Weakly coordinating bulky anions designed by ef®cient useof poly¯uoro-substitution

Hiroshi KobayashiInstitute of Advanced Material Study, Kyushu University Kasuga, Fukuoka 816-8580, Japan

Abstract

The molecular design and applications of weakly coordinating tetrakis(3,5-bis(tri¯uoromethyl)phenyl)borate (TFPB) and tetrakis(penta-

¯uorophenyl)borate (FTPB) are reviewed. # 2000 Elsevier Science S.A. All rights reserved.

Keywords: Weakly coordinating bulky anions; Tetrakis(3,5-bis(tri¯uoromethyl)phenyl)borate; Tetrakis(penta¯uorophenyl)borate; Phase-transfer catalysis

Tetraphenylborate anions with many ¯uoro- and tri¯uor-

omethyl substituents on the phenyl parts have attracted

wide-ranging interests at present as bulky and weakly

coordinating anions (Fig. 1) [1±3]. The compounds of this

group were designed originally in the mid-1970s so as to be

useful as a phase-transfer catalyst for electrophilic and acid-

mediated reactions, and since then their chemistry have been

taken over in our laboratory as a successful prototype, where

many features characteristic of the poly¯uoro-substitution

are multiply combined in the anionic species [4].

The studies of phase-transfer catalysis (PTC) had already

attained to an established status at that time, however, most

of the successful applications were con®ned within those to

nucleophilic and base-mediated reactions by use of cationic

catalysts.1 Under such circumstances we had an idea of

extending the application of this technique to electrophilic

and acid-mediated reactions, which was combined with a

part of my formal task in the institute to develop new organic

catalyses.

Catalysts useful to the above-mentioned purpose had to

satisfy the following requisites: (1) negatively charged

permanent or stable ionic species; (2) three-dimensionally

bulky molecular structure; (3) no or negligible nucleophi-

licity to cationic species; (4) high lipophilicity of its salts,

irrespective of the kind of counter ion; and (5) chemical

stability under acid and oxidative conditions. According to

these requisites, our attention was focused to tetraphenyl-

borate (TPB) anion as a prospective compound by simple

analogy of the structural features of useful cationic counter-

part, quaternary ammonium ion. TPB was already very

popular in the analytical use, and was known to be fatally

labile against the above requisites, decomposing rapidly in

the presence of acids or oxidants, even under aeration.2

To improve the drawbacks of TPB, we immediately

conceived the use of ¯uorine-substitution. The peculiarities

due to poly¯uoro-substitution impacted on me, once dated

back to early 1960s, when 1,1,1,5,5,5-hexa¯uoro- and 1,1,1-

tri¯uoropentan-2,4-dionato-metal chelate complexes were

veri®ed separable by gas-chromatography according to the

kind of central metal ion [5]. Metal chelate complexes were

thitherto regarded to be far from volatile matter and it

sounded earthshaking for me that the poly¯uoro-substitution

afforded them to dissolve in such an inert solvent as tetra-

chlorocarbon. Since then we had been dealing with the

search for the peculiarities induced by poly¯uoro-substitu-

tion and some synthetic applications obtaining 1,2-bifunc-

tional poly¯uorobenzene derivatives of analytical use.

During such works, our group gained knowledge and experi-

ence on the chemistry of organo¯uorine compounds.

Meanwhile some partly ¯uorinated tetraphenylborate ions

appeared in literatures, and indicated a tendency that the

introduction of ¯uoro- or tri¯uoromethyl substituents onto

the phenyl groups actually improved the stability against

acid and increased hydrophobicity of their salts [6±8].

Use of the ¯uorine-substitution for improving TPB

seemed effective in one way to suppress its lability [9±

10],3 and in another way to prepare the required organo-

Journal of Fluorine Chemistry 105 (2000) 201±203

1 Some of the then current books concerning phase-transfer catalysis are

listed in Ref. [1] of [4].

2 The lability of TPB was reported in some papers listed in Ref. [3] of

[4].3 Effects induced by polyfluoro-substitution and some of their applica-

tions were reviewed previously. Some physical data were recently revised

in [10].

0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.

PII: S 0 0 2 2 - 1 1 3 9 ( 0 0 ) 0 0 2 7 4 - 8

magnesium intermediates on the synthetic sequence. Thus,

the ®rst target was tetrakis(penta¯uorophenyl)borate

(FTPB) ion, whose synthesis had been then reported, but

few necessary physical properties were described [11]. In

spite of the repeated attacks with various contrivances, the

schemes going through a monohydro derivative from hexa-

¯uorobenzene failed in obtaining the target product, which

was, however, found most recently to be successfully iso-

lated in a stable form by an improved scheme starting with

bromopenta¯uorobenzene [12].

Parallel to the unsettled works for FTPB, we turned our

view to the use of tri¯uoromethyl groups for the improve-

ment of TPB, especially in respect to durability against

protic acids and oxidants and solubility in hydrophobic

organic solvents. The conceivable phenyl moieties contain-

ing more than one tri¯uoromethyl group were so narrowed,

that no choice except 3,5-bis(tri¯uoromethyl)phenyl one

[13].4 The corresponding xylene derivative which we started

with, was provided to us by the Daikin Industries, Osaka,

who produced the compound as an intermediate of some

agrochemicals. We were greatly indebted for their favor,

which afforded us a successful synthetic scheme leading to

tetrakis(3,5-bis(tri¯uoromethyl)phenyl)borate (TFPB) ion

(Fig. 2).

