Photochemical dehydrogenation

Sharma HK, Arias-Ugarte R, Metta-Magana AJ, Pannell KH (2009) Dehydrogenative dimerization of di-tert-butyltin dihydride photochemically and thermally catalyzed by iron and molybdenum complexes. Angew Chem Int Ed 48 6309-6312... 

Dichlorodithicno[3,2-A2, 3 -/,<]thiophene-2,5-carboxanilide 121 synthesized from compound 120 has been cyclized into the diquinolone 122 by double photochemical dehydrogenation (Scheme 10) <2005MOL279>. 

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

At this point it became clear that, although a wide variety of complexes could be employed for both transfer dehydrogenation and photochemical dehydrogena-... 

In the presence of an oxidizing species (oxygen, iodine, tetracyanoethylene and others) DHP is dehydrogenated into phenanthrene. This oxidation can also be thermal or photochemical. In general, meta substituted stilbenes give rise to two isomeric substituted phenanthrenes ortho-substituted stilbenes can lose the substituent on cyclization. 

Few examples of photochemical dehydrogenation reactions accompanied by cyclization are reported in the literature. 2,3,5-Triphenyl-tetrazolium chloride (CV), which is used biochemically as reduction indicator, on irradiation in ethanol with ultraviolet light, undergoes dehydrogenation accompanied by cyclization to yield 2,3 (2,2 -bi-phenylene)-5-phenyltetrazolium chloride (CVI).106 167 Light-induced cyclization of substituted derivatives of CV to the corresponding bi-phenylene derivatives has been reported recently. sl 33... 

The Ic pattern for carbazole synthesis arises primarily in the dehydrogenative cyclization of diphenylamines. This cyclization can be accomplished photolytically, with an oxygen sensitive intermediate having been detected (equation 58) (81JA6889). Preparative work with substituted diphenylamines has resulted in rather variable yields, depending upon particular substituents. This variability may be the result of alteration in the dominant photochemical process with substituent changes. 

Table XXXVI is a list of some catalytic photochemical redox transformation of organic reactants by (Q or H)3PW 204o. In the presence of UV light, Q3PW12O40 reacts with paraffins, arenes, alcohols, alkyl halides, ketones, nitriles, thioethers, and water. Under either anaerobic or aerobic conditions, decarboxylation, dehydrogenation, dimerization, polymerization, oxidation, and acylation takes place.

The synthesis of 9-phenylphenanthrene (Expt 6.14) illustrates the formation of the phenanthrene system by the cyclisation of a 1,2-diphenylethylene (stilbene). The process involves an allowed photochemical cyclisation which gives initially a dihydrophenanthrene. This is readily dehydrogenated in situ by molecular oxygen in the presence of iodine. 

The platinum-catalyzed reaction of alkanes with chlorine leads to alkyl chlorides and alcohols (Table 6, entry 46) with modest rates and conversions [50], Cydooctane can be easily dehydrogenated (Table 6, entry 47) in the presence of a stabilized vinylalkane by use of the neutral rhenium compound ReH7(PR3)2 [51]. By employing an iridium-based catalyst, the photochemical dehydrogenation of methylcydohexane to methylenecyclohexane is performed at room temperature... 

With inflexible substrates, this photochemical functionalization process showed significant selectivity. As our first example, photolysis of steroid ester 70 led to carbonyl insertion into the C-H bonds at C-7 and C-14 [149,150]. Other steroid esters afforded selective functionalizations in various positions [e.g. 151, 152], and compound 71 directly dehydrogenated the steroid to form the product olefin 72 [150]. 

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