A Photocatalytic C(sp)–B Bond Formation Employing SOMOphilic Alkynyl Sulfones and Nucleophilic Boryl Radicals

Boron-containing compounds are privileged building blocks in synthetic chemistry with broad applications in academia and industry.[1] This integral relevance arises from the ability of carbon–boron bonds to be easily converted into C–C, C–O, and C–N via cross-coupling reactions like the Nobel prize-winning Suzuki–Miyaura reaction, or the Chan–Lam process.[1] Indeed, almost 40% of all the transformations used by the pharmaceutical sector for the construction of C–C bonds involve the use of organoboron partners[2] and the Suzuki–Miyaura cross-coupling is overall the 5th most used reaction in pharma.[2b]

A particular family of interesting borylated compounds are alkynylborons.[3] The most direct application of these entities pertains to the direct alkynylation of electrophilic species by activation of the C(sp)–B bond (Scheme [1a]).[4] They have been also used for cycloaddition and benzannulation reactions[5] where the alkyne motif behaves as a reactive π-system, enabling the access to (hetero)aromatics with site-selectivity orthogonal to typical direct borylation methods.[6] Additionally, the alkyne unit can be also functionalized by means of carbometalation approaches to obtain stereodefined multisubstituted olefins.[7]

Scheme 1 a) Importance of alkynylborons in organic chemistry; b) Selected methods for the formation of C(sp)–B bonds; c) Trapping of carbon radicals with SOMOphiles for alkynylation reactions. EBX: Ethynylbenziodoxolone; d) This work: application of the SOMOphilic alkynylation concept employing nucleophilic boryl radicals.

In contrast to the research of sp³- and sp²-containing C–B bonds, the development of methods for C(sp)–B bond formation has been comparatively less explored, mainly due to the competing reactivity of the C(sp)–H bond of terminal alkynes versus the borylation of the C≡C bond.

Traditionally, compounds with C(sp)–B bond have been prepared by using the approach developed by Brown in the 70’s by lithiation/borylation of terminal alkynes (Scheme [1b]).[8] However, this approach requires the use of strong bases (e.g., ⁿ BuLi) to form the reactive lithium acetylide. Other polar approaches consist of the transmetalation between nucleophilic sources of alkynes, such as alkynylstannanes[9] or alkynylsilanes[10] with haloborons (Scheme [1b]). In recent years, the use of transition metals has been more widely employed harnessing the potentially high catalytic turnover exerted by these systems. Several examples have been described using dehydrogenative processes with Ir,[11] Ag,[12] Zn,[13] Cu,[14] Mg,[15] and Fe,[16] enabling the access to stable alkynyl-Bpin (pin = pinacolate) products where the boron atom behaves as a Lewis acid (Scheme [1b]). Organocatalytic methods are also known, but they are less explored and require the post-functionalization of the boron unit.[17]

Table 1 Reaction Development and Optimization Studies

Entry	Photocatalyst	Solvent	Base	Conversion of 1a (%)^a	Yield (%)^b of 3a
1	4-CzIPN	organic aprotic solvent^c	Cs₂CO₃	>98	–
2	4-CzIPN	MeOH	Cs₂CO₃	>98	34
3	fac-Ir(ppy)₃	MeOH	Cs₂CO₃	>98	3
4	[Ir(dF(CF₃))₂(bpy)]PF₆	MeOH	Cs₂CO₃	>98	16
5	[Ir(dF(Me)ppy)₂(dtbbpy)]PF₆	MeOH	Cs₂CO₃	>98	13
6	Ir[p-F(t-Bu)-ppy]₃	MeOH	Cs₂CO₃	>98	10
7	Ir[(dF(OMe)ppy)₂(5,5′-dCF₃bpy)]PF₆	MeOH	Cs₂CO₃	>98	14
8	4-CzIPN	tBuOH	Cs₂CO₃	>98	12
9	4-CzIPN	iPrOH	Cs₂CO₃	>98	21
10	4-CzIPN	EtOH	Cs₂CO₃	>98	29
11	4-CzIPN	isoamyl alcohol	Cs₂CO₃	>98	19
12	4-CzIPN	MeOH	pyridine, lutidine, Et₃N, DABCO, TMG, ^t Bu-TMG	>98	–
13	4-CzIPN	MeOH	KOAc	>98	36
14	4-CzIPN	MeOH	K₃PO₄	>98	40
15	4-CzIPN	MeOH	CsHCO₃	>98	48
16	4-CzIPN	MeOH	Na₂CO₃	>98	37
17	4-CzIPN	MeOH	K₂CO₃	>98	61
18	–	MeOH	K₂CO₃	26	–
19	4-CzIPN	MeOH	–	51	–
20^d	4-CzIPN	MeOH	K₂CO₃	>98	–
21^e	4-CzIPN	MeOH	K₂CO₃	>98	43
22^f	4-CzIPN	MeOH	K₂CO₃	>98	60

^a Determined by ¹H NMR spectroscopy of the reaction crude.

^b Isolated yield.

^c Including CH₂Cl₂, THF, toluene, PhCF₃, EtOAc, acetone, DMF, and cyclohexane.

^d No irradiation.

^e Reaction run using 1.5 equiv. of 2.

