Cyclohexadiene from benzene derivatives

Miscellaneous.- New optically active phosphites (103) and (104) derived from R- or 5-2,2 -binaphthol have been prepared by standard methods and used as ligands for asymmetric hydroformylation or hydrocyanation reactions. Low to moderate asymmetric induction was achieved in the synthesis of S,6-disubstituted cyclohexadienes from benzene chromium complexes when the optically active phosphites (105) were ligands. ... 

The pioneering work of Gibson3 on the isolation and mutation of Pseudomonas strains that oxidatively degrade aromatic compounds has led, 25 years later, to the application of cyclohexadiene cis-diols in asymmetric synthesis. The first applications of these types of compounds to synthesis were the use of meso-diol derived from benzene for production of polyphenylene4 by ICI and in the synthesis of racemic... 

As a related reaction, Yamamoto reported a novel allylative dearomatization to give 6-allyl-3-mettiylene-l,4-cyclohexadiene (373) by the reaction of benzyl chloride and allylstaimane under mild conditions. In this reaction, the 7r-benzylpalla-dium species 370 is generated from benzylpalladium 369. The bis-7r-allylpalladium species 371 and 372 are generated by the reaction of tt-benzylpalladium 370 with allylstaimane. Finally the dearomatization product 373 is formed from 372 by reductive elimination. It is surprising that the dearomatization product 373 is rather stable and no isomerization to a benzene derivative was observed at room temperature [142]. 

Benzene was introduced in Chapter 5 (Section 5.10). Chapter 21 will discuss many benzene derivatives, along with the chemical reactions that are characteristic of these compounds. In the context of dissolving metal reductions of aldehydes, ketones, and alkynes, however, one reaction of benzene must be introduced. When benzene (65) is treated with sodium metal in a mixture of liquid ammonia and ethanol, the product is 1,4-cyclohexadiene 66. Note that the nonconjugated diene is formed. The reaction follows a similar mechanism to that presented for alkynes. Initial electron transfer from sodium metal to benzene leads to radical anion 67. Resonance delocalization as shown shordd favor the resonance contributor 67B due to charge separation. 

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,... 

In case of benzene, the potassium salt of its anion-radical can be separated as a precipitate after benzene reduction by potassium in the presence of low concentrations of 18-crown-6-ether. For benzene, the heavy-form content is greatest in the solution, not in the precipitate. It is in the solution where most of the nonreduced neutral molecules remain. Since the neutral molecules are inert toward protons, the anion-radicals combine with the protons to give dihydro derivatives (products of the Birch reaction). Therefore, it is possible to conduct the separation chemically. The easiest way is to protonate a mixture after the electron transfer, than to separate the aromatic compounds from the respective dihydroaromatics (cyclohexadiene, dihydronaphthalene, etc.) (Chang and Coombe 1971, Stevenson and Alegria 1976 Stevenson et al. 1986a, 1986c, 1988). 

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

In all cases, 4-acetyl-5-Z-8-methyl-ll-oxatricyclo[6.3.0.01,4]undeca-2,5-di-enes (Z = OMe, F, CH3, CF3), resulting from syn addition, were the predominant photoproducts, as determined by direct NMR analysis. Following thermal treatment, the cyclooctatriene and/or cyclohexadiene derivatives were then isolated in 60-70% yield in the ratios shown in Scheme 21. With 2-methyl, 2-methoxy, and 2-trifluoromethyl substitution, the reaction is 100% regioselective syn, addition of the remote double bond occurring only at the 3,4-positions of the benzene ring, not the 4,5. The 2-fluoro derivative alone provides both anti and syn regioisomers in a 15 85 ratio. The 2-cyano derivative did not react. The authors surmised that the... 

In the formation of tetraenes from bicyclo[4.2.0]octa-2,4-dienes, two bonds are broken. This may occur in one concerted reaction which can be regarded as a retro [2 + 2] cycloaddition. It is also possible that the central bond, being part of a cyclohexadiene system, is the first one to break in a thermal, concerted disrota-tory process that leads to a 1,3,5-cyclooctatriene derivative. Ring opening of the cyclooctatriene then might take place photochemically, again disrotatory, to produce a tetraene. This two-step sequence was first observed by Mirbach et al. [114] in their study of the photocycloaddition of the two parent molecules benzene and ethene. The same explanation for the formation of a tetraene was given by Nuss et al. [160] in their report on the intramolecular ortho photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile (see Scheme 40). 

The reaction of electron-poor butadienes with enamines can afford the corresponding benzenes by treatment of the initial 1,3-cyclohexadiene derivatives with some oxidants or by heating96. An example is the preparation of sulfonyl benzenes 186 from disulfonyl dienes 183 and enamines 184 giving adduct 185, which then aromatizes (equation 38)97,98. The presence of an additional unsaturation like in trimethylsilyl... 

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