Cyclohexadiene structures, thermal

Thermal scission of cyclohexadiene structures formed via the intramolecular Diels-Alder route (49,50) constitutes an attractive mechanism... 

Thus, widely used aromatic polyamides with high mechanical characteristics and thermal stability [45] are rather reactive relative to NO. This is because of the specific ion-radical initiation. In this process, chemically inert aromatic rings are transformed into active cyclohexadiene structures, and in this way simple synthesis of spin-labeled... 

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. 

Prior to the work of Cheng et al. Rubin et al. reported a similar [4 + 4] pho-tocyclization reaction [299]. Rubin has shown that a cyclohexadiene derivative underwent a very facile photochemically promoted rearrangement to the stable bridged bisfulleroid 198 (Scheme 76). This process occurs via the initial [4 + 4] photoadduct (not observed), which undergoes a thermally allowed [2 + 2 + 2] cycloreversion to afford a bis-methano[12]annulene structure 198. Alternatively, compound 198 can be obtained by photolysis of the allylic alcohol 197 under reflux and acidic conditions in good yields. The allylic alcohol is obtained by acidic... 

Figure 2 Classification of the six-electron (6e ) electrocyclizations is based on the structure of the resulting cyclohexadiene derivatives (Types I-XVU) irrespective of the method of ring closure, whether thermally, photochemically or otherwise induced...

The 1,4-diyl species, 73 +, is obtained also upon electron transfer from 1,5-cyclohexadiene, 77, both in cryogenic matrices [143] and in solution [201]. Solution experiments provided chemical evidence for the cyclohexane-1,4-diyl structure type in an elegant study of the electron-transfer-initiated photochemistry of 2,5-diphenylhexa-1,5-diene, 78, and derivatives in the presence of molecular oxygen [201-203]. The intermediate 1,4-cyclohexanediyls, 79 , were intercepted by O2 the stereochemistry of the en /o-peroxide products, 80, showed that the initial cycloaddition occurred in the same stereospecific manner established for the thermal rearrangement of the neutral parent [204]. 

The interconversions of the corresponding dimethylcyclohexadienes and the 2,4,6-octatrienes are also stereospecific (Fig. 29.13). Here, too, thermal and photochemical reactions differ in stereochemistry. If we examine the structures closely, we see something else the stereochemistry of the triene-cyclohexadiene interconversions is opposite to that of the diene-cyclobutene inlerconversions. For the thermal reactions, for example, cis methyl groups in the cyclobutene become cis and trans in the diene cis methyl groups in the cyclohexadiene are trans and trans in the related triene. 

Some information regarding thermal stability of this type of polymer is available in literature. These polymers typically generate CO2, some maleic anhydride, and fragments related to the olefin. For example, poly(maleic anhydride-co-frans-stilbene) (alternating), when heated from ambient to 500° C, generates CO2, styrene, maleic anhydride small amounts of benzene, cyclohexadiene, dimethylbutene, ethylcyclobutanol, and 4-methoxystyrene. Major decomposition products for this polymer are chain fragments (79%), including aromatic, ketonic and unsaturated structures [5],... 

Poly(para-phenylene), with its rodlike structure composed of highly delocalized n-electron orbitals, satisfies most of the requirements for high thermal stability. Possessing Ti/2 > 400°C and an estimated melting point of 1400°C, PPP constitutes a reference for heat-resistant polymer. Poly(para-phenylene) can be synthesized by a number of routes, with the best heat-resistant material obtained by polymerization of 1,3-cyclohexadiene, followed by dehydrogenation of the polymer formed according to Eq. (35) [16]. 

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