^f Reaction run for 16 h.

Under this scenario, an alternative, yet missing approach, would be the use of radicals to promote the formation of the C(sp)–B bond. However, this is especially challenging since C(sp)–centered radicals are not stable. In contrast, extensive research has been developed in recent years for the exploration of boryl radicals as a new way of introducing this functionality,[18] including the addition to aromatics,[19] π-deficient olefins,[20] and styrenes.[21] Additionally, they are known to undergo easy addition to C≡C bonds, leading to the anti-hydroboration[22] of these structural platforms, therefore posing additional hurdles to the construction of C(sp)–B bonds. Nevertheless, the utilization of boryl radicals for the formation of alkynylborons is an unmet challenge in the literature.

In order to develop such a method through a radical strategy, we focused on the concept of SOMOphilic alkynylation.[23] This strategy exploits electrophilic alkynes bearing a leaving group attached at the acetylenic carbon that favors the α-addition of open-shell, radical species. This renders the alkynylated product after homolytic extrusion of the leaving group (Scheme [1c]). Typical SOMOphilic acetylenes are alkynyl sulfones, EBX- and iodo-containing alkynes, which have been employed in combination with different sources of carbon radicals (e.g., alkyl NHPI esters, Katritzky salts or alkyl halides among others).[23] Since this reactivity is enhanced by the presence of nucleophilic radicals, we decided to explore the use of amine-ligated boryl radicals (R₃N–BH₂ ^•) owing to their increased nucleophilicity in comparison with other ligated boryl radicals, such as N-heterocyclic carbenes or pyridines.[24]

Herein we report the application of boryl radicals as borylating species of SOMOphilic acetylenes as a new way to form organoboron compounds (Scheme 1d). The method exploits, for the first time, a photoredox strategy employing aminocarboxylic acids under visible light, rendering the corresponding borylated products, which are stable at room temperature and isolable by typical chromatographic methods. Additionally, we include mechanistic studies by means of theoretical calculations to understand the regioselectivity during the addition of the boryl radical to the SOMOphile, and the plausible reaction mechanism.

Reaction Development and Optimization

At the outset we decided to use the aminocarboxylic acid 2, recently developed by the Leonori group[25] as a way of forming the desired boryl radical (see Table [1]). This species can be deprotonated by bases (pK _a ~8),[26] followed by SET (E _ox = +0.38 V vs SCE in CH₃CN)[25] with an excited photocatalyst to form the corresponding boryl radical and CO₂. As a SOMOphile we decided to use the alkynyl sulfone 1a, which can be accessed in one step from the corresponding terminal alkyne using non-toxic reagents.[27] Unfortunately, we did not observe any borylation product between 1a and 2 using 4-CzIPN and Cs₂CO₃ in the presence of different aprotic organic solvents, including CH₂Cl₂, THF, toluene, PhCF₃, EtOAc, acetone, DMF, and cyclohexane (Table [1], entry 1, for additional details see the Supporting Information). The use of protic solvents such as MeOH led to full conversion of the starting material 1a with formation of the desired alkynyl borane 3a in 34% yield (entry 2). Therefore, we decided to study other potential photocatalysts, but none of them enabled to increase the yield for the formation of 3a (entries 3–7). Then, we explored other protic solvents such as ^t BuOH, ⁱ PrOH, EtOH, and isoamyl alcohol, leading to lower yields (entries 8–11). Finally, we decided to use different bases. The addition of organic bases such as pyridine, 2,6-lutidine, Et₃N, DABCO, TMG, or ^t Bu-TMG led to complete conversion of the alkynyl sulfone 1a but inhibited the formation of the alkynyl borane 3a (entry 12, for additional details see the Supporting Information). Inorganic bases such as KOAc, K₃PO₄, CsHCO₃, or Na₂CO₃ were more effective (entries 13–16). Delightfully, the use of K₂CO₃ led to a 61% yield of 3a after 3 hours (entry 17). Control experiments demonstrated that both photocatalyst and base were necessary for the reaction (entries 18 and 19, respectively), and that light irradiation was required to promote the process (entry 20). The use of less amount of the Me₃N–BH₂CO₂H reagent led to a lower yield of 43% (entry 21), while running the reaction for prolonged times did not affect the reaction outcome suggesting that once 3a is formed, it remains stable under the reaction conditions (entry 22).

Scheme 2 Reaction scope for the radical borylation of alkynyl sulfones. ^a Reaction run for 3 h. ^b Reaction run for 5 h. ^c Reaction run for 16 h.

Substrate Scope

Having found optimal conditions for the borylation reaction, we further explored the substrate scope of the process (Scheme [2]).

