A Flexible Approach to Strained Sandwich Compounds: Chiral [1]Ferrocenophanes with Boron, Gallium, Silicon, and Tin in Bridging Positions

Abstract

The enantiomerically pure dibromoferrocene 3 [(Sp,Sp )-1,1'-dibromo-2,2'-di(isopropyl)ferrocene], equipped with two iPr groups in α positions, was prepared using known "Ugi amine" chemistry. Species 3 was targeted in order to gain access to new [1]ferrocenophanes ([1]FCPs) to be used as monomers for ring-opening polymerization. The iPr groups on the sandwich unit were introduced to stabilize bridging moieties, as well as to increase solubilities of targeted metallopolymers. The planar chiral dibromide 3 can quantitatively be lithiated at 0°C [2 equiv nBuLi, hexanes/thf (9:1), 30 min]. Salt-metathesis reactions with respective element dichloride species gave chiral [1]FCPs with a variety of bridging moieties [ERx =Ga[2-(Me2NCH2)C6H4] (4 a), SiMe2 (4 b), SntBu2 (4 c), BNiPr2 (4 d)]. The new [1]FCPs were fully characterized including single-crystal X-ray analysis. The stabilizing iPr groups on the Cp rings increase the thermal stabilities of 4 b-d compared to known [1]FCPs, equipped with the same bridging moieties. All three compounds 4 b-d are volatile and could be isolated by vacuum sublimation. Our new approach to [1]FCPs has the potential to overcome many of the existing difficulties in ferrocenophane chemistry, such as limited stability of starting monomers and low solubilities of resulting polyferrocenes.

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

“…As expected, the molecular structure of 3 3-Pen did not reveal any surprises. Its structure is very similar to that of 3 iPr , [11] including the conformation of the alkyl groups, a fact that will become important for the later discussion (see mechanistic insights below). Both dibromides, 3 iPr and 3…”

“…Recently, we had shown that addition of 2.1 equivalents of nBuLi to 3 iPr in a solvent mixture of thf/hexanes (1:9) results in a clean lithium-bromide exchange (Scheme 1); [11,19] . It seems that the lack of formation of crystals of the latter two species prohibited further purification. )…”

“…For example, the [1]FCP 4 b iPr was prepared before using the "low temperature" (0 8C to room temperature) procedure with a yield of isolated product of 20 %. [11] With the improved proce- [a]…”

“…This yield was matching the result obtained from 1 H NMR spectroscopy of the crude reaction mixture, where the 1,1'-bis(boryl)ferrocene 5 b iPr (Scheme 2) was the major product. [11] In the first attempts to prepare the new , respectively, at 0 8C in mixtures of thf and hexanes (1:9), the ice bath was removed and a solution of the aminoborane was added dropwise during 10 min through a cannula from a second Schlenk flask. Table 1 shows approximate ratios between targeted [1]FCPs (4; Scheme 1) and the unwanted bis(boryl)ferrocenes (5; Scheme 2) as determined by 1 H NMR spectroscopy from reaction mixtures.…”

“…Recently, we tackled both problems simultaneously through the introduction of isopropyl groups adjacent to the bridging element, which provide steric protection and act as solubilizing groups (B; Figure 1). [11,12] A particularly interesting family of strained sandwich compounds is that of boron-bridged [1]FCPs: they are the only known [1]FCPs bridged by a 2nd period element.…”