Ligand exchange and reaction mechanisms offluorinated compounds

Structure-reactivity studies on hypervalent square-pyramidal dithieno[3,2-b:2',3'-d]phospholes

A series of neutral pentacoordinate dithieno[3,2-b:2',3'-d]phosphole compounds were synthesized by [4 + 1] cycloaddition with o-quinones. Counter to the expected trigonal bipyramidal geometry, the luminescent hypervalent dithienophospholes exhibit square pyramidal geometry with inherently Lewis acidic phosphorus center that is stabilized via supramolecular π-stacking interactions in the solid state and in solution. Due to their Lewis-acid character, the compounds react with nucleophiles, suggesting their potential as mediator in organic transformations. The new species thus present an intriguing structural plaform for the design of neutral P(v) Lewis acids with useful reactivities.

Bis(perchlorocatecholato)silane-A Neutral Silicon Lewis Super Acid

No neutral silicon Lewis super acids are known to date. We report on the synthesis of bis(perchlorocatecholato)silane and verify its Lewis super acidity by computation (DLPNO-CCSD(T)) and experiment (fluoride abstraction from SbF₆^- ). The exceptional affinity towards donors is further demonstrated by, for example, the characterization of an unprecedented SiO₄ F₂ dianion and applied in the first hydrodefluorination reaction catalyzed by a neutral silicon Lewis acid. Given the strength and convenient access to this new Lewis acid, versatile applications might be foreseen.

On the preparation of fluorine-18 labelled XeF(2) and chemical exchange between fluoride ion and XeF(2)

A recent report claims to have prepared [18F]XeF2 by exchange between a large stoichiometric excess of XeF2 and no-carrier-added 18F-, as salts of the [2,2,2-crypt-M+] (M = K or Cs) cations, in CH2Cl2 or CHCl3 solvents at room temperature. Attempts to repeat this work have proven unsuccessful and have led to a critical reinvestigation of chemical exchange between fluoride ion, in the form of anhydrous [N(CH3)4][F] and [2,2,2-crypt-K][F], and XeF2 in dry CH2Cl2 and CH3CN solvents. It was shown, by use of 19F and 1H NMR spectroscopies, that [2,2,2-crypt-K][F] rapidly reacts with CH3CN solvent to form HF2-, and with CH2Cl2 solvent to form HF2-, CH2ClF, and CH2F2 at room temperature. Moreover, XeF2 rapidly oxidizes 2,2,2-crypt in CH2Cl2 solvent at room temperature to form HF and HF2-. Thus, the exchange between XeF2 and no-carrier-added 18F- reported in the prior work arises from exchange between XeF2 and HF/HF2-, and does not involve fluoride ion. However, naked fluoride ion has been shown to undergo exchange with XeF2 under rigorously anhydrous and HF-free conditions. A two-dimensional 19F-19F EXSY NMR study demonstrated that [N(CH3)4][F] exchanges with XeF2 in CH3CN solvent, but exchange of HF2- with either XeF2 or F- is not detectable under these conditions. The exchange between XeF2 and F- is postulated to proceed by the formation of XeF3- as the exchange intermediate.

Chelated fluoroboron cations. I. Synthesis and NMR studies involving the tertiary-amine ligands N,N,N',N'-tetramethylethylenediamine and N,N,N',N",N''-pentamethyldiethylenetriamine

Several possible methods of synthesis of chelated fluoroboron cations are explored, using the tert-amines N,N,N',N'-tetramethylethylenediamine (Me4en) and N,N,N',N",N"-pentamethyldiethylenetriamine (Me5dien) as model chelating ligands. Both ligands displace pyridine from (pyr)2BF2+ (as its PF6- salt) to form the bidentate (Me4en)BF2+ and (Me5dien)BF2+ cations. The same cations, as well as the corresponding BFCl+ and BFBr+ cations, can also be prepared by displacement of the donor molecule (D = pyridine or isoxazole) and the heavy halide ion (Cl- or Br-) from the neutral D·BF2X and D·BFX2 adducts. The central nitrogen of Me5dien becomes chiral when it and one terminal nitrogen are coordinated, and the prochiral and magnetically nonequivalent fluorines of (Me5dien)BF2+ give 19F NMR signals separated by 1.2 ppm. In (Me5dien)BFCl+ the boron is a second chiral centre and the two diastereomers, distinguishable by NMR with 19F chemical shifts differing by 3.0 ppm, form in a 3:1 ratio. The bidentate BFBr+ cations of Me4en and Me5dien are insoluble in non-coordinating solvents but have been detected by positive ion FAB mass spectrometry and 11B MAS NMR. The tridentate complex (Me5dien)BF+2 does not form under our conditions.Key words: N,N,N',N'-tetramethylethylenediamine, N,N,N',N",N"- pentamethyldiethylenetriamine, chelated fluoroboron cations, fluorine-19 NMR, boron-11 NMR, pyridine, isoxazole, chiral, magnetically nonequivalent.

