Borylation of fluorinated arenes using the boron-centred nucleophile B(CN)₃²⁻ – a unique entry to aryltricyanoborates

Abstract

The potassium salt of the boron-centred nucleophile B(CN)₃²⁻ (1) readily reacts with perfluorinated arenes, such as hexafluorobenzene, decafluorobiphenyl, octafluoronaphthalene and pentafluoropyridine, which results in KF and the K⁺ salts of the respective borate anions with one {B(CN)₃} unit bonded to the (hetero)arene. An excess of K₂1 leads to the successive reaction of two or, in the case of perfluoropyridine, even three C–F moieties and the formation of di- and trianions, respectively. Moreover, all of the 11 partially fluorinated benzene derivatives, C₆F_6−nH_n (n = 1–5), generally react with K₂1 to give new tricyano(phenyl)borate anions with high chemo- and regioselectivity. A decreasing number of fluorine substituents on benzene results in a decrease in the reaction rate. In the cases of partially fluorinated benzenes, the addition of LiCl is advantageous or even necessary to facilitate the reaction. Also, pentafluorobenzenes R–C₆F₅ (R = –CN, –OMe, –Me, or –CF₃) react via C–F/C–B exchange that mostly occurs in the para position and to a lesser extent in the meta or ortho positions. Most of the reactions proceed via an S_NAr mechanism. The reaction of 1,4-F₂C₆H₄ with K₂1 shows that an aryne mechanism has to be considered in some cases as well. In summary, a wealth of new stable tricyano(aryl)borates have been synthesised and fully characterized using multi-NMR spectroscopy and most of them were characterised using single-crystal X-ray diffraction.

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Introduction

Anionic boron-centred nucleophiles are of growing interest as boron centres are usually electrophilic (Lewis-acidic). Therefore, boron-centred nucleophiles have unusual reactivity and are expected to have large synthetic potential.^1–4 The isolation of the first boron-centred nucleophile in 2006, the lithiated boryl anion A that is stable in THF at −45 °C for months, was an important development in this field that stimulated intensive further research (Scheme 1).⁵ The chemistry of the lithiated boryl anion A and closely related species^3,6–8 was studied in detail, and these compounds were found to be versatile starting compounds for the introduction of boryl moieties.^1,3–14 Although some other boron-centred nucleophiles have been described, there is still a limited number of them.^15–26 Boryl anions were found to be stabilized by carbene ligands such as NHCs (NHC = N-heterocyclic carbene) and cAACs (cAAC = cyclic (alkyl)(amino)carbene), as demonstrated in B²⁷ and C²⁸ (Scheme 1). An alternative approach for the stabilization of boron-centred nucleophiles is the introduction of cyano groups at the boron atom, as demonstrated in C²⁸ and the tricyanoborate dianion B(CN)₃²⁻ (1).^29–32 The latter is the only dianionic boryl anion known to date, and its alkali metal salts are indefinitely stable at room temperature under an inert atmosphere. The first syntheses towards the alkali metal salts of 1 started from M[B(CN)₄] (M = alkali metal).²⁹ Recently, we reported convenient, high-yield and large-scale entries towards the salts of 1 starting from the readily available tricyanoborates M[BF(CN)₃]^33,34 and M[BH(CN)₃]^35,36 (Scheme 1).^30–32 The unprecedented formation of dianion 1 from [BH(CN)₃]⁻ is the first example of the deprotonation of a hydridoborate anion.^31,32 The conversion of a hydridoborane into monoanion C is the only related reaction²⁸ known to date.

Scheme 1 Selected boron-centred nucleophiles (top), the different types of conversion of C–F into C–B moieties of arenes and heteroarenes (middle), and convenient syntheses of K₂1 (bottom); (dipp = 2,6-diisopropylphenyl, cat = catecholato, pin = pinacolato, and neop = neopentyl glycolato).

