Photoextrusion Reactions

Use of the insertion of an arylnitrene into a C—H bond has been made of in the synthesis of 9H-pyrimido[4,5-6]indole (2.27) 244). 

Azidoformates have been used as starting material for the synthesis of 1,3-oxazine derivatives (2.29)246). 

Vinyl azides decompose to 3H-azirines. This reaction has been reviewed recently 247). 

The initial step in a photochemical extrusion reaction is again cleavage of a bond to give a pair of radicals or a diradical, but now a second bond is broken immediately after the first one, thus leading to the extrusion of a small molecule as CO, C02, N2 or S02. As in a recent review on photoextrusion of small molecules 248) only such reactions will be outlined wherein the small molecule that is to be expelled is initially part of a ring, and where the product formed is again a cyclic system. 

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Three types of photoextrusion reactions have been identified in the irradiation of aryl-substituted 1,3,2-dioxathiolane 2-oxides [5 1-1-2 4-2] cycloelimination to produce a carbonyl compound, a carbene and sulfur dioxide extrusion of sulfur dioxide accompanied by a pinacol-like rearrangement to yield an aldehyde or ketone and extrusion of sulfur trioxide to give an alkene <72JOC2589>. Sensitization and quenching experiments indicate that a singlet state is responsible for the cycloelimination reaction, whereas the rearrangement and sulfur trioxide extrusion reactions arise from a triplet state <82JCR(S)175>. 

Examples are known of photoextrusion reactions which occur with ring contraction and provide bicyclo[3.3.0]octane derivatives. 1,2-Quinone diazides (e. g., 225)189 190) and bicyclo[3.3.1]nonan-9-ones (e. g., 126)191,192) are particularly... 

Givens, R. S., Matuszewski, B., Photoextrusion Reactions Comparative Mechanistic Study of S02 Photoextrusion, Tetrahedron Lett. 1978, 861 864. 

The general synthetic methods for [2.2] CPs have been already estabhshed. Most of them employ the ring contraction reactions of the 2,11-diheteroatom-substituted [3.3]CPs. Photochemical ehmination of sulfur or selenium atoms from [3.3] CP-2,11-disulfide or diselenide and flash vacuum pyrolysis of sulfur dioxide obtained by the oxidation of the corresponding sulfides are the most general methods. 2,11-Diaza[3.3]CPs are converted to the corresponding [2.2]CPs via their nitroso derivatives by reductive elimination of nitrogen. " Photoextrusion reactions of COj from cychc diesters can also be appHed to the synthesis of [2.2]paracyclophanes (PCPs)"- and [2.2]heterophanes, as described below. "... 

The photoextrusion of sulphur dioxide to form cyclophanes or other novel aromatic molecules has been reviewed and studied by Givens208-210, while the photodecomposition of aromatic sulphones to form products of radical coupling reactions has recently also received attention211. 

With the introduction of ring contraction reactions for the preparation of [2.2] — phanes such as the sulfone pyrolysis [71] or the photoextrusion of sulfur in thiophilic solvents [72], the synthesis of thia[3 ]cyclophanes gained in significance. 

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... 

Photoextrusion of small stable molecules is a well-known phenomenon, with SO2 being one of the standard leaving groups for that type of reaction. Photochemical extrusion of SO is also known, though less conunon, and is reviewed here. Loss of SO appears to be an uncommon process of sulfinyl radicals, at least near room temperature. This was foreshadowed by the earliest photolysis of dibenzyl sulfoxide vide supra), in which sulfenate-derived products clearly dominate any loss of SO from the benzylsulfinyl radical, in marked contrast to the ketone case. The loss of SO from CH3SO- is estimated to be endothermic by 50 kcal/mol [62,63], so it is clear that near simultaneous formation of a stable structure in the carbon portion of the molecule is a critical component in the design of extrusion reactions. 

Very recently, a promising new reaction has been discovered, alkane borylation, illustrated by Eq. 2.41 [123]. This can be driven by light, or, being exothermic, it can be carried out as a thermal process. Photoextrusion of CO from the tungsten carbonyl precursor is believed to be followed by oxidative addition of the alkane CH bond, followed by reductive elimination of the R-BR2product (Eq. 2.42) [123a]. 

Graham and coworkers" " looked at the related dicarbonyl analogue, Cp Ir(CO)2, in which photoextrusion of CO leads to very similar chemistry. Methane at 8 atm in a perfluorohexane solution also reacts to give the methyl hydride", a reaction which proceeds even at 12 K, illustrating the low activation energy for the C—H oxidative addition. The derivatization of the alkyl hydride with CCI4 to give the alkyl chloro complex was particularly useful because the latter is much more kinetically stable than the former. 

Photochemical alkane carbonylation with RhCl(CO)(PMe3)2 is also possible. This seems to operate by initial photoextrusion of CO from the catalyst, oxidative addition of the alkane C—H bond, addition of CO to the metal, followed by insertion, and then reductive elimination as shown in Figure 3. Preferential reaction at the 1 ° or 2° C—H bond is found. Here the initial product does not seem to isomerize, but Norrish type II photoreactions tend to degrade the aldehyde product. Moving to longer wavelengths minimizes the Norrish degradation problem, but the selectivity of the catalytic system then falls off No more than 30 turnovers have been observed to date (e.g. equation 24)... 

Photoextrusion of isocyanides from monoimines (705 R = Ph, cyclohexyl, or n-propyl) has been observed on irradiation in furan. No reaction occurs when cyclohexane is used as solvent. The ketone (706) is formally an adduct of furan with tetramethylcyclopropanone or its derived 1,3-dipoIe (c/ the formal reversal of this in the synthesis of iminocyclobutanones, p.73). 

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