The synthetic sequence and spectroscopic data of TFPB

supported consistently the assigned structure [14],5 which

was independently con®rmed by the single-crystal X-ray

analysis of lithium TFPB tetrahydrate [15].

Synthetic scheme was further improved for a method

affording a higher yield with easier processing, which

was successfully applied to the syntheses of some higher

homologues of tetraarylborate ions substituted with many

tri¯uoromethyl groups [16].

Both ¯uoro- and tri¯uoromethyl-substitution afforded

drastic improvements to TPB in the ef®cacy as a phase-

transfer catalyst, though in somewhat different manner. The

former favored to the suppression of lability under acid and

oxidative conditions, while the latter to the increased solu-

bility in hydrophobic organic solvents [12].

In the course of synthesis of TFPB, even before obtaining

a de®ned specimen, we were impatient to examine its

catalytic effect by a simple test of diazo-coupling of N-

ethylcarbazole with 4-nitrobenzene-diazonium tetra¯uoro-

borate in a dichloromethane±water two-phase system after

the preceding examples.6 Encouraged by the positive results

of the preliminary test, diazo-coupling reactions of arene-

diazonium ions with various diazophile components were

investigated in two-phase systems in the presence of a

catalytic amount of TFPB [17,18].

Kinetic investigations proved that TFPB catalyst turned

over, as being expected, to promote the reaction under PTC

conditions. The diazonium ions were dehydrated in the

hydrophobic organic phase, to become more reactive than

in the aqueous phase and meanwhile, retained free from the

attack by various nucleophilic agents, so that complex side

reactions were suppressed, affording a satisfactory yield of

coupling products [19,20].

Sodium TFPB placed in a dichloromethane±aqueous

sulfuric acid two-phase system, could incorporate oxonium

ion into the hydrophobic organic phase quantitatively by

exchanging sodium ion. The oxonium ion was regarded to

stay in the form of ion-pair with TFPB. The analytical

concentration of oxonium ion in the organic phase, there-

Fig. 1. The structures of fluorinated tetraphenylborates, TFPB and

FTPB.

Fig. 2. Synthetic scheme of TFPB.

4 The disposition of two trifluoromethyl groups at 3- and 5-positions of

the phenyl part had been known as one of those which induced the

strongest electron-withdrawing effect, when we made our choice without

noticing the important report concerned.

5 Dojin Chemical Laboratory, Kumamoto, offers reagent-grade TFPB

with the trade name of `TFPB/The Kobayashi's reagent'.6 Reports on diazo-coupling reactions in two-phase systems at that time

are listed in Ref. [2] of [4].

202 H. Kobayashi / Journal of Fluorine Chemistry 105 (2000) 201±203

fore, was de®ned by that of counter anion, TFPB. Its acidity

changed over a wide range depending upon the aqueous acid

concentration, conceivably due to the varied degree of

hydration of oxonium. The TFPB-catalyzed two-phase sys-

tem could be regarded as a quite unique one affording

oxonium ions of high acidity even at a lower concentration

[4].

The oxonium of high acidity in the organic phase served

therein to generate electrophiles of higher order, E�, by

oxonium-catalyzed dehydration from the corresponding

precursors, EOH, according to a general scheme as follows:

EOH � H3O�TFPBÿ ! E� � 2H2O

where TFPB carried oxonium ions catalytically to the

reaction sites, to promote the overall electrophilic reactions

of E� or its equivalent in the two-phase systems. Such a

scheme was exemplified by Friedel±Crafts alkylations [21],

diazotization and nitrosation [22,23].

In the structures of TFPB and FTPB ions, the negative

charge localized at the central boron is sterically shielded as

well as stabilized by strong electron-withdrawing effect due

to the shell of ¯uorine atoms arrayed on the surface of

molecule. Therefore, the anions would interact with cationic

species solely in an electrostatic manner at a longer distance

off, no more in the covalent or the coordinate one. The

cationic species in the hydrophobic solution phase, on the

other hand, are retained under inert environments with least

in¯uence by the counter ions and the solvent molecules. It

can display therein its own inherent power in a case as it is,

while in another case after converted to a cationic species of

higher order [22,23].

Poly¯uoro-substitution successfully brought forth the

weakly coordinating bulky anions such as TFPB and FTPB,

which are prospective to develop novel ®elds of solution

chemistry of cationic species. Extensive reports7 on their

application as the effective counter ions in homogeneous

olefin polymerization catalysts seem indicative of such

prospects in near future.8

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7 The applications reported independently from the groups other than

ours are listed in Ref. [7] of [12].

8 Some of the reports on polymerization catalysts are listed in Ref. [5] of

[12].

H. Kobayashi / Journal of Fluorine Chemistry 105 (2000) 201±203 203