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

It should be noted that, in contrast to well-known cis-bicyclo[6.1.0]nonatrienes, the thermal behavior of trans-bicyclo[6.1.0]nonatrienes has been insufficiently explored76 – 78. It was found that the thermolysis of trans-nonatriene 206a and the parent compound 206b occurs surprisingly easily to form the previously unknown 3,4-homoheptafulvene system 210. Using an optically active compound 206a, a degenerate cyclopropane-walk- rearrangement which precedes the structural isomerization was detected. In both processess the diradical exo-207 was considered to be a possible intermediate (equation 66)77. The thermolysis of triene 206a (180 °C, degassed benzene, 3 h, conversion 100%) and 206b (200 °C, degassed toluene, 10 min, 90% conversion) gives in every case only the sole product 210a or 210b. Compounds 210a and 210b are very sensitive to air and prone to undergo polymerization. The probable mechanism for process 206 ! 210

R

(210) R

(a) R = Me

(b) R = H

includes a cleavage of distal bond in cyclopropane ring of 206 to give the diradical intermediate 207, which undergoes an electrocyclic ring closure to form the diradical 208. A subsequent hydrogen shift leads to the homotropylidene 209 which relieves the excessive internal strain by a rapid Cope rearrangement into the homoheptafulvene 210.

Interesting transformations in which the phosphorus analogues of bicyclo[6.1.0]nona- 2,4,6-triene undergo various rearrangements were reported. Thus, dipotassium cyclooctatetraenide reacts with dichlorophenylphosphine in THF to give an adduct 211 which isomerizes upon heating to bicyclic triene 212 (equation 67)79. The same approach was utilized to prepare the bridged phosphonium barbaralanes 215a 215b which are degenerate Cope systems (equation 68)80 (see also Section IV.C.2.d). The alkylation of the 9-phospha derivatives 213 with trialkyloxonium salts leads to products 215, presumably via the intermediate phosphonium salts 214 which undergo a disrotatory ring opening, followed by successive conversions of cationic bicyclic ylides. The P-barbaralane 215 demonstrates the rapid degenerate Cope rearrangement above 25 °C which can be frozen below 72 °C80.

2−

(67)

The reaction of phenyllithium with the bridged phosphine oxide 216 gives the rearrangement product 217 in 60% yield (equation 69)81. The behavior of the heterocyclic system 218 toward oxidizing reagents and a new pathway for its skeletal rearrangement were described82. Treatment of compounds 218 with hydrogen peroxide or tert-butyl hydroperoxide at 15 °C leads to a cleavage of the C C bond of the three-membered ring to form the relatively instable phosphonin ring system 219. When the latter was warmed to 25 °C it was completely rearranged to dihydrophosphindole 220 (equation 70)82.

3. Cyclooctatetraenes

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83 – 85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of 1,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170 –180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as 1,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86.

The interaction of cyclooctatetraene as a dienophile with the diazadiene, 3,6-bis(trifluo- romethyl)-1,2,4,5-tetrazine 226, is accompanied by nitrogen elimination and gives rise to the 1,1-adduct 227. The latter displays interesting thermal rearrangements depending on the solvent polarity and temperature (equation 73)87. In toluene solution a [1,3]-carbon