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Cite This: J. Am. Chem. Soc. 2018, 140, 8119−8123
Remote Stereochemistry of a Frustrated Lewis Pair Provides Thermal and Photochemical Control of Reactivity Louie Fan,‡ Andrew R. Jupp,‡ and Douglas W. Stephan* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
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S Supporting Information *
Scheme 1. Reactions of B/S FLPs, Hydroalumination of Phosphino-Alkynes and Outline of Present Work
ABSTRACT: Hydroboration of t BuCCSR with (C6F5)2BH generate the B/S FLPs, (C6F5)2B(RS)C CH(tBu) (R = ptol, Me). For R = ptol the product Z-1 exhibits Z stereochemistry, is monomeric in solution and is capable of both classical Lewis acid−base and FLP reactivity. Photoisomerization of Z-1 affords E-1, which features an intramolecular S−B interaction promoted by the remote steric influence of the tert-butyl group. E-1 is not active as an FLP under ambient conditions but reacts with phenylacetylene when activated with either heat or light. The stereochemical outcome of the product is dependent on the stimulus employed. he concept of “frustrated Lewis pairs” (FLPs) was articulated over a decade ago. This concept emerged from the finding that combination of sterically demanding Lewis acids and bases that precluded classical adduct formation effect the reversible activation of dihydrogen.1 This breakthrough has spawned numerous studies in metalfree hydrogenation catalysis,2 and FLP reactivity with small molecules.3 More recently, the concept of FLP reactivity has also broadened beyond the main group, finding relevance and applications in transition metal chemistry, as well as surface, polymer and materials chemistry.4 Typically, FLP reactions commence on addition of the substrate to the FLP system. External stimuli, such as heat or light, to initiate such reactions grants an additional level of temporal control. There are a number of FLP reactions that require heating to proceed.5 For example, Hoshimoto, Ogoshi and co-workers described that the combination of a phosphine-oxide derived carbene and B(C6F5)3 form a classical Lewis adduct at room temperature, but this system can be thermally stimulated to react with H2 via the corresponding FLP.6 The notion of photoinitiation of FLP chemistry has not been explored, although Piers and coworkers synthesized a “photo Lewis acid generator” that affords B(C 6 F 5 ) 3 on irradiation. 7 Furthermore, Ozin described an FLP-type reduction of CO2 on the surface of In2O3−x(OH)y, which was more effective in the light.8 Within currently known FLP systems, the majority employ a borane and a sterically encumbered P, N or carbene donor.3c Sulfur-based donors have drawn less attention. Combinations of a thioether, such as Me2S and (PhCH2)2S, and B(C6F5)3 have been reported to add to terminal alkynes (Scheme 1).9 The weak nature of the B−S interaction was exploited by Gabbaı̈ et al. to develop sensors for cyanide and
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fluoride ions in water.10 The dimeric methylene-linked thioether/borane (BH2CH2SMe)2 was described by Nöth and co-workers11 whereas Wagner et al.12 described the related dimer (PhBrBCH2SMe)2. In 2011, we reported the reaction of PhSCH2B(C6F5)2 with alkynes affording fivemembered zwitterionic heterocycles (Scheme 1).13 In considering other synthetic routes to FLPs, we noted that Uhl and co-workers14 have described a variety of reactions of hydroaluminations of phosphino-alkynes to generate geminal P/Al systems (Scheme 1). Though cisAlH addition affords the Z-isomers, the less sterically congested products are subsequently