Plain alkyl groups could be introduced at the aromatic unit at ortho-, meta-, and para-positions. This was exemplified by the synthesis of alkylated products 3b, 3c, and 3d, respectively, with medium to good yields. These examples showcase the tolerance of the method to benzylic positions, which could be activated by hydrogen atom transfer (HAT). Next, we studied the effect of electronically different aromatics. Electron-rich substituents such as p-OMe (3e) were tolerated, albeit the reaction yield was slightly lower (33%) and required more reaction time (5 h), likely because of the lower electrophilicity of the acetylenic unit. Contrarily, electron-poor substituents such as p-CO₂Me and p-CF₃ (3f and 3g, respectively) reacted well. Remarkably, we did not observe boryl radical-mediated defluorination of the CF₃ group in the latter case.[28] Then, we explored π-extended systems like 1-naphthyl (3h) and p-Ph (3i) rendering the desired borylated products in 56% and 68% yield, respectively. Subsequently, we studied the tolerance of the method to the presence of halides, obtaining the corresponding products with p-Cl (3j) and m-F (3k) in good yields. Importantly, the formation of 3k highlights the applicability of the method to incorporate fluorinated aromatic scaffolds, since boryl radicals are known to undergo addition to these entities.[19] We expanded the scope to electron-rich heteroaromatics (3l) and non-aromatic alkynes (3m), forming the desired products in 69% and 52% yield, respectively. Finally, we could apply these conditions for the access to borylated complex substrates such as ethynyl estradiol derivative 3n in 24% yield. Despite the low yield obtained, this example showcases the applicability of the method to complex substrates. Importantly, we did not detect the product from hydroboration of the triple bond in any case. Unfortunately, we could not apply the method for the borylation of challenging π-systems such as pyrene (1o), or π-deficient heteroaromatics such as pyridine (1p) or quinoline (1q). In these cases, we observed full decomposition of the starting material, but we could not isolate any by-product (Scheme [2], bottom part).

Computational Studies

Then, we decided to study the plausible mechanism for this new borylation reaction. Generally, for carbon radical additions to SOMOphilic species, there are two mechanistic possibilities postulated in the literature:[23] (1) the first one is the ipso-addition of the radical with respect to the sulfonyl group (α-pathway). This mechanism would lead to a conjugated radical at the β-position, which may suffer sulfinate extrusion to render the final product and the p-TolSO₂ radical. Alternatively, (2) the radical may add to the β-position of the alkyne through a Michael-type process, to form an sp²-hybrized alkenyl radical α to the sulfonyl group. In this mechanism, the excision of the C–S bond to extrude the sulfinate radical would generate a carbene-type species, from which migration of the R group takes place to form the alkynylation product.

In order to shed light on this aspect, we carried out DFT calculations (Scheme [3]). For this study, we considered the model substrate 1a (R = Ph) and the Me₃N–BH₂ radical formed after SET-induced decarboxylation comparing both α-(blue pathway) and β-additions (red pathway) (Scheme [3a]). We found that the α-addition (ΔG ^≠ = 1.8 kcal mol^–1) is slightly more favorable than the β-addition (ΔG ^≠ = 3.2 kcal mol^–1). This result is in good agreement with the reactivity predicted by the corresponding Fukui function,[29] which showed that the C_α position is more susceptible to a radical attack (C_α 0.18 vs C_β 0.05).

Scheme 3 Computational studies. a) Reaction profile for the addition of the boryl radical to 1a at the α-(blue pathway) or β-position (red pathways); b) Evolution of spin density along the reaction pathway. The calculations were done at the UM06-2X/cc-pVTZ/SMD(MeOH)//UM06-2X/cc-pVDZ/gas level of theory.

While both additions are exergonic and display low energy barriers, ruling out the reversible addition of the boryl radical to the alkyne, there is a much larger gap in the relative energies of the radical intermediates. In particular, the intermediate coming from the α-addition (I2α, –49.8 kcal·mol^–1) is ca. 20 kcal·mol^–1 more stable than its β counterpart (I2β, –31.7 kcal·mol^–1). The analysis of the spin densities of both structures revealed that this large energy difference can be related to the amount of delocalization of the unpaired electron (Scheme [3b]). In I2β, this ‘extra’ electron is entirely localized at the C_α position. However, in I2α, due to the linear arrangement of the Ph–C=C moiety, the unpaired electron is delocalized over the whole π-conjugated system. All attempts to optimize a transition state that would lead from I1 to the formation of the Z-isomer of I2β failed, always collapsing to the E-isomer.

Lastly, regarding the formation of 3a, we found that the reaction from I2α is almost thermoneutral (ΔG _r = 1.2 kcal·mol^–1) and with a rather low activation barrier (ΔG ^≠ = 6.6 kcal·mol^–1) (Scheme [3a]). For I2β the reaction is clearly exergonic (ΔG _r = –16.9 kcal·mol^–1) but with a much larger barrier (ΔG ^≠ = 28.7 kcal·mol^–1). Strikingly, and opposed to previous proposals,[23] IRC analysis showed that the conversion of I2β to I3 takes place in a single step, with no detection of a carbene intermediate. The reaction follows a concerted but asynchronous pathway: first the C–S bond is cleaved, and migration of the boryl group occurs afterwards. All together, we propose that the reaction involves the α-addition of the boryl radical to sulfone followed by homolytic C–S cleavage, due to the low activation barriers and the higher stability of the radical intermediate. The same conclusions are obtained when the reaction profile for the formation of the alkyl-derivative 3m was studied by DFT calculations (see the Supporting Information for details).

