Cyclisation reactions photochemical

A full paper has appeared expanding on the previous report concerning the photo-Fries rearrangements of N-aroylcarbazoles. In polar solvents mixtures of carbazole and both (311) and (312) are obtained, while in non-polar solvents carbazole and low yields of (311) only are isolated instead. An intramolecular cyclisation reaction competed successfully with rearrangement in the case of N-(ortho-chlorobenzoyl)carbazole to yield (313) as the major product. In a report from a different laboratory the photo-Fries rearrangement of N-sulphonylcarbazole is described and the products identified as (314) and (315) in contrast to the results described above, these authors report that N-benzoylcarbazole is photochemically inert. 

Electrophilic Opening 20 Nucleophilic Opening 22 Cyclisation Reactions 26 Reduction and Elimination Reactions 28 Photochemical Reactions 30 Reactions with Organometallic Compounds 30 Miscellaneous Reactions 34... 

L = [14]ane N4) haAre been used to make RS thiyl radicals, which in turn unexpectedly react with [EtCo(L)(H2P)]2+ to foim C2H4 and RSH.239-241 Addition of base to [(tacn)Co(L)] (tacn = 1,4,7-tiiazacyclononane L = l,6-diamino-3-thiahexane) causes deprotonation of L and formation of an N,C,N-bonded ligand, in equilibrium with its N,S,N-bonded fonn. > 3 other Co(III) alkyls have been produced by photochemical decarboxylation of chelate carboxylate complexes. The synthesis and structure of the nl-C 02 polymer [ Co(en)2(C02)) QO4 H20 ]n has been reported.246 The structure of [Co(salen)(Pr )(py)] has been determined,247 and tandem cyclisation reactions involving organocobalt salen and salophen complexes have been described.248.249... 

In case of butadiene, the photochemical cyclisation reaction produces two products—cyclobutene and bicyclobutane. The product obtained depends on geometry of butadiene. 

Cyclisation of malonates is one of the few reliable ionic reactions giving four-membered rings. Hence when the ais and trans ketones (38) were wanted for a photochemical study, acids (39) were the obvious starting materials as these could be made by cyclisation of (40). Analysis... 

Presumably the silyl enol ether of 37 adds in a conjugate fashion to the unsaturated ester 39 and the intermediate enolate then cyclises onto the cation 40 to give 38. This will happen only if the stereochemistry of 40 is the same as that of the product 38 as the 4/5 and 4/6 ring fusions must both be cis. This suggests that the first step is reversible. The formation of the cyclobutane requires that particular relationship between ketone and unsaturated ester so this kind of reaction is less versatile than photochemical cyclisation. Asymmetric versions of these reactions are also known.14 Probably the most versatile thermal method to make cyclobutanes uses ketenes and is the subject of the next chapter. 

The classic precursors used for generating diradicals are cyclic, bicychc and polycyclic diazenes. However, diradicals have also been made by Norrish type I photochemical extrusion of CO from cychc ketones, by thermal cleavage of vinyl and divinylcyclopropanes, by pinacol reactions of diketones, by Bergman-type cyclisations of endiynes, by several types of photoelectron transfer reactions and in other ways. Most synthetic applications have started with a derivative of 2,3-diazabicyclo[2.2.1]hept-2-ene which on heat-... 

The unsaturated 6-lactone (84) undergoes intramolecular photochemical (2 + 2)-cycloaddition to give (85)." The scope of the process has been evaluated and the 5-lactones (86-88) all behave similarly, affording the products shown in Scheme 2. The lactones (86) and (87) both cyclise in the two possible modes. Intramolecular cycloaddition reactions within polymethyldia-minebis(4-methyl-7-coumarinyl)oxyacetamides have been described. Zhu and Wu have reported that a biscoumarin system linked by a phenanthroline unit does not undergo photodimerisation. Instead, intramolecular addition of a coumarin unit to the phenanthroline occurs. 

An electron-transfer mechanism is proposed to account for the photochemical cyclisation of the fluorinated aryl amines (36) and (37). Photochemical oxidative cyclisation has also been reported as an efficient path to some benzo[h]thienothienoquinolines. A study of the photochemical reactivity and degradation of diclofenac and meclofenamic acid has been published. The bis(diphenylamino)butane (38) is photochemically reactive and irradiation at 313 nm in an air-saturated solution brings about cyclisation which yields the biscarbazole (39). It is clear from the results obtained that the reaction is stepwise with initial cyclisation affording (40). The quantum yields for the cyclisation of (38) to (40) is 0.3 while that for (40) to (39) is 0.02. 

In the second step the reaction is less efficient because of quenching of the excited state of the diphenylamine component by the adjacent carbazole. Other cyclisations such as the conversion of (41) into (42) by irradiation in acetic acid have also been reported. As mentioned earlier cw-azobenzene derivatives cyclise under conditions where the nitrogens are protonated or complexed. A series of heavily substituted azobenzenes has been shown to undergo photochemical cyclodehydrogenation on irradiation in methylene dichloride solution with added SnCU- The reaction has been shown to be an efficient path to the heavily methylated cinnolines in good yield. 

Irradiation of the thiopyran derivative (292a) results in extrusion of HNCS and the formation of the pyridine (292b, 63.3%) as the major product. The photochemical reactivity of the pyranthione (293) is concentration dependent in 3-methylpentane as solvent. At low concentrations the thione reacts with the solvent but at higher concentrations the main reaction is the production of thiyl radicals. Laser flash photolysis has been used to identify that the triplet excited state of (294) is involved in addition reactions which occur to electron deficient alkenes such as acrylonitrile giving (295), for example. Other compounds related to thiourea are also photochemically reactive. Thus the photochemical cyclisation of (296) to afford (297) has been reported. ... 

The photocyclisation of diarylethylenes in which a positively charged nitrogen atom is present yields fused polycyclic azonia arenes further examples of such reactions have appeared. Thus compound (212) upon ultra-violet light irradiation yields (213), presumably by way of sequential photochemical reactions in which (214) is an intermediate. If (213) is phenyl substituted then (215) is obtained also. The initial cyclisation of (212) is regio-selective and none of (216) is formed. Analogous reactions proceed for the systems (217), which yields (218), (219), which yields (220), " and (221), which yields (222). A closely related reaction is reported for (223) which gives (224). ... 

A Michael Reaction Coupled to a Photochemical Cyclisation Copper Again Michael Additions of Heteroatom Nucleophiles... 

Cyclobutene (3) was used on page 269 in a photochemical cycloaddition. It is made by ionic reactions from readily available adipic acid (27) (see Chapter 27). This is something of an exception and cyclisation by carbonyl condensations is not normally a recommended route to four-membered rings... 

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