converted to the Eisomers.14d These advances inspired us to probe the impact on reactivity of stereoisomerism of FLPs derived from the hydroboration of alkynyl-thiolates. Herein, we report the first example of interconvertible stereoisomers of geminal B/S species where differing reactivity results from the orientation of a remote substituent. Moreover, the E-isomer does not act as an FLP under ambient conditions, but offers stereochemical control of alkyne activation products depending on the nature of the stimuli. Equimolar combination of HB(C6F5)2 and (tert-butylacetynyl)(p-tolyl)sulfide (tBuCCSptol) reacts to give the cis-hydroborated product, Z-1, as a yellow solid in 87% yield (Scheme 2). The 1H NMR spectrum of Z-1 shows a characteristic olefinic resonance at 6.66 ppm, and 2D-NMR experiments were consistent with the formulation as ZReceived: May 17, 2018 Published: June 14, 2018 8119
DOI: 10.1021/jacs.8b05176 J. Am. Chem. Soc. 2018, 140, 8119−8123
Communication
Journal of the American Chemical Society Scheme 2. Synthesis and Reactivity of Z-1
trigonal planar geometry, with bond angles around the boron center of 120.2(3)°, 120.5(3)° and 119.4(3)°. The B−C−S angle is 118.7(3)°, typical of an sp2-hybridized carbon center, giving a B···S separation of 2.858 Å. The boron center of Z-1 is accessible to Lewis bases. This is evidenced by the reactions of pyridine or acrylonitrile with Z-1, which result in the immediate formation of the Lewis acid−base adducts Z-tBu(H)CC(Sptol)(B(C 6 F 5 ) 2 (NC 5 H 5 )) Z-2 and Z- t Bu(H)CC(Sptol)(B(C6F5)2(NCCHCH2)) Z-3, respectively (Scheme 2). Z-2 and Z-3 display upfield resonances in the 11B NMR spectra at −0.4 and +1.9 ppm and a decrease in the meta-para gap to 5.8 and 7.7 ppm by 19F NMR spectroscopy, respectively, consistent with quaternization of the boron centers. The formulation of Z-3 was confirmed by X-ray crystallography (Figure 1b) revealing a BN bond distance of 1.588(3) Å. Frustrated Lewis pair type reactivity was observed upon the addition of phenylacetylene to Z-1, forming the zwitterionic heterocycle Z-tBu(H)CCS(ptol)C(Ph)C(H)B(C6F5)2 Z-4 (Scheme 2). The 11B{1H} NMR spectrum showed a sharp peak at −6.8 ppm, indicating a four-coordinate bond center, and six distinct resonances in the 19F NMR spectrum resulting from two chemically inequivalent C6F5 rings. The structure of Z-4 was confirmed crystallographically (Figure 1c), revealing that the sulfur has added to the substituted carbon. Z-4 shows the expected contraction of the B−C−S bond angle to 109.6(1)° from 118.7(2)° in Z-1. The p-tolyl substituent on sulfur is crucial to the isolation of Z-1 as a monomeric product. By analogy with Z-1, the Smethyl analogue Z-5 was derived from MeSCC(tBu) and HB(C6F5)2. Z-5 is predominantly a dimer in solution, as evidenced by the upfield shift of the 11B{1H} NMR signal at 5.4 ppm. The solid-state structure of Z-5 was also confirmed by X-ray crystallography (Figure 2) and shows the dimer
(C6F5)2B((ptol)S)CCH(tBu) (see SI). A downfield chemical shift of 57.5 ppm in the 11B{1H} NMR spectrum of Z-1 and the relatively large meta-para gap of 13.4 ppm in the 19 1 F{ H} NMR spectrum15 is indicative of a three-coordinate boron center. These data also demonstrate that Z-1 is a monomer in solution, in contrast to the previously reported geminal B/S systems.13 An X-ray crystallographic study of the monomeric Z-1 (Figure 1a), confirmed the geminal orientation of boron and sulfur. The boron center shows minimal deviation from a
Figure 2. POV-ray depiction of the molecular structure of Z-5: C, black; F, pink; S, yellow; B, yellow-green. Hydrogen atoms are omitted for clarity.
