Hepta-l,6-diene

The parent compound, 69, has been synthesized and characterised <2003ZFA1475>. 4-Chloro-hepta-l,6-diene was reacted with Mg. No Grignard rearrangement was noticed but instead the Grignard reagent was converted into l-allyl-3-butenylphosphonous dichloride by reaction with PC13. Reduction with LiAlH. produced l-allyl-3-butenyl-phosphane. Radical-initiated cyclization led to the product, l-phosphabicyclo[3.3.0]octane. Four derivatives were similarly prepared and characterized (70-73). Compound 74 was similarly prepared via a radical reaction < 1997PS(123)141 >. 

The decomposition of this compound occurs between 426 and 464° C by two concurrent paths to yield as initial products cyclopentene plus ethylene and hepta-l,6-diene. 

In the conversion of hepta-l,6-dien-3-ols to bicyclo[3.2.0]heptan-3-ols 8 the encfo-alcohol is usually formed selectively.8... 

There are three double bonds each in a (3,y-position to the alcohol group. Two give identical products as shown in the top line of Figure S29 but the third gives a quite different pair of products from the same electrocyclic mechanism. Statistically, one might expect the yield of trimethylcyclohexene and hepta-l,6-diene-4-one to be 33%. In fact they only occur to the extent of 20%, indicating that there is some preference for the desired reaction pathway. The reasons for this probably lie in stereoelectronic factors. 

Devasena, T., Rajasekaran, K.N., and Menon, V.P., Bis-l,7-(2-hydroxyphenyl)-hepta-l,6-diene-3,5-dione (a curcumin analog) ameliorates DMH-induced hepatic oxidative stress during colon carcinogenesis, P/jarroaco/. Res., 46 (1), 39 5, 2002. 

Kapitan P, Gracza T. Stereocontrolled oxycarbonylation of 4-benzyloxy-hepta-l,6-diene-3,5-diols promoted by chiral palladium(II) complexes. Tetrahedron Asymm. 2008 19 38 4. 

A small amount of this mixture can also be obtained when the isomeric 3-oxatetracy-clo[3.1.1.02,4.06,7]heptene is heated to 145°C. The major product, however, is 2-oxabi-cyclo[3.2.0]hepta-3,6-diene.113 2,3,5,6,9-Pentamethyl-4-oxa-9-azatetracyclo[5.3.0.02,6.03 5]dec-l(7)-ene-8,10-dione readily isomerizes to the corresponding oxepino[4,5-c]pyrrole. 14... 

The facile, photoinduced valence isomerization of ethyl 1//-azepine-l-carboxylate to ethyl 2-azabicyclo[3.2.0]hepta-3,6-diene-2-carboxylatehas been studied as a potential solar energy storage system.101102 Unfortunately, the system proved to be inefficient due to build up of polymeric material during the thermally induced, exothermic retro-reaction. 

Azabicyclo[3.2.0]hepta-3,6-dienes are the 4 5 bicyclic valence isomers of 1//-azepines. In some cases there is an equilibrium between the bicycle and the azepine, whereas with other 1//-azepines photolysis yields an azabicycloheptadiene that can be isolated and characterized. For example, ethyl 2-azabicyclo[3.2.0]hepta-3,6-diene-2-carboxylate (1), the photoinduced valence isomer of ethyl l//-azepine-l-carboxylate (2), undergoes a clean, exothermic, first order, electrocyclic ring opening (AG = 20 kJ mol-1) to the parent 1//-azepine 2 on heating at 113-143C in an inert solvent (e.g., hexadecane).101... 

However, if the azepine is C-monosubstituted, e.g. 14, or unsymmetrically substituted, then two isomeric 2-azabicycloheptadienes, e. g. 15 and 16, may result corresponding to electrocyclic ring closure involving C2-C5 or C4-C7 of the azepine ring. In practice, the ratio of the two isomers formed (which may be separated by vapor phase chromatography) varies with the position of the substituent.236 In contrast, irradiation of methyl 2,5-di-tm-butyl-l//-azepine-l-carboxylatein methanol yields only methyl 3,5-di-tert-bulyl-2-azabicycIo[3.2.0]hepta-3,6-diene-2-carboxylate (81 %).70... 

