Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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The Addition of a Cyclopropyl Alkyne to an AsymmetricallySubstituted Disilene: A Mechanistic Study Andrew T. Henry,† Jeremy L. Bourque,† Isabell Vacirca,‡ David Scheschkewitz,‡ and Kim M. Baines*,† †
Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7 Krupp-Chair of General and Inorganic Chemistry, Saarland University, 66123 Saarbrücken, Germany
‡
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S Supporting Information *
ABSTRACT: The addition of (2-ethynyl-3-methoxy-2methylcyclopropyl)benzene to Tip2SiSiTipPh, a disilene with an asymmetric substitution pattern, was investigated. The regiochemistry of the ring-opened products indicates a stepwise mechanism with a biradical intermediate. The results are consistent with those obtained in similar experiments with symmetrically substituted disilenes.
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INTRODUCTION Since the discovery of the first isolable disilene almost four decades ago,1 the cycloaddition reactions of disilenes have been investigated because of the possibility of forming complex heterocycles in one reaction.2 The addition of alkynes to disilenes to give 1,2-disilacyclobutenes has been shown to be a reliable, high-yielding reaction and as such has been used to trap transient disilenes,3 to test the reactivity of new stable disilenes,4 and to serve as the bond-forming reaction in the synthesis of organic−inorganic hybrid polymers.5 It has also been utilized as a reliable means to attach organic groups to the Si(100) 2 × 1 surface, which has silicon dimers that mimic the reactivity of molecular disilenes.6 Because of the critical role that this reaction plays in disilene chemistry, an understanding of the mechanism of alkyne cycloaddition is critical. Early mechanistic studies, reported by Sakurai7 and West,8 revealed that the reaction proceeds through a stepwise mechanism. Sakurai proposed the formation of a biradical intermediate, whereas West speculated that the intermediate was zwitterionic in nature since the disilene tested reacted only with terminal alkynes with polarizing substituents. To distinguish between the formation of a biradical or a zwitterionic intermediate in the addition of alkynes to disilenes, we have utilized a specially designed9 mechanistic probe.10 Compound 1 features phenyl and methoxy substituents on the cyclopropyl ring. Upon the formation of a vinyl radical adjacent to the ring, regioselective ring opening toward the phenyl substituent occurs; however, upon formation of a vinyl cation, ring opening toward the methoxy substituent takes place. Both rearrangements have rate constants in the range of 1010−1012 s−1 (Scheme 1).11 By elucidation of the regiochemistry of the phenyl and methoxy functional groups in the structure of the products, the nature of the intermediate can be inferred. The mechanisms of alkyne cycloaddition to two disilenes, tetramesityldisilene (2)12 and tetrakis(tert-butyldimethylsilyl)disilene (3),13 have been studied using alkyne 1 as a mechanistic probe. While there were © XXXX American Chemical Society
Scheme 1. Ring-Opening Pathways of 1 under Radical and Cationic Conditions
some differences in the reactivities of the two disilenes with 1, both the tetra-aryl-substituted12 and tetrasilyl-substituted13 disilenes react with 1 to give a biradical intermediate. While these studies are consistent, more examples are needed to determine the generality of the mechanism. Scheschkewitz et al. studied the addition of phenylacetylene to two asymmetrically-substituted disilenes, 45 and 514 (Scheme 2). The reaction was found to be regiospecific, suggesting the selective formation of an intermediate. It was Scheme 2. Reaction of Asymmetrically Substituted Disilenes 4 and 5 with Phenylacetylene (Tip = 2,4,6-iPr3C6H2; TMOP = 2,4,6-(MeO)3C6H2)
Received: January 25, 2019
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DOI: 10.1021/acs.organomet.9b00054 Organometallics XXXX, XXX, XXX−XXX
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Organometallics speculated that the polarization of the SiSi bond may lead to the formation of a zwitterionic intermediate.5 The isolated zwitterion 6 derived from the addition of an N-heterocyclic carbene (NHC) to an asymmetrically substituted disilene (Scheme 3) was provided as evidence for the formation of a Scheme 3. Reaction of an NHC with a Disilene15
zwitterion;15 however, the nucleophilicity of an NHC is much higher than that of an alkyne.16 The question arises whether the change in the aryl substituent from Tip to phenyl is sufficient to change the nature of the reaction intermediate in the reaction with alkynes. Thus, the addition of cyclopropyl alkyne 1 to disilene 4 has been examined, and the results are reported herein.
