1.4- bicyclo hexadiene

Two intramolecular [2 + 2] photocycloadditions of note have been reported. The quantitative conversion of 7-oxanorbornadiene to the corresponding oxaquadri-cyclane has now been achieved despite many earlier efforts. Few syntheses of prismane derivatives can match the efficiency displayed in the cyclization to afford (45) which results from cycloaddition across the 1,4-diene system of the bicyclo-hexadiene, being best effected with light of wavelength 350 nm, which the product... 

Bicyclo[2.2.0[hexadiene, hexamethyl-[Bicyclo[ 2.2.0] hexa-2,5-diene,... 

Bicyclo[2.2.0]hexadienes and prismanes are valence isomers of benzenes. These compiounds actually have the structures that were proposed for benzenes in the nineteenth century. Prismanes have the Ladenburg formula, and bicyclo[2.2.0]-hexadienes have the Dewar formula. Because of this bicyclo[2.2.0]hexadiene is often called Dewar benzene. On page 32 it was mentioned that Dewar formulas are canonical forms (though not very important) of benzenes. Yet, they also exist as separate compounds in which the positions of the nuclei are different from those of benzenes. 

It was pointed out earlier that a Cope rearrangement of 1,5-hexadiene gives 1,5-hexadiene. This is a degenerate Cope rearrangement (p. 1380). Another molecule that undergoes it is bicyclo[5.1.0]octadiene (105). At room temperature the NMR... 

The thermolysis of ladderanes has been studied in detail (Scheme 1). On heating, bicyclo[2.2.0]hexane and its derivatives exhibit skeletal inversion and cleavage to 1,5-hexadiene derivatives.26 The thermolysis of anti- and yyft-tricyclo[4.2.0.02,5]octanes and their derivatives gives cis,cis- and cis, trans-1,5-cyclooctadienes, cis- and trans-1,2-divinylcy clobutanes, and 4-vinylcyclohexene as ring-opening products.27-29 Furthermore, syn-tricyclo-[4.2.0.02,5]octane isomerizes to aw//-tricyclo[4.2.0.02,5]octane.29c,d The thermodynamic parameters and the reaction mechanisms for these thermal reactions have been discussed. 

Diisobutylaluminium hydride catalyses the ring-closure of various dienes. It is proposed that the process involves addition of the aluminium hydride to a terminal double bond, followed by ring-closure and, finally, elimination of the catalyst (equation 106). Thus 1,5-hexadiene gives methylenecyclopentane (213) (equation 107), 1,6-heptadiene gives methylenecyclohexane (214) (equation 108), 4-vinylcyclohexene gives bicyclo[3.2.1]oct-2-ene (215) (equation 109) and the spiro compound 217 is obtained from 5-methylene-l,8-nonadiene (216) (equation 110)112. 

Anwendung dieser Methode auf konjugierte Diene (z. B. 2,4-Hexadien, 1,3-Cyclooctadien) mit Anilinen als Amin-Komponenle fuhrt zur oxidativen Cycloaminierung, z.B. unter Bildung des betreffenden 1-AryI-2,5-dimethyl-2,5-dihydro-pyrrols bzw. 9-Aryl-9-aza-bicyclo[4.2.1]non-2-ensI. Das 1,3-Dienwird bei dieser Reaktion im UberschuB (2 equiv.) eingesetzt. 

Hexadien und 1,5-Cyclooctadien reagieren unter sonst gleichen Bedingungen mit 2-Aquivalenten Thallium(III)-acetat und 3-Aquivalentcn Anilin zu 2,5-Bis-[anilinome-thyl]- 1-phenyl-pyrrolidin (75%) bzw. 2,6-Dianilino-9-phenyl-9-aza-bicyclo[3.3.1]-nonan <70%>-... 

Support for these mechanistic proposals was boosted by the successful isolation of related valence isomers from benzenes or heteroaromatic compounds carrying bulky (t-butyl) or fluorinated groups. 1, 3,5-Tri-t-butvl benzene gives rise to a benzvalene 13.26), but fluorinated substituents or more severe steric crowding (e.g. 3.271 lead to a preference for bicyclo[2.2.0]hexadiene formation Ithese latter isomers are sometimes called Dewar benzenes I. as does the presence of nitrogen in the ring (3.28. ... 

Nonconjugated dienes and polyenes have triplet photochemistry which may be considered to arise from intramolecular interaction of one excited double bond with an isolated ground-state double bond. For example, the photocyclization of enrfo-dicyclopentadiene can be effected using acetone as a sensitizer.286 Other more flexible 1,5-dienes, when sensitized to triplet states, cross couple to yield bicyclo[2.1.1]-hexane structures. For instance, triplet mercury atoms convert both 1,5-hexadiene and 1,5-cyclooctadiene to such structures.267 Irradiation of the cyclooctadiene in the presence of cuprous chloride produces the tricyclo derivative in good yield266 but recent evidence again indicates that this latter reaction may proceed via free-radical intermediates.269... 

