Cyclohexadienes from 1,3,5-trienes

A similar analysis also correctly predicts the stereochemistry of the formation of a cyclohexadiene from a triene For the thermal reaction the HOMO is ti3. Examination of this MO shows that disrotation is necessary for the overlap forming the new sigma bond to be bonding ... 

In the projected synthesis of vitamin B12, the plan called for the construction of a key intermediate by the stereospecific cyclization of a stereochemically well-defined 1,3,5-triene to the corresponding 1,3-cyclohexadiene. From the inspection of molecular models, Woodward and his colleagues were confident that the minimization of angle strain coupled with appropriate orbital overlap would favor a conrotatory cyclization. While the reaction was indeed found to be highly stereospecific, it took the disrotatory path instead. To explain the observed contradiction, it was necessary to recognize a new control element that Woodward and Hoffmann christened conservation of orbital symmetry [2, 3]. 

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

There is a very significant difference between the rate of aromatization of trans- and c/i-hexatriene (Table III), which shows that geometrical isomerization prior to cyclization may be rate limiting. Since this occurs via half-hydrogenated species (60), it is promoted by the presence of hydrogen, and so is benzene formation. It should be noted that cyelohexane and cyclohexene are produced from cw-triene. The hydrogenation of cyclohexadiene may explain their formation here and in other cases of stepwise Cg dehydro-cyclization. 

Consequently, it is not surprising that the rate ratio for solvolysis of 99 100 was found to be greater than 2.5 x 106 and that at 150°C 100 did not solvolyze at all.395 This evidence is kinetic. Unlike the cases of the cyclobutene—1,3-diene and cyclohexadiene—1,3,5-triene interconversions, the direct product here is a cation, which is not stable but reacts with a nucleophile and loses some of its steric integrity in the process, so that much of the evidence has been of the kinetic type rather than from studies of product stereochemistry. However, it has been shown by investigations in super acids, where it is possible to keep the cations intact and to study their structures by nmr, that in all cases studied the cation that is predicted by these rules is in fact formed.396... 

Pyranylidene complexes, which are easily obtained from (l-alkynyl)car-bene complexes, prove to be most valuable building blocks for the generation of open-chain l-metalla-l,3,5-trienes as well as of cyclohexadiene annulation products. 

From a thermodynamic standpmnt, one can expect that for the thermal conversion of (1) to (2) and related derivatives, unless the temperature is sufficiently high or strain factors become predominant, cycloreversion will not be observed. A more complete discussion of the factors affecting the thermal equilibration of strained hexatrienes and cyclohexadienes will be discussed in Section 6.2.2.2.I. A kinetic study of the pericyclic transformation of the parent triene (1) to 1,3-cyclohexadiene (2) provided an activation energy of 29.9 kcal mol. Taking into account the heats of formation of reactant and product (40.6 kcal mol for (Z)-l,3,5-hexatriene and 25.4 kcal mol" for 1,3-cyclohexadiene), the... 

Only a few cases of thermal electrocyclizations of [1,2 3,4] bis-annulated trienes have been described. That die central double bond is part of a ring system eliminates the possibility of secondary thermal processes that were operating in the precursors to the simpler type IV cyclohexadienes. For example, the cyclization of diketone (239) to (240) takes place smoothly without interference from other thermal processes. ... 

In the course of their synthesis of Vitamin B12, R. B. Woodward and co-workers were puzzled by the failure of certain cyclic products to form from apparently appropriate starting materials—in particular, the stereochemistry of interconversions of cyclohexadienes with conjugated trienes in thermal and photochemical reactions. Woodward, in collaboration with Roald Hoffmann (ca. 1965), discovered that the course of such reactions depended on identifiable symmetries of the participating molecular orbitals. The principle of conservation of orbital symmetry can be stated thus ... 

The cobalt and PTC carbonylation of dienes and trienes give acylated products [147]. Cyclohexa-1,3-diene is transformed to 1-acetylcyclohexa-1,3-diene and a mixture of isomeric diacylated cyclohexadienes. 1-Vinylcyclohexene and other substituted dienes are acylated at the dienic moiety to give the tranj -isomers of the corresponding dienones [153]. Carbonylation of a-vinylcinnamic acid in the presence of nickel cyanide gives a-ketolactone, resulting from a decarboxylation-double carbonylation reaction [154]. 

The olefin synthesis has been extended to the preparation of conjugated dienes from the tosylhydrazoncs of /3-unsatnrated ketones.61 The reaction is particularly useful for preparation of 1,3-cyclohexadienes as shown by the preparation of (8). /3-lononc tosylhydrazone was converted into the triene (9). The method cannot be used to prepare allenes. Further experiments indicated that the eliminated a-hydro-gen must be part of a methylene or methyl group. Methyllithium was found to be superior to -butyllithium. 

The pattern that emerges from the experimental and computational studies of the conjugated dienes and trienes is the involvement of CIs having certain features in common. The singlet CI for the dienes appears to be a tetraradicaloid with the potential for several re-bonding schemes. In the absence of steric problems, it is structurally compact. The hexatriene-cyclohexadiene system also appears to involve a tetraradicaloid structure, one component of which is an allylic system. These structures can account for the major product types in both systems. 

The reaction hexenes —> hexadienes was demonstrated without using radiotracers both on oxide and metal catalysts, Nil 1 and Ptj l Mixtures containing [ " CJ-hexene contributed to the clarification of the further reaction pathway. These studies showed that neither the hexene cyclohexane nor the hexene —> cyclohexene ring closure pathway took place.Table 2 indicates that radioactivity appeared in both the hexatriene and 1,3-cyclohexadiene fractions when their inactive form was admixed to radioactive hexene. The aromatisation of both inactive components was much more rapid than that of hexene, therefore their specific radioactivities showed very low absolute values, however, these were still higher than that of benzene produced mainly from these non-radioactive precursors. The true precursor of ring closure should have been cis-cis-1,3,5-hexatriene. Its ring closure takes place without any catalyst from 513 The stepwise dehydrogenation of open-chain hydrocarbons produces cis- and trans-isomers of alkenes and alkadienes. Any c s-c s-triene... 

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