Cyclohexadienes, isomerization

The hexatriene/cyclohexadiene isomerization has been extensively studied and has been the topic of numerous reviews and monographs this section will attempt to deal only with applications of these reactions to synthesis, and in particular the use of these reactions for the synthesis of natural products. Much of the early work in this area was done by Havinga and coworkers during the course of their detailed work on the stereochemical consequences of the thermal and photochemical conversions in the vitamin D field this work provided much of the impetus for the development and elaboration of the Woodward-Hoffman mles (Scheme 7). The reversible photochemical ring opening of provitamin 30 to precalciferol (31) and the photochemical ring closure of 31 to lumisterol 32 can be explained by consecutive photochemically allowed conrotatory processes. ... 

Formation of 1,3-Cyclohexadienes. Isomerizations of 1,4-cyclohexadienes with KOt-Bu in DMSO furnish the thermodynamically more stable conjugated 1,3-cyclo-hexadienes. 

With Fe(CO)5, the coupling reaction and the isomerization of alkenes or aikynes proceed. Carbonyl insertion tends to proceed during these reactions [17]. In the reaction of alkenes, the reaction with conjugate dienes is especially liable to proceed. Therefore, non-conjugated dienes such as 1.4-cyclohexadiene isomerize to the conjugate dienes such as 1,3-cyclohexadiene to which Fe(CO)5 is easily coordinated as shown in eq. (15.8) [13,15,17-21]. 

Diacetoxylation of various conjugated dienes including cyclic dienes has been extensively studied. 1,3-Cyclohexadiene was converted into a mixture of isomeric l,4-diacetoxy-2-cyclohexenes of unknown stereochemistry[303]. The stereoselective Pd-catalyzed 1,4-diacetoxylation of dienes is carried out in AcOH in the presence of LiOAc and /or LiCI and beiizoquinone[304.305]. In the presence of acetate ion and in the absence of chloride ion, /rau.v-diacetox-ylation occurs, whereas addition of a catalytic amount of LiCl changes the stereochemistry to cis addition. The coordination of a chloride ion to Pd makes the cis migration of the acetate from Pd impossible. From 1,3-cyclohexadiene, trans- and ci j-l,4-diacetoxy-2-cyclohexenes (346 and 347) can be prepared stereoselectively. For the 6-substituted 1,3-cycloheptadiene 348, a high diaster-eoselectivity is observed. The stereoselective cij-diacetoxylation of 5-carbo-methoxy-1,3-cyclohexadiene (349) has been applied to the synthesis of dl-shikimic acid (350). 

The photo-induced exo selectivity was observed in other classic Diels-Alder reactions. Data relating to some exo adducts obtained by reacting cyclopentadiene or cyclohexadiene with 2-methyl-1,4-benzoquinone, 5-hydroxynaphtho-quinone, 4-cyclopentene-l,3-dione and maleic anhydride are given in Scheme 4.13. The presence and amount of EtsN plays a decisive role in reversing the endo selectivity. The possibility that the prevalence of exo adduct is due to isomerization of endo adduct under photolytic conditions was rejected by control experiments, at least for less reactive dienophiles. 

As described ( 6.5.3.1) l-phenylbora-2,5-cyclohexadiene (cyclic divinylborane) reacts with FeCCOj under irradiation to the Fe(CO)3 complex. The dinuclear borabenzene complex [(C5H5BPh)Fe(CO>2]2 is found as a by-product. It is to be compared with the well-known [Tj -CpFe(CO)2]2. CjHjBPh reacts spontaneously with Co2(CO)u to form three products. Above 60°C (C5H5BPh)Co(CO)2 is the only product . At 30°C yellow isomers III and IV having partially hydrogenated borabenzene ligands form above 40°C III isomerizes to IV. 

Reactions favoring [2 + 2] cycloaddition tended to be those that had strongly electronegative groups on the sp2-hybridized silicon but only H and the neopentyl group on the sp2-hybridized carbon atom. Butadiene and cyclohexadiene generally favored [2 + 2] cycloaddition with these silenes. The [2 + 2] adducts with cyclohexadiene appear to be kinetic products, since they cleanly isomerized to the Diels-Alder adducts over time.182... 

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

Finally, thermally induced isomerizations which generate carbon-centered biradical organic molecules have been shown to serve as alternative for conventional chemical and photochemical methods [71]. A straightforward procedure to accomplish such biradicals was described by Myers using a thermal conversion of yne-allenes [72]. According to this scheme, Wang and coworkers [73] heated 3-178 in 1,4-cyclohexadiene to 75 °C and obtained 3-181 in 22% yield via the biradicals 3-179 and 3-180 (Scheme 3.48). 

It is well known that, when treated with complex substrates, cyclopenta-diene can form complexes with cyclopentadiene, cyclopentadienyl, and even cyclopentenyl ligands. The same possibilities are found for bora-2,5-cyclohexadienes, but with the additional complexity of ligand isomerism. 

The bora-2,5-cyclohexadienes 25, 43, and 44 have been used in experimental studies. Photochemical reaction of 25 and 43 with Fe(CO)s produces the robust complexes 45 (29) and 46 (92) with 7)5-divinylborane structures (92), whereas thermally induced complex formation of 44 with Fe2(CO)9 is accompanied by ligand isomerization and affords complex 47 (67). We note in passing that S(UB) = 38.8 ppm for 47 is at rather low field. The only strictly comparable boraolefin known is l-methoxy-6-(trimethylsilyl)bora-2,4-cyclohexadiene [8(nB) = 47.1 ppm] (26). On this basis, the high field shift upon complexation (only 8.3 ppm) indicates weak Fe-B interaction. 

As with the isomeric cyclohexadienes, there are a variety of data to present. Let us start with the 1,5-isomer, 26 and remind the reader there are three forms of this species,... 

We wish to argue that experimental error is the case. Pedley cites liquid phase enthalpies of formation of —12.7 and —58.7 kJmol-1 for the isomeric 3-methylenecyclohexene and 2-methyl-l,3-cyclohexadiene. The difference of these two numbers, -46 kJ mol-1, is meaningfully... 

Our uncertainty is derived in part from the lack of a measured enthalpy of vaporization, cf Reference 67. However, what triggered our skepticism is the observation that the isomeric 1,2-and 1,4-dihydronaphthalenes have reported enthalpies of formation that differ by ca 13 kJ mol-1 while the corresponding species lacking the benzene ring, the isomeric 1,3- and 1,4-cyclohexadienes, are almost isoenergetic (see Section V.D of this chapter). From J. F. Liebman, in The Cyclophanes (Eds. P. M. Keehn and S. M. Rosenfeld), Academic Press, New York, 1983,... 

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

Palladium-catalyzed oxidation of 1,4-dienes has also been reported. Thus, Brown and Davidson28 obtained the 1,3-diacetate 25 from oxidation of 1,4-cyclohexadiene by ben-zoquinone in acetic acid with palladium acetate as the catalyst (Scheme 3). Presumably the reaction proceeds via acetoxypalladation-isomerization to give a rr-allyl intermediate, which subsequently undergoes nucleophilic attack by acetate. This principle, i.e. rearrangement of a (allyl)palladium complex, has been applied in nonoxidative palladium-catalyzed reactions of 1,4-dienes by Larock and coworkers29. Akermark and coworkers have demonstrated the stereochemistry of this process by the transformation of 1,4-cyclohexadiene to the ( r-allyl)palladium complex 26 by treatment... 

An intermediate containing the l-aza-2,3-cyclohexadiene system, which is isomeric to the skeleton of 311, was described for the first time in 1997. The action of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) on the carbacephalosporin triflate 322 in... 

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