Pyridine 20 has been prepared by the silver- or acid-catalyzed isomerization of 1-aza-bicyclo[2.2.0]hexadiene 22, which in turn was obtained in a [2 + 2]-cycloaddition reaction of tris(to7-butyl)azete with hexafluorobut-2-yne. Irradiation of Dewar pyridine 22 gives a high yield of azaprismane 23, which isomerizes thermally to rearranged pyridine 24. Finally, pyridine 24 yields 1-aza Dewar pyridine 25 photolytically.34... 

In using the resonance method, we assume that all the resonance structures contributing to a given resonance hybrid have exactly the same spatial arrangements of the nuclei but different pairing schemes for the electrons. Therefore 11, 12, and 13 are not to be confused with bicyclo[2.2.0]-2,5-hexadiene, 15, because 15 is a known (albeit not very stable) molecule with different atom positions and therefore vastly different bond angles and bond lengths from benzene ... 

Bicyclo[2.2.0]-2,5-hexadiene is much less stable than its isomer, benzene, yet it does not rearrange to benzene except at elevated temperatures. Give a reason for this observation. 

Photolysis of nor camphor gives rise to carbon monoxide, 1,5-hexadiene and bicyclo [2.1.1] hexane (XXIV). The stoichiometry of the products... 

In the photolysis of bicyclo [3.2.0] heptanone-3 (XXIII) the important products are carbon monoxide, 1,5-hexadiene and a polymer. A small amount of bicyclo [2.2.0] hexane (XXVII) is also formed. Although the... 

Das Bicyclo[2,l.l]hexan-Derivat 38 mit der gleichen Stereochemie wie in 36 stellt 90 % des fliichtigen Produktes der durch Benzophenon sensi-bilisierten Isomerisierung des Nerylacetats (Formel 37) (70) dar. Ebenso laBt sich l,5-Hexadien-3-on (Formel39) glatt in Bicyclo[2,l,l]hexan-2-on (Formel 40) uberfuhren (26). 

Tetraphosphahexadienes substituted with either O or N groups at the bridging C atoms show, with respect to reactivity, the ruling influence of substituents in these positions. The O-substituted compounds undergo [2 + 2] cycloadditions, whereas N-substituted compounds do not. A cross-test with a partially 0-, partially N-substituted compound demonstrated both possibilities, with the bicyclos and the tetraphospha-hexadiene interconnected via an equilibrium [Eq. (49)]. 

Isopropyliden-l-(2-methyl-propen-yliden)-E19b, 144 (Carben + En) Cyclopropan-(spiro-3)-bicyclo 2.2.0 hexan-<2-spiro>-cyclopropan E17c. 2236 (Cyclobuten + Bi-cyclopropyliden) Cyclopropan--bicyclo 2.2.2 oct-2-en El7b, 1531 (1,3-Cyclo-hexadien + En)... 

Woodward and Hoffmann have recently invited speculation on the possible existence of raws-bicyclo[2,2,0]hexadiene (62) b. The as-isomer 63 is a relatively stable compound owing its existence to the fact that the only geometrically simple paths of rearrangement to its preferred valence isomer benzene are s5umnetry-forbidden. The question about the possible existence... 

The capacity of a metal system to remove orbital symmetry restrictions invites similar speculation on the survival capabilities of certain metal-coordinated species. The existence, for example, of quadricyclene coordinated to certain transition metals 29) in a bidentate configuration rests upon the ligand field restrictions described by the nonreacting ligands. When these restrictions are removed, the existence of quadricyclene becomes as tenuous as that of [Pg.89]

At low photochemic conversions the reversible transformation to the (2D hexatriene may dominate. However, the composition of the photostationary state depends upon the substitution pattern of the cyclo-hexadiene which controls the preferred conformation. It is believed that a planar 1,3-cyclohexadiene produces preferentially a bicyclo[2.2.0]hex-2-ene, e.g. molecules of the type (374), while dienes with skewed structures form hexatrienes. Another factor that changes dramatically the composition of the photolysis mixture is the wavelength of irradiation. At 254 nm the photostationary state mixture of cis-bicyclo[4.3.0]nona-2,4-diene (58) and ( ,Z,Z)-l,3,5-cyclononatriene (59) is 40% and 60%, respectively. At 300 nm, irreversible formatitm of tricyclo[4.i0.0 ]non-3-ene (374) becomes the preferred pathway. The ratio of extinction coefficients of (58) and (59) at the wavelength used would explain the shift in the photoequilibrium mixture. ... 

The Marcus equation provides a nice conceptual tool for understanding trends in reactivity. Consider for example the degenerate Cope rearrangement of 1,5-hexadiene and the ring-opening of Dewar benzene (bicyclo-[2,2,0]hexa-2,5-diene) to benzene. Figure 15.29. The experimentally observed activation energies are 34 kcahmol and 23 kcal/mol, respectively. The Cope reaction is an example of a Woodward-Hoffmann... 

In recent years there has been a sharp decline in the number of reports dealing with photoisomerizations of benzenes into fulvenes, benzvalenes, bicyclo[2.2.0]hexadienes (Dewar-benzenes), and prismanes. There is now... 

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