Quantum photoaddition

Photoaddition reactions can destroy the original sensitizer, form a new sensitizer, or form a substance that quenches the desired reaction completely. Since substrate would also be consumed, its measured rate of disappearance will be anomalously high and that of product appearance too low. The importance of ensuring that the sensitizer acts as a true catalyst in a photo-reaction is demonstrated by a study88 in which 1,2-benzanthracene (13) was used to sensitize the dimerization of cyclohexadiene. The quantum yield of dimer formation was found to decrease at higher sensitizer concentration ... 

Micellar Catalysis. Non-polar molecules such as aromatic hydrocarbons are practically insoluble in water. In a micellar suspension they concentrate in the non-polar interior of the micelles where they can reach relatively high concentrations, even though their overall concentration may be very low. The quantum yields of bimolecular reactions like photoadditions are therefore greatly increased in micellar suspensions. 

Quantum yield. The relative quantum yield for the photoaddition of cyclohexenone is shown in Figure 5. It is given in terms of measured cyclohexenone concentrations by the relationship... 

Scheme 24 Quantum yields of intramolecular photoaddition products from various 5 -(4-trifluoro acety lpheny 1) -1-pentenes.

A kinetic scheme was proposed [122] with the fluorescent exciplex as precursor of the photoproducts (ortho as well as meta adducts). Quantum yields of adduct formation, exciplex emission, and benzene fluorescence were measured as a function of alkene concentration. The kinetic data fit the proposed reaction scheme. The authors have also attempted to prove the intermediacy of the exciplex in the photoaddition by adding a quencher to the system benzene + 2,2-dimethyl-... 

In fact, alkylated succinamides were isolated in some cases, though in very poor yields, and result from radical combination, which is a chain termination step. The experimental observations, i.e. the formation of (a) 1 1 adducts, (b) telomeric products, (c) alkylated succinamides, and (d) oxamide (when an olefin is absent), are consistent with a free radical mechanism. The telomeric products obtained support the assumption that we deal here with a chain reaction, because they are characteristic products of this type of reaction. Another proof for the chain reaction mechanism is the fact that when benzophenone is used as a photoinitiator (vide infra), the amount of benzpinacol formed is smaller than the amount of the 1 1 addition product of formamide and olefin (16). Quantum yield determinations will supply extra evidence for the validity of a chain reaction mechanism for this photoaddition reaction. 

The photoreactions of 3-ethoxyisoindolenone with 1,1-dimethoxyethene, tetra-methylethylene, and cts-2-butene in methylene chloride solvent were chosen for quantum yield studies50, s9 With 0.06 M 1,1-dimethoxyethene the quantum yield of cycloadduct formation is 0.72, and with 0.06 M tetramethylethylene the total quantum yield of adduct formation is 0.59. Over the concentration range 0.06—2.00 M, the quantum yields of product formation from addition to 1,1-dimethoxyethene and tetramethylethylene are inversely related to the olefin concentration. Plots of reciprocal of quantum yield of product formation vs. reciprocal of olefin concentration one non-linear however, plots of reciprocal of quantum yield vs. olefin concentration are linear with slopes and intercepts as shown in Table 5. For photoaddition of 3-ethoxyisoindolenone to tetramethylethylene and c/s-2-butene, the product ratios (56 57 58 and 59 60 61 62) are independent of olefin concentra-... 

The fact that plots of reciprocal of quantum yield of photoaddition vs. olefin concentration are linear with positive slope (Table 5) suggests that olefin quenches the singlet state and that all triplets formed are captured by olefin in the olefin concentration range examined in competition with unimolecular decay. Inefficiency... 

Using the temperature effect, it was proven that non-emitting exciplex intermediates were also involved in the cycloaddition. For example, in the reaction of naphthalene with diphenylacetylene, with increasing temperature both the quantum yields of photoaddition and the quenching of the naphthalene fluorescence by acetylene decrease by the same magnitude [65],... 

As has been mentioned earlier, it is often very difficult to distinguish between and identify the roles of exciplexes (and excimers) and biradicals in cycloaddition reactions. Caldwell and Creed (1978b) have studied the cycloaddition of dimethyl fumarate to phenanthrene and found that the quantum yield of the cyclobutane photoaddition product is increased in the presence of oxygen. It was suggested that oxygen enhances intersystem crossing in the triplet biradical formed between the two reactants. Nitroxide radicals have also been found to increase intersystem crossing (Sj -> Tj) in carbocyanines when nonpolar solvents are used (Kuzmin et al., 1978). When polar solvents are employed full electron transfer takes place. 

Table 3. Free energy change, AG°ct, and rate constants, of photoinduced electron transfer from group 14 organometallic electron donors to C, observed rate constants, obs, triplet quenching rate constants, kq, and limiting quantum yields, Oco, in the photoaddition of the donors to in benzonitrile at 298 K [212].

In contrast to these efficient intramolecular reactions most intermolecular photoreactions involving acyclic imines proceed with very low quantum yields. This is probably due to the rapid deactivation of the excited imine chromophore by C-N bond rotation. Cyclic imines, however, which lack this possibility, can undergo photoaddition to alkenes. 

