Peculiar Reactivity of Isothiocyanates with Pentaphenylborole

Article pubs.acs.org/IC

Peculiar Reactivity of Isothiocyanates with Pentaphenylborole Kexuan Huang and Caleb D. Martin* Department of Chemistry and Biochemistry, Baylor University, One Bear Place #97348, Waco, Texas 76798, United States S Supporting Information *

ABSTRACT: The reactions of isothiocyanates with the antiaromatic pentaphenylborole were investigated, revealing significantly different outcomes than the analogous reactions with isocyanates. The 1:1 stoichiometric reaction products isolated include a seven-membered BNC5 heterocycle and a fused bicyclic 4/5-ring system. Studies suggest that the sevenmembered ring undergoes an intramolecular [2 + 2] electrocyclic ring closure to produce the bicyclic system. The only derivative for which stoichiometry influenced the reaction outcome was 4-methoxyphenylisothiocyanate. The reaction of borole with an excess of 4-methoxyphenylisothiocyanate resulted in the formation of a fused tetracyclic species with two equivalents of isothiocyanate incorporated into the product. Rational pathways for these unusual transformations are presented.

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INTRODUCTION Although the antiaromatic borole was first reported in 1969, new reactivity of this unusual heterocycle continues to be uncovered.1,2 The diverse reactivity has shown the threecoordinate boron center behave as a Lewis acid, the butadiene moiety act as a dienophile in Diels−Alder chemistry, the central ring undergo reduction by one or two electrons, and the activation of a variety of bonds.3−25 In many cases, the Diels− Alder and Lewis adducts can undergo further rearrangements, resulting in a ring expansion of the five-membered borole ring, which has emerged as an effective method to generate larger boracycles. Eisch and co-workers first described the ring expansion of borole in the reaction of pentaphenylborole (1) with diphenylacetylene to produce the seven-membered borepin 2 via rearrangement of the bicyclic Diels−Alder adduct (Int1; Scheme 1).26,27 With respect to ring expansions of borole proceeding through coordination of the substrate to the Lewis acidic boron center, Piers and co-workers recognized this pathway in the reaction of the same alkyne with the very electrophilic perfluoropentaphenylborole (3), which produced a six-membered heterocycle (4) as the major product via a 1,1carboboration reaction28−35 and the analogous borepin from the Diels−Alder pathway as the minor product.36,37 The sixmembered boracycle 4 was formed by a mechanism initiated by coordination of the alkyne to the boron center (Int3), migration of an aryl group (Int4), and subsequent attack of the endocyclic B−C bond to the carbon center. Since this work, other 1,1-insertion reactions have been observed as well as 1,2and 1,3-insertion reactions via the same pathway to construct six-, seven-, and eight-membered boracycles with high degrees of unsaturation.38−47 Appealing features of the insertion chemistry with boroles are that the reactions usually take place under mild conditions and are typically high yielding. Boron-containing conjugated heterocycles of this type have © XXXX American Chemical Society

been identified as attractive species for electronic materials because of their interesting photophysical properties, but they are synthetically challenging targets.5,48−66 1,2-Dipolar substrates such as aldehydes, ketones, imines, nitriles, and isocyanates have been shown to undergo 1,2insertion reactions with borole to yield seven-membered heterocycles.44,46 The reactions proceed through a similar mechanism in which the Lewis basic site of the substrate coordinates to the boron center, followed by nucleophilic attack of the endocyclic B−C bond to the electrophilic site of the 1,2dipole. The chemistry with 1-adamantylisocyanate and 4methoxyphenylisocyanate showed both molecules reacting as 1,2-dipoles with borole to produce seven-membered rings; however, the products differed as C−N was inserted into the ring for the 1-adamantyl derivative (5; Scheme 2) and C−O was inserted for the 4-methoxyphenyl variant (6).46 The different reaction outcomes were rationalized by the change in the nucleophilic site of the two derivatives. This unusual reactivity with isocyanates prompted us to explore the sulfur variants, namely, isothiocyanates. The chemistry described herein demonstrates that C−N insertion is observed exclusively, although further reactivity of the BNC5 ring can occur.

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RESULTS AND DISCUSSION The 1:1 reaction of 1-adamantylisothiocyanate with borole 1 in CDCl3 immediately gave a brown solution. Acquiring an in situ 1 H NMR spectrum after 5 min revealed the formation of a major species (>90% yield) indicated by the aryl resonances from the borole integrating in a 25:15 ratio to the adamantyl resonances of the isothiocyanate, consistent with a 1:1 reaction Received: October 25, 2015

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DOI: 10.1021/acs.inorgchem.5b02469 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Examples of Ring-Expansion Reactivity of Boroles via Diels−Alder (a) and Coordination Pathways (b)

