Chiral Bora[1]ferrocenophanes: Syntheses, Mechanistic Insights, and Ring‐Opening Polymerizations

Abstract

A series of new boron-bridged [1]ferrocenophanes ([1]FCPs) was prepared by salt-metathesis reactions between enantiomerically pure dilithioferrocenes and amino(dichloro)boranes (Et2 NBCl2 , iPr2 NBCl2 , or tBu(Me3 Si)NBCl2 ). The dilithioferrocenes were prepared in situ by lithium-bromine exchange from the respective planar-chiral dibromides (Sp ,Sp )-[1-Br-2-(HR2 C)H3 C5 ]2 Fe (R=Me or Et). In most of the cases, mixtures of the targeted [1]FCPs 4 and the unwanted 1,1'-bis(boryl)ferrocenes 5 were formed. The product ratio depends on the bulkiness of the amino group, the speed of addition of the amino(dichloro)borane, the alkyl group on Cp rings, and in particular on the reaction temperature. The formation of strained [1]FCPs is strongly favored by increased reaction temperatures. Secondly, CHEt2 groups at Cp rings favored the formation of the targeted [1]FCPs stronger than CHMe2 groups. These discoveries open up new possibilities to further suppress the formation of unwanted byproducts by a careful choice of the reaction temperature and through tailoring the bulkiness of CHR2 groups on ferrocene. Thermal ring-opening polymerizations of selected boron-bridged [1]FCPs gave metallopolymers with a Mw of 10 kDa (GPC).

Unique Bora[1]ferrocenophanes with Sterically Protected Boron: A Potential Gateway to Helical Polyferrocenes

“…In our recent report on the new family of bora[1]ferrocenophanes, we thoroughly investigated the outcome and mechanism of the salt‐metathesis reaction that is illustarted in Scheme . The starting dibromoferrocene derivatives 1 were applied as enantiopure species with ( S p ,S p ) planar chirality so that the resulting [1]FCPs were obtained as chiral compounds . We discovered that the 1,1′‐bis(boryl)ferrocene species 3 (Scheme ) is the main byproduct in the synthesis of the strained [1]FCPs 2 .…”

“…However, monomers C are thermally robust up to ≈220 °C and only produced polymers through thermal ROP around that threshold ( M w ≈10 kDa). One can speculate that amino groups at the bridging boron atom contribute significantly to the stability of these [1]FCPs . Furthermore, these amino groups would prevent effective conjugation between boron and ferrocene moieties in polymers because the acceptor orbital at the boron would almost exclusively be involved in π‐bonding with nitrogen.…”

Unique Bora[1]ferrocenophanes with Sterically Protected Boron: A Potential Gateway to Helical Polyferrocenes

“…[17,18] A new synthetic strategy was adopted by the research groups of Jäkle and Wagner, where fc(BBr 2 ) 2 [fc = Fe(C 5 H 4 ) 2 ]w as applied for apolycondensation to obtain poly(ferrocenylborane)s with BBr spacers. [21,22] These alkyl groups increased the solubility of the resulting polymers. [19] Theonly other report on poly(ferrocenylborane)s came from our laboratory and was based on an ew family of enantiopure boron-bridged [1]FCPs (C,F igure 1) equipped with alkyl groups on the Cp moieties.…”

“…[19,20] Post-polymerization modifications of these materials gave the corresponding BMes-and B[O(CH 2 ) 4 Br]-bridged polymers.H owever,t he method only gave polymers of M w up to % 7kDa. [21] However,m onomers C are thermally robust up to % 220 8 8Ca nd only produced polymers through thermal ROP around that threshold (M w % 10 kDa). [21,22] These alkyl groups increased the solubility of the resulting polymers.…”

“…[21,22] These alkyl groups increased the solubility of the resulting polymers. [21] Furthermore,t hese amino groups would prevent effective conjugation between boron and ferrocene moieties in polymers because the acceptor orbital at the boron would almost exclusively be involved in p-bonding with nitrogen. One can speculate that amino groups at the bridging boron atom contribute significantly to the stability of these [1]FCPs.…”