An essentially quantitative yield (98%) of the thermally stable hexafluoro-2-aza-bicyclo[3.2.0]hepta-3,6-diene 18 is obtained from the small-scale photolysis of the hexafluoro-l//-azepine 17.230 Hexafluoro-l//-azepine-l-carbonitrileand-l-carboxamidebehave similarly. On scaling up the process for the l-(ethoxycarbonyl) derivative 17, however, a much lower yield (59%) of bicycle 18 resulted, although this was improved to 81% by carrying out the photolysis in hexane or rm-butyl alcohol in the presence of a triplet sensitizer, e. g. ben-zophenone. 

A few examples are known of the thermally reversible, photoisomerization of 4//-azepines, e.g. 22, to l-azabicycIo[3.2.0]hepta-2,6-dienes, e.g. 23.83 The yields are high (65 95%) and the alternative isomers, the 6-azabicycIo[3.2.0]hepta-3,6-dienes, resulting from C2-C6 ring closure, do not appear to be formed. 

Acyl-3.4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes 1, on heating at 250-280 C for a short time without solvent, rearrange to the 1-acyl-1-benzazepines 2 (Method A).23-38 In some cases, rearrangement is accompanied by minor amounts of Ar-aeyl-l-naphthylamine and, at higher temperatures, the acylnaphthylatnine can become the major product (see Section 3.2.2.6.). In the presence of silver(I) tetrafluoroborate (Method B) rearrangement takes place at lower temperatures but the yields of benzazepine are inferior as the silver(I) ion also catalyzes the reverse reaction (see Section 3.2.2.1.). 

Benzoyl-l-methyl-3,4-benzo-2-azabicyclo[3.2.0]hepta-3.6-diene (3) at 250"C yields exclusively l-benzoyl-2-mcthyl-l//-l-benzazepine (4).23 In contrast, silvcr(I) ion catalyzed ring expansion of 3 yields a mixture of the rearranged bicycle 5 (43 %), starting material (43 %), and the isomeric 1 H-l-benzazepines 4 (8%) and 6 (4%).23... 

The thermally induced electrocyclic ring opening of 2-alkyl- and 2-acyl-3.4-benzo-2-azabicyclo-[3.2.0]hepta-3,6-dienes 2 to l//-l-benzazepines 1 (see Section 3.2.1.4.1.1.) are photorever-sible.23,37 38 Also, l-acyl-1//-benzazepines 1 (R = acyl), in refluxing xylene in the presence of silver(I) tetrafluoroborate, are in thermal equilibrium with their valence isomers the 2-acyl-3,4-benzo-2-azabicyclo[3.2.Oj nepia-3,6-uienes 2 (R = acyl).23-38... 

Another photochemical ring enlargement leading to a 1,3-oxazepine is the formation of 2-phenyl-l,3-oxazepine (13) by irradiation of 4-phenyl-2-oxa-3-azabicyclo[3.2.0]hepta-3,6-diene (11).12 It is proposed that the process involves the azirine 12 as an intermediate. 

A valence bond isomer of pentakis-(trifluoromethyl)-l, 3-diazepine (44) was prepared from (43) (81TL1113) (44) can be transformed thermally or photochemically to a 2,4-diazabicyclo(3.2.0)hepta-2,6 diene (45), which was subsequently photolysed to an imidazole in an anionic process. Compound (45) is highly acidic arising out of the bishomoaromaticity of the anion and forms a salt with Et3N (81TL1369). 

The Dewar benzene of hexafluorobenzene formed an adduct with pheny-lazide that gave a polyfluoro-l//-azepine on pyrolysis. R=C02Et (47) was obtained when ethylazidoformate was decomposed in C6F6 [82JCS(P1)2101]. Photolysis of (47) yielded a 2-aza-bicyclo(3.2.0.)hepta-3,6-diene, which, in contrast to its nonfluorinated analogue, showed excellent thermal stability (3 h, 200°C, 88% recovered) [82JCS(P1 )2105]. 