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B{1 H} NMR spectroscopic signature indicative of a tricoordinate boron center (52.2 ppm). Crystals suitable for an X-ray diffraction study were grown via vapor diffusion of pentane into a toluene solution, and the product was identified as the B,N containing bicyclo[3.2.0] hept-6-ene analogue 8PhCH3 (see Figure 2). The bicyclic product differed significantly from the sevenmembered rings 5 and 6 obtained in the reactions of isocyanides with borole. Perplexed by the new product, we explored other aryl isothiocyanates, namely, 4-methoxyphenyland 4-(trifluoromethyl)phenyl-substituted isothiocyanates, to determine the effect of electron-donating or -withdrawing substituents on the aryl group. The analogous color transformation from blue to orange then yellow was observed as well as similar resonances in the 11B{1H} NMR spectra [4methoxyphenyl 51.6 ppm and 4-(trifluoromethyl)phenyl 50.2 ppm]. X-ray crystallographic studies on both reaction products showed the same fused bicyclic systems were produced and, after workup, isolated in high yields [4-(trifluoromethyl)phenyl 81%, 8PhCF3; 4-methoxyphenyl 89%, 8PhOCH3], indicating that electron-withdrawing and -donating substituents on the 4position of the aryl group were not influential on the reaction outcome. In all cases, the products (8) were stable in solution and in the solid state under a nitrogen atmosphere. The new bicyclic product led us to probe the transformation further. Because in situ monitoring of the 4-tolylisothiocyanate reaction showed that the intermediate formed rapidly and the conversion to 8PhCH3 took 24 h, the Lewis base pyridine was added to the solution to trap the intermediate (Scheme 4). An X-ray diffraction study of crystals grown via vapor diffusion of pentane into a toluene solution identified the product as the pyridine complex of the seven-membered BNC5 ring-expanded product 7PhCH3·pyr with the C,N-unit of the isothiocyanate

Scheme 2. Reactivity of Borole 1 with Isocyanates

(Scheme 3). This species decomposed in solution over time to a complex mixture of compounds that we were unable to identify. Conducting the experiment on a larger scale in CH2Cl2 permitted characterization of the compound initially formed. An X-ray diffraction study identified the product as the ringexpanded, seven-membered BNC5 heterocycle 7, with a thioamido group introduced into the borole ring (see Figure 1). The complex is isostructural to the 1-adamantylisocyanate product 5 with the exception of a sulfur atom in place of oxygen but is much less stable than the oxygen congener. Given the contrasting reactivity between the aryl and alkyl isocyanates, we investigated the 1:1 reaction of 4-tolylisothiocyanate with borole 1. In situ 1H NMR monitoring of the reaction in C6D6 was informative via the methyl protons of the tolyl group as a spectroscopic handle. This indicated an initial species was formed (δCH3 = 1.78) that cleanly converted to a second product after 24 h (δCH3 = 2.03) and was accompanied by the color change of the solution from the deep-blue solution of 1 to orange that became yellow. The final product was isolated in high yield after workup (86%) and features a

Scheme 3. Isolated Products in Reactions of Borole 1 with 1 equiv of Isothiocyanate

B


Article

Inorganic Chemistry Scheme 4. Proposed Mechanism to Yield the Fused Bicyclic Systems

Scheme 5. Proposed Mechanism for the Reaction of Borole 1 with 4-Methoxyphenylisothiocyanate and Additional Aryl Isothiocyanate

introduced into the borole ring akin to 7Ad (see Figure 1). Although we were unable to isolate the free species for the tolyl reaction, crystals could be grown from the 4-methoxyphenylisothiocyanate reaction, concluding the formation of the sevenmembered ring intermediate (7PhOCH3). The process to form the seven-membered BNC5 thioamido complexes (7) is reminiscent of the C,N-insertion of imines with borole, which has been modeled theoretically. By analogy, the mechanism to form 7 occurs by coordination of the nitrogen of the isothiocyanate to boron (Int5), and then the endocyclic B−C bond attacks the electrophilic carbon of the isothiocyanate, forming the seven-membered ring 7 (Scheme 4). However, the sequence to the bicyclic species 8 has not been established. The cis-phenyl groups on the bridgeheadcarbons in all species are consistent with a photochemically driven disrotatory [2 + 2] electrocyclic cyclization.67−70 The reaction time decreases significantly upon irradiation with UV light (λ = 254 nm), going to completion in an hour, whereas in ambient light the reaction takes 24 h. The photochemical acceleration of the reaction further supports an intramolecular [2 + 2] mechanism.71

