Cycloheptatriene photochemically

Photochemical decomposition of diazomethane yields methylene, which reacts with benzene to form toluene and cycloheptatriene (via norcaradiene) (Scheme 5).65 66... 

The products formed in these reactions are very sensitive to the functionality on the carbenoid. A study of Schechter and coworkers132 using 2-diazo-1,3-indandione (152) nicely illustrates this point. The resulting carbenoid would be expected to be more electrophilic than the one generated from alkyl diazoacetate and consequently ihodium(II) acetate could be used as catalyst. The alkylation products (153) were formed in high yields without any evidence of cycloheptatrienes (Scheme 33). As can be seen in the case for anisole, the reaction was much more selective than the rhodium(II)-catalyzed decomposition of ethyl diazoacetate (Scheme 31), resulting in the exclusive formation of the para product. Application of this alkylation process to the synthesis of a novel p-quinodimethane has been reported.133 Similar alkylation products were formed when dimethyl diazomalonate was decomposed in the presence of aromatic systems, but as these earlier studies134 were carried out either photochemically or by copper catalysis, side reactions also occurred, as can be seen in the reaction with toluene (equation 36). 

Ultraviolet irradiation of [Cr(CO)3(ty6-l,3,5-cycloheptatriene)] (41) in the presence of conjugated dienes causes no CO evolution. A photochemical [6 + 4] cycloaddition takes place with the dienes la-Id, li, and 2,3-dimethyl-1,3-butadiene (It) (69-71) [Eq. (21)]. Within the coordination sphere of chromium, two C—C bonds are formed between C-l and C-6... 

Ultraviolet activation of the complexes hexacarbonylbis( / 5-cyclopen-tadienyl)dimolybdenum and -ditungsten (135) has been studied in detail (136-138). In addition to homolytic cleavage of the metal-metal bonds, loss of carbon monoxide has also been observed. The products of the photochemical reactions of [(t/5-CH3C5H4)M(CO)3]2 (M = Mo, W) with the dienes la- 1c, lg and It and 1,3,5-cycloheptatriene (68a) differ markedly from those obtained from the thermal reaction of [(f/s-C5Hs)Mo(CO)B]2 (n = 2,3) with dienes (139,140). 

While 154-157 are specific products for la, complexes of the type of 153a are the predominate products when 152 is reacted photochemically with acetylene (221), olefins (750,222,225), dienes, trienes, and tetraenes (224-226). In contrast to a previous report (227), 1,3,5-cycloheptatriene reacts with 152 like the other unsaturated hydrocarbons. Similar reaction products are obtained with lc as the diene component [Eq. (76)]. Although complexes comparable with 154,156, and 157 are missing, a trinulear species is formed... 

The photochemical reactions of FeCl3 and RuC13 with IPrMgBr and mixtures of 1,5-cyclooctadiene and cycloheptatriene, or 1,5-cyclooctadiene and... 

Diene iron tricarbonyl complexes are prepared by thermal or photochemical reaction of conjugated dienes with iron pen-tacarbonyl in the presence of TMANO, triiron dodecacarbonyl, ()]" -benzylidenacetone)iron tricarbonyl, diiron nonacarbonyl, or diiron nonacarbonyl absorbed on silica gel in the absence of solvent. The latter method is particnlarly usefiil for the preparation of complexes from polar electron-rich dienes and heterodienes. A reductive complexation of cycloheptatrienes using iron tricarbonyl and sodium borohydride to give cyclo-heptadiene iron tricarbonyl has been developed (Scheme 126). 

Other results in this area point to an initial [4 + 4] cycloaddition step followed by a themnally or photochemically initiated rearrangement of the initial cycloadduct to give a [4 + 2] species as the final observed pi ucL Irradiation of 9-cyanoanthracene and cycloheptatriene at low temperature initially produced the [4 -i- 4] adduct (106) which promptly rearranged to (107) at temperatures exceeding 0 C. Related diene-arene photocycloadditions have been reported in which comparable tandem processes were apparently operative. " ... 

