Irradiation triplet sensitizer

In addition, there are many surface modification processes that use triplet sensitizers to permit oxidation reactions. In a typical process, polyisocyanate is applied on a polyolefin together with a sensitizer such as benzo-phenone and then irradiated with UV light. As shown in Eq. (15) the sensitizer has an oxidizing effect to produce hydroxyl groups over the polymer surface. These hydroxyl groups finally react with isocyanate to provide a functional polymer [56,57]. 

Direct irradiation or thermolysis of a diazo-compound (2) is believed to generate the carbene initially in its singlet spin state. Triplet sensitization (5) is presumed to give the triplet carbene directly without first forming its singlet state via the triplet diazo-compound. In some cases, careful comparison of the results of direct irradiation experiments with those from triplet sensitization can provide useful information to identify the spin state initiating a reaction. 

The triplet-sensitized irradiation of DABA in neat isopropyl alcohol does not give a detectable amount of the ether. For comparison, triplet sensitization of DAF or DAX (see below) gives the appropriate ethers in a high yield which is essentially unchanged from the direct irradiation. 

The significance of a value of AG 5.2 kcal mol -1 is that most of the bimolecular reactions of the triplet carbene will be faster than the formation of the singlet from the triplet. Thus intersystem crossing from BA to 3BA is irreversible, and if 3BA is formed first (triplet sensitization), then there will be no reaction at all of BA. However, in the direct irradiation of DABA, some... 

In contrast to the behavior of BA, triplet sensitized formation of XA in the presence of alcohol gives exactly the same result as does the direct irradiation ether is formed in very high yield. Thus, even when 3XA is formed first, the chemical behavior observed is that of XA. This finding is consistent with a carbene having a singlet ground state a view at least supported by the absence of an epr spectrum for the triplet carbene as noted above. 

The species identified as XA reacts with styrene to give the expected cyclopropane. The rate constant for this reaction is ca 200 times less than the corresponding rate constant for 3BA (Table 6). Also, use of the deuterium-labeled a-methylstyrene reveals that the cyclopropanation occurs with essentially total retention of stereochemistry. Moreover, precisely the same result is obtained when this carbene is formed by triplet sensitization rather than by direct irradiation. These findings also point to a reaction originating from a singlet carbene. 

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. 

Irradiation of DAAN in benzene gives 3AN. This carbene reacts with oxygen very rapidly to give an intermediate believed to be the carbonyl oxide. The triplet carbene reacts with labeled a-methylstyrene to give the cyclopropane with total loss of stereochemistry (Table 6). Direct irradiation in neat isopropyl alcohol gives the ether in low yield (relative to the yields from XA, DMFL, FL, and BFL). The other products are those expected to result from hydrogen-atom abstraction. Triplet-sensitized irradiation of DAAN in the alcohol does not give a detectable amount of the ether. 

The chemical behavior of PM is characteristic of a carbene for which equilibration of spin-states is faster than irreversible reaction. In particular, direct and triplet-sensitized irradiations of 2-n-butylphenyldiazomethane give exactly the same products (Baer and Gutsche, 1971). A similar conclusion was reached in the study of ANM (Hadel et al., 1983). 

Irradiation of vinylallene (2) in the presence of the triplet sensitizer benzophe-none in pentane at low temperatures with a medium-pressure mercury lamp induces [2 + 2] photoaddition, leading to the primary products 311-313 (Scheme 5.47) [127]. 

Less complex non-conjugated diene systems also lead to cubane-like derivatives as in the diene 175. Here the outcome of the reaction is dependent upon the excited state. Thus, direct irradiation brings about fragmentation with the formation of 1,4-difluorobenzene and excited-state naphthalene while triplet-sensitized irradiation follows a different path with the formation of the cage compound 17682. 

To confirm this hypothesis the photoreactivity of aldehyde 23, previously reported as unreactive in the ODPM mode [47], was reinvestigated. The results obtained show that wi-methoxyacetophenone-sensitized irradiation of 23 (2 hr) brings about the formation of the cyclopropyl aldehyde 11, resulting from an ODPM rearrangement, in 57% isolated yield. Further support for the above postulates was obtained from studies of triplet-sensitized irradiation of aldehydes 22, 35, and 36. These substances undergo the ODPM rearrangement to yield the corresponding cyclopropyl aldehydes 37 (82%), 38 (83%) and 39 (96%), respectively [51]. 

