Bicyclo nona-1,3,5,8-tetraene

Bicyclo[4,3,0]nona-2,4,8-triene(C9Hio) gives a brick-red, diamagnetic complex [Mo(CO)3(C9Hl0)] which absorbs one molecule of hydrogen (HI, 142). The complex is thus given structure (VIII), in which the ligand is the monocyclic tautomer cyclonona-l,3,5,7-tetraene, rather than structure (IX). 

A scheme that would seem to be consistent with all of the facts is shown in Figure 43. Alternative routes to the all-cis-cyclonona-l,3,5,7-tetraene are possible but seem implausible. Thus competitive disrotatory ring-opening of either 13 or the bicyclo[5.2.0]nona-2,5,8-triene would provide pathways to the all-cis-cyclononatetraene but the activation parameters for these processes would have to be very unusual since it would be necessary to explain how they occur at negligible rates relative to the pathway shown in Figure 43 when the temperature is 31°C (where C(9) epimerization and trapping... 

The photochemistry of 2,7-bridged 3,5-cycloheptadienones parallels that of monocyclic systems. The benzo system 22 a underwent smooth decarbonylation on direct photolysis in hexane to form the benzocycloocta-2,4,6,3-tetraene 23a,while sensitized photolysis of 22a gave the barbaralone 25a. Photochemistry studies of the parent compound, the bicyclo[4.2.1]nona-... 

The analogous amine oxide derived from cw-bicyclo[6.1.0]nona-2,4,6-triene, i.e. 5, rearranged to give a mixture of two bicyclic tetraenes 7 and 8, probably via c/5-9-methylenebicy-clo[6.1.0]nona-2,4,6-triene (6) as an intermediate. " ... 

Spiro[2.6]nonatetraene to Bicyclo[5.2.0]nona-l,3,5,8-tetraene to Indene... 

Thus, cis,trans,cis,cis-l,3,5,7-cyclononaXcirsiQnQ could be trapped during the thermolysis of the 6.1.0 triene. Subsequently it was found that this tetraene could be trapped with high concentrations of TCNE in a 2 + 2 fashion with a rate independent of TCNE concentration, which indicates rate-determining formation of the tetraene.Adducts of this tetraene were isolated earlier, and these were subsequently shown to be of the trans stereochemistry. Further, it was also found that the syn and anti isomers of 9-deuterio-cw-bicyclo[6.1.0]nonatriene interconvert prior to the isomerization to the dihydroindenes. The most likely pathway to the tetraene is a divinylcyclopropane 3,3-shift to bicyclo[5.2.0]nona-2,5,8-triene followed by a conrotatory cyclobutene ring opening (Scheme 10.11). ... 

Finally, bicyclo[5.2.0]nona-2,4,8-triene has been shown to give c/ -8,9-dihydroindene in a flow system at temperatures above 360°C. This reaction could be the result of ring opening to the c, ,c,c-nonatetraene (but not the t,c,c,c-tetraene - see earlier) followed by closure of this to the bicyclic materials shown earlier to give mostly c/5 -8,9-dihydroindene (Scheme 10.14). 

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