Friedel-Crafts reaction photo

Relatively little is known about the photochemistry of phosphorus containing organic substrates. Continuing our photochemical work on compounds of structure 1a and b which leads to thio- and selenoxanthones 3a,b via thiol ester-thiopyrone-, selenol ester-seleninone-transformations ( 1, 2, 3), or photo-Friedel-Crafts-reactions ( 3-6J, respectively, "(ScFeme 1) we have extended our interest to aroyl diphenyl phosphines 1c - k (Schemes 2-5) in which X in 1a and b stands for P-phenyl. 

As 1c behaves photochemically similar to certain thiol and selenol esters (e.g. 1a and b), yielding heterocycles of type 3 in the first photo-Friedel-Crafts-reaction of an organic phosphorus compound (Scheme 2), the aroyl diphenyl phosphines id - k show other reactions on u.v. irradiation (7, 8, 9J. 

The A-heterocyclic carbene precursor 49 is a photo-switchable catalyst for amidation reactions. The synthesis of this catalyst begins with two Friedel-Crafts reactions firstly, 2-melhyl-5-phenylthiophene was 3-acetylated with acetic anhydride using SnCLj, then product ketone oxidised to a glyoxal by SeOa- The glyoxal itself was used as acylating agent to react with 2-methyl-5-phenylthiophene with the assistance of SnCLj. After several further steps, the salt 49 was obtained (Scheme 51) [69]. 

The isomer distribution for 5-ArDMU s was analogous to those typically observed in aromatic photosubstitution reactions. On the other hand, few reports have appeared on the meta-oriented aromatic photosubstitution reaction, the primary example being the photoinduced proton exchange in benzenes that proceeds via an electrophilic aromatic photosubstitution reaction. In the case of either the 5-chloro-or 5-fluoro-l,3-dimethyluracil (5-ClDMU, 5-FDMU), > addition of TFA simply promoted 5-arylation (photo-Friedel-Crafts reaction), while the addition of a bromide (p-dibromobenzene, 1,2-dibromoet-hane, benzyl bromide, HBr) to the solution induced the meta-oriented 6-arylation (Scheme 2)."... 

Aromatic photosubstitution, including acid-catalyzed photosubstitution reaction (photo-Friedel-Crafts reaction)... 

Cyclopropene and its deuterium-labelled derivatives can be obtained by the photo-decarbonylation of the corresponding furan at 254 nm but the method is of strictly limited value because of the photolability of many cyclopropenes (Section IV.B.2). West and his coworkers have shown that aryltrihalo- and diaryldihalocyclopropenes are available from classical Friedel-Crafts aromatic substitution reactions employing the cyclopropenyl cation as electrophile. Thus tetrachlorocyclopropene is converted to its derived cation which is then allowed to react with an aromatic compound. The exothermic reaction provides monoarylcyclopropene at low ( 0°C) temperature and the diaryl derivative at higher ( > 50° C) temperature (equation 26). In this way 2-phenyl-1,3,3-trichlorocyclopropene can be obtained in 58 % yield and the p-fluorophenyl analogue in... 

In contrast to these observations, Snell studied the photo-Fries rearrangement of sulphonate esters of 6-substituted 4-hydroxypyrimidines (equation 9) and found efficient conversion for a variety of alkane- and arenesulphonates [e.g. R1 = Me, Et, Bu, (CH2)3C1, Ph, 4-Tol, 4-ClC6H4, 4-BrC6H4, 4-An, 2,5-Me2C6H3, 2,4,6-Me3C6H2]47. Yields vary between 20-60%, and the reaction does not proceed for R3 = NH2 or NHAc. The reaction has synthetic utility, since the product sulphones are inaccesible via the Friedel-Crafts procedure. 

Following the original protocols described by Kraus,Mattay and co-workers examined numerous photoacylations of 1,4-quinones for which they introduced the term Photo-Friedel-Crafts acylation . In aU cases, the corresponding acylated products were obtained in moderate to good yields (Table 88.17). Because these authors were particularly interested in a large-scale solar chemical outdoor application of the acylation reaction, they replaced the toxic solvents (benzene or acetonitrile) with nontoxic f-butyl alcohol. 

For other Photo-Friedal-Crafts reactions , see (a) Martens, J., Praefcke, K., and Schulze, U, Intramoleulare Photo-Friedel-Crafts-Reaktionen ein neues Syntheses-Prinzip fUr Heterocyclen, Synthesis, 532-533, 1976 (b) Bryce-Smith, D., Deshpande, R., Gilbert, A., and Grzonka, ]., Acid-catalysis of photochemical reactions, Chem. Commun., 561-562, 1970. 

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