SUMMARY / RELATED TOPICS

Stilbene photocyclization

Stilbene photocyclization is the coupling of two aromatic carbons in stilbenes upon ultraviolet irradiation. The reaction can be used to form polycyclic aromatic heteroaromatics. Under UV irradiation and its derivatives undergo intramolecular cyclization to form dihydrophenanthrenes. In the presence of an oxidant, the dihydrophenanthrene aromatize to give polycyclic aromatics result; the dihydrophenanthrenes themselves are unstable, revert to cis-stilbenes in the absence of a hydrogen-trapping agent. Suitably substituted stilbenes may undergo irreversible, rearomatizing elimination or -shift processes in the absence of an oxidant. Aryl enynes, heteroatomic stilbene derivatives, substrates containing a single heteroatom in place of the stilbene double bond undergo the reaction. Regardless of the presence or absence of an oxidant, the first step of the reaction is excitation and formation of a dihydrophenanthrene intermediate; the excited stilbene may undergo reversible cis-trans isomerization. Orbital symmetry considerations dictate that the relative configuration at the newly bound centers must be trans After cyclization, oxidation of the dihydrophenanthrene intermediate to phenanthrene occurs.

Oxygen and iodine are the most employed oxidants. For most substrates, in the absence of an oxidant, the dihydrophenanthrene intermediate may reversibly open to the corresponding cis-stilbene. However, suitably substituted stilbenes cyclize irreversibly if an aromatizing elimination or hydrogen shift process can take place. Examples of these transformations are provided below. Photocyclization can be carried out with ortho-, meta-, para-substituted stilbene substrates. Ortho-Substituted substrates give 1-substituted phenanthrenes, unless the substituent is a good leaving group, in which case elimination to form unsubstituted phenanthrene occurs. Meta- Substituted substrates give mixtures of 2- and 4-substituted products. Substitution of the exocyclic double bond is well tolerated. Polycyclic aromatic compounds can be synthesized using substrates containing multiple aromatic rings. Stilbene derivatives containing fused aromatic systems may cyclize using either of two nonequivalent ortho carbons.

Which carbon reacts depends on both steric and electronic factors. Electronically, the dihydrophenanthrene intermediate exhibiting greater aromatic stabilization is preferred. For instance, in 1-naphthyl-2-phenylethylene, electronic factors favor the formation of 1 over 2 in a ratio of 98.5:1.5. Ortho-Terphenyl substrates cyclize to the corresponding triphenylenes in the presence of an oxidant, such as iodine. Oxygen is unsatisfactory because ring-opening to stabilized terphenyl is faster than oxidation when oxygen is used. Amides may cyclize to form lactams. Esters, which exist in the trans conformation about the C-O single bond, do not undergo this process efficiently. Photocyclization can form five-membered rings. In the vinyl naphthalene series, both oxidative and non-oxidative processes are possible. Cyclization of arylvinyl - or diarylamines provides carbazoles, respectively. In one interesting example, the use of circularly polarized light provided 3 in slight enantiomeric excess. Photocyclization can be used as the final step of a sequence to generate a fused aromatic ring at a benzylic position.

After benzylic bromization with N-bromosuccinimide, transformation to the phosphonium salt, a Wittig reaction with anaromatic aldehyde, photocyclization fuses the aromatic rings. Iteration of this sequence results in helicenes. Several other methods are available to synthesize the phenanthrene ring system; the Haworth and Wagner-Meerwein syntheses below are two examples. Like the Diels-Alder reaction, photocyclization forms six-membered rings.

Andis Shala

Andis Shala is a German footballer, playing as a forward for Kickers Offenbach. His father is former Croatian international Kujtim Shala. Having begun career in the youth team at Hannover 96, Shala joined VfR Mannheim in January 2008 on a short-term deal. Shala joined Dundee United in pre-season for the 2008–09 season, with manager Craig Levein calling him a "decent prospect" and stated that he had "high hopes for him". Shala arrived in mid June 2008 with fellow new signing Michael McGovern, he made his debut as a substitute in an August 2008 Scottish League Cup match. He made his league debut in February 2009, with his first touch of the ball, set up Lee Wilkie to score the equalising goal in a 1–1 draw against Inverness Caledonian Thistle, he was released by Dundee United in the summer of 2011. Following his release, Shala returned to Germany to sign for Hallescher FC. After earning promotion to the 3. Liga in the 2011–12 season, he left Halle to sign for Carl Zeiss Jena in January 2013. Eighteen months he signed for Berliner FC Dynamo.

Shala is of Kosovar Albanian descent. In late December 2009, Shala was the subject of interest from the Albania national team, with team manager Josip Kuže confirming he was intending to watch the player. In an interview for the Albanian newspaper Sporti Shqiptar, Shala stated his willingness to play for Albania, stated that if he never got called up for the squad, that he would not consider playing for any other country. Andis Shala at Soccerbase Andis Shala at fussballdaten.de