A series of micro2-fluoro-bridged heteronuclear bidentate Lewis acid complexes [K(18-crown-6)THF]+ [Fc(BMeF)(SnMe2Cl)F]- (1-2F), [K(18-crown-6)THF]+ [Fc(BMeF)(SnMe2F)F]- (1-3F), [K(18-crown-6)THF]+ [Fc(BMePh)(SnMe2Cl)F]- (2-F), and [K(18-crown-6)THF]+ [Fc(BMePh)(SnMe2F)F]- (2-2F) (Fc=1,2-ferrocenediyl) was prepared. Compounds 2-F and 2-2F were obtained as a mixture of diastereomers, which arise due to the generation of a stereocenter at boron in addition to their inherent planar chirality. All compounds have been studied in the solid state by single-crystal X-ray diffraction analysis and by multinuclear NMR spectroscopy in solution. As a result of bridging-fluoride interactions, tetrahedral boron and distorted trigonal-bipyramidal tin centers are observed. Comparison with the corresponding monofunctional ferrocenylborates further supports the bridging nature of the fluoride anion. Two-dimensional exchange spectroscopy 19F NMR studies provide evidence for facile intermolecular and intramolecular fluorine exchange processes. All complexes display reversible one-electron oxidation events at lower potentials than those of the tricoordinate ferrocenylborane precursors, which is typical of ferrocenylborate complexes.

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.

-N-methyl pseudoephedrine, and (4S,5S)-(-)-4,5-dihydro-4-methoxymethyl-2-methyl-5-phenyloxazole) were purchased from Sigma Aldrich and dichlorophenylborane from Acros. All chemicals were used without further purification. (1R,2R)-(-)-N-methyl pseudoephedrine [1] , Me 3 SnAll [2] , and 1,2-Fc(SnMe 2 Cl)(BClMe) [3] were prepared according to literature procedures. Deuterated chloroform (CDCl 3 >99.7%) was obtained from Cambridge Isotope Laboratories (CIL). The solvent was stirred for several days over anhydrous CaH 2 , then degassed via several freeze pump thaw cycles and stored over 3Å molecular sieves. All reactions and manipulations were carried out under an atmosphere of prepurified nitrogen using either Schlenk techniques or an inert-atmosphere glove box (MBraun Glovebox Technology). Hydrocarbon and chlorinated solvents were purified using a solvent purification system (Innovative Technologies; alumina / copper columns for hydrocarbon solvents) and the chlorinated solvents were subsequently degassed via several freeze pump thaw cycles.All 499.9 MHz 1 H, 125.7 MHz 13 C, 186.4 MHz 119 Sn, and 160.3 11 B NMR spectra were recorded on a Varian INOVA NMR spectrometer (Varian Inc., Palo Alto, CA) equipped with a 5 mm dual broadband gradient probe (Nalorac, Varian Inc., Martinez, CA). Solution 1 H and 13 C NMR spectra were referenced internally to the solvent signals. 119 Sn and 11 B NMR spectra were referenced externally to SnMe 4 (δ = 0) and BF 3 . Et 2 O (δ = 0) in C 6 D 6 , respectively. Splittings of NMR signals are abbreviated as pst (pseudo-triplet), dpst (doublet of pseudo-triplet), nr (not resolved).Two-dimensional 1 H NOESY [4] measurements were obtained with the standard pulse sequence that was followed by a 90° pulse flanked by two 5 G/cm gradient for dephasing any residual transverse magnetization and suppressing potential artifacts, before the relaxation delay. Spectra were recorded in the phase sensitive mode by employing the TPPI improvement [5] of the States-Haberkorn-Ruben Hypercomplex method. [6] Typically, 256 t1 increments of 2K complex data points over 5.0 kHz spectral widths were collected with 32 scans per t1 increment, preceded by 16 or 32 dummy scans, and a relaxation delay of 2 s. Data sets were processed on a Sun Blade 100 workstation (Sun Microsystems Inc., Palo Alto, CA) using the VNMR software package (Varian Inc., Palo Alto, CA). In order to decrease t1 ridges arising from incorrect treatment of the first data point in the discreet Fourier transform (FT) algorithm, the spectrum corresponding to the first t1 value was divided by 2 prior to FT along t1. [7] Unshifted Sine Bell window functions were used in both dimensions. Data sets were zero-filled in the t1 dimension yielding 1K x 1K final matrices.Elemental analyses were performed by Quantitative Technologies Inc. Whitehouse, NJ. Optical rotation analysis was performed on an Autopol II polarimeter, Rudolph Research Analytical, using a tungsten-halogen light source operating at λ = 589 nm. GC analysis was performed on Hewlett Packard ...

