Prototropic acetylene-allene isomerization in planarly chiral ferrocenes. Intramolecular acymmetric induction in the course of the formation of chiral axis

Racemic 2-trimethylsilyl- and 2-trimethylstannyl-1-(3-phenyl-2-propynyl)ferrocene (rac-1a,b) as well as the dextrorotatory specimen of the latter, (+)-1b, were synthesized in two steps from racemic 1-formyl-2-trimethylsilyl- and 1-formyl-2-trimethylstannylferrocenes (2a,b) or from the levorotatory specimen of the latter, (–)-2b, respectively. On the contact with strongly alkaline alumina compounds1a,b and (+)-1b undergo diastereoselective prototropic acetylene-allene rearrangement to give predominantly one of the two possible stereoisomers of 2-trimethylsilyl- 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 of the axially chiral fragment during the transformation of (+)-1b to (+)-4b is estimated at 38 %.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1111– 1115, June, 1994.The work was carried out with financial support from the Russian Fundamental for Basic Research (Project No.93-03-5827).