Cyclohexadienes desymmetrization

The binuclear /x-oxo compound (Scheme 330) has been prepared and is readily isolated by chromatography. Subsequent treatment with SiXMe3 (X = C1, I, OTf) followed by transmetallation and desymmetrization reaction with the lithium salt of cyclohexadiene affords an excellent reagent for the transfer of cyclohexadienyl groups to various aldehydes.503... 

The experimental enthalpy of activation for disrotatory thermal isomerization of cis-l,3,5-hexatriene to 1,3-cyclohexadiene in the gas phase at 100 C is 29.2 kcal/mol [13]. The reaction is exothermic by 14.5 kcal/mol [14, p. 127], so of the reverse reaction is 43.7 kcal/mol, but - in spite of its high activation energy - it is characterized as allowed by all of the common orbital symmetry criteria. In norcaradiene ([4.1.0]hepta-2,4-diene), the cyclopropane ring bridging Cl and Ce of cyclohexadiene has built the disrotation into the molecule, desymmetrizing it - and its monocyclic isomer, cycloheptatriene - to C, in which the 61 and ai orbitals correlate directly (Fig. 5.3). The rate of isomerization is so much faster that it had to be measured at low temperature (ca. 100 K) in a hydrocarbon glass [15] is only 6.3 kcal/mol ... 

The first step is a disrotatory cyclohexadiene-hexatriene isomerization. Its product, cf5-dihydrobenzocyclooctatetraene, is less stable than the trans dimer and is known to isomerize to it, [27] the isomerization presumably taking place via an a" displacement that reduces symmetry to Ci, in which no reaction is forbidden. At the higher temperatures at which fragmentation occurs, the first product should be in equilibrium with the reactant, and its eight-membered ring is sufficiently flexible that a similar desymmetrization would allow it to serve as an unstable intermediate. The activation parameters cited above, which - for the postulated mechanism - measure the enthalpy and entropy differences between the reactant and the transition state of the second step, are not inconsistent with concerted electrocyclic rupture of both bonds via a relatively unconstrained transition state. 

Desymmetrization via bromolactonization has been attained in the case of the prochiral cyclohexadiene (19) on reaction with NBS catalysed by (DHQD)2PHAL (21) the corresponding fi- and y-bromolactones (20) were thus obtained with <93% ee. The latter reaction can also be used for the kinetic resolution of racemic cyclic ene-carboxylic acids. A catalytic asymmetric bromocyclization of trisubstituted olefinic sulfonamides (22) has been attained on reaction with iV-bromophthalimide (23), catalysed by the C2-symmetrical selenium derivative of mannitol (24) (Scheme 2). The reaction is believed to proceed via formation of the Br-Se+ species (26) that attacks the C=C bond with a subsequent 5 N) -exo-trig ring closure (27). ... 

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