Fluoroborate

Addition of l,3-bis(methylthio)allyllithium to aldehydes, ketones, and epoxides followed by mercuric ion-promoted hydrolysis furnishes hydroxyalkyl derivatives of acrolein5 that are otherwise available in lower yield by multistep procedures. For example, addition of 1,3-bis-(methylthio)allyllithium to acetone proceeds in 97% yield to give a tertiary alcohol that is hydrolyzed with mercuric chloride and calcium carbonate to raws-4-hydroxy-4-methyl-2-pentenal in 41% yield.5 Addition to an epoxide and hydrolysis affords a 5-hydroxy-a,/S-un-saturated aldehyde.8 Similarly, addition of l,3-bis(methylthio)allyl-lithium to an epoxide, acetylation of the hydroxyl group, and hydrolysis with mercuric chloride and calcium carbonate provides a 5-acetoxy- raw.s-a,/9-unsaturated aldehyde,6 as indicated in Table I. Cyclic cis-epoxides give aldehydes in which the acetoxy group is trans to the 3-oxopropenyl group. 

Erickson, Ph. D. Thesis, Harvard University, Cambridge, Mass., 1970. 

Submitted by Mitchell J. Bogdanowicz and Baery M. Trost1 Checked by Tsutomu Aoki and Watahu Naoata 

Preparation of Cyclopropyldiphenylsvlfonium Fluoroborate. A suspension of 118.7 g. (0.339 mole) of 3-chloropropyldiphenylsulfonium fluoroborate (Note 2) in 500 ml. of dry tetrahydrofuran (Note 12) is placed in a 2-1., one-necked flask equipped with a magnetic stirring bar and nitrogen inlet tube under nitrogen. Then 5-g. portions of 55% sodium hydride-mineral oil dispersion (15.2 g., 0.350 mole) are added in 30-minute intervals. The resulting mixture is stirred (Note 13) at room temperature for 24 hours. An aqueous solution of 25 ml. of 48% fluoroboric acid (Note 14), 15 g. of sodium fluoroborate (Notes 7, 15), and 400 ml. of water is added to the well-stirred reaction to destroy residual hydride and swamp out chloride ion (Note 16). After 5 minutes 300 ml. of methylene chloride is added, and the top methylene chloride layer is removed from the lower aqueous layer (Note 17). The methylene chloride solution is then extracted with 100 ml. of water. This water layer is combined with the first aqueous layer, and the combined water layers are extracted with an additional 100 ml. of methylene chloride. The methylene chloride portions are combined, dried over anhydrous sodium sulfate, and evaporated at reduced pressure until precipitation occurs. Addition of 1 1. of ether completes the precipitation of the salt. 

The crystals are collected, washed with ether, and recrystallized from hot absolute ethanol (approximately 400 ml.) (Note 18). After drying under reduced pressure the yield is 79.5-88.0 g. (75-83%), m.p. 137— 139° (Note 19). 

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If triphenylmethyl chloride in ether is treated with sodium, a yellow colour is produced due to the presence of the anionic spiecies PhsC". Alternatively, if triphenylmethyl chloride is treated with silver perchlorate in a solvent such as THF, the triphenylmethyl cation is obtained. More conveniently, triphenylmethyl salts, PhsC X", can be obtained as orange-red crystalline solids from the action of the appropriate strong acid on triphenylcarbinol in ethanoic or propanoic anhydride solution. The perchlorate, fluoroborate and hexafluoro-phosphate salts are most commonly used for hydride ion abstraction from organic compounds (e.g. cycloheptatriene gives tropylium salts). The salts are rather easily hydrolysed to triphenylcarbinol. 

For very strong acids, it is usually possible to use a solvent of a more conventional kind thus, for example, the acid HBF, tetra fluoroboric acid, is extremely strong, because attachment of the hydrogen to the tetrafluoroborate group BF is essentially ionic, H BF and hence dissociation to an acid is very easy. Hence HBF behaves as a strong acid in, for example, an organic solvent, in which it can be used. 

The Bart reaction has now been extended to the synthesis of arylphosphonic acids by the interaction of the diazonium fluoroborate and phosphorus trichloride (Doak and Freedman, 1951). 

It has been mentioned ( 4.4.2) that nitronium tetraffuoroborate reaets with pyridine to give i-nitropyridinium tetraffuoroborate. This compound and several of its derivatives have been used to effect what is called the transfer nitration ofbenzeneandtoluene. i-Nitropyridinium tetraffuoroborate is only sparingly soluble in acetonitrile, but its homologues are quite soluble and ean be used without isolation from the solution in which they are prepared. i-Nitropyridinium tetra-fluoroborate did nitrate toluene in boiling aeetonitrile slowly, but not at 25 In eontrast, i-nitro-2-pieolinium tetraffuoroborate readily... 

From these results it appears that the 5-position of thiazole is two to three more reactive than the 4-position, that methylation in the 2-position enhances the rate of nitration by a factor of 15 in the 5-position and of 8 in the 4-position, that this last factor is 10 and 14 for 2-Et and 2-t-Bu groups, respectively. Asato (374) and Dou (375) arrived at the same figure for the orientation of the nitration of 2-methyl and 2-propylthiazole Asato used nitronium fluoroborate and the dinitrogen tetroxide-boron trifluoride complex at room temperature, and Dou used sulfonitric acid at 70°C (Table T54). About the same proportion of 4-and 5-isomers was obtained in the nitration of 2-methoxythiazole by Friedmann (376). Recently, Katritzky et al. (377) presented the first kinetic studies of electrophilic substitution in thiazoles the nitration of thiazoles and thiazolones (Table 1-55). The reaction was followed spec-trophotometrically and performed at different acidities by varying the... 

Diazonium salt chemistry provides the principal synthetic method for the prepara tion of aryl fluorides through a process known as the Schiemann reaction In this pro cedure the aryl diazonium ion is isolated as its fluoroborate salt which then yields the desired aryl fluoride on being heated... 

A standard way to form the aryl diazonium fluoroborate salt is to add fluoroboric acid (HBF4) or a fluoroborate salt to the diazotization medium... 

Diazotization of an arylamine followed by treatment with fluoroboric acid gives an aryl diazonium fluoroborate salt Heating this salt converts it to an aryl fluoride... 

Schiemann reaction (Section 22 17) Preparation of an aryl fluonde by heating the diazonium fluoroborate formed by addition of tetrafluorobonc acid (HBF4) to a diazonium ion Schiffs base (Section 17 10) Another name for an imine a compound of the type R2C=NR ... 

FLUORINECOMPOUNDS,INORGANIC - BORON - FLUOROBORIC ACID] (Vol 11) -in pharmaceuticals [PHARMACEUTICALS] (Vol 18)... 

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