The lack of conversion to the 4,5-fused ring system with the adamantyl derivative was puzzling. The analogous reaction with the smallest alkyl isothiocyanate, methylisothiocyanate, was investigated and cleanly produced the bicyclic species 8CH3. This suggests the steric influence of the encumbering adamantyl group impedes orbital alignment of the sevenmembered ring for the pericyclic process. The smaller aryl and methyl isothiocyanates do not encounter this hurdle. The isothiocyanates studied reacted independent of stoichiometry to produce 1:1 species with the sole exception of 4-methoxyphenylisothiocyanate. When borole 1 was treated with an excess of 4-methoxyphenylisothiocyanate, a species with two methoxy groups integrating in a 1:1 ratio shifted upfield with respect to the shift of the methoxy group for the bicyclic compound (δ = 3.71 and 3.66, c.f. δ = 3.86) was generated in high yield (Scheme 5). An X-ray diffraction study performed on crystals grown via vapor diffusion of pentane into a toluene solution identified the product as the fused tetracyclic complex 9 incorporating a second equivalent of 4-methoxyphenylisothiocyanate (Figure 3). The mechanism is proposed to occur via reaction of the BNC5 heterocycle 7PhOCH3 with a second equivalent of 4C


Article

Inorganic Chemistry

Figure 1. Solid-state structures of 7Ad (a), 7PhOCH3 (b), and 7PhCH3·pyr (c). Hydrogen atoms have been omitted for clarity. Ellipsoids are drawn at the 50% probability level. (d) Diagram illustrating the dihedral planes θprow and θstern, defining the deviation of the ring from planarity into a boatlike conformation. Selected bond lengths and angles can be found in Table 1.

the range of typical single bonds, and the C−S bond lengths for the thioamido group are double bonds. Boron and nitrogen are both essentially planar, consistent with sp2 hybridization [B = 359.9(2)°, 359.8(2)°, N = 357.7(1)°, 357.5(1)°]. With respect to the boat-shaped ring, both B1 and C3/C4 rise out of the plane defined by N1−C1−C2−C5; the angles between this plane and those defined by N1/B1/C1 (θ prow ) differ substantially with 66.3(1)° for 7Ad and 50.9(1)° for 7PhOCH3. The greater angle is likely a result of the larger adamantyl group and could rationalize why 7Ad does not cleanly undergo an electrocyclic cyclization. The other interplanar angle defined by C2−C3−C4−C5 and N1−C1− C2−C5 (θstern) is virtually identical for both compounds [7Ad = 54.3(1)° and 7PhOCH3 = 54.1(1)°]. The pyridine adduct 7PhCH3·pyr has a similar boat-type structure with the exception of a quaternary boron center at the prow position from the coordination of pyridine. The bond lengths at boron are elongated due to the coordination number increasing from three to four. The bicyclic family of compounds 8 are all isostructural featuring a fused 4/5-ring system with the phenyl groups on the two bridgehead-carbon atoms in a cis conformation and interplanar angles between the rings in the range of 66.89(8)−68.99(7)° (Figure 2). The endocyclic B−N bonds are much shorter than the seven-membered ring systems [1.4377(19)−1.444(2) Å], indicating delocalization; the endocyclic B−C distances are in the range of B−C single bonds [1.582(3)−1.590(2) Å], and the exocyclic carbon−sulfur bonds have double-bond character [1.6296(17)−1.638(2) Å]. The boron and nitrogen centers are planar in all complexes [∑anglesB = 359.9(2)−360.0(2)°; ∑anglesN = 359.6(1)− 360.0(2)°], consistent with a double bond between boron and nitrogen. The tetracyclic species 9 and 10 feature a central BNC5 ring with C1−C2, C3−C4, and C5−N1 bond distances consistent with double-bond character while C2−C3 and C4−C5 are on the order of single bonds (Figure 3 and Table 3). The 4methoxyphenyl group from the isothiocyanate introduced into the seven-membered ring has a fourth group bound to the ipsocarbon (C61), and the ortho-carbon (C62) is bound to the sulfur of the isothiocyanate. The nitrogen of the second isothiocyanide is coordinated to boron, making a quaternary center. A noteworthy change is the lengthening of the C5−S1 bond in comparison to the seven-membered rings upon binding to the aryl group, consistent with the conversion of the thioamido group to a thioether.

methoxyphenylisothiocyanate, because the bicyclic products do not react with additional isothiocyanate. The transformation likely goes through a boron-mediated Friedel−Crafts reaction facilitated by the 4-methoxyphenyl group of the sevenmembered heterocycle attacking the second equivalent of isothiocyanate (Int6). The adjacent CS double bond can trap the 4-methoxy-stabilized intermediate (Int7) to form the fused ring product 9. Acquiring an in situ 1H NMR spectrum of the reaction showed only one diastereomer, indicating stereoselectivity for the reaction. The in situ generated 4methoxyphenyl seven-membered heterocycle 7PhOCH3 can also react with other isothiocyanates such as 4-tolyl isothiocyanate to generate a similar tetracyclic complex 10. This underscores that the 4-methoxy group on the sevenmembered heterocycle is a critical substituent for the reaction. X-ray Crystallographic Studies. The structures of 7Ad and 7PhOCH3 are similar to each other with nonplanar sevenmembered BNC5 rings (Figure 1). There is distinct bondlength alternation within the ring, with C1−C2 and C3−C4 exhibiting shorter distances than C2−C3 and C4−C5 (Table 1). The B−N bonds are intermediary between single and double bonds, while the endocyclic B−C bond distances are in Table 1. Salient Bond Lengths (Å) and Angles (deg) in Compounds 7Ad, 7PhOCH3, and 7PhCH3·pyr