The anthracenes (188), which have a 3,5-dialkoxybenzyloxymethyl substituent on the 9-position, undergo quantitative (4jt + 4ji) intramolecular photocycloaddition to yield (189). " The process is thermally reversible, and (189) is readily converted to the diketone (190) on treatment with acid. In the presence of acid, the linked naphthyl and resorcinyl moieties in (191) undergo (2ji -I- 2jt) photoaddition with 300 nm radiation to give the tetrahydrofuran derivatives (192) by the route outlined in Scheme 3." The reaction also occurs in the absence of acid for (191) with R = -0-(CH2)2-0Me, but the quantum efficiency is reduced by 35-fold. The products (192) are labile under 254 nm radiation and undergo a novel photoextrusion of acetaldehyde to yield (193) by the pathway... 

The quantum yield for photodimerization of thianaphthene 1,1-dioxide is enhanced in the presence of bromoethane.188 The increase is not due to solvent polarity it does, in fact, appear to be the result of enhanced intersystem crossing to the reactive triplet. Cycloaddition reactions of the thione group have also attracted attention, particularly those occurring in pyrimidine thiones. The isolation of thietans, previously proposed as intermediates in the photoaddition of electron-deficient alkenes to thiouracil derivatives, has now been realized 187 in this way, for example, the dihydrouracil (259) is converted into the adduct... 

There is little doubt that the addition of acrylonitrile and related compounds to a variety of substrates provides a useful synthetic path for the formation of many bicyclic compounds. Typical of this is the photoaddition of malononitrile or fumaronitrile to 3-methylcyclohex-2-enone which provides four adducts in a ratio of 29 15 21 35. The structure of the predominant isomer was determined by X-ray crystallography and was established to be the cyclobutane 279. The ratio of the products obtained from these experiments was shown to be independent of the dose of irradiation. Quantum yield... 

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... 

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... 

Example 3.6 Photoaddition with quenching of the triplet intermediate Taking into account the quenching of A" the following rates, quantum yields, and the concentration of quencher, which reduces the quantum yield to one half, are obtained ... 

In both the photoaddition reactions the quantum yield depends on the concentrations of the second reactant C as well as of the quencher B. 

Example 5.11 Photochemical quantum yields of a photoaddition reaction In Example 3.4 the photo-addition had been treated according to eq. (3.3). One finds for the quantum yield... 

The rates or efficiencies of bimolecular photoreactions such as excimer formation, photodimerization, and photoaddition can be strongly affected by using micellar solvents when the reactants associate with the micelles When low concentrations of reactants are solubilized in solutions of high concentrations of micelles, the reactants will be separated by association to different micelles and the reaction will be inhibited. Under opposite conditions high local concentrations will cause an increase of quantum yields compared to homogeneous solutions of equal analytical concentration. Thus in micellar solution the efficiency of bimolecular... 

These intramolecular addition reactions are remarkable in that they have no intermolecular counterpart. In fact, A/,W-dialky-lamides and tetraalkyl ureas fail to quench styrene fluorescence. However, photoaddition of some 1,1-diarylethylenes and tetra-methylurea has been reported. The intramolecular reactions are proposed to occur via weakly bound nonfluorescent singlet exciplex intermediates, which undergo a-C-H transfer to yield the biradical precursors of the observed products. A triplet mechanism was excluded based on the failure of sensitization by xanthone or quenching by 1,3-pentadiene. The involvement of charge transfer is consistent with the requirement of polar solvents for these reactions. The quantum yields for adduct formation from 19 and 25 are much higher than those of their p-methoxy derivatives, in which the styrene is a much weaker electron acceptor. ... 

After photoinduced electron-transfer from the singlet-excited phenan-threne ( Phen ) to / -dicyanobenzene (/ -DCB) in the presence of KCN or [Bu4N]CN a novel chain photoaddition of 2,5-dimethyl-2,4-hexadiene (159) with /7-DCB took place to yield (160). It was found that the quantum yields for the formation of adducts were remarkably enhanced at lower temperatures. Similar intermolecular photoadditions with active methylene compounds instead of CN were reported. ... 

Photoreactions of MA with 1,2-polybutadiene, 1,4-polybutadiene, poly(styrene-co-butadiene), poly(styrene-co-isoprene), polystyrene, and poly(styrene-co-methyl methacrylate) have been studied in air. " In homogeneous solutions, MA addition to the polymers proceeds efficiently by a chain mechanism, where the quantum yield of the photoaddition was greater than unity under irradiation at A >310 nm. From the effects of solvent and photosensitizers and spectroscopic data, a radical chain mechanism was proposed to account for addition and crosslinking of the polymers by MA molecules. The photoaddition reaction was applied to the surface of polymer films. The photoreactions were conducted at the interphase between solid polymer and acetone solution of anhydride and also at the interphase between solid polymer and gaseous anhydride. Irradiation with a 300-W high-pressure lamp brought about considerable surface modification, as shown by wettability and dyeability properties. 

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