Planar Chiral Organoborane Lewis Acids Derived from Naphthylferrocene

Abstract

Enantiomerically pure metalated 2-(1-naphthyl)ferrocene (NpFc) derivatives NpFcM (M=SnMe(3), HgCl) were prepared and characterized by multinuclear NMR and UV/Vis spectroscopy, cyclic voltammetry, and elemental analysis. Optical rotation measurements were performed and the absolute configuration of the new planar chiral ferrocene species was confirmed by single-crystal X-ray diffraction analysis. The mercuriated species NpFcHgCl proved suitable as a reagent for the preparation of the chiral organoborane Lewis acid NpFcBCl(2), which can in turn be converted to other ferrocenylboranes by replacement of Cl with nucleophiles. The highly Lewis acidic perfluoroarylborane derivatives NpFcB(C(6)F(5))Cl and NpFcB(C(6)F(5))(2) were successfully prepared by treatment with CuC(6)F(5). The structures were studied by single-crystal X-ray diffraction and variable-temperature (19)F NMR spectroscopy, which suggested that pi stacking of a C(6)F(5) group on boron with the adjacent naphthyl group is energetically favorable. UV/Vis absorption spectroscopy and cyclic voltammetry measurements were performed to examine the electronic properties of these novel redox-active chiral Lewis acids.

Frustrated Lewis Pair Chemistry: Development and Perspectives

“…[64] Erker et al have briefly investigated the catalytic hydrogenation of prochiral bulky imines with the enantiomerically highly enriched chiral ferrocene derivative 73 bearing a borane and a phosphine substituent (Scheme 28) at one Cp ring (Cp = h-C 5 H 5 ) but obtained only rather moderate ee values (max. 26 %).…”

Structure and reactivity of a planar chiral naphthylferrocenylcopper heteroaggregate

“…In fact, the reagent (C 6 F 5 Cu) 4 has been successfully employed in areas ranging from molecular Lewis acid chemistry [9,10] to polymeric Lewis acids [11], conjugated materials [12], and even amphiphilic borate block copolymers [13]. With the ultimate goal of developing new chiral naphthylferrocenylborane Lewis acids, we decided to explore the reactivity of the heteroaggregate 2 with boron halides.…”

“…Further examination of the peak patterns in the 1 H NMR spectrum revealed that not the expected 1,2-isomer 3a, but a mixture of the 1,3-and 1,1 0 -isomers 3b and 3c in a 45:55 ratio was obtained. The different isomers were identified by comparison with literature data [10].…”

“…In this regard, we are currently exploring heteronuclear bidentate Lewis acids, in which an organotin and an organoborane Lewis acid group are attached adjacent to one another at one of the Cp rings of ferrocene [15], as well as planar chiral 2-naphthylferrocenylborane species [10,16]. We describe here the preparation of a planar chiral 2-naphthylferrocenylcopper species and its reactivity toward boron halides.…”

“…However, pentafluorophenyl copper also serves as a mild reagent in organometallic synthesis, specifically in (a) metal exchange reactions with formation of other arylcopper species and (b) metathesis reactions that involve transfer of the C 6 F 5 groups to other metals and metalloids (Scheme 1) [7,8]. Of particular interest is the aryl group transfer to boron with generation of perfluoroarylboranes [9][10][11][12][13]. This class of compounds has attracted tremendous interest in areas ranging from Lewis acid catalysis to activators in olefin polymerization, and as components of so-called ''frustrated Lewis pairs'', which are able to promote metal-free activation of small molecules, most notably H 2 [14].…”