B(1)−N(1) B(1)−N(2) B(1)−C(71) B(1)−C(1) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(4)−C(5) C(5)−N(1) C(5)−S(1) N(1)−B(1)−C(1) C(1)−B(1)−C(71) N(1)−B(1)−C(71) B(1)−N(1)−C(5) C(5)−N(1)−C(61) B(1)−N(1)−C(61) dihedral planes θprow dihedral planes θstern

7Ad

7PhOCH3

7PhCH3·pyr

1.524(2) N/A 1.541(3) 1.585(3) 1.352(2) 1.496(2) 1.367(2) 1.518(2) 1.338(2) 1.6700(17) 110.53(15) 127.23(16) 122.15(16) 108.39(14) 127.19(14) 122.15(13) 66.3(1) 54.3(1)

1.503(2) N/A 1.539(2) 1.586(2) 1.345(2) 1.493(2) 1.353(2) 1.499(2) 1.361(2) 1.6477(18) 110.53(14) 129.27(15) 120.00(15) 115.23(14) 121.19(14) 121.11(13) 50.9(1) 54.1(1)

1.600(3) 1.621(3) 1.620(3) 1.626(3) 1.356(3) 1.483(3) 1.353(3) 1.495(3) 1.347(3) 1.685(2) 109.08(18) 123.1(1) 124.2(1) 122.50(19) 116.39(19) 121.09(18) 49.8(2) 36.7(1) D


Article

Inorganic Chemistry

Figure 2. Solid-state structures of 8PhCH3 (a), 8PhCF3 (b), 8PhOCH3 (c), and 8CH3 (d). Hydrogen atoms and solvates (where applicable) have been omitted for clarity, and ellipsoids are drawn at the 50% probability level. (e) Diagram illustrating the interplanar angles for the two rings θ. Selected bond lengths and angles are listed in Table 2.

heterocycles bearing a thioamido group are very reactive species.

Table 2. Salient Bond Lengths (Å) and Angles (deg) in Compounds 8PhCH3, 8PhCF3, 8PhOCH3, and 8CH3 B(1)−N(1) B(1)−C(1) C(1)−C(4) C(4)−C(5) C(5)−N(1) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(5)−S(1) N(1)−B(1)− C(71) C(1)−B(1)− C(71) C(1)−B(1)− N(1) B(1)−N(1)− C(5) C(5)−N(1)− C(61) B(1)−N(1)− C(61) N(1)−C(5)− S(1) S(1)−C(5)− C(4) C(4)−C(5)− N(1) interplanar angle (θ)

8PhCH3

8PhCF3

8PhOCH3

8CH3

1.4377(19) 1.590(2) 1.6129(18) 1.526(2) 1.3901(17) 1.5405(18) 1.3470(19) 1.535(2) 1.6327(14) 126.49(13)

1.444(2) 1.588(2) 1.612(2) 1.526(2) 1.389(2) 1.540(2) 1.348(2) 1.535(2) 1.6296(17) 126.49(15)

1.438(3) 1.586(3) 1.613(2) 1.522(3) 1.393(2) 1.539(3) 1.344(3) 1.532(3) 1.6301(19) 126.71(17)

1.438(3) 1.582(3) 1.613(3) 1.526(3) 1.386(3) 1.547(3) 1.348(3) 1.533(3) 1.638(2) 122.9(2)

125.95(12)

126.91(15)

126.20(17)

129.39(19)

107.50(12)

106.52(13)

107.08(16)

107.70(19)

114.47(11)

114.83(13)

114.71(15)

114.34(18)

118.75(11)

117.63(13)

118.29(15)

119.29(19)

126.73(11)

127.17(13)

126.95(15)

126.4(2)

124.84(10)

124.59(12)

124.25(14)

124.69(17)

125.64(10)

125.74(12)

126.24(14)

125.67(17)

109.49(11)

109.64(13)

109.43(15)

109.59(18)

67.59(13)

68.99(15)

66.89(17)

68.82(19)

