Phenylborates

Phenylboric acid (benzeneboronic acid) [98-80-6] M 121.9, m -43", 215-216" (anhydride), 217-220", pKj 8.83. It recrystallises from H2O, but can convert spontaneously to benzeneboronic anhydride or phenylboroxide on standing in dry air. Possible impurity is dibenzeneborinic acid which can be removed by washing with pet ether. Heating in an oven at 110°/760mm Ih converts it to the anhydride m 214-216°. Its solubility in H2O is 1.1% at 0° and 2.5% at 25° and in EtOH it is 10% (w/v). [Gilman and Moore J Am Chem Soc 80 3609 1958.] If the acid is required, not the anhydride, the acid (from recrystallisation in H2O) is dried in a slow stream of air saturated with H2O. The anhydride is converted to the acid by recrystallisation from H2O. The acid gradually dehydrates to the anhydride if left in air at room temperature with 30-40% relative humidity. The melting point is usually that of the anhydride because the acid dehydrates before it melts [Washburn et al. Org Synth Coll Vol IV 68 7965]. 

Standard ionic potentials Ajy can be calculated from the ionic distribution coefficients or transfer energies see Eq. (30). In order to perform such calculations, an appropriate nonthermodynamic assumption that allows division of the E> mx) or electrolyte function into ionic constituents has to be made. At the present time, the assumption about the equality of the transfer energies of tetraphenylarsonium cations (TPhAs ) and tetra-phenylborate anions (TPhB ) is considered as most appropriate [2,36]. It can be presented in the following form ... 

The organic solvent should feature a low solubility in water and a high dielectric constant. Numerous studies have been reported for liquid-liquid junctions involving DCE [43,62,70,71,73], nitrobenzene [67,68,74,75], and nitrophenyloctylether (NPOE) [56]. Various hydrophobic electrolytes have been employed in these solvents. Tetraphenylarsonium (TPAs+) [[71,75,76], bis-triphenylphosphoranylidene (BTPPA+) [43,50], and hydrophobic tetra-arylammonium [77,78] are among the cations used in the organic phase. The choice for anions has been mostly restricted to borate derivatives, tetraphenylborate (TPB ) [70,79,80], tetrakis(4-chlorophenyl)borate (TPBCH) [43,81,82], and tetrakis(penta-fluoro)phenylborate (TPFB ) [49,83], as well as dicarbollyl-cobaltate [75]. 

The redox stability organic of phase cations is commonly very high in comparison to the anionic phenylborate derivatives. It has been proposed that Fc is reduced to Fc in DCE in the presence of TPB (Ph4B ) [71],... 

DCE interface in the presence of TPBCl [43,82]. The accumulation of products of the redox reactions were followed by spectrophotometry in situ, and quantitative relationships were obtained between the accumulation of products and the charge transfer across the interface. These results confirmed the higher stability of this anion in comparison to TPB . It was also reported that the redox potential of TPBCP is 0.51V more positive than (see Fig. 3). However, the redox stability of the chlorinated derivative of tetra-phenylborate is not sufficient in the presence of highly reactive species such as photoex-cited water-soluble porphyrins. Fermin et al. have shown that TPBCP can be oxidized by adsorbed zinc tetrakis-(carboxyphenyl)porphyrin at the water-DCE interface under illumination [50]. Under these conditions, the fully fluorinated derivative TPFB has proved to be extremely stable and consequently ideal for photoinduced ET studies [49,83]. Another anion which exhibits high redox stability is PFg- however, its solubility in the water phase restricts the positive end of the ideally polarizable window to < —0.2V [85]. 

Samec et al. [15] used the AC polarographic method to study the potential dependence of the differential capacity of the ideally polarized water-nitrobenzene interface at various concentrations of the aqueous (LiCl) and the organic solvent (tetrabutylammonium tetra-phenylborate) electrolytes. The capacity showed a single minimum at an interfacial potential difference, which is close to that for the electrocapillary maximum. The experimental capacity was found to agree well with the capacity calculated from Eq. (28) for 1 /C,- = 0 and for the capacities of the space charge regions calculated using the GC theory,... 

Among cations, potassium, acetylcholine, some cationic surfactants (where the ion-exchanger ion is the / -chlorotetraphenylborate or tetra-phenylborate), calcium (long-chain alkyl esters of phosphoric acid as ion-exchanger ions), among anions, nitrate, perchlorate and tetrafluoro-borate (long-chain tetraalkylammonium cations in the membrane), etc., are determined with this type of ion-selective electrodes. 

