Binding Cooperativity of Two Different Lewis Acid Groups at the Edge of Ferrocene

Abstract

The binding properties of heteronuclear bidentate Lewis acids, in which an organoboron and an organotin moiety are attached adjacent to each other at one of the Cp rings of ferrocene, have been studied. Treatment of [1,2-fc(SnMe2Cl)(BClMe)] (1-Cl) (fc = ferrocenediyl) with one equivalent of pyridine or 4-dimethylaminopyridine (DMAP) resulted in diastereoselective complexation of boron. Adducts 2 and 3 have been studied by multinuclear NMR, and the stereoselectivity of complexation was further confirmed by single crystal X-ray diffraction of 2. The importance of cooperative effects that involve an intramolecular B-ClSn interaction on the diastereoselectivity is evident from comparison with binding studies on the phenyl-substituted analogue [1,2-fc(SnMe2Cl)(BPhMe)] (1-Ph). Complexation of 1-Ph led to diastereomeric mixtures of adducts 4 and 5, respectively, which were identified by multinuclear NMR including NOESY experiments. The solid-state structure of one of the diastereomers of 5 was confirmed by X-ray crystallography. Facile isomerization was found in solution and the barrier of activation was determined by VT NMR studies (4: Delta(#)(298) = 54.9+/-0.4 kJ mol(-1); 5: Delta(#)(298) = 70.3+/-0.1 kJ mol(-1)). Competitive binding of pyridine to 1-Cl and [FcB(Cl)Me] (Fc = ferrocenyl) showed that cooperative effects between tin and boron lead to significant Lewis acidity enhancement. Binding of a second nucleophile in the presence of excess of base occurred also at boron. The novel zwitterionic complexes [1,2-fc(BMe(py)2)(SnMe2Cl2)] (6) and [1,2-fc(BMe(dmap)(2))(SnMe(2)Cl2)] (7) formed, which consist of boronium cation and stannate anion moieties. The structure of 7 in the solid-state was confirmed by X-ray crystallography. Multinuclear NMR data and competition experiments indicate weak binding of chloride to tin in 7 and partial dissociation in solution.

Synthesis and oxidation of hexafluoroisopropoxyborylferrocenes

“…NMR spectra were obtained in d 2 -methylene chloride at room temperature on a BRUKER AVANCE II + 400 spectrometer. 1 H and 13 C chemical shifts are given with respect to TMS, 19 F NMR spectra to fluorotrichloromethane, and 11 B NMR spectra to the boron-trifluoride-diethyl-ether-complex. IR spectra were recorded on a ZnSe or Diamond ATR unit on a Nicolet Magna IR Spectrometer with 1 cm -1 resolution or on a Diamond ATR unit (200-30 000 cm -1 ) on a BRUKER alpha Fourier transform-infrared (FT-IR) spectrometer with a resolution of 4 cm -1 .…”

“…BÖHRER et al 13 C HMBC NMR spectrum, because, besides the characteristic shift to higher field, the 2 J( 1 H, 13 C) cross-signals to the ring protons are observable. In the 19 F NMR spectrum, a doublet at -75.5 ppm and in the 11 B NMR spectrum a singlet at 31.2 ppm were observed (Δν 1/2 = 365 Hz).…”

“…In this redox-switchable Li scavenger, the ion is not coordinated by main-group Lewis bases. Bifunctional Lewis acids like Carpenters 1,1'-[B(C 6 F 5 ) 2 ] 2 Fc are highly electrophilic, also 1,2-bifunctional ferrocenylborane-based Lewis acids are known [17][18][19]. Ansa-ferrocenes take advantage of the self-assembly and reversible breaking of B-N or B-P Lewis acid-base pairs as interannular bridge.…”

Bifunctional Indenyl‐Derived Receptors for Fluoride Chelation and Detection

“…The four structurese ach display at etra-coordinate boron centref eaturing either ab ound cyanideo rf luoride ion. Interestingly,f or both receptors, the binding of cyanide increases the intramolecular B-P distance (e.g.,f rom 3.483(4) to 3.5594 (18) (15) and 2.6936 (15) , respectively] whichf all within the sum of the relevant Vand er Waals radii (3.27 ). [24] Thatt hese contacts are structurally significant is consistent with the observed contraction in the B-P distance in each case, and with the 1 J(F,P) coupling observedi nt he respective 31 PNMR spectra.G abbaï and co-workersr eport as imilar P-F contact [2.666 (2) ] for the fluoride adduct XIIÀF, together with ac lose-to-linearF -P-C phenyl angle [176.4(1)8]a nd am arked elongation of the P-Ph bond.…”

Allylation of Ketones with a Ferrocene‐Based Planar Chiral Lewis Acid

“…[6] Unusual binding phenomena are observed for 1-Cl, including the diastereoselective coordination of pyridine from the exo side of ferrocene, which is aided by a bridging chlorine substituent between the neighboring boron and tin centers. [7] However, since 1-Cl is obtained in racemic form, it has not been applied to stereoselective synthesis. We report herein a facile route for the chiral resolution of 1-Cl and describe the application of the resulting enantiomerically pure planar chiral bidentate Lewis acids in the stereoselective allylation of ketones.…”