PROTOTROPIC ACETYLENE-ALLENE ISOMERIZATION IN PLANARLY CHIRAL FERROCENES - INTRAMOLECULAR ACYMMETRIC INDUCTION IN THE COURSE OF THE FORMATION OF CHIRAL AXIS

Racemic 2-trimethylsilyl- and -trimethylstannyl-1-(3-phenyl-2-propynyl )ferrocene (rac-1a,b) as well as the dextrorotatory specimen of the la tter, (+)-1b, were synthesized in two steps from racemic 1-formyl-2-tr imethylsilyl- and 1-formyl-2-trimethylstannylferrocenes (2a,b) or from the levorotatory specimen of the latter, (-)-2b, respectively. On the contact with strongly alkaline alumina compounds 1a,b and (+)-1b unde rgo diastereoselective prototropic acetylene-allene rearrangement to g ive predominantly one of the two possible stereoisomers of 2-trimethyl silyl- or 2-trimethylstannyl- 1-(3-phenyl-1,2-propadienyl)ferrocenes, rac-4a,b or (+)-4b, depending on the starting material (d. e. 30-40 %) . The extent of intramolecular asymmetric induction in the formation o f the axially chiral fragment during the transformation of (+)-1b to ( +)-4b is estimated at 38 %.