Scheme 4 Tentative mechanistic proposal for the radical borylation of alkynyl sulfones

Therefore, a plausible catalytic cycle for the overall transformation is depicted in Scheme [4]. First, the excited photocatalyst (E*_red 4-CzIPN*/4-CzIPN^•– = + 1.35 V vs SCE in CH₃CN)[30] would oxidize the carboxylate anion from aminocarboxylic acid 2, which is deprotonated in presence of the base (E _ox = +0.38 V vs SCE in CH₃CN).[25] After that, the extrusion of CO₂ would generate the nucleophilic boryl radical, which would then undergo α-addition to the SOMOphilic sulfone 1 to give the transient species I, where the radical is delocalized through the whole π-system. Following this, the elimination of the sulfinyl radical (p-TolSO₂ ^•) would lead to the formation of the desired borylated product. Finally, the reduced radical anion from the photocatalyst (E _red 4-CzIPN/4-CzIPN^•– = –1.21 V vs SCE in CH₃CN)[30] would react with the sulfinate radical (E _red Ts^•/Ts^– = –0.50 V vs SCE in CH₃CN)[31] by SET to recover the photocatalyst in the ground state and form the sulfinate anion, thereby closing the catalytic cycle.

Finally, we explored the potential synthetic applicability of the resulting alkynyl amine-boranes in preliminary experiments (Scheme [5]). From these studies we concluded that the activation of the C(sp)–B bond enables the Tsuji–Trost-type alkynylation of allylic systems catalyzed by Ni (4), oxidation (5), and Suzuki–Miyaura cross-coupling (6).

In conclusion, we have developed the first method for the formation of C(sp)–B bonds by a photocatalytic radical approach. The key to the method lies in the use of Me₃N–BH₂CO₂H as a source of nucleophilic boryl radical, in combination with electrophilic alkynyl sulfones as SOMOphiles. The reaction renders the corresponding alkynylated amine-boranes with medium to good yields, which can be isolated by typical chromatographic methods and are stable under air. Additionally, we have determined the plausible regioselective outcome of the boryl radical attack onto the π-system through an α-addition pathway by means of DFT studies. Further photochemical studies and functionalization of the borylated products are currently ongoing in our group and will be reported in the future.

Scheme 5 Synthetic transformations

All chemicals were used directly without purification. All air and moisture sensitive reactions were carried out under N₂ atmosphere using standard Schlenk manifold technique. All solvents were bought from Acros with 99.8% purity. ¹H and ¹³C NMR spectra were acquired at various field strengths as indicated and were referenced to CHCl₃ (7.26 and 77.16 ppm for ¹H and ¹³C respectively). ¹H NMR coupling constants are reported in hertz (Hz). Data are reported as follows: chemical shift, integration, multiplicity (s = singlet, br s = broad singlet, d = doublet, br d = broad doublet, t = triplet, q = quartet, p = pentet, m = multiplet, dd = doublet of doublet, etc.). ¹¹B NMR spectra were recorded and reported unreferenced. High-resolution mass spectra were obtained using a JEOL JMS-700 spectrometer or a Fissions VG Trio 2000 quadrupole mass spectrometer. Spectra were obtained using electron impact ionization (EI), positive electrospray (ESI) or atmospheric-pressure chemical ionization (APCI). Analytical TLC: aluminum backed plates pre-coated (0.25 mm) with Merck Silica Gel 60 F254. Compounds were visualized by exposure to UV-light or by dipping the plates in permanganate (KMnO₄) stain followed by heating. Flash column chromatography was performed using Merck Silica Gel 60 (40–63 μm). All mixed solvent eluents are reported as v/v solutions.

Alkynyl sulfones were prepared according to known procedures and are known compounds.[27] Me₃N–BH₂CO₂H was prepared according to reported procedure.[25]

In all boron containing compounds, the carbon attached to boron was not observed due to quadrupole broadening caused by the ¹¹B nucleus.[32]

C(sp)–B Borylation; General Procedure

An oven-dried 8 mL microwave vial equipped with a stirring bar was charged with the alkynyl sulfone 1 (0.1 mmol, 1.0 equiv.), 4-CzIPN (4 mg, 5 μmol, 5 mol%), K₂CO₃ (28 mg, 0.2 mmol, 2.0 equiv.), and the aminocarboxylic acid 2 (23 mg, 0.2 mmol, 2.0 equiv). The vial was capped with a Supelco aluminum crimp seal with septum (PTFE/butyl), evacuated and refilled with N₂ (3 ×). Anhyd and degassed MeOH (1.0 mL) was added. The vial was sealed with parafilm and was placed approximately 4 cm from blue LEDs (440 nm Kessil lamp). The blue LEDs were switched on with a fan and the contents of the vial were stirred at rt and the reaction followed by TLC analysis. Once the starting material was consumed, the tube was opened, and the solvent was evaporated. The resulting residue was purified by column chromatography on silica gel to give the product.

(Phenylethynyl)borane Trimethylamine Complex (3a)

Following the general procedure using 1-methyl-4-((phenylethynyl)sulfonyl)benzene (25.6 mg, 0.1 mmol, 1.0 equiv.) gave 3a (10.6 mg, 61%) as a white solid; mp 110–112 °C; R_f = 0.51 [n-pentane/EtOAc (6:1)].