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EXPERIMENTAL SECTION

General Considerations. All manipulations were performed under an inert atmosphere in a nitrogen-filled MBraun Unilab glovebox. Solvents were purchased from commercial sources as anhydrous grade, dried further using a JC Meyer Solvent System with dual columns packed with solvent-appropriate drying agents, and stored over molecular sieves. The isothiocyanates were purchased from commercial sources and used as received (methylisothiocyanate, Sigma-Aldrich; 4-methoxyphenylisothiocyanate, Alfa Aesar; 1-adamantylisothiocyanate, Enamine Ltd.; 4-tolylisothiocyanate and 4(trifluoromethyl)phenyl, Oakwood Chemical). Borole 1 was prepared via the literature procedure.1 Solvents for NMR spectroscopy (C6D6 and CDCl3) were purchased from Cambridge Isotope Laboratories or Sigma-Aldrich and dried by stirring for 3 days over CaH2, distilled prior to use, and stored over 4 Å molecular sieves. Multinuclear NMR spectra were recorded on a Bruker 600 MHz spectrometer. FT-IR spectra were recorded on a Bruker Alpha ATR FT-IR spectrometer on the solid samples. Single-crystal X-ray diffraction data were collected on a Bruker Apex II-CCD detector using Mo Kα radiation (λ = 0.71073 Å). Crystals were selected under paratone oil, mounted on micromounts, and then immediately placed in a cold stream of N2. Structures were solved and refined using SHELXTL.72 For compounds 8PhCF3, 9, and 10, solvates were found to be disordered to an extent that could not be modeled, and the solvent contributions were removed from the reflection data using the SQUEEZE function in the PLATON software suite.73 Synthesis of 7Ad. At room temperature, a solution of 1adamantylisothiocyanate (26.1 mg, 0.135 mmol) in CH2Cl2 (1 mL) was added to a solution of borole 1 (60.0 mg, 0.135 mmol) in CH2Cl2 (1 mL). After stirring for 5 min, the solvent was removed in vacuo, giving a yellow solid. Crude yield: 79.5 mg, 93% (>90% purity). This species is unstable in solution, precluding isolation of pure material. Single crystals for X-ray diffraction studies were grown from a diethyl ether solution via vapor diffusion into hexanes at −30 °C. d.p. = 112 °C (turned dark red); 1H NMR (600 MHz, CDCl3) δ 8.40−8.11 (br, 1H, C6H5), 7.71−7.65 (m, 1H, C6H5), 7.60−7.46 (br, 2H, C6H5), 7.31−7.27 (m, 2H, C6H5), 7.25−7.17 (m, 3H, C6H5), 7.15−7.08 (m, 2H, C6H5), 7.14−6.97 (m, 4H, C6H5), 6.94 (t, J = 7.3 Hz, 1H, C6H5), 6.85−6.75 (m, 7H, C6H5), 2.37−2.20 (m, 6H, Ad), 1.84 (s, 3H, Ad), 1.48−1.30 (m, 6H, Ad); 13C{1H} NMR (151 MHz, CDCl3) δ 193.80 (CS), 142.58, 142.21, 141.62, 139.08, 138.10, 137.45, 137.18, 137.12, 134.77, 131.58, 131.53, 131.08, 130.70, 129.97, 129.61, 128.60, 128.33, 127.91, 127.61, 127.60, 127.34, 127.25, 127.00, 126.92, 126.89, 126.69, 61.32 (NC, Ad), 39.01 (CH2, Ad), 36.28 (CH2, Ad), 29.66 (CH, Ad); A 11B{1H} NMR resonance could not be observed; FT-IR (cm−1 (ranked intensity)): 3052(14), 2906(4), 2050(10), 1595(8), 1489(5), 1440(6), 1381(15), 1304(2), 1026(9), 917(11), 796(13), 746(7), 694(1), 541(12), 517(3); high-resolution mass spectrometry

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CONCLUSIONS The reactions of pentaphenylborole with isothiocyanates led to the production of BNC5 seven-membered rings that could react further to yield new fused cyclic systems. The 1:1 stoichiometric reactions of aryl- and methylisothiocyanates converged to bicyclic complexes that are believed to form via an intramolecular [2 + 2] cycloaddition of the butadiene moiety of the BNC5 heterocycle. The reactions were independent of stoichiometry except for 4-methoxyphenylisothiocyanate, which produced a tetracyclic species from the reaction of the sevenmembered ring with a second equivalent of aryl isothiocyanate. The 4-methoxy group likely serves a role in activating the aryl group, ultimately resulting in dearomatization of the phenyl ring. These studies reveal more insight into the chemistry of antiaromatic boroles as well as that unsaturated BNC 5 E