As mentioned previously, boron-centred nucleophiles have large synthetic potential. Surprisingly, only very few examples of reactions of such nucleophiles with fluorinated arenes have been reported. Boryl lithium A was found to react with C₆F₆ or C₆H₅F to obtain the corresponding monoborylated benzenes.⁶ Similar reactions have been reported for B²⁷ and 1³⁰ with C₆F₆ only (Scheme 1). In general, only a limited number of transformations of C–F into C–B bonds of arenes or heteroarenes are known. They are either metal-catalyzed^37–42 or metal-mediated⁴³ reactions, or photoinduced radical borylations^44,45 (Scheme 1).

Herein, we report on the S_NAr reactions of the boryl dianion B(CN)₃²⁻ (1) with selected fully and partially fluorinated arenes^46–51 including perfluorinated pyridine, naphthalene and biphenyl as well as fluorinated arenes with a functional group. All of the reactions proceeded via the exchange of fluorine with the tricyanoboryl moiety and most of them were chemo- and regioselective. Even multiple exchange reactions that provide access to bis- and tris(tricyanoborate) anions have been achieved.

Results and discussion

Reactions of K₂B(CN)₃ (K₂1) with perfluoropyridine

As outlined in the introduction, K₂B(CN)₃ (K₂1) was found to react with an excess of C₆F₆ at room temperature in THF to give the salts of the [C₆F₅B(CN)₃]⁻ anion (B6a) in 67% yield.³⁰ A related reaction of perfluoropyridine with K₂1 was found to start at temperatures below 0 °C, as evidenced by the rapid decolorization of the suspension. The [4-{(NC)₃B}-C₅F₄N]⁻ anion (Py1) was obtained as the main product of the reaction with an excess of perfluoropyridine (Table 1 and Fig. 1). The exchange of the fluorine in the 4-position is typical for the S_NAr reactions of C₅F₅N.^52–55 The dianion [2,4-{(NC)₃B}₂-C₅F₃N]²⁻ (Py2), a different isomer of Py1, most likely [2-{(NC)₃B}-C₅F₄N]⁻, and small amounts (<1%) of further tricyano(fluoropyridinyl)borate anions, including the trianion [2,4,6-{(NC)₃B}₃-C₅F₃N]³⁻ (Py3), were obtained as the side products.

