Boronic acids, fluorination

There are only a few examples of displacement of a fluorine atom in a cross-coupling reaction. With tricarbonylchromium complexes of fluoroarenes as substrates, cross-coupling with both boronic acids and vinylstannanes has been realized. Interestingly, only PMe3 is effective as ligand in this reaction (89).2" See Section 9.6.3.4.10 for an example of the involvement of unactivated fluoroarenes in cross-coupling reactions. 

While most reporter molecules have been designed to interact with cations, Plenio and Diodone [326] reported fluorine containing cryptands, which interact with perchlorate. London and Gabel [327] reported fluorobenzene boronic acid, which interacted with specific sugars. 

With boron trifluoride-etherate, methanethiol and acetic acid fluorine is incorporated in almost quantitative yields. For example, the bis-dimethylamino-derivative 164 can be converted to l-fluoro-l-dimethylamino-2.4.6-triphenyl-phoshorin 172. ... 

A novel method for the convenient synthesis of alkenyl fluorides 15, as well as diflu-oromethyl-substituted alcohols 16 and amides 17, via electrophilic fluorination with one equivalent of F-Teda BF4 (6) of alkenyl boronic acids and trifluoroborates, has been reported.87 The alkenyl fluorides 15 are obtained as Z/E mixtures when the reaction is carried out with one equivalent of F-Teda BF4 in acetonitrile at room temperature. When the reaction is performed with two equivalents of F-Teda BF4 in water or a nitrile solvent the difluoromethyl-substituted alcohols 16 or amides 17, respectively, are obtained. 

As noted above, the hardness or softness of an acidic or basic site is not an inherent property of the particular atom at that site, but can be influenced by the substituent atoms The addition of soft, polarizable substituents can soften an otherwise hard center and the presence of electron-withdrawing substituents can reduce the softness of a site. The acidic boron atom is borderline between hard and soft. Addition of three hard, electronegative fluorine atoms hardens the boron and makes it a hard Lewis acid. Conversely, addition of three soft, electropositive hydrogens54 softens the boron and makes it a soft Lewis acid. Examples of the difference in hardness of these two boron acids are... 

The benzene derivatives containing the fluorinated sulfone have been prepared either by nucleophilic substitution of the 4-fluorophenyl derivative (e.g. 1) or by starting with the appropriately substituted sodium thiophenoxide and reacting with perfluoroalkyl iodide follow by oxidation with either MCPBA or chromium oxide (12. li.) The biphenyl derivatives have been prepared by palladium catalyzed cross coupling chemistry of the 4-bromophenyl derivative (e.g. 2) with substituted phenyl boronic acid (yields 37-84%) (JLH, .). Compound 16 has been prepared by palladium catalyzed cross coupling of (4-bromophenyl)perfluorohexyl sulfone with vinyl anisole in 37 % yield (JJL). The vinyl sulfones, 7 and 9, have been prepared by condensation of CH3S02Rf (JJL) with the appropriate aldehyde (yields 70,and 73%) following a literature procedure (1 ). Yields were not optimized. 

An efficient aqueous phase Suzuki-Miyaura reaction of activated aryl chlorides with aryl boronic acids has been reported. The method uses a new D-glucosamine-based dicyclohexylarylphosphine ligand for the palladium catalyst and works well with nitro-and cyano-activated chlorides.32 The aryl fluoride bond has been considered inert to palladium-catalysed substitution reactions. However, a computational study, backed up by experiment, shows that the presence of a carboxylate group ortho to fluorine will allow reaction both with phenylboronic acids in a Suzuki-type reaction and with organotin reagents in a Stille-type reaction the presence of the adjacent oxyanion stabilizes the transition state.33... 

Theoretical support was obtained to explain the experimental results observed with a-chloro- or a-bromo-substituted pinacolboronates (5)6 (Scheme 3.IX). When calculations were performed on the reaction between the a-fluoro-substituted allyl-boronic acid and formaldehyde, transition state A, in which fluorine atom occupies an axial position, was found to be more stable than transition state B by 3.5 kcal/mol. 

