Insight into the Formation of Highly Strained [1]Ferrocenophanes with Boron in Bridging Position

Abstract

Four planar-chiral, enantiomerically pure ferrocene dibromides (3 R1R2 ; [(CHR 1 R 2 )BrH 3 C 5 ] 2 Fe) equipped with two CHR 1 R 2 groups in α position to bromine were synthesized. From the four C 2 symmetrical species, two are already known [CHR 1 R 2 = CHMe 2 (3 MeMe ), CHEt 2 (3 EtEt )] and two are new compoundsThe dibromides 3 R1R2 were in situ converted into dilithio ferrocene derivatives and reacted with Cl 2 BNiPr 2 resulting in mixtures of bora[1]-ferrocenophanes (4 R1R2 ) and 1,1′-bis(boryl)ferrocenes (5 R1R2 ). The aim of this investigation was to test the hypothesis that the alkyl group that is oriented approximately perpendicular to the Cp ring, i.e., R 2 , affects the outcome of the salt-metathesis reaction. The obtained product ratios 4 R1R2 :5 R1R2 were determined by 1 H NMR spectroscopy and revealed that systems with the same R 2 group gave similar 4 R1R2 :5 R1R2 ratios (1.0:0.51 and 1.0:0.49 for R 2 = Me; 1.0:0.30 and 1.0:0.27 for R 2 = Et), confirming the hypothesis. Shown by DFT calculations (B3PW91/6-311+G(d,p)), reaction paths resulting in either product 4 R1R2 or product 5 R1R2 are both concerted steps.

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

“…[27,28] Forinstance,asteady change in color is observed on going from ferrocene to sila [1]ferrocenophane to phospha [1]ferrocenophane (l max = 440, 478, and 498 nm, respectively) as the a angle gradually increases. [29] Thed ark purple color of bora [1]ferrocenophane 2b shows af urther bathochromic shift with l max = 516 nm, which is higher than the corresponding values of both the known bora-[1]ferrocenophane 2 MeEt [22] and the sulfur-bridged [1]FCP D [28] (Table 2). TheT ip-substituted [1]FCP 2b shows the lowest-energy absorption maximum of any strained FCP reported.…”

“…Single crystals of the known compound 2 MeEt [22] were obtained by crystallization from hexanes at À80 8 8C; the result of its single-crystal X-ray analysis is illustrated in Figure 5. We applied the B3PW91/6-311 + G(d,p) method because it gives realistic geometries and can also be used to predict thermodynamic data for ROP.…”

“…[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.…”

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 .…”

“…[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. [21,22] These alkyl groups increased the solubility of the resulting polymers. [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).…”