Table 1 Reactions of K₂1 with selected fluoro(hetero)arenes

Entry	Substrate	LiCl^a	K21^b	Conditions	[BH(CN)3]^−^c	Major tricyanoborate anion(s) formed		Isolated yield
a Whether LiCl was added to the reaction mixture. b Equivalents of K21. c The percentage that was formed as a side product; [BH(CN)3]^− was removed during the work-up. d The ratio B2c : B2b was 6 : 4 in the reaction mixture and 7 : 3 in the isolated material. e 11% of K[BH(CN)3]. f [Et3NH]^+ salt. g The internal yield was 75% Py1, 16% Py2, 8% another isomer (probably [2-{(NC)3B}-C5F4N]^−), and 1% Py3 and unknown tricyano(fluoropyridinyl)borate anions. h Purity ca. 85% (^11B/^19F NMR); it contained 15% other tricyano(fluoropyridinyl)borates. i The internal yield (^11B/^19F NMR) was 60% Py2, 30% Py3, and 10% another tricyano(fluoropyridinyl)borate anion. j Purity ca. 75% (^11B/^19F NMR); it contained 15% K3Py3 and 10% another tricyano(fluoropyridinyl)borate. k N2 : N3 = 1.0 : 0.8 (N3 = [2,7-{(NC)3B}2-C10F6]^2−). l The ratio of the reaction mixture: N1 : N2 : N3 = 0.4 : 1.0 : 0.7; N2 is hardly soluble and was obtained as a pure K^+ salt. m K[B(CN)4] was mostly removed via fractional precipitation.
1	C6FH5	Yes	<1	80 °C, 2 d	25%	[1-{(NC)3B}-C6H5]^− (B1)	Sole isomer	45%
2	1,2-C6F2H4	Yes	<1	r.t., 16 h	9%	[1-{(NC)3B}-2-F-C6H4]^− (B2a)	Sole isomer	58%
3	1,3-C6F2H4	Yes	<1	r.t., 3 d	<5%	[1-{(NC)3B}-3-F-C6H4]^− (B2b)	Sole isomer	70%
4	1,4-C6F2H4	Yes	<1	75 °C, 30 h	28%	[1-{(NC)3B}-4-F-C6H4]^− (B2c) + B2b (6 : 4^d)^e		45%^e
5	1,2,3-C6F3H3	Yes	<1	r.t., 3 d	<5%	[1-{(NC)3B}-2,3-F2-C6H3]^− (B3a) + [1-{(NC)3B}-2,6-F2-C6H3]^− (B3b) (4 : 1)		66%
6	1,2,4-C6F3H3	Yes	<1	r.t., 2 h	<5%	[1-{(NC)3B}-2,5-F2-C6H3]^− (B3c)	Sole isomer	76%
7	1,3,5-C6F3H3	Yes	<1	r.t., 16 h	<5%	[1-{(NC)3B}-3,5-F2-C6H3]^− (B3d)	Sole isomer	63%
8	1,2,3,4-C6F4H2	Yes	<1	r.t., <1 h	<5%	[1-{(NC)3B}-2,3,6-F3-C6H2]^− (B4a)	Sole isomer	77%
9	1,2,3,5-C6F4H2	Yes	<1	r.t., 2 h	<5%	[1-{(NC)3B}-2,3,5-F3-C6H2]^− (B4b)	Sole isomer	63%
10	1,2,3,5-C6F4H2	No	<1	75 °C, 30 h	10%	B4b	8% of other isomers	51%
11	1,2,4,5-C6F4H2	Yes	<1	r.t., 30 min	30%	[1-{(NC)3B}-2,4,5-F3-C6H2]^− (B4c)	Sole isomer	50%
12	1,2,4,5-C6F4H2	Yes	2	r.t., 16 h	n.d.	[1,4-{(NC)3B}2-2,5-F2-C6H2]^2− (B4d)	Sole isomer	42%^f
13	C6F5H	Yes	<1	r.t., 10 min	10%	[1-{(NC)3B}-2,3,5,6-F4-C6H]^− (B5)	6% of one other isomer	62%
14	C6F5H	No	<1	r.t., 2 d	48%	B5	20% of other anions	39%
15	C6F6	No	<1	0 °C	—	[1-{(NC)3B}-C6F5]^− (B6a)^29	Sole isomer	67% (ref. 30)
16	C6F6	No	2.2	Reflux, 20 h	—	[1,4-{(NC)3B}2-C6F4]^2− (B6b)	Sole isomer	74%
17	C5F5N	No	0.33	r.t., 12 h	—	[4-{(NC)3B}-C5F4N]^− (Py1)^g	Other isomer(s), Py2, Py3	81%^h
18	C5F5N	No	2	r.t., 4 d	—	[2,4-{(NC)3B}2-C5F3N]^2− (Py2)^i	Other isomer(s), Py3	59%^j
19	C5F5N	No	3	r.t., 4 d	—	[2,4,6-{(NC)3B}3-C5F2N]^3− (Py3)	<10% Py1 and Py2	31%
20	C12F10	No	0.8	r.t., 2 h	—	[4-{(NC)3B}-C12F9]^− (BP1)	<20% of BP2	49%
21	C12F10	No	2.2	50 °C, 1 h	—	[4,4′-{(NC)3B}2-C12F8]^2− (BP2)	Sole isomer	90%
22	C10F8	No	0.5	r.t., 3 d	—	[2-{(NC)3B}-C10F7]^− (N1)	8% of other isomers^k	55%
23	C10F8	No	2	60 °C, 16 h	—	[2,6-{(NC)3B}2-C10F6]^2− (N2)	Mixture of N1, N2 and N3^l	24%
24	F3C–C6F5	No	<1	r.t., 3 h	—	[1-F3C-4-{(NC)3B}-C6F4]^− (B7)	Sole isomer	68%
25	Me–C6F5	No	<1	90 °C, 3 d	—	[1-Me-4-{(NC)3B}-C6F4]^− (B8a) + [1-Me-3-{(NC)3B}-C6F4]^− (B8b) (9 : 1)		n.d.
26	1-F3C-4-H-C6F4	No	<1	r.t., 2 h	75%	Unidentified borate anions		n.d.
27	1-F3C-6-H-C6F4	No	<1	r.t., 4 d	2%	[1-F3C-4-{(NC)3B}-2,3,5-F3-C6H]^− (B9)	7% of other isomers	78%
28	MeO–C6F5	No	<1	60 °C, 16 h	—	[1-MeO-4-{(NC)3B}-C6F4]^− (B10a) + [1-MeO-3-{(NC)3B}-C6F4]^− (B10b) (1 : 1)		82%
29	NC-C6F5	No	<1	r.t., <1 h	—	[1-NC-4-{(NC)3B}-C6F4]^− (B11a) + [1-NC-2-{(NC)3B}-C6F4]^− (B11b) + [B(CN)4]^− (2.5 : 1.5 : 1)^m		n.d.
30	Cl–C6F5	No	<1	r.t., <1 h	—	[BCl(CN)3]^−, [B2(CN)6]^2−, …		n.d.
31	O2N–C6F5	No	<1	r.t., 16 h	—	[B2(CN)6]^2−, …		n.d.