The fluorination of A-methyldiethanolaminc esters 1 of aromatic boronic acids with cesium fluoroxysulfate in acetonitrile at room temperature results in the formation of fluoroaromatic compounds 2 in low to fair yield. 

Since the first asymmetric reduction of ketones with chiral borohydrides by Itsuno et al. [ 1 ], a number of studies on the asymmetric reduction of ketones with chiral borane reagents have been demonstrated [2]. Corey s oxazaborolidines are some of the most successful reagents [3 ]. The effect of fluorine substituents was examined in the asymmetric reduction of acetophenone with LiBH4 by the use of chiral boronates (73) obtained from substituted phenyl boronic acid and tartaric acid [4]. Likewise, 3-nitro, fluorine, and trifluoromethyl groups on the 3- or 4-position provided enhanced stereoselection (Scheme 5.20). 

Boron has a high affinity for fluoride, and boronic acids can be converted, via reaction with KHF2, into trifluoroborates (RBF3K), the fluorine analogues of the boronate anion. These compounds are very stable, but can be reactive under the appropriate conditions and are very useful in palladium-catalysed couplings. 

Cesium fluoroxysulfate (CSSO4F) is moderately stable, although potentially explosive, and is particularly useful for ip o-fluorination of stannanes and boronic acids. °... 

LEAD PROTOXIDE (1317-36-8) PbO An oxidizer. Explosive reaction with 90% peroxyformic acid, mbidium acetylide. Reacts violently with strong oxidizers, aluminum carbide boron, chlorine, fluorine, dichloromethylsilane, calcium sulfide, hydrogen peroxide, hydrogen trisulfide (ignition), hydroxylamine (ignition), lithium carbide, metal acetylides, metal powders (e.g., aluminum, molybdenum, sodium, zirconium, etc.), perchloric acid, red phosphoms, selenium oxychloride, sodium. 

EXPLOSION and FIRE CONCERNS combustible NFPA rating Health 3, Flammability 2, Reactivity 0 spontaneous explosive reactions with dibenzol peroxide, fluorine nitrate, nitrosal perchlorate, red fuming nitric acid, and tetranitromethane reacts violently with boron chloride, peroxyformic acid, fluorine, trichloronitromethane, acetic anhydride, chlorosulfonic acid, perchromates, oleum and n-halomides ignites on contact with sodium peroxide and water forms heat-or shock-sensitive explosive mixtures with hydrogen peroxide, nitromethane and 1 -chloro-2,3-epoxypropane reactions with ozone and perchloric acid form explosive products decomposition emits highly toxic fumes of NOx use alcohol foam, dry chemical, water spray, or carbon dioxide for fire-fighting purposes. 

Very recently, the Ritter group presented two examples of the silver-mediated fluorination of aryl stannanes and aryl boronic acids (Scheme 6) [23, 24]. Good yields were reported in both cases. The authors proposed the transmetallation... 

Boron, atomic number 5, occurs naturally as two isotopes, and B, with natural abundances of 19.9% and 80.1%, respectively, (a) In what ways do the two isotopes differ from each other Does the electronic configuration of differ from that of B (b) Draw the orbital diagram for an atom of B. Which electrons are the valence electrons (c) Indicate three major ways in which the Is electrons in boron differ from its 2s electrons, (d) Elemental boron reacts with fluorine to form BF3, a gas. Write a balanced chemical equation for the reaction of solid boron with fluorine gas. (e) AHf for Bp3(g) is —1135.6 kl/mol Calculate the standard enthalpy change in the reaction of boron with fluorine, (f) When BCI3, also a gas at room temperature, comes into contact with water, the two react to form hydrochloric acid and boric add, H3BO3, a very weak acid in water. Write a balanced net ionic equation for this reaction. 

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