Boron trifluoride polarity

In contrast to this generally high preference for. tyn-products for boron trifluoride mediated reactions between 3-a//c v/-Substituted allylstannanes and aldehydes, //-products are preferred for reactions involving 3-p/ cn>7-substituted allylstannanes. This stereoselectivity was observed for a range of aldehydes, and was explained in terms of the increased propensity for the tin-allylie carbon bond to be polarized when the -substituent is able to stabilize a positive charge so favoring a cyclic transition state73. 

Predict whether (a) a boron trifluoride molecule, BF3, and (b) an ozone molecule, O, are polar. 

Nakane ei al. (1964) established equilibrium constants of boron isotope exchange between boron trifluoride gas on one side and boron trifluoride methyl fluoride, methyl chloride, isopropyl chloride and t-butyl chloride. The value of the equilibrium constants, which represents the thermodynamic isotope effect, was related to the polarity, stability and catalytic activity of the complexes. 

Neopentyl sulfides have been prepared by alkylation of sodium sulfide with neopentyl tosylate in high-boiling polar solvents,4,5 or in low yields by reduction of alkyl 2,2-dimethylpropanethioate with the combination of lithium aluminum hydride and a large excess of boron trifluoride-etherate. ... 

The action of strong aprotic Lewis acids (antimony(V) fluoride, arsenic(V) fluoride etc.) provokes the ionization of xenon difluoride, leading to the formation of fluoroxenonium salts XcF + MFn or Xe2F3 MFn less strong acceptors of the fluoride ion (hydrogen fluoride, boron trifluoride, etc.) polarize the xenon difluoride molecule. 

The addition of boron trifluoride as a catalyst has been found to be essential in the fluorination of carboxylic groups of poly(acrylic acid) and poly(methacrylic acid) with sulfur tetrafluoride.12 Boron trifluoride as a Lewis acid is necessary in catalytic amounts to polarize the C = 0 bond before fluorination with molybdenum(VI) hexafluoride takes place.1314... 

Room-temperature fluorination of aromatic compounds with cesium fluoroxysulfate is generally run in the presence of a polar solvent like acetonitrile and a catalyst like boron trifluoride. Thus benzene yields fluorobenzene (22). 

Studies by Nakane et al. also support the two-step mechanism when alkylation is carried out with alkyl halides under substantially nonionizing conditions. It was further shown that in nonpolar organic solvents carbocations rather than the polarized complexes participate directly in the formation of the first n complex. BF3—H20 catalyzes ethylation,130 isopropylation,131 and benzylation132 through the corresponding carbocations. Accordingly, ethylbenzene equally labeled in both a and p positions was obtained when [2-14C]-ethyl halides were reacted in hexane solution in the presence of boron trifluoride, BF3—H20, or aluminum... 

Figure 6.24 illustrates a similar situation in boron trifluoride, BF3, where three fluorine atoms are oriented 120° from one another around a central boron atom. Because the angles are all the same, and because each fluorine atom pulls on the electrons of its boron—fluorine bond with the same force, the resulting polarity of this molecule is zero. 

SAMPLE SOLUTION (a) Boron trifluoride is planar with 120° bond angles. Although each boron-fluorine bond is polar, their combined effects cancel and the molecule has no dipole moment. 

When 2-hydroxymethyl-5,6-methylenedioxybenzo[6]thiophene is heated in a polar solvent with a catalytic amount of mineral acid, the polycyclic compound (166) is formed in 75% yield, probably by way of the intermediate (251).190 3-Hydroxymethylbenzo[6]thiophene and di(3-benzo[6]thienyl)methane react together in the presence of boron trifluoride to give a compound with the probable structure 252.486... 

Similar results have been obtained in the reactions of 1 and sodium salts of p-cresol and - and /J-naphthols. Under SN2 conditions (DMF or DMSO solvent), the alkylation of sodium cresolate occurs exclusively at the oxygen atom. The addition of a protic solvent causes C-alkylation, though the yields of C-alkylated products are low. Thus in acetone-water or dioxane-water, the yield of C-alkylated products 251 and 252 increases only up to 2%. C-Alkylation has also been observed in the reactions catalyzed by trifluoroacetic acid or boron trifluoride etherate at room temperature. The observed C-alkylation in protic media may be a reflection of a mechanism that involves a protonated epoxide or a more polarized transition state than in an SN2 pathway. 

The structure of the complex (96) between benzaldehyde (95) and boron trifluoride (equation 15) was investigated by X-ray crystallography169. In 96, BF3 is in the anti position to the phenyl ring and this geometry remains also in solutions, as tested by the 19F-NMR spectrum in CD2CI2. An ab-initio study170 on interactions between formaldehyde and boron trihalides showed that these complexes (mainly donor-acceptor complexes) affect spectroscopic properties and the reactivity of the carbonyl group the polarization of the C=0 bond favours the attack of nucleophiles. 

In particular, 1,3-dithiane prepared from dimethoxymethane (methylal) and pro pane-1,3-dithiol in the presence of boron trifluoride-etherate,237 and 2-alkyl-1,3-dithianes prepared similarly from aldehydes,2383 are important acyl anion equivalents. These and other uses are discussed in Sections 5.7.5, p. 596, and 6.6.1, p. 909. A wide-ranging review of the reversal of polarity of the carbonyl group through the formation of these sulphur-containing reagents has emphasised their value in organic synthesis.2388... 

For instance, in cases where the mechanism of the propagation of the polymer chain is by means of cationic polymerization, the rate increases with the polarity of the solvent. Thus, when the boron trifluoride-diethyl ether complex is used as the catalyst for styrene polymerization, then at 0°C the rate equation for a series of solvents takes the simple form of dependence on the solvent polarity (Heublein 1985) ... 

Fatty acid analysis of a fat is nowadays a relatively routine analytical operation. After methylation of the fat using reaction with boron trifluoride/methanol, boron trichloride/methanol, methanolic hydrogen chloride solution, diazomethane or, if free fatty acids are not present, alkaline catalysts such as sodium methoxide/methanol, the prepared methyl esters are then analysed by GC on a polar column such as CpSil 88, BPX70 or SP2340. The high polarity of the column is necessary to separate the saturated and unsaturated fatty acids fully. The fatty acid composition of a milk fat sample is thus relatively easily obtained, and was therefore one of the first techniques investigated for authentication purposes. 

Alkyl halides are common alkylating agents in this reaction. Benzene is converted to toluene at atmospheric pressure by methyl chloride in the presence of aluminum chloride. Nitroparaffins have been used as solvents for the aluminum chloride catalyst. An amalgamated aluminum Catalyst is more effective than aluminum chloride in certain alkylations by alkyl chlorides. Boron trifluoride must be accompanied by water, alcohol, or some other polar compound in order to be effective in similar alkylations. Hydrogen chloride, hydrogen fluoride, ferric chloride, and beryllium chloride also have been used as catalysts. 

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