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Fig. 1 Top: the reactions of K₂1 with pentafluoropyridine to give Py1, Py2 and Py3 (^acontained 15% other tricyano(fluoropyridinyl)borates; ^bcontained 15% K₃Py3 and 10% another tricyano(fluoropyridinyl)borate). Bottom: the anions in crystals of their K⁺ salts (the displacement ellipsoids are at the 25% probability level, except for Py3 where they are at 50%).

An excess of K₂1 yielded larger amounts of the dianion Py2 and the trianion Py3 and the reaction of perfluoropyridine with two equivalents of K₂1 gave K₂Py2 as the major product. Three equivalents of the potassium salt resulted in K₃Py3 in 31% yield. A related, successive replacement of 1, 2 and 3 fluorine substituents of perfluoropyridine via nucleophilic replacement has been previously reported for the methoxy anion.⁵⁴ In contrast, alkyl and aryl Grignard reagents were reported to result in only mono-substitutions in the para position of perfluoropyridine.⁵⁵ The decreasing solubility of the potassium borates KPy1, K₂Py2 and K₃Py3 with the increasing charge of the anion enabled the enrichment of K₂Py2 and the purification of K₃Py3 by precipitation from the THF solutions via the slow addition of CH₂Cl₂. Pure KPy1 and K₂Py2 were obtained via crystallization. Single-crystals of KPy1, K₂Py2·OC(CH₃)₂ and K₃Py3·3THF·1.04 H₂O were studied using X-ray diffraction (Fig. 1). Selected experimental bond distances of the three related anions Py1, Py2 and Py3 were compared to the calculated bond lengths in Table S3.†

Monosubstitution of partly fluorinated benzenes with K₂1

In addition to the reactions of K₂1 with hexafluorobenzene³⁰ and pentafluoropyridine, K₂1 was found to undergo C–F/C–B exchange reactions with all of the partly fluorinated benzenes C₆F_6−nH_n (n = 1–5) (Table 1 and Fig. 2). Unprecedentedly, the nucleophilic attack of 1 at almost all of the partially fluorinated benzenes was regio- and chemoselective. As expected for electron deficient partly fluorinated benzenes, deprotonation was observed for the strong base 1, yielding [BH(CN)₃]⁻ as a side product. The amount of [BH(CN)₃]⁻ formed reflects the Brønsted acidity of the hydrogen atoms of the respective fluorobenzene.^56,57 1,2,4,5-tetrafluoro- and pentafluorobenzene gave the largest amounts of [BH(CN)₃]⁻ with up to 48% for C₆F₅H (Table 1). The addition of LiCl at the start of the reaction of K₂1 with C₆F₅H resulted in a reduction in the amount of [BH(CN)₃]⁻ to 10% and an increased yield of [1-{(NC)₃B}-2,3,5,6-F₄-C₆H]⁻ (B5) of 62%. Since the salts [(alkyl)₃NH][BH(CN)₃] (alkyl = Me, Et) are water soluble, purification was achieved via the precipitation or extraction of the respective trimethyl- or triethylammonium phenylborates from aqueous solutions. The ammonium salts were easily back-converted into K⁺ salts with K₂CO₃.