Article

Inorganic Chemistry

Figure 3. Solid-state structures of 9 and 10. Hydrogen atoms except on the chiral carbon C62 have been omitted for clarity. Ellipsoids are drawn at the 50% probability level. Selected bond lengths and angles are listed in Table 2. hexanes (2 × 0.3 mL) and dried in vacuo to give the products as yellow powders. Single crystals for X-ray diffraction studies for all three aryl compounds were grown via vapor diffusion of pentane into a toluene solution and from a diethyl ether solution via vapor diffusion into hexanes for 8CH3. 8PhCH3. 4-Tolylisothiocyanate (20.1 mg, 0.135 mmol); yield: 68.9 mg, 86%; mp >240 °C; 1H NMR (600 MHz, CDCl3) δ 7.75−7.71 (m, 2H, aryl), 7.43−7.38 (m, 2H, aryl), 7.31−7.05 (m, 15H, aryl), 7.04− 6.91 (m, 10H, aryl), 2.42 (s, 3H, CH3); 13C{1H} NMR (151 MHz, CDCl3) δ 145.32, 144.07, 139.61, 139.57, 139.55, 137.86, 137.13, 135.35, 132.06, 131.58, 130.24, 129.98, 129.95, 129.72, 129.70, 129.04, 128.59, 128.55, 128.40, 128.37, 128.20, 128.12, 128.08, 128.04, 128.04, 127.75, 127.74, 127.54, 127.42, 127.36, 126.52, 125.83, 79.13 (bridgehead carbon), 67.60 (bridgehead carbon), 21.50 (PhCH3); 11 1 B{ H} NMR (193 MHz, CDCl3) δ = 52.2; FT-IR (cm−1 (ranked intensity)): 3022(7), 1597(10), 1492(5), 1440(4), 1341(13), 1240(2), 1072(14), 1020(6), 918(15), 831(11), 741(9), 693(1), 578(3), 552(12), 500(8); high-resolution mass spectrometry (HRMS) electrospray ionization (ESI): calcd. for C42H33BNS [M + H]+: 594.2349; found: 594.2426. 8PhCF3. 4-Trifluoromethylphenylisothiocyanate (27.4 mg, 0.135 mmol); yield: 70.8 mg, 81%; mp 171−172 °C; 1H NMR (600 MHz, CDCl3) δ 7.75−7.68 (m, 4H, aryl), 7.41−7.33 (m, 4H, aryl), 7.31− 7.19 (m, 7H, aryl), 7.18−7.14 (m, 2H, aryl), 7.10−6.99 (m, 7H, aryl), 6.98−6.91 (m, 5H, C6H5); 13C{1H} NMR (151 MHz, CDCl3) δ 145.41, 145.18, 144.12, 139.27, 139.03, 136.53, 135.08, 131.94, 131.76, 130.00, 129.96, 129.68, 129.32, 129.28, 129.01, 128.61, 128.56, 128.42, 128.33, 128.19, 128.14, 127.88, 127.80, 127.77, 127.53, 127.50, 126.71, 126.66, 126.63, 126.61, 126.05, 79.31 (bridgehead carbon), 67.75 (bridgehead carbon); 11B{1H} NMR (193 MHz, CDCl3) δ = 50.2; 19 1 F{ H} NMR (565 MHz, CDCl3) δ −62.3; FT-IR (cm−1 (ranked intensity)): 3055(13), 1595(5), 1492(7), 1437(8), 1321(2), 1235(14), 1064(6), 846(10), 776(9), 756(3), 735(12), 692(1), 615(15), 590(11), 554(4); high-resolution mass spectrometry (HRMS) electrospray ionization (ESI): calcd. for C42H30BF3NS [M + H]+: 648.2066; found: 648.2133. 8PhOCH3. 4-Methoxyphenylisothiocyanate (22.3 mg, 0.135 mmol); yield: 73.2 mg, 89%; mp = 215−216 °C; 1H NMR (600 MHz, CDCl3) δ 7.74−7.70 (m, 2H, aryl), 7.42−7.38 (m, 2H, aryl), 7.30−7.09 (m, 13H, aryl), 7.06−6.90 (m, 12H, aryl), 3.86 (s, 3H, OCH3); 13C{1H} NMR (151 MHz, CDCl3) δ 158.98, 145.34, 144.07, 139.55, 139.54, 137.17, 135.35, 135.03, 132.03, 131.61, 129.94, 129.69, 129.34, 129.03, 128.55, 128.39, 128.38, 128.21, 128.08, 127.75, 127.59, 127.43, 126.53, 125.83, 114.69 (MeOC6H4), 79.07 (bridgehead carbon), 67.57 (bridgehead carbon), 55.55 (PhOCH3), 79.07, 67.57, 55.55; 11B{1H} NMR (193 MHz, CDCl3) δ = 51.6; FT-IR (cm−1 (ranked intensity)): 2907(3), 2029(6), 2011(15), 1595(7), 1508(4), 1438(8), 1239(2), 1028(10), 913(13), 796(14), 746(9), 693(1), 552(5), 434(11), 413(12); high-resolution mass spectrometry (HRMS) electrospray

Table 3. Salient Bond Lengths (Å) and Angles (deg) in Compounds 9 and 10 B(1)−N(1) B(1)−N(2) B(1)−C(1) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(4)−C(5) C(5)−N(1) N(1)−C(61) C(5)−S(1) S(1)−C(62) N(2)−C(6) C(6)−S(2) C(61)−C(62) N(1)−B(1)−N(2) C(5)−S(1)−C(62)

