SYNTHESIS, STRUCTURE, AND DYNAMIC BEHAVIOR OF ANSA-FERROCENES WITH PYRAZABOLE BRIDGES

A variety of stereorigid ansa-ferrocenes 2 with o-phenylene-type bridg es have been obtained by the reaction of 1,1'-diborylferrocenes (1,1'- Fc(BBrR)(2); Fc = ferrocenyl; R = CH3, Br, OEt, NC4H8) with selected p yrazole derivatives. Subsequent intramolecular B-N adduct formation es tablishes the interannular bridges. The synthetic approach is based on the replacement of certain critical carbon-carbon links by the isoele ctronic, self-assembling, dative boron-nitrogen bonds and thus follows the principles of noncovalent synthesis. The pyrazabole bridge does n ot seem to cause any significant distortion of the ferrocene core. Mos t substituents at boron or pyrazole lead to an ansa-ferrocene structur e. However, the presence of both strong electron donors (i.e. pyrrolid ine) at boron and strongly electron-withdrawing pyrazole substituents (i.e. ester groups) results in an open chain structure 2i with three-c oordinate boron atoms. The tetravalent boron centers in ansa-1,1'-Fc[B (Br)(mu-pz)](2), 2a, show an unusually high tendency to undergo nucleo philic substitution reactions, even though they are located at bridgeh ead positions. NMR spectroscopy, as well as chemical evidence argue ag ainst an unbridged open chain intermediate in the course of these reac tions.