. While triplet sensitized reactions of other cyclic stilbene analogues (e.g., 5-9) have not been reported, it appears quite likely that they should occur. 

In this case, only a thermal pathway for the formation of the cyclooctatetraene is proposed, in agreement with many other publications (for references, see Sec. II and Table 3). Tinnemans and Neckers [62], however, describe the ring opening of the ortho adduct from methyl phenylpropiolate and benzene as well as the reverse reaction as photochemical processes. The formation of this ortho adduct can also be accomplished in a xanthone-sensitized photoreaction [63], and in that case, the authors consider the ring opening as a thermal or a triplet-sensitized reaction and the reverse reaction as one proceeding via the singlet. 

On the basis of triplet sensitization experiments, this rearrangement was shown to occur from the excited singlet state and to involve the biradical intermediate 111. Interestingly, vinyl cyclopropane esters 113-114 underwent geometric isomerization at a much faster rate than rearrangement on direct excitation (Scheme 8.34) [57, 58]. However, the triplet-sensitized reaction of these esters give both cyclopentene and isomerization products. 

Lamps and filters. The combination of radiation source and filter accounts for the actual radiation wavelength to which the reaction mixture is exposed. An unavoidable filter is the glass type of the reaction vessel. In the direct irradiation of substrates not containing functionality other than the phenyl groups, wavelengths in the 280 nm region are needed. Typically a Hanovia 450 W medium-pressure mercury lamp and a 2 mm Corex cylindrical filter (X > 270 nm) are employed. Triplet-sensitized reactions need a higher cut-off, which can be obtained by the use of the same lamp with a 2 mm Pyrex filter (X > 300 nm). The Rayonet photoreactor can be equipped with 254, 300 and 350 nm lamps. 

The triplet-sensitized reactions of cyclopropenes have been widely studied and show great variety depending on substituents. They have been reviewed. The photochemical halogena-tion of cyclopropenes to form 1,2-dihalocyclopropanes has been discussed in Section 1.1.6.1.4. along with some thermally induced counterparts. 

It is particularly informative to compare the results of direct and triplet-sensitized irradiation of DCDAF in acetonitrile containing both methyl alcohol and a-methylstyrene. As the reactions shown in Scheme 5 clearly demonstrate, both ether and cyclopropane products are to be expected. The relative yields of these products, however, could depend on whether the reaction is direct or sensitized, and on the rate of reaction with methyl alcohol and a-methylstyrene in comparison with intersystem crossing (/ sT and Ts)- ratio of ether to cyclopropane for the triplet-sensitized reaction of DCDAF should vary according to (29). This prediction is verified by experiments where the alcohol concentration is held constant and the a-methylstyrene concentration is varied. Combination of this result with the rate constant for the reaction of the triplet carbene with a-methylstyrene and assuming that DCFL reacts with methyl alcohol at approximately the diffusion-limited rate (cf. XA and DMFL) gives X, = KMX) which corresponds to ACs = 4 kcal mol". ... 

Loyola University of Chicago), found that Hammond s estimate is accurate, and the fluorescent lifetime of 2 and 40 ate 180 ps. Triplet-sensitized reactions of 2-pyridones have not been extensively studied. However Sharp and Hammond found no evidence for 2-pyri-done photoproducts when xanthone is used as a photosensitizer. Using tethered pyridones, Nakamura found [2+2] and [4+2] cycloaddition products with benzophenone sensitization (see Figure 14). This is consistent with 2-pyrone photochemistry,but the generality of these results await further experimentation. 

The formation of two of the four possible oxa-di-7c-methane products on sensitized irradiation of (+)-3-ethyl-3-methyl-5-phenylpent-4-en-2-one has been interpreted in terms of a stepwise, rather than a concerted, mechanism.Evidence for a stepwise pathway in the oxa-di-7i-methane rearrangement is also available from investigations on ( —)-(378). Triplet-sensitized reaction of (—)-(378) gave ( —)-(lS,9S)-(379) and (—)-(lS,9R)-(380) enantiomeric purities were 42% for recovered (378) and 48% each for (379) and (380). The essentially full retention of enantiomeric purity during the rearrangement and the formation of endo- and exo-products with identical C-1 configuration eliminate both a-cleavage and concerted mechanisms. 

While direct irradiation of , E-2,4-hexadiene 5 gives only , Z-2,4-hexadiene from singlet excited state, triplet-sensitized reaction gives both E,Z- and Z,Z-2,4-hexadienes. The singlet state reaction proceeds with just one terminal double bond rotation involving allyUc methylene or cyclopropane methylene diradical with just one double bond rotation, whereas the triplet excited state reaction proceeds with double double bond rotation [11]. 

The photochemical reactions of allenes via an electron-transfer pathway have also been reported. The triplet-sensitized reaction of tetraphenylallene 16 in the presence of xanthone was reported by Klett and Johnson to afford only vinyl ether 43. They also reported that the photochemical reaction (X, >330 nm) of triphenylallene 20 in methanol in the presence of 1,4-dicyanonaphthalene (1,4-DCN) yields vinyl ethers 44-46 via an electron-transfer pathway, together with aUyl ether 47 and alkyne 33. In this reaction, the radical cation of allene 48 is assumed to be the key intermediate. 

The photochemistry of carbonyl compounds has been one of the main areas of research in organic photochemistry for many years. Among aU the different types of carbonyl compounds, P,y-unsaturated ketones have been the subject of extensive studies. The results obtained from these efforts, conducted over a 30-year period, show that, in general, direct irradiation of P,y-unsaturated ketones yields products resulting from 1,3-acyl migration, while triplet-sensitized reactions of these compounds affords cyclopropyl ketones by oxa-di-n-methane (ODPM) rearrangement pathways. Alternative reaction routes, such as decarbonylation, ketene formation, epimerization, [2+2]-intramolecular cycloadditions, Norrish Type I and Norrish Type II reactions, cis-trans isomerizations, and reductions of the C-C double bond, have also been described in some instances, depending on some particular structural features present in the P,y-unsaturated ketone. However, the photoreactivity of these compounds is dominated by the two main processes mentioned above. 

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