¹H NMR (600 MHz, CDCl₃): δ = 7.44–7.40 (m, 2 H), 7.25–7.18 (m, 3 H), 2.73 (s, 9 H), 2.16 (q, J = 98.7 Hz, 2 H).

¹³C NMR (151 MHz, CDCl₃). δ = 131.6, 128.1, 127.7, 126.9, 52.3, 29.9.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.64.

HRMS (ESI): m/z calcd for C₁₁H₁₆BN [M]⁺: 173.1376; found: 173.1374.

(o-Tolylethynyl)borane Trimethylamine Complex (3b)

Following the general procedure using 1-methyl-2-(tosylethynyl)benzene (27.0 mg, 0.1 mmol, 1.0 equiv.) gave 3b (13.2 mg, 71%) as a yellowish oil; R_f = 0.50 [n-pentane/EtOAc (6:1)].

¹H NMR (600 MHz, CDCl₃): δ = 7.41 (dd, J = 7.4, 1.6 Hz, 1 H), 7.16 (d, J = 6.3 Hz, 1 H), 7.11 (td, J = 7.5, 1.6 Hz, 1 H), 7.08 (td, J = 7.4, 1.6 Hz, 1 H), 2.73 (s, 9 H), 2.46 (s, 3 H).

¹³C NMR (151 MHz, CDCl₃): δ = 139.4, 131.8, 129.2, 126.8, 125.4, 52.2, 21.3.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.45.

HRMS (ESI): m/z calcd C₁₂H₁₉BN [M – H]⁺: 188.1611; found: 188.1615.

(m-Tolylethynyl)borane Trimethylamine Complex (3c)

Following the general procedure using 1-methyl-3-(tosylethynyl)benzene (27.0 mg, 0.1 mmol, 1.0 equiv.) gave 3c (12.7 mg, 68%) as an oil; R_f = 0.50 [n-pentane/EtOAc (6:1)].

¹H NMR (600 MHz, CDCl₃): δ = 7.29–7.20 (m, 2 H), 7.14 (t, J = 7.6 Hz, 1 H), 7.02 (d, J = 7.7 Hz, 1 H), 2.72 (s, 7 H), 2.30 (s, 3 H).

¹³C NMR (151 MHz, CDCl₃): δ = 137.6, 132.2, 128.6, 128.0, 127.8, 52.2, 21.3.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.57.

HRMS (ESI): m/z calcd C₁₂H₁₉BN [M – H]⁺: 188.1611; found: 188.1612.

((4-Propylphenyl)ethynyl)borane Trimethylamine Complex (3d)

Following the general procedure using 1-methyl-4-(((4-propylphenyl)ethynyl)sulfonyl)benzene (29.8 mg, 0.1 mmol, 1.0 equiv.) gave 3d (7.3 mg, 34%) as a white solid; mp 95–98 °C; R_f = 0.50 [n-hexane/EtOAc (4:1)].

¹H NMR (600 MHz, CDCl₃): δ = 7.34 (d, J = 8.1 Hz, 2 H), 7.06 (d, J = 8.1 Hz, 2 H), 2.71 (s, 9 H), 2.54 (t, J = 7.7 Hz, 2 H), 2.15 (q, J = 98.7 Hz, 2 H), 1.61 (hept, J = 7.4 Hz, 2 H), 0.92 (t, J = 7.3 Hz, 3 H).

¹³C NMR (151 MHz, CDCl₃): δ = 141.5, 131.4, 128.3, 123.0, 52.2, 38.0, 24.5, 13.9.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.54 (t, J = 100.9 Hz).

HRMS (ESI): m/z calcd C₁₄H₂₁BN [M – H]⁺: 214.1767; found: 214.1764.

((4-Methoxyphenyl)ethynyl)borane Trimethylamine Complex (3e)

Following the general procedure using 1-methoxy-4-(tosylethynyl)benzene (28.6 mg, 0.1 mmol, 1.0 equiv.) gave 3e (6.7 mg, 33%) as a white solid; R_f = 0.17 [n-hexane/EtOAc (7:3)].

¹H NMR (600 MHz, CDCl₃): δ = 7.35 (d, J = 8.8 Hz, 2 H), 6.78 (d, J = 8.7 Hz, 2 H), 3.77 (s, 3 H), 2.70 (s, 9 H), 2.13 (q, J = 102.5 Hz, 2 H).

¹³C NMR (151 MHz, CDCl₃): δ = 158.7, 132.9, 118.3, 113.8, 55.4, 52.3.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.49 (t, J = 100.7 Hz).

HRMS (ESI): m/z calcd C₁₂H₁₇BNO [M – H]⁺: 202.1398; found: 202.1395.

Methyl 4-(Boraneylethynyl)benzoate Trimethylamine Complex (3f)

Following the general procedure using methyl 4-(tosylethynyl)benzoate (31.4 mg, 0.1 mmol, 1.0 equiv.) gave 3f (13.2 mg, 57%) as a white solid; mp 102–106 °C; R_f = 0.51 [n-hexane/EtOAc (4:1)].
¹H NMR (600 MHz, CDCl₃): δ = 7.92 (d, J = 7.9 Hz, 2 H), 7.46 (d, J = 8.0 Hz, 2 H), 3.90 (s, 3 H), 2.73 (s, 9 H), 2.16 (q, J = 99.5 Hz, 2 H).

