Rearrangements photochemical induced

The photoreactions of four-membered nitrogen containing heterocycles have been less well investigated and meaningful results are confined to certain azetidines and azetidinones. The photochemically induced rearrangement of azetines 148 to 1,4-diazepines 149 has been rationalized in terms of a process equivalent to a di-7t-methane rearrangement116 a more... 

Rearrangements arising by photochemically induced 1,3-acyl migrations have been reported in the 3tf-azepine239,181 and in certain l,3-dihydro-2JT-... 

Rearrangements. It will have been obvious from the Cr(C0)5... X discussion above that the matrix technique is particularly well suited to the examination of photochemically induced rearrangements and isomerization since the matrix holds the participating species in fixed positions and hence allows spectroscopic study of each, yet allows ready photochemical interconversion. 

Subsequent irradiation of S-14 with k > 570 nm in the presence of a nitrogen molecule in the same matrix cage results as in the case of T-10 in the recapture of nitrogen (which is also possible by annealing the matrix at 25 K). Parallel to this reaction, a photochemically induced rearrangement of S-14 to N-cyano-1 W-azirene (12) — perhaps via the ring-opened carbene 11 — and Af-cyanoketenimine occurs. 

In the list of diazoketones studied by us95 mostly derivatives were included which have in solution no or only a small tendency for a Wolff rearrangement. Nevertheless we found not a single diazoketone 71 which enabled us to identify a ketocarbene 72, only the corresponding ketenes 73 could be detected. The same observation was made when we studied in collaboration with Yannoni et al." the photochemically induced deazotation of l-diazo-2-propanone in an organic matrix at 77 K, using 13C CPMAS NMR spectroscopy as the analytical tool. 

In some way formally similar to the benzidine rearrangement is the Wallach rearrangement of azoxybenzene 23 to give 4-hydroxyazobenzene 24 in concentrated (typically 95%) H2SO4. The 2-hydroxy isomer is sometimes formed in low yield with some substituted azoxybenzenes, and it is the main product in the photochemically induced reaction. Much of what is known about the reaction has been covered in earlier review articles28-30. This contribution will report work published since 1981. 

Spontaneous isomerization is difficult to vahdate since traces of ligand are almost impossible to avoid. Photochemically-induced rearrangements may go via a tetrahedral intermediate (Sec. 7.2.1), although there is no evidence for this. 

Dialkylquinazolines 825 are available by microwave-assisted amination and ring closure of 2-acylamino phenyl-ketones 824 with ammonium formate <20070L69>. The 2-aminophenyl ketone precursors of the amides 824 are available by a photochemically induced Fries rearrangement of anilides 823, which enables a variety of different alkyl substituents to be incorporated at the 4-position of the quinazoline <20070L69>. 

Several photochemically induced vinylcyclopropane to cyclopentene rearrangements of nor-carene derivatives to form bicyclo[3.2.0]heptenes can be understood as ring contractions of cyclohexenes to cyclobutanes. Upon direct irradiation of norcar-2-ene (bicyclo[4.1.0]hept-2-ene) at 214 nm (pentane solution), however, complex product mixtures were obtained containing only small amounts of bicyclo[3.2.0]hept-2-ene, while toluene sensitized photolysis in 50 millimolar solution in degassed pentane at 254 nm gave mainly the cyclobutane derivative 13 in addition to EjZ-isomeric hepla-l,3,6-trienes.72... 

Photochemically induced rearrangement of the cycloadduct (251) results in either ring expansion to give benzazepines or ring contraction to aminomethyleneindenes, depending upon the reaction conditions (B-77MI30500). 

Electron transfer may also be important in a series of photosolvolysis/rearrangement of certain bridged polycyclic compounds. The efficiency of photochemically induced solvolyses (186), for example eq. 60,... 

Photochemically induced ring cleavage reactions have also been reported in a,/i-epoxydiazomethyl ketones" and in a,/f-epoxy esters (glycidic esters).100 Two separate photoreactions are involved, however, in the rearrangement of phenyl epoxycinnamate (118) to the flavone (119), as shown in Scheme 7.101... 