Resolution of the planar-chiral bidentate Lewis acid Fc[B(Cl)Me][SnMe2Cl] (Fc = 1,2-ferrocenediyl) (1) by complexation with pseudoephedrine derivatives was studied. Compound 1 was first converted to the methoxy derivative Fc[B(OMe)Me][SnMe2Cl] (2) by treatment with Me3SiOMe. The latter was fully characterized by multinuclear NMR and single-crystal X-ray diffraction analysis, both of which suggest a significant interaction between the oxygen of the B(OMe)Me substituent and the neighboring tin center. Complexation with (1S,2S)-pseudoephedrine under release of MeOH proceeded smoothly at RT, but gave a 1:1 mixture of the R p and S p complexes, even when a deficiency of the pseudoephedrine was used. The complexes were separated by fractional crystallization and analyzed by multinuclear NMR, 2D NOESY, and X-ray crystallography. In contrast, with the sterically more demanding ligand N-methylpseudoephedrine (MPE) highly stereoselective complexation was achieved. Reaction of 2 with 0.5 equiv of the (1S,2S)-derivative gave a complex with S p-2, leaving behind the uncomplexed isomer R p-2, while treatment with 0.5 equiv of the (1R,2R)-derivative furnished the isomeric complex of R p-2 and uncomplexed S p-2. The complex between (1S,2S)-MPE and S p-2 was fully characterized, and the structure was examined by 2D NOESY and single-crystal X-ray analysis. In all cases the central chirality at boron was found to correlate with the chirality of the pseudoephedrine derivative; that is, with (1S,2S)-pseudoephedrine or (1S,2S)-N-methylpseudoephedrine the S B isomer was formed, while (1R,2R)-pseudoephedrine led to R B chirality at boron. Finally, a sample of enantiomerically pure R p-2 was reacted with (1S,2S)-MPE to answer the question why complexation is less favorable for this combination. Two species were detected in solution, corresponding to a mixture of the S B and R B isomers, which were found to be in fast equilibrium on the NMR time scale at RT. An X-ray diffraction analysis showed that in the solid state only the S B isomer was present. The crystal structure revealed a very unfavorable steric interaction between the NMe2 group and the free Cp ring of ferrocene, leading to major distortions in the structure.

Transmetalation reactions where organotin reagents are used for the transfer of organic groups to boron halides are among the most selective and hence synthetically useful methods for the preparation of organoboranes. We describe here a rare example of the reverse reaction, where migration of the organic group from boron to tin is promoted by addition of fluoride. The bidentate Lewis acids Fc(BMeR)(SnMe 2 Cl) (Fc ) 1,2-ferrocenediyl; R ) phenyl (Ph), thienyl (Th), allyl (All)) react with KF at 45 °C to give the rearranged fluoroboranes Fc(BMeF)(SnMe 2 R) as oily liquids. With an excess of KF, the fluoroborate complexes K[Fc(BMeF 2 )(SnMe 2 R)] are obtained, which are readily isolated as light yellow solids in good to high yields. For the phenyl derivative it was demonstrated that the borate salt can be converted back to the fluoroborane by treatment with Me 3 SiOTf or addition of pyridine, which gives the adduct Fc(BMeF)(SnMe 2 Ph) • Py as a crystalline solid. The fluoride and pyridine complexes have been characterized by multinuclear NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction (for R ) Ph). Preliminary mechanistic studies suggest an intramolecular process and indicate that Lewis acid induced Sn-F bond activation may play an important role.