¹³C NMR (151 MHz, CDCl₃): δ = 167.1, 131.5, 130.8, 129.5, 128.3, 52.5, 52.3.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.65 (t, J = 101.4 Hz).

HRMS (ESI): m/z calcd C₁₃H₁₉BNO₂ [M + H]⁺: 232.1503; found: 232.1500.

((4-(Trifluoromethyl)phenyl)ethynyl)borane Trimethylamine Complex (3g)

Following the general procedure using 1-methyl-4-(((4-(trifluoromethyl)phenyl)ethynyl)sulfonyl)benzene (32.4 mg, 0.1 mmol, 1.0 equiv.) gave 3g (9.2 mg, 38%) as a yellowish oil; R_f = 0.48 [n-hexane/EtOAc (4:1)].

¹H NMR (600 MHz, CDCl₃): δ = 7.50 (s, 4 H), 2.73 (s, 9 H), 2.15 (q, J = 99.3 Hz, 2 H).

¹³C NMR (101 MHz, CDCl₃): δ = 131.7, 125.2, 52.5.

¹⁹F NMR (565 MHz, CDCl₃): δ = –62.6.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.69 (t, J = 101.3 Hz).

HRMS not found due to decomposition of material.

(Naphthalen-1-ylethynyl)borane Trimethylamine Complex (3h)

Following the general procedure using 1-(tosylethynyl)naphthalene (30.6 mg, 0.1 mmol, 1.0 equiv.) gave 3h (12.5 mg, 56%) as an oil; R_f = 0.50 [n-pentane/EtOAc (6:1)].

¹H NMR (400 MHz, CDCl₃): δ = 8.46 (dd, J = 8.2, 1.4 Hz, 1 H), 7.81 (d, J = 7.4 Hz, 1 H), 7.73 (d, J = 8.2 Hz, 1 H), 7.65 (dd, J = 7.1, 1.2 Hz, 1 H), 7.50 (dddd, J = 19.7, 8.1, 6.9, 1.4 Hz, 2 H), 7.38 (dd, J = 8.3, 7.1 Hz, 1 H), 2.79 (s, 9 H), 2.29 (dd, J = 198.5, 97.1 Hz, 2 H).

¹³C NMR (101 MHz, CDCl₃): δ = 133.7, 133.4, 129.8, 128.2, 127.2, 126.9, 126.3, 126.1, 125.4, 123.7, 52.4.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.33 (t, J = 102.8 Hz).

HRMS (ESI): m/z calcd C₁₅H₁₉BN [M – H]⁺: 224.1611; found: 224.1615.

([1,1′-Biphenyl]-4-ylethynyl)borane Trimethylamine Complex (3i)

Following the general procedure using 4-(tosylethynyl)-1,1′-biphenyl (33.2 mg, 0.1 mmol, 1.0 equiv.) gave 3i (16.9 mg, 68%) as a solid; mp 98–101 °C; R_f = 0.50 [n-pentane/EtOAc (6:1)].

¹H NMR (600 MHz, CDCl₃): δ = 7.61–7.56 (m, 2 H), 7.50 (s, 4 H), 7.42 (t, J = 7.7 Hz, 2 H), 7.35–7.31 (m, 1 H), 2.74 (s, 9 H), 2.19 (q, J = 105.7 Hz, 2 H).

¹³C NMR (151 MHz, CDCl₃): δ = 140.9, 139.6, 132.0, 128.9, 127.4, 127.1, 126.8, 124.9, 52.3.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.60.

HRMS (ESI): m/z calcd for C₁₇H₂₀BNNa [M + Na]⁺: 272.1581; found: 272.1577.

((4-Chlorophenyl)ethynyl)borane Trimethylamine Complex (3j)

Following the general procedure using 1-chloro-4-(tosylethynyl)benzene (29.0 mg, 0.1 mmol, 1.0 equiv.) gave 3j (11.4 mg, 55%) as an oil; R_f = 0.52 [n-pentane/EtOAc (6:1)].

¹H NMR (600 MHz, CDCl₃): δ = 7.34 (d, J = 8.1 Hz, 2 H), 7.21 (d, J = 8.1 Hz, 2 H), 2.72 (s, 9 H), 2.14 (dd, J = 200.9, 99.3 Hz, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 132.8, 132.7, 128.4, 124.4, 52.3.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.66 (t, J = 100.7 Hz).

HRMS (ESI): m/z calcd for C₁₁H₁₆BClN [M + H]⁺: 208.1064; found: 208.1072.

((3-Fluorophenyl)ethynyl)borane Trimethylamine Complex (3k)

Following the general procedure using 1-fluoro-3-(tosylethynyl)benzene (27.5 mg, 0.1 mmol, 1.0 equiv.) gave 3k (9.9 mg, 52%) as an oil; R_f = 0.46 [n-pentane/EtOAc (6:1)].

¹H NMR (600 MHz, CDCl₃): δ = 7.23–7.16 (m, 2 H), 7.10 (d, J = 9.9 Hz, 1 H), 6.94–6.88 (m, 1 H), 2.72 (s, 9 H), 2.49–1.82 (m, 2 H).