In addition to hydroboration, haloboration and phenylboration have also been used in the synthesis of 7r-conjugated organoboron polymers (9)29 (Fig. 9) and polymers containing B—N bonds.30... 

Figure 9 The -77-conjugated organoboron polymer (9) produced by haloboration-phenylboration polymerization between 2,7-diethynylflourene and Ph2BBr. (Adapted from ref. 29.)...

Conjugated organoboron polymers were prepared by haloboration-phenylboration polymerization between diyne monomers and bromodiphenylborane (scheme 29).54 The polymerization was carried out by adding a slight excess of bromodiphenylborane to a tetrachloroethane solution of diynes at room temperature... 

The polymers prepared by phenylboration polymerization of diynes exhibit relatively high stability against air and thermal oxidation (scheme 30).55... 

Parshall (65IC52) carried out this reaction with bis(hydroxyalkyl)phos-phine obtained from phosphine and hexafluorocyclobutanone and diethyl ester of phenylboric acid [Eq. (53)]. 

Interaction between tris(a-hydroxy-/3,/3,/3-trichloroethyl)phosphines and the anhydride of phenylboric acid gave rise to 1,3,2,5-dioxaphosphori-nanes (102) with 5-a-hydroxy-j8,j8,j8-trichloroethyl substituents [Eq. (59)]. 

But in the presence of pyridine, l-borata-2,6,7-trioxa-4-phosphabi-cyclo[2,2,2]octane (109) has been obtained [Eq. (65)]. The course of the reaction does not depend on the replacement of phenylboric acid anhydride by its ester (85IZV469 86JZVI641). 

The possibility of carrying out a multistep synthesis makes it possible to obtain P,B-containing derivatives from unstable intermediate a-hydroxyalkylphosphines. Thus, phenylphosphine, salicylic aldehyde, phenylboric acid anhydride, and triethylamine interact to give a bicyc-lic product—2,8,9-trioxa-1 -borata-4-phospha-6,7 -benzobicyclo [3,3,3] nonane (115) [Eq. (74)] (87IZV2118 89IZV946). In this case an aldehyde takes part in the reaction opening up new synthetic possibilities. 

More significant differences from the properties of cyclic boryloxyalkyl-phosphines were revealed for 5,6-benzo-4-diphenylphosphino-2-phenyl-l,3-dioxa-2-boracyclohexane (126), obtained by the interaction of diphe-nylphosphine, salicylic aldehyde, and phenylboric acid ester [Eq. (83)] (92IZV196). 

Some chemical properties of boryloxyphenylenephosphines (192) have been studied [Eq. (141)]. In Balueva et al. (91IZV2397) the intramolecular trans-esterification of phenylboric acid, a phenyl ester with a-... 

Recently, addition of organorhodium species to nitriles has been reported.420 4203 4201 Intermolecular reaction of benzonitrile with phenylborate (accompanied with r//w-aryiation) (Equation (65)), arylative cyclization of acetylenic nitriles (Equation (66)), and cyclization of 2-cyanophenylboronic acid with alkynes or strained alkenes (Equation (67)) are proposed to proceed via this process. 

Quite recently, an asymmetric synthesis of alkoxyaminosulfonium salt 122 and diaminosulfonium salt 125 was accomplished (163) by way of menthoxysulfonium salt 126, obtained by treating A -p-toluene-sulftnylmorphoUne with 1-chlorobenzotriazole (NCBT), followed by menthol. The subsequent addition of methanol and sodium tetra-phenylborate gave the optically active salt 122 (Scheme 7). When 126 was treated with piperidine instead of methanol, the optically active diaminosulfonium salt 125 was formed. 

The formulas for compounds described in volume 34 are entered in alphabetical order. They represent the total composition of the compounds, e.g., BF24KC38H21 for potassium tetra-3,5-bis(trifluoro-methyl)phenylborate. The elements in the formulas are arranged in alphabetical order, with carbon and hydrogen listed last. All formulas are permuted on the symbols other than carbon and hydrogen representing organic groups in coordination compounds. Thus potassium tetra-3,5-bis(trifluoro-methyl)phenylborate can be found under B, F, and K in this index. 

The structures of methyhnagnesium tris(3-ferr-butylpyrazolyl)phenylborate (230) and ethyknagnesium tris(3-terr-butylpyrazolyl)phenylborate (231) have been determined by X-ray crystallography and are shown schematically (Figure 94) . Their structures show large similarities with that of 227 and 228 and only differ in the presence of a boron-bonded phenyl group in 230 and 231. 

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