1.3- dienes from photochemical ring-opening

Pleiadiene (9), a naphtho[a,fi]cycloheptatriene resulting from photoinduced ring opening (unusual two photon process) or thermal isomerization at 250°C (forbidden sigmatropic reaction) -of l,8-naphthobicyclo[3.2.0]hepta-2,6-diene (10), can alternatively be obtained by photolysis of l,8-naphthotricyclo[4.1.0.0 ]heptene (11). All three of these 1,8-naphtho (C4H4) hydrocarbons 9,10 and 11 undergo thermal and photochemical interconversions. - 34... 

The products from cyclopropene ring opening have the advantage of easy conversion into tricarbonyl(Ti -diene)iron complexes (34), its regioisomer and (35), by thermal loss of carbon monoxide, or photochemically without CO evolution (eqs 19 and 20). ... 

In a combination of photochemical cyclization and a radical reaction Yoshimatsu et al synthesized 2-azabicyclo[33.0locta-3,7-diene 169 from the trienal hydrazone 166.1891 The domino process was initiated by irradiation of 166 at 400-500 nm in benzene. The transformation may include an intermolecular [2+2]-cyclization, followed by ring opening to give... 

In the formation of tetraenes from bicyclo[4.2.0]octa-2,4-dienes, two bonds are broken. This may occur in one concerted reaction which can be regarded as a retro [2 + 2] cycloaddition. It is also possible that the central bond, being part of a cyclohexadiene system, is the first one to break in a thermal, concerted disrota-tory process that leads to a 1,3,5-cyclooctatriene derivative. Ring opening of the cyclooctatriene then might take place photochemically, again disrotatory, to produce a tetraene. This two-step sequence was first observed by Mirbach et al. [114] in their study of the photocycloaddition of the two parent molecules benzene and ethene. The same explanation for the formation of a tetraene was given by Nuss et al. [160] in their report on the intramolecular ortho photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile (see Scheme 40). 

Slobodin reported the photodimerization reaction of allene. Irradiation of 1,1-dimethylallene 49 affords dimers 63 and 64. The ring-opening reaction of cyclic bisallene 65 was reported by Kaupp to yield butatriene 66 on irradiation in a low-temperature matrix,and the reverse reaction from butatriene 67 to cyclic bisallene 68 in the solid state was also reported by Berkovitch-YeUin and co-workers. The photochemical racemization of optically active penta-2,3-diene 69 in hexane was reported by Rodriguez and Morrison. Kuhn and Schulz reported the photochemical ds,trans-isomerization reaction between cis-butatriene 70 and trans-butatriene 71 and also reported a 39.5% 60.5% cis-trans ratio at equilibrium upon irradiation at 313 nm. ... 

Cyclopropane rings. Synthesis of (diene)Fe(CO)3 complexes, and of cyclohexenones, lactones, ketones, or sulfones, can follow from the opening of vinylcyclopropanes with Fe(CO)s under either thermal or photochemical conditions. Reactions of the vinylcyclopropane (15) with Fe(CO>5 under thermal conditions resulted in ring opening with concomitant hydrogen shift to form the diene complex (16) (eq 7) 2.63,64... 

Pioneering progress in the field of [5+1] cycloadditions was made by Sarel et al. [9]. When they treated 1,1-dicyclopro-pylethylene 1 with Fe(CO)s, they obtained unexpectedly cyclohexenone-Fe complex 3, in addition to the desired diene 7t-complex 2 (Scheme 17.1). Cyclohexenone-Fe complex 3 comes from [5+1 ] carbonylation of 1 with CO from Fe (CO)s followed by opening of the second cyclopropane ring. To achieve the [5+1] cycloaddition under mild conditions, they changed the original thermal reaction to a photochemical [5+1] reaction (Scheme 17.2). 

---