1.2.5- Hexatriene, rearrangements

In an important extension of 3,3-shifts and hexatriene rearrangements, Bergman demonstrated the pairwise interchange of Cl and C3 with C4 and C6 of cis-, 2-diethynylethene at 200°C in the gas phase (Scheme 7.1). ... 

In solution, an initial photoequilibrium is established between the Z- and -isomers, while the rearrangement products 117 and 118 are formed along with traces of cyclohexadiene (CHD) over much longer irradiation times (equation 46). In solution, the major products are 3-vinylcyclobutene (117) and bicyclo[3.1.0]hex-2-ene (118) Z-l,2,4-hexatriene (119), which is a major product in the gas phase176,211, is formed in relatively low yields. The quantum yields for ,Z-photoisomerization of Z- and -l,3,5-hexatriene in pentane solution (265 nm excitation) are /, r = 0.034 and E—Z = 0.016, respectively188. 

It should be noted that products like 443 and 447 are the normal products of photochemical reactions of acyclic 1,3,5-hexatrienes, as well as of divinyl aromatics, but are quite unusual for thermal transformations of such substrates. Presumably, the electrostatic repulsion between CF2 groups prevents the formation of conformation 450 which is necessary for the electrocyclic ring closure (i.e. 438 — 439 and 445 -> 446). Instead, it leads to conformation 451 which is favorable to generate the diradical and then the fused vinyl-cyclopropane intermediates 452 (equation 170). Note that the rearrangement 452 —> 453... 

There is no unity of opinion in the literature concerning a classification, i.e, whether to call these transformations aza-Claisen or aza-Cope rearrangements. It is accepted that the term aza-Claisen should be reserved only for those processes in which a carbon atom in the allyl vinyl ether system has been replaced by nitrogen357. Three different types of aliphatic 3-aza-Cope reactions which were studied theoretically are the rearrangements of 3-aza-l,5-hexadienes (610, equation 262), 3-azonia-l,5-hexadienes (611, equation 263) and 3-aza-l,2,5-hexatrienes (612, equation 264) (the latter is a ketenimine rearrangement )357. 

The (diphenylmethylene)aminocyclobutenecarboxylates 109 obtained by rearrangement of the DMPA-H adducts of 1-Me, 2-Me, contain a 2-azadiene unit and a cyclobutene moiety. Indeed, the parent compound 109 a reacted with 4-phenyl-l,2,4-triazoline-3,5-dione (PTAD, [80]) at room temperature in a [4-1-2] cycloaddition mode to yield the tricyclic tetraazaundecene 132 in almost quantitative yield (Scheme 44) [8]. As substituted cyclobutenes, compounds 109 should be capable of opening up to the corresponding butadienes [1, 2b, 811. When compounds 109 were subjected to flash vacuum pyrolysis, the dihydro-isoquinolines 135 were obtained, presumably via the expected ring-opened intermediates 133, which subsequently underwent bn electrocyclization followed by a 1,5-shift, as is common for other 3-aza-l,3,5-hexatrienes [82]. 

The third mechanism of isomerization, photoinduced rearrangements of radical cations, has been pursued in a variety of systems. Matrix isolated radical cations have been noted to undergo some rigorous reorganizations as well as subtle ones. For example, the ring opening of cyclohexadiene to hexatriene radical cation and the interconversion of its different rotamers have been achieved by irradiation with UV or visible light [173-174]. 

Direct photolysis of l,l-dicyano-3,3-dimethyl-l,4,5-hexatriene 29 gives an allenyl-cyclopropane 30 via DPM rearrangement in 57% yield, while acetone-sensitization yields a hausan 31 via an intramolecular [2 + 2] cycloaddition (Scheme 4.15) [20]. 

The combination of the spin-coupled formulation of modem valence bond theory with intrinsic reaction coordinate calculations provides easy-to-interpret models for the electronic rearrangements that occur along reaction pathways. We survey here the information revealed by such studies of the mechanisms of various gas-phase six-electron pericyclic reactions the Diels-Alder reaction between butadiene and ethene, the electrocyclization of cis-l,3,5-hexatriene, the 1,3-dipolar cycloaddition between fulminic acid and ethyne, and the 1,3-dipolar cycloaddition of diazomethane. The fully-variational CASVB strategy proves particularly efficient for such studies. 

A mixture of cis- and trans-2-methylethynylcyclopropanes 48 and 49, prepared by the catalyzed (CuCl) addition of diazomethane to 3-penten-l-yne, upon heating to 530 °C underwent rearrangement to 3- and 4-methylenecyclopentenes 51 and 52, likely via the intermediate allylallene JO, and to 1,3-cyclohexadiene 54 and benzene 55, likely via the intermediate 1,3,5-hexatriene 53, Eq. (16) 26). 

Pericyclic reactions are commonly divided into three classes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. An electrocyclic reaction forms a sigma bond between the end atoms of a series of conjugated pi bonds within a molecule. The 1,3-butadiene to cyclobutene conversion is an example, as is the similar reaction of 1,3,5-hexatriene to form 1,3-cyclohexadiene ... 

Measurement of activation energy and entropy has not distinguished between antara-antara Cope rearrangement and other concerted processes in the photochemical rearrangement of y-thujaplicin O-methyl ether to the bicyclohep-tadienones (157) and (158).255 The hexatriene-cyclohexadiene disrotatory elec-... 

Cyclopropyl-substituted allenes open the door to yet another reaction mode. When treated with aryl iodides in the presence of a typical Heck-catalyst system and a dienophile, cyclohexene derivatives 77 were obtained (Scheme 11) [53,54]. Thus, the initially formed arylpalladium iodide car-bopalladates 72 to form a a-allylpalladium intermediate 73. It swiftly undergoes the cyclopropylcarbinyl to homoallyl rearrangement yielding the ho-moallylpalladium species 74 which finally suffers /1-hydride elimination. The thus formed 2-aryl-1,3,5-hexatrienes 75 are prone to undergo polymerization, but can be efficiently trapped by an appropriate dienophile at the least steri-... 

Pericyclic reactions are the ones where the electrons rearrange through a closed loop of interacting orbitals, snch as in the electrocyclization of 1,3,5-hexatriene (88). Lemal pointed ont that a concerted reaction could also take place within a cyclic array, bnt where the orbitals involved do not form a closed loop. Rather, a disconnection occnrs at one or more atoms. At this disconnection, nonbonding and bonding orbitals exchange roles. Such a reaction has been termedpseudopericyclic. 

For the parent (20 l,3,S-hexatriene (1), the main pathways competing with the desired electrocycliza-tion to (2) are the isomerization of the central double bond to give the ( )-triene (50) as well as die formation of other cyclized products (51) and (52) and the vinylallene (53). The sensitized irradiation of isomeric trienes results primarily in rapid (Z)-(E) isomerization about the central double bond and dimerization. By contrast to singlet state photochemistry, no electrocyclic or sigmatropic rearrangements occur under sensitized conditions. ... 

---