Study on the synthesis of nonracemic C2-symmetric 1,1′-binaphthyl-2,2′-diyl bridged ferrocene. Stereochemical result of the cross-coupling reactions controlled by Pd(II) or Pd(IV) complex intermediacy

Palladium catalyzed Negishi, Suzuki and Stille cross-coupling reactions of enantiopure 2,2′-diiodo-1,1′-binaphthyl with the corresponding 1,1′-dimetalloferrocenes gave the C2-symmetric binaphthyl bridged ferrocene 1-1,1′-(1,1′-binaphthyl-2,2′-diyl)ferrocene (1). The latter was obtained by Stille coupling with the bis(trimethylstannyl) derivative but not with the bis(tributylstannyl) one. Products of alkyl group transfer from tin to binaphthyl were obtained as the main products in both cases. The stereochemical result of these cross-coupling reactions in the positions 2 and 2′ of 1,1′-binaphthyl depends on the reactivity of 1,1′-dimetalloferrocenes. Negishi coupling proceeds stereoconservatively (affording enantiopure product 1). Complete racemization of binaphthyl moiety occurs during the reactions with less reactive boron and tin organometallics. Proposed different reaction pathways include C1-symmetric palladium(II) intermediate in the former and configurationally unstable C2-symmetric pallada(IV)cyclic intermediate in the latter cases. In contrast to the cross-coupling reactions, free radical arylation of ferrocene with enantiopure 1,1′-binaphthyl-2,2′-bisdiazonium salt gave predominantly oligomeric binaphthyl bridged ferrocenes and only traces of the partially racemized product 1.