9

10

1.556(3) 1.587(3) 1.615(3) 1.364(3) 1.491(3) 1.370(3) 1.452(3) 1.291(3) 1.474(3) 1.736(2) 1.886(2) 1.340(3) 1.664(2) 1.533(3) 95.16(16) 90.77(10)

1.559(3) 1.580(3) 1.613(3) 1.360(3) 1.492(3) 1.361(3) 1.453(3) 1.294(3) 1.467(3) 1.733(2) 1.882(2) 1.340(3) 1.656(2) 1.532(3) 95.20(16) 90.54(10)

(HRMS) electrospray ionization (ESI): calcd. for C45H41BNS [M + H]+: 638.2975; found: 638.3046. Synthesis of 7PhCH3·pyr. At room temperature, a solution of 4tolylisothiocyanate (20.1 mg, 0.135 mmol) in CH2Cl2 (1 mL) was added to a solution of borole 1 (60.0 mg, 0.135 mmol) in CH2Cl2 (1 mL). After stirring for 5 min, a solution of pyridine (11.1 mg, 0.14 mmol) in CH2Cl2 (0.5 mL) was added, and the solution was kept stirring for 10 min. The solvent was removed in vacuo, giving a yellow solid. The solids were washed with hexanes (2 × 0.3 mL) and dried in vacuo to give 7PhCH3·pyr as a yellow solid. Yield: 88.4 mg, 95% (89% purity). Further purification failed due to decomposition. Single crystals for X-ray diffraction studies were grown via vapor diffusion of pentane into a toluene solution. Crystallization of 7PhOCH3. At room temperature, a solution of 4methoxyphenylisothiocyanate (22.3 mg, 0.135 mmol) in CH2Cl2 (1 mL) was added to a solution of borole 1 (60.0 mg, 0.135 mmol) in CH2Cl2 (1 mL). After stirring for 5 min, the solvent was removed in vacuo, giving a red solid. Single crystals for X-ray diffraction studies were grown as a mixture of 7PhOCH3 and 8PhOCH3 from a CH2Cl2 solution via vapor diffusion into hexanes at −30 °C (in the dark). General Synthesis for 8PhCH3, 8PhCF3, 8PhOCH3, and 8CH3. Quantities, purification details, and characterization follow. At room temperature, a solution of aryl- or methylisothiocyanate (0.135 mmol) in CH2Cl2 (1 mL) was added to a solution of borole 1 (60.0 mg, 0.135 mmol) in CH2Cl2 (1 mL). After stirring for 24 h, the solvent was removed in vacuo, giving a yellow solid. The solids were washed with F