¹³C NMR (151 MHz, CDCl₃): δ = 162.5 (d, J = 245.2 Hz), 129.6 (d, J = 8.5 Hz), 127.8 (d, J = 9.7 Hz), 127.4 (d, J = 3.0 Hz), 118.2 (d, J = 21.8 Hz), 114.1 (d, J = 21.2 Hz), 52.3.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.31 (t, J = 101.2 Hz).

HRMS (ESI): m/z calcd for C₁₁H₁₆BFN [M + H]⁺: 192.1360; found: 192.1357.

(Thiophen-3-ylethynyl)borane Trimethylamine Complex (3l)

Following the general procedure using 3-(tosylethynyl)thiophene (26.2 mg, 0.1 mmol, 1.0 equiv.) gave 3l (12.4 mg, 69%) as an oil; R_f = 0.46 [n-pentane/EtOAc (6:1)].

¹H NMR (600 MHz, CDCl₃): δ = 7.31 (dd, J = 3.0, 1.2 Hz, 1 H), 7.19 (dd, J = 5.0, 3.0 Hz, 1 H), 7.09 (dd, J = 5.0, 1.2 Hz, 1 H), 2.70 (s, 9 H), 2.13 (d, J = 98.0 Hz, 2 H).

¹³C NMR (151 MHz, CDCl₃): δ = 130.4, 126.7, 124.9, 124.6, 52.2.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.63.

HRMS (ESI): m/z calcd for C₉H₁₅BNS [M + H]⁺: 180.1018; found: 180.1023.

(3,3-Dimethylbut-1-yn-1-yl)borane Trimethylamine Complex (3m)

Following the general procedure using 1-((3,3-dimethylbut-1-yn-1-yl)sulfonyl)-4-methylbenzene (23.6 mg, 0.1 mmol, 1.0 equiv.) gave 3m (8.0 mg, 52%) as an oil; R_f = 0.58 [n-pentane/EtOAc (6:1)].

¹H NMR (600 MHz, CDCl₃): δ = 2.64 (s, 9 H), 1.96 (q, J = 99.2 Hz, 2 H), 1.24 (s, 9 H).

¹³C NMR (101 MHz, CDCl₃): δ = 52.0, 32.0, 28.2.

¹¹B NMR (193 MHz, CDCl₃): δ = –10.68 (t, J = 97.7 Hz).

HRMS (ESI): m/z calcd C₉H₁₉BN [M – H]⁺: 152.1605; found: 152.1602.

(8R,9S,13S,14S,17R)-17-Ethynyl-3-methoxy-13-methyl-7,8,9,11,12, 13,14,15,16,17-decahydro-6H-cyclopenta[a]phenanthren-17-ol)borane Trimethylamine Complex (3n)

Following the general procedure using the corresponding ethynyl estradiol derivative[33] 1n (46 mg, 0.1 mmol, 1.0 equiv.) gave 3n (9.2 mg, 24%) as an oil; R_f = 0.35 [n-pentane/EtOAc (4:1)].

1H NMR (600 MHz, CDCl₃): δ = 7.22 (d, J = 8.6 Hz, 1 H), 6.72 (dd, J = 8.6, 2.8 Hz, 1 H), 6.63 (d, J = 2.7 Hz, 1 H), 3.78 (s, 3 H), 2.88–2.82 (m, 2 H), 2.75 (s, 9 H), 2.40–2.33 (m, 1 H), 2.28 (ddd, J = 13.8, 9.7, 5.7 Hz, 1 H), 2.19 (td, J = 10.6, 9.7, 3.2 Hz, 1 H), 2.02 (dd, J = 10.9, 2.8 Hz, 1 H), 1.99 (dd, J = 10.9, 2.7 Hz, 1 H), 1.90–1.86 (m, 1 H), 1.83 (td, J = 13.1, 4.2 Hz, 1 H), 1.79–1.74 (m, 1 H), 1.74–1.69 (m, 1 H), 1.66–1.60 (m, 1 H), 1.48–1.37 (m, 4 H), 0.88 (s, 3 H).

¹³C NMR (151 MHz, CDCl₃): δ = 157.6, 138.1, 132.6, 126.5, 113.9, 111.7, 90.7, 85.7, 79.8, 65.4, 55.3, 49.8, 47.5, 43.9, 39.6, 39.1, 33.1, 31.7, 27.4, 26.6, 23.0, 13.0.

¹¹B NMR (192 MHz, CDCl₃): δ = –9.5 (br s).

HRMS (ESI): m/z calcd C₂₄H₃₇BNO₂ [M – H]⁺: 382.2917; found: 382.2920.

Ni-Catalyzed Tsuji–Trost Reaction; General Procedure

An oven-dried 8 mL microwave vial equipped with a stirring bar was charged with the alkynyl amine-borane 3a (17 mg, 0.1 mmol, 1.0 equiv.), NiCl₂(dppe) (8 mg, 15 μmol, 15 mol%), methyl (E)-(4-phenylbut-3-en-2-yl)carbonate (56 mg, 0.25 mmol, 2.5 equiv.), and KOH (17 mg, 0.3 mmol, 3.0 equiv.). The vial was capped with a Supelco aluminum crimp seal with septum (PTFE/butyl), evacuated and refilled with N₂ (3 ×). Anhyd and degassed 1,2-dichloroethane (0.90 mL) and H₂O (0.1 mL) were added and the mixture was allowed to stir at 100 °C in a preheated oil bath for 24 h. Then, the reaction was allowed to cool to r.t. and EtOAc (10 mL) was added. The organic phase was washed with H₂O (2 × 10 mL) and dried (MgSO₄). After removing the solvent in vacuo, the crude was further purified by column chromatography to give 4 as an oil (6 mg, 26%); R_f = 0.42 (n-pentane).