Of the cyclic 1,3,5-trienes, only 1,3,5-cycloheptatriene and compounds derived from this ring system have been studied extensively. In almost all the cases the primary photochemical process has been a valence tautomerization which, in cycloheptatriene itself, can be written as ... 

Since the extent and scope of this reaction has been recently reviewed, it is adequate to discuss here the quantitative aspects of the photochemical primary processes, which have been studied only in 1,3,5-cycloheptatriene itself. 

In this reaction, triplet methylene inserts in the ethene molecule to produce an energized cyclopropane molecule, which may then either isomerize to propene or may be collisionally deactivated to give cyclopropane. Molecules may also be activated photochemically. Although most photochemical processes involve more than one electronic state of the molecule, it is possible in some cases to produce molecules in their electronic ground states with high vibrational excitation, and these may subsequently isomerize or dissociate. A typical example is the photoexcitation of cycloheptatriene followed by its isomerization to toluene. 

Buchner method of ring expansion Thermal or photochemical reaction of ethyl diazoacetate with benzenes and its homologs to give the isomeric esters of cycloheptatriene carboxylic acid. 68... 

Although photodecarboxylations of esters and carboxylic acids are widely known, loss of CO2 from lactones is usually only a minor photochemical pathway. When pyrone (64) (Scheme 15) was irradiated in benzene solution with 356 nm light under conventional, low-intensity conditions, no products were formed, but under high-intensity, laser-jet conditions, the carboxylic acid (66) and cycloheptatriene (67) were obtained. The products can be accounted for, as shown in Scheme 15, on the assumption of initial C-O bond cleavage to... 

Decarboxylation of (175) occurs on its irradiation in an argon matrix at 10 K using 254 nm light. Spectroscopic analysis of the resulting matrix indicates the presence of a complex between carbon dioxide and the carbene (176). Tiaprofenic acid (177) undergoes facile photochemical decarboxylation, and this is reported to take place from an upper triplet excited state." A study of the transient photochemistry of 5-(p-toluyl)-l-methyl-2-pyrrolylacetic acid has been reported. Decarboxylation results in the formation of a carbanion in its triplet state. A laser-flash study using irradiation at 266 nm of the xanthene-9-carboxylate (178) has shown that the radical (179) is formed. This study used NaY zeolites and studied the oxidation of the radical within the cage structure. Calculations have indicated that decarboxylation of (180) and (181) and deprotonation of cycloheptatriene and cyclopentadiene affords the same anions (182) and (183), respectively. ... 

Numerous examples for the synthesis of cyclopropanes by [2-f 1] cycloaddition of photochemically generated acylcarbenes to alkenes, allenes, 1,3-dienes, and cycloheptatriene are given in Houben-Weyl Vol. 4/5b, pp 1158-1257 and in Vol. E19b, pp 1099-1107 and pp 1300-1303. 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

For photochemically generated (2-biphenylcarbonyl)phenylcarbene, several competing intramolecular reactions are observed, namely cyclopropanation of an aromatic nucleus (followed by norcaradiene to cycloheptatriene tautomerization), Wolff rearrangement, C-H insertion, and a carbene-to-carbene rearrangement (see Houben-Weyl, Vol. E19b, pl282). 

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 photochemical reaction of phosphorous derivatives with p -benzoquinones has been studied. The Argon matrix isolation of cyclopentadienone has been achieved using a variety of photo -precursors such as o - and p -bezoquinones. />-Benzoquinone and 1,4-naphthoquinone add to cycloheptatriene to afford spirocyclic... 

A study of photochemical 1,7-hydrogen migrations within the cycloheptatriene derivatives (127) has been reported. Hansen and his coworkers have demonstrated that irradiation of the heptalenes (128) brings about reversible isomerization into a mixture of isomers composed of (129) and starting material (128). To some extent, the ratio of photo-product starting material is dependent upon the substitution as indicated by the yields shown under the appropriate structure. ... 

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