When covalently attached to electron transfer active subunits, the DHA-VHF couple can facilitate chemical and physical switching of electronic properties, as a result of photochemically induced rearrangement accompanied by a change in the redox potential. An interesting example of such a switching system is the compound containing a dihydroazulene component and a covalently attached anthraquinone moiety.1311 This system is able to act as a multimode switch, assisted by various processes such as photochromism, reversible electron transfer, and protonation-deprotonation reactions (Scheme 8). 

Photochemically induced rearrangement of the azole ring in the fused isoxazole (231) leads to the corresponding oxazole (232) in low to moderate yields (78CPB2497). 

The photochemically induced addition of bulky disilanes to C6o has already been discussed in Section IV.A.l.h, as it affords the unusual inherently chiral 1,16-addition pattern (( )-72, Figure 1.20). In this reaction some phenyl-substituted disilanes give rearranged adducts of the type ( )-73 (Figure 1.20).197,404 Further [60]fullerene derivatives with stereogenic centers in the addends were obtained in photoinduced addition-rearrangement reactions with 3,4-bis(alkylidene)-l,2-disilacyclobutanes 405 and with octaaryl-substi-tuted cyclotetrasilanes and -germanes406... 

The unhomogeneous composition of the products generated by the photochemical reaction is due to another mechanism. While the thermal isomerization of 1,5-dienes proceeds via a cyclic transition state in a synchronous sense, the photochemically induced transformation causes a reorientation of the allyl radicals generated from the educts. Warming up the reaction mixture to 100°C activates a complete transfer from 4c to 5c) of all isomers. This step may be explained by a radical CC bond split of the 1,2-diphenylethylene unit. Since the isomerization of the diastereomeric compound 4c to 5c is activated at much lower temperatures than for the Cope rearrangement (from 3c to 4c), it is clear that the thermal transfer exclusively forms the twofold changed product. 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

Electron transfer to or from a conjugated tr-system can also induce pericyclic reactions leading to skeletal rearrangements. A typical example is the Diels-Alder cycloaddition occurring after radical-cation formation from either the diene or the dienophile [295-297], The radical cation formation is in most cases achieved via photochemically induced electron transfer to an acceptor. The main structural aspect is that the cycloaddition product (s Scheme 9) contains a smaller n-system which is less efficient in charge stabilization than the starting material. Also, the original radical cations can enter uncontrollable polymerization reactions next to the desired cycloaddition, which feature limits the preparative scope of radical-type cycloaddition. 

In many thermally induced rearrangements, it is often difficult or impossible to distinguish between a thermally induced fluorine-atom shift and the type of fluoride-induced rearrangement that we have just exemphfied. However, evidence for photochemically induced 1,3-shifts of fluorine in the equilibrium between 7.42A and 7.42B is very convincing [139] (Figure 7.42). 

Another synthetic pathway recently used is the cyclization of linear triazenes, like (224), which in aprotic media with Lewis base catal) is yield 5-amino-1-ary 1-1,2,3-triazoles (225). However, in protic media Dimroth rearrangement affords the isomeric 5-(arylamino)-l,2,3-triazoles (226) (70JOC2215, 81JOC856). The 2-tetrazene (227) gives methyl l-benzyl-1,2,3-triazolecarboxylate (228) by photochemically induced homol) is of a single N—N bond (81TL227). 

Rearrangements.—Various examples of Z,E-photoisomeri7ation about the carbon-nitrogen double bond have been reported. The E-esters (1) have been prepared by irradiation of the Z-isomers (2) in pentane-benzene, and quantum yields for the photochemically induced interconversions of the four stereoisomers of l-(methoxyimino)-3-phenylprop-2-ene have been determined. Particular interest has been shown in the isomerization of azines the E — Z photoisomerization of benzophenone-9-anthraldehyde azine is quenched by dyes with the appropriate singlet energy," with the quantum yield for this transformation being influenced by solvent polarity." The composition of the photostationary state has been determined for a number of arylazines "... 

Subsliluted 3-acyloxyquinazoline-4(3/f)-thiones 5 rearrange very easily in a photochemically induced radical reaction to give the corresponding 4-alkylsulfanylquinazolines 6. ... 

---