¹H NMR (600 MHz, CDCl₃): δ = 7.21–7.46 (m, 10 H), 6.77 (d, J = 16.1 Hz, 1 H), 6.24 (dd, J = 6.5, 16.0 Hz, 1 H), 3.5 (m, 1 H), 1.45 (d, J = 7.4 Hz, 3 H).

The spectroscopic data are in accordance with the literature.[34]

Oxidation Reaction; General Procedure

Following a reported procedure,[35] an oven-dried 8 mL microwave vial equipped with a stirring bar was charged with the alkynyl amine-borane 3a (17 mg, 0.1 mmol, 1.0 equiv.), Oxone (300 mg, 10 mmol, 10.0 equiv.), DMF (1.0 mL), and EtOH (1.0 mL). The vial was capped with a Supelco aluminum crimp seal with septum (PTFE/butyl) and the mixture was allowed to stir at 120 °C in a preheated oil bath for 24 h. Then, the reaction was allowed to cool to rt and EtOAc (10 mL) was added. The organic phase was washed with H₂O (3 × 10 ml), brine (10 mL) and dried (MgSO₄). After removing the solvent in vacuo, the crude was further purified by column chromatography to give 5 as an oil (12.7 mg, 71%); R_f = 0.42 (n-pentane); R_f = 0.50 (n-pentane/EtOAc 10:1).

¹H NMR (400 MHz, CDCl₃): δ = 7.31–7.35 (m, 2 H), 7.22–7.26 (m, 3 H), 4.17 (q, J = 7.2 Hz, 2 H), 3.00 (t, J = 7.9 Hz, 2 H), 2.66 (t, J = 7.9 Hz, 2 H), 1.27 (t, J = 7.2 Hz, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 172.9, 140.6, 128.5, 128.3, 126.3, 60.4, 36.0, 31.0, 14.2.

The spectroscopic data are in accordance with the literature.[36]

Suzuki–Miyaura Coupling; General Procedure

Adapted from a reported procedure:[25] A tube equipped with a stirring bar was charged with KOH (17 mg, 0.30 mmol, 3.0 equiv.), RuPhos (9.3 mg, 0.02 mmol, 20 mol%), Pd₂(dba₃) (5 mg, 5 μmol, 5 mol%), the amine-borane 3a (17 mg, 0.1 mmol, 1.0 equiv.), and 4-bromoanisole (47 mg, 0.25 mmol, 2.5 equiv.). The tube was capped with a Supelco aluminum crimp seal with septum (PTFE/butyl), evacuated and refilled with N₂ (3 ×), then toluene–H₂O (0.8–0.2 mL; total 0.1 M) was added. The mixture was warmed to 120 °C and stirred for 24 h. The mixture was cooled to rt, diluted with brine (5 mL) and EtOAc (5 mL) and shaken vigorously. The layers were separated, and the aqueous layer was extracted with EtOAc (2 × 10 mL). The combined organic layers were dried (MgSO₄), filtered and evaporated. The crude was purified by flash column chromatography on silica gel to give 6 as an oil (10.6 mg, 51%); R_f = 0.42 (n-pentane), R_f = 0.50 (n-pentane/EtOAc 10:1).

¹H NMR (400 MHz, CDCl₃): δ = 7.53–7.44 (m, 4 H), 7.37–7.30 (m, 3 H), 6.87 (dt, J = 8.9, 2.7 Hz, 2 H), 3.82 (s, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 159.6, 133.1, 131.5, 128.3, 127.9, 123.6, 115.4, 114.0, 89.4, 88.1, 55.3.

The spectroscopic data are in accordance with the literature.[37]

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Figures

Scheme 1 a) Importance of alkynylborons in organic chemistry; b) Selected methods for the formation of C(sp)–B bonds; c) Trapping of carbon radicals with SOMOphiles for alkynylation reactions. EBX: Ethynylbenziodoxolone; d) This work: application of the SOMOphilic alkynylation concept employing nucleophilic boryl radicals.

Scheme 2 Reaction scope for the radical borylation of alkynyl sulfones. ^a Reaction run for 3 h. ^b Reaction run for 5 h. ^c Reaction run for 16 h.

Scheme 3 Computational studies. a) Reaction profile for the addition of the boryl radical to 1a at the α-(blue pathway) or β-position (red pathways); b) Evolution of spin density along the reaction pathway. The calculations were done at the UM06-2X/cc-pVTZ/SMD(MeOH)//UM06-2X/cc-pVDZ/gas level of theory.

Scheme 4 Tentative mechanistic proposal for the radical borylation of alkynyl sulfones

Scheme 5 Synthetic transformations

Supplementary Material

Supporting Information