A 0-0 C3P-C3PS 2-Thia-l-phospha-bicyclo [3.2.0]hepta-3,6-diene 2, 493, 502 ... 

Scheme 6.53 Products formed by cycloadditions of 3<52-l H-naphthalene (221), liberated from 220 by KOtBu, with furan, 2,5-dimethylfuran and spiro[2.4]hepta-4,6-diene.

Figure 11.5 Ball-and-stick transition state models for (i) the 5-dig-cyclization of the hexa-4,5-dien-l-yl radical (left) and the 5-exo-trig-cyclization of the hepta-5,6-dien-l-yl radical (right).

Whereas alkylation of activated methylene systems by classical methods produces a mixture of mono- and dialkylated products, with the latter frequently predominating, phase-transfer catalytic procedures permit better control and it is possible to obtain only the monoalkylated derivatives. Extended reaction times or more vigorous conditions with an excess of the alkylating agent lead to dialkylated products or, with dihaloalkanes, carbocyclic compounds as the technique mimics dilute concentration conditions, e.g. the resonance stabilized cyclopentadienyl anion, generated under solidiliquid two-phase conditions, or under liquiddiquid conditions, reacts with 1,2-dihaloethanes to form spiro[2,4]hepta-4,6-diene (70-85%) [1-3]. Reaction with dichloromethane produces bis(cyclopenta-2,4-dien-l-yl)methane (60%) [4],... 

Hept-6-en-2-ol, (E) -5 -iso-propyl- 7 - (2-methy-tetrahydro-fur-2-yl) Lf Hept-6-en-2-one, (E)-5-iso-propyl-7-(2-methyl-tetrahydro-fur-2-yl) Lf Hepta-4-(E)-6-dienoic acid, 3-iso-propyl-6-methyl, methyl ester Cured Lf 0.075 Hepta-4-6-dien-l-ol, 3-iso-propyl-6-methyl, (E) Lf ... 

Methylenetetrahydropyrans are formed regioselectively in good yields from the Ru-catalysed reaction of prop-2-yn-l-ols with allyl alcohol <99JOC3524>, whilst 2-vinyltetrahydropyrans result from a Pd-catalysed intramolecular hydrocarbonation of alkoxyallenes <99TL1747>. Various hepta-5,6-dien-l-ols 13 undergo a Ru-catalysed... 

Recently, a new reactivity index has been proposed (80H(14)1717> which predicts accurately the site selectivity of photocyclization of substituted cycloheptatrienes to their bicyclic valence tautomers. Unfortunately, application of the method to substituted lH-azepines is far less successful. For example, for 2-methyl-l-methoxycarbonyl-lH-azepine (37 R = 2-Me) AGrs values for C-2—C-5 and C-4—C-7 cyclization are calculated as 0.093 and 0.040 kJ mol-1, respectively, i.e. predicting the 1-methyl isomer (39) as the major product. Experimentally, however, the reverse is true, the yields being 93.5% for 3-methyl (38 R = Me) and 6.5% for 1-methyl (39 R = Me). The corresponding photoinduced valence isomerizations of 1-benzazepines to 3,4-benz-2-azabicyclo[3.2.0]hepta-3,6-dienes (38a) have been recorded (80JOC462). These isomerizations have also been achieved thermally in the presence of silver ion (80TL3403). 

A few examples of the photoisomerization of AH- azepines to l-azabicyclo[3.2.0]hepta-2,6-dienes have been noted, e.g. (49) -> (50) (80TL595). 

Initial thermal sigmatropic 1,5-carbon shifts are believed to account for the product formation from spiro[2.4]hepta-4,6-diene (l) 200 and spiro[cyclopropyl-l, 2 -2 //-indene] (6),201 obtained by photodecarbonylation of dione 5. In the first case, a 1,5-hydrogen shift and an electrocyclic ring opening completes the formation of 3,4-dimethylenecyclopent-l-ene (4),200 and in the second case, the initially formed benzoannulated bicyclo[3.2.0]hepta-l,3-diene 7 dimerizes to 8 as mixture of two stereoisomers.201... 

---