Fig. 2 Reactions of K₂1 with fluorinated benzenes (top). The main products; dashed boxes show the anions that were derived from the same arene. In the cases of C₆F₆ and 1,2,4,5-C₆F₄H₂, the different anions were obtained from different entries (bottom left). The tricyanophenylborate anions in the crystal structures of their K⁺ (B2a, B2b, B3a, B3c, B3d, B4a, and B4b), [Et₄N]⁺ (B6b), [Et₃NH]⁺ (B4d) and [Me₃NH]⁺ (B4c) salts (the displacement ellipsoids are at the 50% probability level, except for B3a where they are at 25%, and the H atoms are depicted with arbitrary radii) (bottom right).

The regioselectivities of the C–F/C–B exchange reactions were found to be high, and in most cases one major isomer had formed (Table 1). Most of the new tricyanoborates were characterized using single-crystal X-ray diffraction (Fig. 2) and the details of the experimental and calculated bond parameters are summarized in Table S3.† The exchange of a fluorine with a tricyanoboryl group in the para position to a fluorine substituent is unfavoured. Replacement was found to be preferred for fluorine substituents in the meta position to one or two further fluorine substituents, which is typical for the S_NAr reactions of fluorobenzenes.^52,53 1,4-difluorobenzene gave a mixture of [1-{(NC)₃B}-4-F-C₆H₄]⁻ (B2c) and [1-{(NC)₃B}-3-F-C₆H₄]⁻ (B2b) in conjunction with 25% of [BH(CN)₃]⁻ (Scheme 2). The formation of anion B2b is rationalized by an aryne mechanism that is similar to related reactions.⁵⁸ In summary, the high regioselectivities that were observed show that an S_NAr mechanism dominates for the C–F/C–B exchange presented herein.

Scheme 2 Reaction of 1,4-F₂-C₆H₄ with K₂1 in the presence of LiCl to give a mixture of KB2 and KB3 (ratio 7 : 3). The [BH(CN)₃]⁻ that formed (25%) is not shown.

The reaction rate of the C–F/C–B exchange strongly depended on the degree of fluorination and therefore on the electron density of the aromatic ring system. C₆F₆ reacted within minutes at room temperature, whereas the conversion of C₆F₅H required two days. In the cases of mono-, di- and trifluorobenzenes, no reaction was observed with K₂1 in THF even at 80 °C. Tetrafluorobenzenes showed some reactivity towards K₂1 in THF depending on the substitution scheme. 1,2,3,5-tetrafluorobenzene reacted at 75 °C within 3 days. The reaction of the 1,2,4,5-isomer required 120 °C, yielding an inseparable brownish mixture. The addition of anhydrous LiCl was found to result in a tremendous increase in the reaction rate. For example, upon the addition of LiCl, a mixture of 1,2,4,5-tetrafluorobenzene and K₂1 immediately became warm and the reaction was complete within minutes. Similarly, the reaction time of the conversion of C₆F₅H into B5 was reduced from 2 days to 10 minutes. The shorter reaction time was accompanied by enhanced chemo- and regioselectivity (Table 1). The LiCl-induced reaction was successfully applied for all of the di- and trifluorobenzenes and C₆FH₅ (Fig. 1). The reactions of all three trifluorobenzene isomers and of 1,2- and 1,3-difluorobenzene were conducted at room temperature. Only for 1,4-F₂-C₆H₄ and C₆FH₅ were higher temperatures necessary. Three different effects may be responsible for the faster reactions and the higher chemo- and regioselectivities in the presence of LiCl: (i) Li₂1 is more soluble in THF than K₂1,³¹ which results in an enhanced availability of the dianion 1; (ii) the high fluoride ion affinity of Li⁺ may lead to a Li⋯F interaction, a weakening of the C–F bond and a lowering of the activation barrier for the nucleophilic replacement; and (iii) a weak Li⋯B or Li⋯N interaction between Li⁺ and the boryl dianion 1 may influence the reactivity of 1.

