Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0739 884

Numerous attempts to experimentally confirm the possibility of the diradical mechanism of the Cope rearrangement were conducted for example, by the introduction of substituents which are capable of stabilizing the radicals (e.g., the cyano or phenyl group) (for a review, see Reference 225). A peculiar approach to develop a reliable model of a non-concerted, diradical mechanism closely related to the Cope rearrangement was described225. It involves the insertion of a seventh carbon atom between positions C(3) and C(4) in 1,5-hexadiene, and was called a ‘frustrated’ Cope rearrangement. The authors believe225 that the resulting system would still be able to undergo the first step of the rearrangement, i.e. the formation of a single C C bond between the terminal atoms of the 1,5-diene, but it would be unable to complete the second step, i.e. the cleavage of the C(3) C(4) bond (equation 158). Heating of 1,6-heptadiene 413 in o-dichlorobenzene at 220 °C for 401 h gave the cyclic olefin 414 (49%) along with the saturated bicycle 415 (19%) and the unconverted precursor 413 (32%). It was concluded that when the lifetimes of the conjectured diradicals become longer due to unavoidable conformational barriers which protect them from instantaneous collapse, the diradicals become the intermediates and can be more or less easily detectable225.

Another example of a similar approach for a novel type of homo-Cope rearrangement by thermolysis of the sterically rigid 1,5-heptadiene derivative endo,endo-dimethanenaph- thalene 411 was described226. Relatively small structural variations in this system seem to bring about a change from a pericyclic to a stepwise mechanism. Gas-phase thermolysis of 411 at 275 °C under carefully controlled conditions (primarily in order to exclude an acid catalysis) leads to isomerization to the polycyclic olefin 416 (31%) besides the expected retro-Diels –Alder products, cyclopentadiene 418 and norbornadiene 393 (formed in 69% combined yield) (equation 159)226. The endo,exo-isomer 417 forms under the same conditions only the latter two products (418, 393) without rearrangement. Thus, one of the possible mechanisms, i.e., via a retro-Diels –Alder step, can be excluded. These results confirm a pericyclic homo-Cope rearrangement of endo,endo-411 to 416. This novel rearrangement was further studied by using structurally related exo-isopropylidene-substituted systems (419, endo,endo-; 423, exo,endo-; and 424, endo,exo-) (equations 160 –162)226. Similarly to compound 411 the asymmetrical isomer 419 having face-to-face arrangement of the two endo-cyclic -bonds was transformed on thermolysis at 220 °C to the rearrangement product 420 and the retro-Diels – Alder products 393, 418, 421 and 422

Another possibility to overcome this lability consists in a release of strain by elongation of the 2,7-bridge, for example, by using compounds 428 and 429 and also dienone 430 (equation 164)228. Triene 429 is almost as labile as compound 426b whereas the halflives of the stellatriene 428 and the dienone 430, both having the two-carbon bridges and forming the polycycles 431 and 432, are 560 h and 102 h at 80 °C, respectively.

O

An influence of phenyl substituents on the geometry of the transition state was studied by using the d,l- and meso-isomers 433 and 434 of a polycyclic diene229. The d,l- diastereoisomer 433 is constrained to undergo Cope rearrangement in the chair conformation whereas the meso-diastereoisomer 434 is constrained to have a boat transition state: The activation free energies for their unimolecular [3,3]-sigmatropic rearrangement to give prod-

ucts 435 and 436 are 25.2 š 2.9 (for 433) and 38.1 š 3.7 (for 434) kcal mol 1 and from them a kd,l/kmeso ratio of 7 ð 106 at 150 °C was determined (equations 165 and 166)229.

Doering and coworkers230 have suggested distinguishing two types of positions in the hypothetical ‘aromatic’ transition state of the thermal Cope rearrangement, designated as ‘a’ (‘active’) and ‘n’ (“nodal”) (equation 167). Previous examinations of radical-stabilizing substituents have concentrated on their influence in the ‘n’ positions. In order to obtain a quantitatively reliable estimate for the effect on the ‘a’ positions, the activation parameters of the degenerate rearrangement of 1,4-diphenyl-1,5-diene (437) 13C-labeled in the C(6) position were evaluated. It was concluded that two phenyl groups in the ‘a’ positions have lowered the activation enthalpy by 32.2 kJ mol 1 relative to the parent 1,5-hexadiene while two phenyl groups in the ‘n’ positions of 3,5-diphenyl-1,5-hexadiene have lowered the activation enthalpy by 71.2 kJ mol 1. These results explain an advantage of using phenyl substituents in order to construct systems which enable one to change the concerted mechanism of the Cope rearrangement toward the two-step, so-called ‘diyl’ mechanism230.

H H

(165)

(435)

The Cope rearrangement mechanism can be also strongly affected by other substituents. Thus, the ‘normal’ electrocyclic process in the thermal isomerization of divinyl aromatics has been suppressed relative to the thermolysis of 1,2-bis(trifluorovinyl)naphthalene 438 (in benzene, at 193 °C, 24 h)231. Three major products 440– 442 were isolated from the reaction mixture, but none of them was the expected product 439. Also formed in low

F

F

(453)

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

The rearrangements of the diastereoisomeric dienes 455 and 456 are compared in the same work232. In accordance with the results of a similar investigation229 (equations 165 and 166) the isomer 455 (R D F) undergoes a Cope rearrangement more easily than its hydrocarbon counterpart (455, R D H) while the Cope rearrangement of isomers 456 (R D H, F) is strongly inhibited (equation 172). The contrasting behavior of compounds 455 and 456, as well as of 433 and 434, is ascribed to steric repulsions of the cis-fluorine C(1) and C(6) substituents in the former pair as well as to the presence of bulky phenyl groups in the latter pair of compounds.

The competition of the Cope rearrangement with cyclization processes was reported for perfluoro-1,5-hexadiene 457233. The cyclizations proceed undoubtedly via the corresponding diradicals 458 and 459 (equation 173). This course of events was revealed by using a