Inorganic Chemistry

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ionization (ESI): calcd. for C42H33BNOS [M + H]+: 610.2298; found: 610.2377. 8CH3. Methylisothiocyanate (9.9 mg, 0.135 mmol); yield: 56.7 mg, 81%; mp =94−95 °C; 1H NMR (600 MHz, CDCl3) δ 7.76−7.73 (m, 2H, C6H5), 7.51−7.46 (m, 2H, C6H5), 7.45−7.40 (m, 2H, C6H5), 7.32 (t, J = 7.6 Hz, 2H, C6H5), 7.29−7.27 (m, 2H, C6H5), 7.24−7.15 (m, 6H, C6H5), 7.09 (t, J = 7.7 Hz, 2H), 7.03−6.96 (m, 3H, C6H5), 6.94− 6.88 (m, 3H, C6H5), 6.87−6.81 (m, 2H, C6H5), 3.87 (s, 3H, CH3); 13 C{1H} NMR (151 MHz, CDCl3) δ 144.08, 143.96, 139.76, 138.34, 134.72, 133.87, 132.34, 130.82, 129.87, 129.48, 129.07, 128.38, 128.36, 128.24, 128.19, 128.06, 128.02, 127.74, 127.42, 126.45, 125.80, 79.37 (bridgehead carbon), 66.84 (bridgehead carbon), 36.15 (NCH3); 11 1 B{ H} NMR (193 MHz, CDCl3) δ = 53.6; FT-IR (cm−1 (ranked intensity)): 2954(9), 1596(7), 1492(8), 1441(5), 1338(10), 1284(1), 1119(6), 1074(12), 1027(15), 1006(3), 915(11), 770(4), 573(14), 552(2), 530(13); high-resolution mass spectrometry (HRMS) electrospray ionization (ESI): calcd. for C36H29BNS [M + H]+: 518.2036; found: 518.2120. Synthesis of 9. At room temperature, a solution of 4methoxyphenylisothiocyanate (66.9 mg, 0.410 mmol) in CH2Cl2 (2 mL) was added to a solution of borole 1 (60.0 mg, 0.135 mmol) in CH2Cl2 (1 mL). After stirring overnight, the solvent was removed in vacuo, giving a red solid. The solids were washed with hexanes (2 × 0.3 mL) and dried in vacuo to give 9 as an orange solid. Yield: 76.9 mg, 74%; single crystals for X-ray diffraction studies were grown via vapor diffusion of pentane into a toluene solution. d.p. = 149 °C (turned red); 1H NMR (600 MHz, CDCl3) δ 8.00 (d, J = 6.9 Hz, 1H), 7.48−7.43 (m, 1H, C6H5), 7.42−7.34 (m, 2H, aryl), 7.32−7.29 (m, 1H, aryl), 7.08−6.35 (m, 23H, aryl), 5.98−5.94 (m, 2H, aryl), 5.85− 5.80 (m, 2H, aryl), 4.88−4.83 (m, 1H, CHS), 3.71 (s, 3H, OCH3), 3.66 (s, 3H, OCH3); 13C{1H} NMR (151 MHz, CDCl3) δ 197.21 (CS), 172.54 (CN), 161.08, 157.69 (MeOC6H4), 155.67, 143.09, 142.40, 141.18, 140.33, 137.91, 135.76, 135.22, 132.39, 130.76, 129.69, 129.15, 128.51, 128.33, 128.02, 127.92, 127.61, 127.57, 127.27, 126.55, 126.38, 125.61, 113.14 (MeOC6H4), 86.56, 85.94, 59.75 (SCH), 55.27 (PhOCH3); 11B{1H} NMR (193 MHz, CDCl3) δ = 3.1; FT-IR (cm−1 (ranked intensity)): 1655(10), 1580(12), 1507(5), 1439(15), 1371(7), 1232(2), 1171(13), 1026(4), 907(8), 829(9), 794(6), 694(1), 611(11), 563(3), 544(14); high-resolution mass spectrometry (HRMS) electrospray ionization (ESI): calcd. for C50H40BN2O2S2 [M + H]+: 775.2546; found: 775.2611. Synthesis of 10. At room temperature, a solution of 4methoxyphenylisothiocyanate (22.3 mg, 0.135 mmol) in CH2Cl2 (1 mL) was added to a solution of borole 1 (60.0 mg, 0.135 mmol) in CH2Cl2 (1 mL). After stirring for 5 min, a solution of 4tolylisothiocyanate (40.3 mg, 0.270 mmol) in CH2Cl2 (1 mL) was added, and the solution was stirred for 24 h. The solvent was removed in vacuo, giving a yellow solid. The solids were washed with hexanes (2 × 0.3 mL) and dried in vacuo to give 10 as a yellow solid. Yield: 78.5 mg, 77%; single crystals for X-ray diffraction studies were grown via vapor diffusion of pentane into a toluene solution. d.p. = 146 °C (turned dark red); 1H NMR (600 MHz, CDCl3) δ 8.00 (d, J = 7.2 Hz, 1H, aryl), 7.48−7.44 (m, 1H, aryl), 7.41−7.31 (m, 3H, aryl), 7.21− 6.42 (m, 22H, aryl), 6.39−6.35 (m, 1H, aryl), 5.99−5.95 (m, 2H, aryl), 5.81 (d, J = 8.3 Hz, 2H, aryl), 4.89−4.84 (m, 1H, CHS), 3.66 (s, 3H, OCH3), 2.23 (s, 3H, CH3); 13C{1H} NMR (151 MHz, CDCl3) δ 197.08 (CS), 172.50 (CN), 161.07, 155.68, 143.06, 142.41, 141.21, 140.34, 140.24, 137.93, 135.86, 135.22, 132.46, 130.76, 129.68, 129.17, 128.64, 128.31, 127.99, 127.91, 127.87, 127.57, 127.55, 127.30, 127.24, 127.20, 126.94, 126.91, 126.88, 126.53, 126.49, 126.35, 125.60, 125.55, 86.59, 86.01, 59.72 (SCH), 55.24 (PhOCH3), 21.30 (PhCH3); 11 1 B{ H} NMR (193 MHz, CDCl3) δ = 2.2; FT-IR (cm−1 (ranked intensity)): 1654(10), 1574(12), 1509(11), 1373(4), 1309(13), 1233(2), 1177(7), 1118(15), 1006(5), 793(9), 759(8), 699(1), 610(14), 562(3), 544(6); high-resolution mass spectrometry (HRMS) electrospray ionization (ESI): calcd. for C50H40BN2OS2 [M + H]+: 759.2597; found: 759.2672.

Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02469. X-ray crystallographic data and multinuclear NMR spectra for 7−10 and stacked NMR plots of in situ monitoring of reactions (PDF) CIF data for all compounds (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was generously supported by the Welch Foundation (Grant No. AA-1846) and Baylor University. REFERENCES

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