Disubstitution of fluorinated benzenes with K₂1

C₆F₆ and 1,2,4,5-F₄-C₆H₂ were reacted with an excess of K₂1 to give salts of the dianions [1,4-{(NC)₃B}₂-C₆F₄]²⁻ (B6b) and [1,4-{(NC)₃B}₂-2,6-F₂-C₆H₂]²⁻ (B4d) (Table 1 and Fig. 2). In the case of the reaction of 1,2,4,5-F₄-C₆H₂, anhydrous LiCl was added to enhance the rate of the reaction. Both of the reactions were found to be highly regioselective and in accordance with an S_NAr mechanism. Therefore, the second C–F/C–B exchange gave the para-{B(CN)₃}₂ derivatives as the sole isomers.

In addition to the multiple C–F/C–B exchange reactions of C₅F₅N (Fig. 1), C₆F₆ and 1,2,4,5-F₄-C₆H₂ (Fig. 2), perfluorobiphenyl and perfluoronaphthalene were successfully applied in related reactions with K₂1. Stoichiometric amounts or a slight excess of K₂1 yielded K₂[4,4′-{(NC)₃B}₂-C₁₂F₈] (K₂BP2) and K₂[2,6-{(NC)₃B}₂-C₁₀F₆] (K₂N2), respectively. In contrast, an excess of the perfluorinated arene predominantly gave the corresponding monoanions [4-{(NC)₃B}-C₁₂F₉]⁻ (BP1) and [2-{(NC)₃B}-C₁₀F₇]⁻ (N1) (Table 1). The successive reactions of perfluorobiphenyl that gave BP1 and BP2 were fully regioselective. K₂BP2 was isolated in an excellent yield of 90% and a single crystal was investigated using X-ray diffraction (Fig. 3 and Table S3†). A slightly lower but still high regioselectivity was found for the first replacement of fluorine with 1 in perfluoronaphthalene to give N1. The introduction of a second {B(CN)₃} moiety resulted in a mixture of N1, N2 and [2,7-{(NC)₃B}₂-C₁₀F₆]²⁻ (N3) with the ratio 0.4 : 1.0 : 0.7. The formation of N2 as the major isomer agrees with the typical substitution scheme for the S_NAr reactions of perfluoronaphthalene.⁵³

Fig. 3 The bis(tricyanoborate) dianions BP2 and N2 in the crystal structures of their K⁺ salts (the displacement ellipsoids are at the 50% probability level).

Reactions of K₂1 with fluorinated benzenes with functional groups

The reactions described so far (vide supra) demonstrate the regioselectivity of the S_NAr reaction of fluorinated (hetero)arenes with K₂1 and show the possibility of synthesising multiple charged tricyanoborate anions. The multiple C–F/C–B exchange reactions are the first examples of the transformations of functionalized fluorinated (hetero)arenes with 1. A series of further selected reactions of polyfluorinated benzenes with a functional group bonded to the benzene ring have been studied (Fig. 4 and Table 1, entries 24–31).

Fig. 4 Reactions of K₂1 with functionalized fluorinated benzenes. Borate anions B9a, B7 and B10a in the crystal structures of their K⁺ salts (the displacement ellipsoids are at the 25% probability level, except for B10a where they are at 50%, and the H atoms are depicted with arbitrary radii).