(Z-tBu(H)CC(SMe)(B(C6F5)2))2. This is due to a combination of the reduced steric demands and the slightly increased basicity of the sulfur center (Scheme 3). The dimer is in equilibrium with the monomer in solution; however, as Z-5 reacts with phenylacetylene to afford the cyclized product Scheme 3. Dimerization of Z-5, and Reaction with Phenylacetylene
Figure 1. POV-ray depictions of the molecular structures of (a) Z-1, (b) Z-3 and (c) Z-4: C, black; H, gray; F, pink; N, blue; S, yellow; B, yellow-green. Hydrogen atoms (except =CH) are omitted for clarity. 8120
DOI: 10.1021/jacs.8b05176 J. Am. Chem. Soc. 2018, 140, 8119−8123
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Journal of the American Chemical Society Scheme 4. Photoisomerization of Z-1 to E-1, and Reactivity with Lewis Bases
Figure 3. Natural bond order (NBO) diagram (0.04 au isosurface) depicting the principal donor−acceptor interaction in E-1 involving a lone pair on sulfur and the vacant orbital on boron.
The ability for E-1 to form this interaction is the driving force for isomerization, and E-1 is correspondingly 4.9 kJ/ mol lower in energy than Z-1. The tert-butyl group, despite being relatively distant from the S−B interaction, is key to this difference. For an efficient S−B interaction, the sulfur lone pair(s) need(s) to be in the correct orientation to donate into the vacant p orbital on boron, but this would necessarily result in a steric clash of the p-tolyl substituent and the tert-butyl group in Z-1. This potential clash is alleviated in E-1, resulting in a stabilizing S−B interaction. This interaction is further facilitated by the steric clash of the tert-butyl group with the C6F5 groups in E-1. This situation is reminiscent of allosterism in enzymes, where remote changes influence the conformation and activity of the active site of the protein. In enzymes, this phenomenon arises from the binding of another compound at a site other than the active site, whereas in E-1/Z-1 the effect arises from the orientation of a peripheral tert-butyl group. By further analogy with allosterism, the profound conformational change also results in a difference of reactivity. Whereas Z-1 reacted readily with acrylonitrile to afford Z-3, E-1 undergoes no such reaction, even after prolonged heating at 50 °C. Nonetheless, the stronger base pyridine does disrupt the S−B interaction in E-1 to yield E-2 (Scheme 4). The solid-state structure of E-2 confirms the formulation and the disposition of the tert-butyl group (Figure 4a). Note the alkenyl B−C−C bond angle of 132.4(4)° is significantly deviated from 120°, highlighting the steric clash between the C6F5 rings and the tert-butyl group. In sharp contrast to Z-1, a C6D6 solution of E-1 and phenylacetylene shows no reaction under ambient conditions. This result prompted us to explore the possibility of inducing FLP reactivity with different stimuli (Scheme 5). Indeed, heating a solution of E-1 for 40 h at 50 °C in the presence of phenylacetylene afforded the zwitterionic heterocycle E-tBu(H)CCS(ptol)C(Ph)C(H)B(C6F5)2 E-4 in 64% isolated yield. The 11B{1H} NMR resonance at −10.9 ppm and the 19 1 F{ H} NMR signals attributable to the inequivalent C6F5 groups were consistent with a molecular structure of E-4 similar to but distinct from Z-4. This was confirmed unambiguously via a crystallographic study (Figure 4b). Conversely, photochemical activation of the same starting mixture establishes an equilibrium between E-1 and Z-1. The latter reacts readily with phenylacetylene at room temperature and drives the reaction to the formation of Z-4. Control experiments show that there is no conversion between the
Z-6. This is consistent with previous reports of geminal B/S FLP systems.13 UV-irradiation of Z-1 for 2−4 h afforded E-1 in 65−87% yields (Scheme 4), as evidenced by 1-D 1H NOESY and 2-D 19 F−1H HOESY NMR experiments (see SI). Note that the prolonged heating of Z-1 at 60 °C in the dark led to eventual decomposition but no evidence of E-1 being formed. This isomerization builds upon the work of Erker and coworkers,16 in which 1,1-carboboration of terminal alkynes with B(C6F5)3 yields a mixture of E and Z isomers, and subsequent irradiation preferentially forms one isomer. We postulate that the vacant p orbital on the boron atom facilitates the formation of an excited state with reduced C C bonding character, allowing free rotation around this bond. This is consistent with the recently reported photoisomerization of