Article

The equilibrium in the Ph2P(O)OC6H4OH-base system may be shifted by varying the base concentration, where base = imidazole, triethylamine, dimethyl sulfoxide, pyridine, and 1,4-dioxane. This equilibrium was studied by 31P and 1H NMR, and information about the symmetry of phosphorus-containing intermediates is provided by the 1H NMR spectrum of the catecholyl ring, with its ABCD spin system. The equilibrium is also affected by trifluoroacetic acid. A mechanism is proposed that involves protonation-deprotonation, cyclic-acyclic equilibria, and selective P-O bond cleavage, with all steps occurring rapidly on the NMR time scale.Key words: Ph2P(O)OC6H4OH-base system, cyclic-acyclic equilibrium, and selective P-O bond cleavage.

Oxidative fluorination in the Ph2SO–XeF2–Cl− system and fluorine exchange in the Ph2S(O)F2–Ph2S(O)F+ system

Oxidative fluorination of diphenyl sulfoxide with xenon difluoride occurs under mild conditions in the presence of chloride ion to give Ph2S(O)F2 in quantitative yield. Chloride ion appears to react with xenon difluoride to generate fluoride ion, and a mechanism of oxidative fluorination is proposed that involves anionic Ph2S(O)F− and radical Ph2S(O)F• intermediates. Addition of cationic Ph2S(O)F+ to Ph2S(O)F2 initiates rapid fluorine exchange, presumably via a symmetrical fluorine-bridged intermediate, and this exchange process was monitored by 13C and 19F NMR spectroscopy. In the presence of chloride ion, Ph2S(O)Cl2 is formed and can be identified by 13C NMR and by its hydrolysis to Ph2SO2. Mechanisms are proposed for these reactions, and ab initio molecular orbital calculations (GAUSSIAN92) were carried out of the postulated intermediates. Key words: preparation of Ph2S(O)F2, Ph2S(O)F+, and Ph2S(O)Cl2; oxidative fluorination in the Ph2SO–XeF2–Cl− system; fluorine exchange in the Ph2S(O)F2–Ph2S(O)F+ system.

Bis- and tris(amidine)fluoroboron cations and mixed tetrahaloborate anions: NMR studies of mixed boron trihalide adduct redistribution reactions involving amidines as strong nitrogen bases

Amidines as strong Lewis bases react with amidine–mixed boron trihalide adduct systems D•BFnCl3−n (n = 0–3) in the presence of excess boron trihalide to give complex mixtures of products including the mixed tetrahaloborate anions BFnCl4−n− and the fluoroboron cations D2BF2+ and D3BF2+, which coexist with the neutral adducts D•BFnCl3−n. Excess amidine rapidly displaces chloride ion from the chlorofluoroborate anions and neutral mixed boron trihalide adducts to give high yields of the fluoroboron cations. The D3BF2+ cations of the amidines 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) and 1,5-diazabicyclo[4,3,0]non-5-ene (DBN) are inert and have been isolated as their hexafluorophosphate salts. (DBN)2BF2+ is inert and has been isolated as its hexafluorophosphate salt, but (DBU)2BF2+ is highly reactive. Key words: amidines, fluoroboron cations, mixed boron trihalide adducts, redistribution reactions, fluorine-19 NMR, boron-11 NMR, 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), 1,5-diazabicyclo[4,3,0]non-5-ene (DBN)

Bis(pyridine)difluoroboron, tris(pyridine)fluoroboron, and other (pyridine)haloboron cations. A systematic NMR study

The adducts pyr•BF2Br and pyr•BFBr2 (pyr = pyridine) form fluoroboron cations by displacement of Br− by excess pyridine, the ease of cation formation being pyr2BF2+ » pyr2BFBr+ » pyr3BF2+•Cl− can be displaced from pyr•BF2Cl and pyr•BFCl2, but much less readily, to form pyr2BF2+, pyr2BFCl+, and, under forcing conditions, a few percent of pyr3BF2+. Non-fluorine-containing mixed boron trihalide adducts of pyridine also form haloboron cations by heaviest-halide-ion displacement, for example pyr•BClI2 giving pyr2BClI+, the ease of displacement always being I− > Br− > Cl−, and displacement always occurring more readily from mixed boron trihalide adducts than from unmixed-halogen adducts. The mechanistic implications of this are discussed. ortho Substituents greatly reduce the ability of pyridine to displace heavy halide ion, so 2-methylpyridine gives 2-Mepyr2BF2+ and 2-Mepyr2BFBr+ but not 2-Mepyr2BFCl+ or 2-Mepyr3BF2+, while 2,6-dimethylpyridine does not form any haloboron cations. 19F spin-lattice relaxation times of the fluoroboron cations are much shorter than those of neutral boron trihalide adducts in the same solution, and provide a further diagnostic test for their presence. Key words: fluoroboron cations, pyridines, mixed boron trihalide adducts, fluorine-19 NMR, boron-11 NMR.