Photocatalyzed isomerization

Hydroxylation of p-coumaric acid, followed by photocatalyzed isomerization of the double bond and lactonization, affords 7-hydroxy coumarin (umbel-liferone). Prenylation at the 6-position, epoxidation, cyclization, and a retro-aldol reaction afford the furanocoumarins (Fig. 14) some (psoralen, bergapten)... 

Mono-fra s-[10]annulene, 7, has been postulated as an unstable intermediate in the conversion of bicydo[6.2.0]deca-2,4,6,9-tetraene, 8. to trans-Q, 10-dihydronaphthalene, 5 >. It is also claimed that [10]annulene is an intermediate in the photocatalyzed isomerization of 10-... 

Earlier examples of copper(I)-photocatalyzed cycloalkene cyclodimerizations have been summarized in Houben-Weyl, Vol. 4/5a, pp 280-292 as was the copper(I) chloride photocata-lyzed isomerization of cycloocta-1,5-diene to tricyclo[3.3.0.02 6]octane (see Houben-Weyl, Vol. 4/5 a, p 231). This same reaction has recently been used for the preparation of 4-oxatctra-cyclo[6.3.0.02,(,.07,1 L]undecanes.10... 

We can infer that the band positions of the irradiated semiconductor are greatly influential in controlling the observed redox chemistry and that formation of radical ions produced by photocatalyzed single electron transfer across the semiconductor-electrolyte interface should be a primary mechanistic step in most such photocatalyzed reactions. Whether oxygenation, rearrangement, isomerization, or other consequences follow the initial electron transfer seem to be controlled, however, by surface effects. 

Qualitative photochemical studies concentrate on diene complexes because of their greater thermal inertness and ease of characterization. Formation of olefin complexes is induced in situ either by photochemical or thermal means and their presence determined by spectroscopy. The photocatalyzed hydrogenation and hydrosilation of 1,3-di-enes the photocatalyzed valence isomerization of norbomadiene to quadricyc-lane and the cis trans photoisomerization of coordinated olefins are potentially usefulHowever, these transformations are not photosubstitution reactions and are not discussed here the reader should consult ref. 1 and references cited therein. Photolysis of olefin complexes leads to olefin loss with high quantum efficiency unless the olefin is a chelating di- or polyene where, as with most chelating ligands, other reactions occur. 

The photocatalyzed hydrosilation of 1-pentene was also performed in the presence of the tetranuclear clusters 15 although they had little activity. The predominant reaction was olefin isomerization. 

Another photocatalytic reaction of some interest is the cyclic dimerization of alkenes, which has been reported for a number of different metal carbonyls including Fe(CO)s, Cr(CO)6, Ni(CO)4 and several substituted analogs [83-85]. Although such dimerizations are catalyzed thermally by some of the same species, the stereochemistries of the products differ depending both upon which complex is utilized and whether the reaction is carried out thermally or photochemically. The dimerization of butadiene to 4-vinylcyclohexene photocatalyzed by the iron dicarbonyl Fe(CO>2(NO)2 has been studied by examining the intermediates formed when the photolysis was carried out in liq. Xe [85]. The key species was found to be the disubstituted species Fe(NO)2(Ti -butadiene)2 which was proposed to undergo isomerization to Fe(NO)2( n -butadiene)(ri2-butadiene) which can undergo an internal Diels-Alder cyclization to the vinyl cyclohexene. 

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