Cyclo-hexadiene, substituted

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Generating 6 from 6,6-dibromobicydo[3.1.0]hexane (35) with methyllithium in the presence of styrene, Moore and Moser [55] observed the first [2 + 2]-cycloaddi-tions of 6. Bottini et al. extended the variety of compounds able to trap 6 to 1,3-cyclo-hexadiene [54, 60], furan, 2-methyl furan, 1,3-cyclopentadiene and methyl-substituted 1,3-butadienes [54], In all these reactions, the dimer 38 of 6 is a byproduct or, as in the case of the less reactive trapping agents, even the main product. Hence it is advisable to use a reaction partner of 6, if it is a liquid, as the solvent. 

The hydrogenation of simple alkenes, such as hexene, cyclohexene, cyclo-hexadiene and benzene, has been extensively studied using biphasic, alternative solvent protocols. These hydrocarbon substrates are more difficult to hydrogenate compared to substrates with electron withdrawing groups. Benzene and alkyl substituted aromatic compounds are considerably more difficult to hydrogenate... 

One of the most general and useful methods of access to (tj6-arene)ruthenium(H) complexes is based on the dehydrogenation of cyclo-hexadiene derivatives by ethanolic solutions of RuC13 xH20 (6). This method has been developed for the synthesis of binuclear complexes 1 by Zelonka and Baird (7) for benzene (la) and by Bennett and co-workers (8-10) for para-cymene (lb), a useful starting material (10), and substituted benzene derivatives, such as xylene, mesitylene (9), or 1,3,5-triphenylbenzene (11) [Eq. (1)]. The chloride ligands of complexes 1 can... 

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. ... 

Thermal isomerizations of 1,3-cyclohexadienes by [1,5] hydrogen shifts were first observed in multiple methyl-substituted substrates29"32. The individual isomers of l-methyl-l,3-cyclo-hexadiene, 1 3, were shown to interconvert31 33 at temperatures above 300°C by sequential [1,5] hydrogen shifts eventually leading to the same equilibrium mixture33 ... 

Correlation diagrams can be constructured in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclo-hexadiene. They lead to the prediction that the disrotatory mode is an allowed process while the conrotatory process is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclization reactions can be analyzed by the same process. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases where the substitution pattern does not correspond in symmetry to that of the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

Use of alcoholic solvents leads to a decreased reaction rate however, tetrahydrofuran as a cosolvent with water can be employed without any noticeable effect on reaction rate or yield. Substituting the less-reactive cyclo-hexadiene for cyclopentadiene in the above equation requires heating (55 C) and results in a substantially lower yield (35%) of the corresponding fused bicyclic [2.2.2] product. 

Non-conjugated terminal 2-substituted-l,5-enynes also react through a 6-endo-dig cyclization to afford 1,3-cyclohexadienes via ruthenium vinylidene intermediates. Nishibayashi and coworkers reported that 2-substituted-l,5-enynes in the presence of 5% methanethiolate-bridged diruthenium complex [Cp RuCl (p2 SMe)]2 and 10 mol% of NH4BF4 afforded the corresponding 1,3-cyclo-hexadienes in good yields (Scheme 30) [144],... 

Cycloaddition reaction of two molecules of alkyne and alkene is a direct route for the synthesis of substituted cyclohexadiene derivatives, which are important components of the Diels-Alder reaction. Therefore, the reaction cycloisomerization of enediyne azamacrocycles represents a pathway to the synthesis of highly functionalized tetra-condensed cyclo-hexadienes in a single step. The synthetic variety of such reactions is... 

Wakatsuki, Y., Kuramitsu, T. and Yamazaki, H. (1974) Cobaltacyclopentadiene complexes as starting materials in the synthesis of substituted benzenes, cyclo-hexadienes, thiophenes, selenophenes and pyrroles. Tetrahedron Letters, 15(51), 4549-4552. 

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