Hexatrienes cyclization

Amino-1 -metalla- 1,3,5-hexatrienes cyclization reactions, 173-174 synthesis, 172-173... 

Ethoxy-l-metalla-l,3,5-hexatrienes cyclization reactions, 175 pyridines via fragmentation, 176... 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

At longer reaction times isomerization to 9-methyldihydrophenanthrene occurred with log = 11.2 - 41 000/2.37 r as determined from the vinylmethyl derivative. A 1,5-shift of the etheno bridge followed by cyclobutene ring opening and hexatriene cyclization can be responsible for this product. 

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

An analogous cyclization to eventually form five-membered rings has also been observed for l-metalla-l,3,5-hexatrienes with an additional heteroatom within the chain, such as in the complexes 157. These are obtained by Michael additions of imines to alkynylcarbene complexes in good to excellent yields (reaction type F in Scheme 4), and their configurations were determined to be Z (>91%) in all cases. Upon warming in THF solution, complexes 157 underwent cyclization with reductive elimination to furnish 2Ff-pyrroles 158 in up to 97% yield (Scheme 34). With two cyclopropyl substituents at the terminus in... 

Scheme 34 Cyclizations of 5-hetera-l-metalla-l,3,5-hexatrienes 157 to mainly yield 2H-pyrroles [37]. For further details see Table 8...

According to this scheme, the catalyst serves primarily to promote dehydrogenation. Cyclization of the hexatriene was shown years ago (JJ.) to occur thermally in the gas phase at temperatures well below these dehydrocyclization conditions. Thus, the overall reaction is projected to be the combination of several catalytic dehydrogenation steps and a non-catalytic cyclization step. This projection implies that the design of the catalytic reactor may be important in order to optimize the ratio of void space for cyclization and catalyst space for dehydrogenation. 

The thermal disrotatory cyclization of hexatrienes leads to cyclohexadienes (equations 11 and 12)11"13. 

The structural variety increases if the second (and further) substituent(s) is (are) not bound to the allene nucleus. For vinylallene (2), the additional vinyl group can be introduced at C-5, leading to 1,2,4,6-heptatetraene (22 only. E-isomer shown) or at C-4, providing 4-methylene-l,2,5-hexatriene (23), the former being an important substrate for cyclization reactions, as will be discussed in Section 5.5 (Scheme 5.2). 

The linear isomer of 225, (E)-l,2,4,6,7-octapentaene (229), is formed in addition to other products on treatment of the bisdibromocarbene adduct to (E)-l,3,5-hexatriene with methyllithium in diethyl ether at -40 C like 226, it is a highly unstable hydrocarbon [90]. Several attempts to characterize the Z-isomer 230 [90, 91] also met with failure. Although very likely generated as an intermediate in these experiments, 230 immediately cyclized to o-xylylene (231), which can be trapped, e.g., as a Diels-Alder addition product. 

The acyclic fluorinated 1,3,5-hexatriene system 454 is also resistant to 6 r-electron electrocyclic ring closure at temperatures up to 200 °C while the analogous hydrocarbons cyclize easily at 160 °C (equation 171)232. 

These processes may be designated conrotatory (16) and disrotatory (17). In practice the isomerization of the appropriately substituted cyclobutenes follow a conrotatory pathway. Thus ci5-3,4-dimethylcyclobutene yields only cia-irans-2,4 hexadiene, and iraws-3,4-dimethyleyclobutene yields only transition state suggested previously, the conrotatory process is in fact the one to be expected. However, the situation is not quite as simple as here implied. By similar arguments the thermal cyclization of hexatrienes would also be expected to be conrotatory, whereas in fact it is disrotatory, viz. ... 

Obviously, once the hexatriene stage is reached, its cis isomer very rapidly gives 1,3-cyclohexadiene (59). Radiotracer studies have confirmed this cyclization step 20). However, tra 5-l,3,5-hexatriene may also be produced with almost the same probability (Scheme I). We do not agree with Rozen-... 

The process is much more rapid over platinum. Dautzenberg and Plat-teeuw (23) assumed the formation and thermal cyclization of hexatriene [similarly to the earlier suggestion with respect to oxides (22)]. However, it is not likely that such an extremely unstable intermediate would leave the catalyst surface just in order to cyclize and then rapidly readsorb to complete aromatization. Still, thermal cyclization cannot be a priori excluded at high temperatures where the equilibrium concentration of triene is higher and its adsorptivity lower, but its appearance may be rather exceptional. We suggest, instead, a surface cyclization step of dj-l,3,5-hexatriene. 

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. 

Essentially the same triple peak system also appeared with Pt-black, but the entire spectrum was shifted toward lower temperatures (r ,a,j = 155°, 175°, and 195°C, respectively). 7>a s-hexatriene gave the same spectrum, but with cw-triene a fourth, low-temperature peak appeared at 140°C. This should correspond to the facile cyclization of cij-triene. The fact that a peak of similar appears when benzene is desorbed from the catalyst is another strong argument against the purely thermal (noncatalytic) character of cyclization of c/s-l,3,5-hexatriene. 

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