The possible competition between ODPM and DPM processes was also studied. Triplet-sensitized irradiation of 44 for 10 min affords the cyclopropyl aldehyde 45 (19%) resulting from an ODPM rearrangement, exclusively. However, under the same conditions, aldehyde 46 yields the cyclopropyl aldehyde 47 (48%), resulting from a DPM rearrangement [51]. 

In an attempt to establish the limits for ODPM reactivity of (B.y-unsaturated aldehydes, we have extended our studies to a series of aldehydes 65, (Scheme 10) which are monosubstituted at C-2. Triplet-sensitized irradiation of 65 leads to the formation of the corresponding cyclopropanecarbaldehydes 66 [59] (Scheme 10). The diphenyl-substituted aldehydes 65b and 65d yield, in addition to the ODPM products, the corresponding alkenes 67a and 67b, resulting from photodecarbonylation. The formation of these alkenes is probably due to stabilization of the radical, formed by allylic cleavage, by diphenyl conjugation. The ODPM rearrangement of aldehydes 65 is diastereoselective, yielding only one diastereoisomer of 66 (Scheme 10). 

The first compound selected for this study was the 2-aza-1, 4-diene 68 (Scheme 11). Triplet-sensitized irradiation of azadiene 68, using acetophenone, for 25 min affords two new products, identified as the cyclopropylimine 69 (11%)... 

The study has been extended to azadienes 82-85. The snbstitntion patterns present in these compounds has proved to be adequate to promote the ODPM rearrangement in [3,7-unsaturated aldehydes [4] and the 1-ADPM reaction in 1-aza-1, 4-dienes [4]. However, triplet-sensitized irradiation of compounds 82-85 yields complex mixtures of products that have not been identified. These results show that a clear difference exists between the DPM reactivity of 2-aza-1,4-dienes and the other DPM processes previously reported [65]. The differences are difficult to explain and further studies are underway to clarify this point. 

Direct or triplet-sensitized irradiations have been used in most of the studies on the photoreactivity of 1,4-unsaturated compounds. However, Zimmerman and Hoffacker have reported the novel cyclization of aryl-substituted 1,4-pentadienes... 

Recently, however, examples of intramolecular triplet sensitization have been described. Alkyl aryl ketones with pendant azido groups in the alkyl moiety were irradiated into the benzoyl chromophore. The excited alkylphenyl ketones undergo intersystem crossing to their triplet states within picoseconds and they can then transfer their triplet energy to the nearby alkyl azido group. This process... 

A mixture of 2-naphthoylazide and sensitizer 2-isopropylthioxanthone ( r = 65 kcal/mol) was irradiated (k > 385 nm), conditions under which the sensitizer absorbed >95% of the light. Laser flash photolysis experiments demonstrated that the triplet sensitizer is quenched by 2-naphthoylazide at a diffusion controlled rate. 

Direct irradiation of 130a-c and 130f led to modest yields of 136a-c and 136f. The yields of the amine could be reduced by the addition of the triplet quencher TMDD. Triplet sensitized photolysis led to increased yields of the amine, even in cases in which the reduction product was not found in direct irradiation experiments. " Triplet sensitized photolysis always led to some of the singlet products 132,133, and/or 134. ... 

Conjugated dienes take part readily in triplet-sensitized photodimerization. and the products obtained from buta-1.3-diene (2.691 include a (4 + 2) adduct as well as slereoisomeric (2 + 2) adducts. The reaction is non-concerted. and a rationalization for the products is provided on the basisof the formation of a biradical intermediate as shown (which is the most stable of the three possible biradicals that might be formed in the first step), by the attack of triplet diene on ground-state diene. Cross-addition takes place in some systems, such as myrcene (2.70) where a triplet diene group attacks the alkene within the same molecule direct irradiation of myrcene gives mainly... 

For related reasons, and because their excited-state energies are lower than for dialkyl ketones, diaryl ketones and simple alkyl aryl ketones do not fragment on irradiation in solution, even at higher temperatures. This leads to a photostability that is one factor contributing to the successful employment of ketones such as benzophe-none tPh-CO) or acetophenone (PhCOMe) as triplet sensitizers. a-Cleavage for ketones in solution at room temperature is promoted if structural factors cause the bond adjacent to the carbonyl group to be somewhat weaker than normal. Hence t-alkyl ketones give decar-borylation products readily (4.5), as do benzyl ketones (4.6 and benzoin derivatives (4.7). 

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