BIS-PARA-SEMIQUINOID TYPE DOUBLE CYCLOPALLADATION IN THE SERIES OF 6-MEMBERED 2-NITROGEN BRIDGED ANNULENE-DIHYDROANNULENE LIGANDS

The cyclometallation reaction known in the series of annulene heteroor ganic ligands was extended to six-membered dihydroannulenes with a din itrogen bridge. Double cyclopalladation of 4-methyl-4-trichloromethylc yclohexa-2,5-dien-1-one azine yielded the first representative of a ne w class of cross-conjugated diazadipalladatetracycles, viz., dro-4b,9b -diaza-5,10-dipalladaindeno[2,1-a]indene, isolated as a diastereomer m ixture of the achiral meso-form (E-isomer) and a racemate (Z-isomer). This reaction offers a method for transition metal-mediated activation of nonreactive C-H bonds at position 2 of cyclohexa-2,5-dienylidene s ystems and a route toward the very rare chiral polyheteroelement syste m with rotational symmetry.