styrenylboranes by Watson and Gilmour.17 Interestingly, the NMR data of E-1 suggest a four-coordinate boron center, as the 11B NMR spectrum exhibits a broad resonance at 24.2 ppm, and the meta-para gap in the 19F{1H} NMR spectrum slightly decreases to 11.6 ppm (13.5 ppm in Z-1). These data suggest a weak S−B interaction; however, diffusion ordered spectroscopy (DOSY) NMR experiments showed similar diffusion coefficients for Z-1 and E-1 (see SI). This infers that E-1 is also monomeric in solution, and suggests that the S−B interaction is intramolecular. The nature of S−B interactions in the isomers of 1 was probed computationally at the B97D3/6-311++G** level of theory (see SI). Efforts to optimize dimeric structures for these species, based on analogy to Z-5, were unsuccessful, which was attributed to steric congestion. The optimized ground state for Z-1 has a S−B distance of 2.900 Å and a B−C−S bond angle of 120.1°, in close agreement with the Xray structure (vide supra), whereas E-1 has a significantly shorter SB distance of 2.525 Å and a smaller B−C−S angle of 96.4°. A local minimum could be located for a “closed” form of Z-1, featuring a S−B distance of 2.473 Å, however this is 12.9 kJ/mol higher in energy than the ground state. In contrast, no minimum could be located for an “open” form of E-1 (see SI). The interaction was further probed using natural bond orbital (NBO) analysis. Examination of the list of donor/acceptor interactions in E-1 reveals the presence of a principal donation of 74.9 kJ/mol from a lone pair on sulfur to the formally vacant orbital on boron (Figure 3). There is also a smaller donation of 19.9 kJ/mol from the second lone pair on sulfur to the same boron orbital. No such interactions are present in Z-1. 8121
DOI: 10.1021/jacs.8b05176 J. Am. Chem. Soc. 2018, 140, 8119−8123
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b05176. Experimental and computational details (PDF) Data for C30H22BF10NS (CIF) Data for C33H23BF10S (CIF) Data for C25H17BF10S (CIF) Data for C28H20BF10NS (CIF) Data for C33H23BF10S (CIF) Data for C19H13BF10S (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Figure 4. POV-ray depictions of the molecular structures of (a) E-2 and (b) E-4: B, pink; C, black; S, yellow; F, neon green. Hydrogenatoms (except =CH) are omitted for clarity.
Douglas W. Stephan: 0000-0001-8140-8355 Author Contributions ‡
These authors contributed equally.
Scheme 5. Reactivity of E-1 and Phenylacetylene under Ambient Conditions, and with Thermal and Photochemical Stimuli
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS D.W.S. gratefully acknowledges the financial support of NSERC of Canada, the award of a Canada Research Chair, and an Einstein Fellowship at TU Berlin. A.R.J. is grateful for the support of a Banting Fellowship. Dr. Darcy Burns is acknowledged for assistance with NMR spectroscopic experiments. L.F. thanks Professor Robert Morris as well as Molly Sung and Chris Seo from the Morris Group. The computational work was made possible by the facilities at SHARCNET and Compute/Calcul Canada.
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REFERENCES
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isomers Z-4 and E-4, under either thermal or photochemical conditions. The above reactivity makes E-1 a particularly interesting system as it is a classical Lewis acid−base adduct under ambient conditions, but it can be activated by different stimuli to undergo FLP reactivity, and crucially the stereochemistry of the ensuing products is determined by the nature of the stimulus. In conclusion, we have shown that the hydroboration of an alkynylsulfide generates a B/S FLP Z-1 that is monomeric. This species is capable of classical Lewis acid−base chemistry and FLP reactivity with alkynes. Z-1 can readily undergo photoisomerization to E-1, and the difference in conformation and reactivity of the two isomers is dependent on the location of the distal tert-butyl group, reminiscent of allosteric behavior in enzymes. E-1 is a classical Lewis adduct in solution, but can be initiated either thermally or photochemically to selectively afford two different isomers of the phenylacetylene-activated product, E-4 and Z-4, respectively. These findings demonstrate it is possible to design photoswitchable FLPs for selective reactivity, and efforts to further the range of FLP reactions by varying the nature of the acid/ base components are ongoing. 8122
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