Perfluorotoluene was found to give [1-F₃C-4-{(NC)₃B}-C₆F₄]⁻ (B7) as the sole isomer and KB7 was isolated in 68% yield. 4-Me-C₆F₅ resulted in a mixture of the isomers [1-Me-4-{(NC)₃B}-C₆F₄]⁻ (B8a) and [1-Me-3-{(NC)₃B}-C₆F₄]⁻ (B8b) in a 9 : 1 ratio and pentafluoroanisole gave [1-MeO-4-{(NC)₃B}-C₆F₄]⁻ (B10a) and [1-MeO-3-{(NC)₃B}-C₆F₄]⁻ (B10b) in equal amounts. The decrease in the regioselectivity in the order F₃C–C₆F₅, Me–C₆F₅ and MeO–C₆F₅ reflects the influence of the electronic properties of the –CF₃, –Me and –OMe substituents on the reactivity of the corresponding fluoroarenes. The trifluoromethyl group is a strong electron withdrawing group, while the methoxy group is an electron donating group. Strong electron withdrawing groups are usually para and ortho directing. It is most likely that steric effects are the reason that the ortho-substituted product [1-F₃C-2-{(NC)₃B}-C₆F₄]⁻ was not observed. The reaction of K₂1 with NC-C₆F₅, which contains the strong electron withdrawing cyano group that is sterically less demanding than the CF₃ group, gave the ortho-substituted anion [1-NC-2-{(NC)₃B}-C₆F₄]⁻ (B11b) together with [1-NC-4-{(NC)₃B}-C₆F₄]⁻ (B11a) and [B(CN)₄]⁻ in a ratio of 1.5 : 2.5 : 1. The formation of the tetracyanoborate anion is due to the nucleophilic attack at the carbon atom of the cyano group. The formation of [B(CN)₄]⁻ starting from K₂1 was reported previously, e.g. from reactions with (CN)₂ and PhOCN.³¹ Single crystals of the potassium salts of [1-F₃C-4-{(NC)₃B}-C₆F₄]⁻ (B7), [1-MeO-4-{(NC)₃B}-C₆F₄]⁻ (B10a) and [1-NC-4-{(NC)₃B}-C₆F₄]⁻ (B11a) were characterized using diffraction experiments (Fig. 4 and Table S3†).

The deprotonation of 1-F₃C-4-H-C₆F₄ to give [BH(CN)₃]⁻ was found to be the most relevant reaction with K₂1, and other unidentified borate anions had formed. In contrast, 1-F₃C-2-H-C₆F₄ and K₂1 gave the [1-F₃C-4-{(NC)₃B}-2,3,5-F₃-C₆H]⁻ anion (B9) in 78% yield and only 2% of [BH(CN)₃]⁻ was observed (Table 1).

The reactions of K₂1 with Cl–C₆F₅ and O₂N–C₆F₅ gave complex product mixtures, and the hexacyanoborate dianion [B₂(CN)₆]²⁻ (ref. 59) was identified. Additionally, the tricyanohaloborate anion [BCl(CN)₃]⁻ (ref. 60) formed from Cl–C₆F₅ (Table 1).

Conclusions

Boron-centred nucleophiles are an emerging class of compounds and are of growing importance for synthetic chemistry because they provide a very convenient entry to boron-functionalized compounds and materials.^1–28 The unprecedented, straightforward, regioselective and transition-metal-free introduction of one, two or even three {B(CN)₃}⁻ units into fluoro(hetero)arenes starting from readily accessible K₂1^30–32 opens unique and convenient access to single and multiple charged anions with tricyanoborate moieties. The observed regioselectivities for the C–F/C–B exchange reactions are almost all in accordance with an S_NAr mechanism, which showed the boron-centred nucleophilic character of the dianion 1, as previously observed for the formation of [B₂(CN)₆]²⁻ starting from K₂1 and K[BF(CN)₃].⁵⁹ So, an alternative radical reaction pathway is highly unlikely, although this type of reactivity was observed for other boron-centred nucleophiles.^61,62 Furthermore, the syntheses highlight the value of K₂1 for the general preparation of a wealth of tricyanoborate anions. Since the salts of the tricyanoborate anions such as the ones described herein are promising building blocks for materials science, for example as components of ionic liquids that are used in electrochemical devices,⁶³ convenient and high yield syntheses are necessary to facilitate applications.

Acknowledgements

The authors are grateful to Merck KGaA (Darmstadt, Germany) for their generous support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional tables, experimental information, analytical data and details of the DFT calculations. CCDC 1548893–1548910. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7sc02249b

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