Bicyclo- -hexatriene

An especially interesting case of the hexatriene-cyclohexadiene type interconversion is the rapid equilibrium between cycloheptatrienes and bicyclo[4.1.0]hepta-2,4-dienes ... 

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

In solution, an initial photoequilibrium is established between the Z- and -isomers, while the rearrangement products 117 and 118 are formed along with traces of cyclohexadiene (CHD) over much longer irradiation times (equation 46). In solution, the major products are 3-vinylcyclobutene (117) and bicyclo[3.1.0]hex-2-ene (118) Z-l,2,4-hexatriene (119), which is a major product in the gas phase176,211, is formed in relatively low yields. The quantum yields for ,Z-photoisomerization of Z- and -l,3,5-hexatriene in pentane solution (265 nm excitation) are /, r = 0.034 and E—Z = 0.016, respectively188. 

In 1975, Washburn et al. investigated the dehydrohalogenation of 2,6-dibromo-bicycIo[3.I.0]hex-3-ene (17) with bases and postulated the intermediate formation of bicyclo[3.1.0]hexatriene (18, Scheme 16.7). 

Theoretical studies have indicated that m-bcnzync is monocyclic with a C(l)-C(3) distance of 2.0 A whereas in tetrafluoro-w-benzyne the increased eclipsing strain between fluorine atoms stabilizes the bicyclo[3.1.0]hexatriene form with a C(l)-C(3) distance of 1.75 A.56 Computational studies coupled with gas-phase experimental studies show that appropriate substituents can be used to tune the reactivity of 1,3-arynes. Thus the presence of NH+ at C(5) makes (13) mildly carbocationic whereas the addition of OH at C(4) in (14) gives a highly reactive (bi)radical.57... 

The importance of conformational effects on photoreaction pathways are clearly seen with substituted hexatrienes. For the opening of hexalins (54), Ae ratio of vinylcyclobutenes (56) to bicyclo[3.1.0]hex-anes (57) depends on Ae size of the substituent R in accordance with a preponderance of one or the other conformer of the photogenerated hexatriene (55). In addition, when the (cZc) conformer is favored, the... 

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. ... 

Oxidation of polyalkyl-substituted 1,2,5-hexatrienes with m-chloroperbenzoic acid produces bicyclo[3.1.0]hexan-2-one derivatives . Certain 1,4-dienes undergo homo-conjugative addition of phosphorus reagents to give 3-phosphabicyclo[3.1.0]hexane derivatives (equation 36) ... 

Dehydrohalogenation. Alkyl-1,3,5-hexatrienes can be prepared conveniently in high purity in 30-40% yield by dehydrohalogenation of dienyl halides with 1,5-diaza-bicyclo[4.3.0]nonene-5 in DMSO (25-80°). Use of potassium r-butoxide-DMSO yields only polymeric material.1... 

The reaction of 3,3-disubstituted cyclopropenes with mono- and 1,2-disubstituted alkenes proceeds only with difficulty and leads to low yields of cyclopropanes. In the case of but-l-ene, an 8% yield, with hex-1-ene and hept-l-ene between 5 and 10% yield, and with cyclooctene about 10% of the cyclopropane product is formed. In these cases, the major product is the formal dimer of the intermediate ethenylcarbene complex, i.e. the corresponding (fj-hexatriene. When copper(I) chloride is used as catalyst rather than the copper halide/phosphane or phosphite system, about half the yield of the [2-f-1] cycloadduct is obtained along with an increased amount of the hexatriene. Mechanistically, these acyclic trienes could also be formed from an (alk-l-enyl)bicyclo[1.1.0]butane intermediate without any carbene being involved. Bicyclo[1.1.0]butanes are low yield (< 20%) byproducts of the thermal dimerization reaction of methyl 3,3-dimethylcyclopropenecarboxylate (1). On the other hand, bicyclo[l. 1. Ojbutanes, such as 3, are known to undergo isomerization to form 1,3-dienes. ... 

Barton has suggested 35> that in photochemical reactions of cyclic conjugated olefins, ring fission will predominate in rings of n annular atoms containing (n/2)-l double bonds and valence tautomerization will occur in other systems. Consequently, it was anticipated that 1,3,5-cyclo-octatriene upon photolysis should give rise to an acyclic tetraene in the same manner that 1,3-cyclohexadiene opens 35> 36> to 1,3,5-hexatriene. When 1,3,5-cyclooctatriene (55) was irradiated in solution (ether or pentane) 37> 39> two isomerization products were isolated, bicyclo[4.2.0]-octa-2,7-diene (56) and tricyclo[5.1.0.0 4> 8]oct-2-ene (57). The formation of 56 is not exceptional. The formation of 57 has been visualized by... 

Am System Hexatrien-( 1,3,5)/Cyclohexadien-(1,3)/Bicyclo [3.1.O.Jhexen-(2) lassen sieh die folgenden elektrocyclisehen Prozesse beobachten, von denen die eon-rotatorischen Ringschlusse (bzw. Ringoffnungen) im angeregten Zustand erlaubt sind2. 

An oxa-analogue of the hexatriene-bicyclo[3,l,0]hex-2-ene conversion has been reported from the study of the photolysis of the isochromenes (142). 5 Irradiation (Pyrex filter) in methanol gave two products (143) and (144), the second of which arises by secondary photolysis of the oxabicyclohexene (143). This product most likely arises by [4 + 2]photoaddition of the quinone methide intermediate... 

Washburn has provided further detailed evidence to substantiate his claim to the synthesis of bicyclo[3,l,0]hexatriene, and cyclopropene intermediates are also involved in the photodecomposition of tosylhydrazide (nO). " Dehydrochlorination of (383) (synthesized by the addition of chloromethylcarbene to chloroethene) with t-butoxide in THF affords (385) the previously unreported parent methylenecyclo-propene (384) is presumably involved. More compelling evidence for the intermediacy of (384) comes from the reaction between (386) and t-butoxide (Scheme 46). Me Cl... 

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