Figure 1. Thermal ellipsoid plot of 7 showing the numbering scheme. Ellipsoids are at the 50% probability level, and all of the hydrogen atoms and Tip groups (except the ipso carbons) have been omitted for clarity. Selected bond lengths [Å] and angles [deg]: Si1−Si2, 2.4795(10); C4−C5, 1.345(4); C3−C13, 1.340(4); Si1−Si2−C5, 97.21(8); Si2−C5−C4, 136.4(2); C5−C4−C3, 129.9(3); C4−C3− C2, 120.8(2); C3−C2−C1, 115.1(2); C2−C1−Si1, 117.05(16); C1− Si1−Si2, 102.07(7).
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RESULTS AND DISCUSSION Cyclopropyl alkyne 1 was added to a yellow hexanes solution of disilene 4 at room temperature to give a red solution. Upon removal of the solvent, subsequent dissolution in DCM, and exposure of the solution to air, the red color of the solution disappeared, yielding a clear and colorless solution. The products were separated by preparative thin-layer chromatography. Four compounds were isolated: 7, 8, 9, and 10 (Scheme 4). Each product was determined to be a 1:1 adduct between
observed correlations to signals at 42.30 and 91.24 ppm, respectively, in the 13C dimension of the 13C−1H gHSQC NMR spectrum of 7. Accordingly, a correlation was observed between the two doublets in the 1H−1H gCOSY NMR spectrum of 7 (also see Tables S1 and S2). The signal at 4.93 ppm in the 1H dimension of the 29Si−1H gHMBC NMR spectrum, assigned to CH−Ph, correlates to the signals at −12 and −32 ppm in the 29Si dimension, demonstrating that the CH−Ph hydrogen is within three bonds of both silicons. In contrast, the signal at 4.08 ppm in the 1H dimension, assigned to CH−OMe, correlates only to the signal at −12 ppm in the 29 Si dimension, and thus, the second silicon must be more than three bonds away. Furthermore, the signal at −32 ppm in the 29 Si dimension correlates to a signal at 8.16 ppm in the 1H dimension, which was assigned to an o-Ph-H on the basis of its chemical shift and correlations to other signals in the aromatic region of the 13C dimension of the 13C−1H gHMBC spectrum of 7, showing that the silicon with a chemical shift of −32 ppm has a phenyl substituent attached. No signals for 7 were evident in the 1H NMR spectrum of the crude product mixture, and therefore, 7 is formed during the separation of the products by preparative chromatography. The structure of 8 was elucidated using 2D NMR spectroscopy and by comparison of 1H, 13C, and 29Si chemical shifts to those of the stereoisomers 11 and 12 derived from the addition of alkyne 1 to tetramesityldisilene (2) (Chart 1 and
Scheme 4. Addition of 1 to 4
disilene 4 and alkyne 1 by electrospray ionization mass spectrometry (ESI-MS). All of the compounds were characterized by 1D and 2D NMR spectroscopy. The structure of 7 was determined unambiguously by singlecrystal X-ray diffraction. While both enantiomers were present in the unit cell, only one is shown in Figure 1. The C4−C5 and C3−C13 bond lengths are consistent with those of typical C C bonds. The molecular structure clearly reveals the regiochemistry of the phenyl and methoxy substituents: the phenyl substituent is located at the position α to the disilyl moiety. The regiochemistry of the phenyl and methoxy substituents is also evident by 29Si−1H gHMBC NMR spectroscopy. The 1H NMR spectrum of 7 contains two doublets at 4.93 and 4.08 ppm, which were assigned to the CH−Ph and CH−OMe hydrogens on the basis of the
Chart 1. Products 11−13 Derived from the Addition of Alkyne 1 to Tetramesityldisilene (2)
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DOI: 10.1021/acs.organomet.9b00054 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table S1).12 Two singlets were observed at 3.71 and 4.02 ppm in the 1H NMR spectrum of 8. The two singlets correlated to signals at 91.7 and 54.1 ppm in the 13C dimension of the 13 C−1H gHSQC NMR spectrum of 8, respectively. While these signals appear as singlets, a strong correlation between the two 1H signals was also observed in the 1H−1H gCOSY NMR spectrum, suggesting that the signals are doublets with small coupling constants. On the basis of the 13C chemical shifts as well as the strong correlation observed in the 1H−1H gCOSY NMR spectrum, the singlets at 3.71 and 4.02 ppm were assigned to the CH−OMe and CH−Ph moieties, respectively. The correlation critical to the elucidation of the structure of 8 was observed in the 13C−1H gHMBC NMR spectrum of 8: the signal at 3.71 ppm in the 1H dimension, assigned to CH−OMe, correlates to the signal at 218.4 ppm in the 13C dimension. The signal corresponding to the CH−Ph hydrogen, at 4.02 ppm in the 1H dimension, does not correlate to the same 13C signal. The signal at 218.4 ppm was assigned to the allenic carbon on the basis of the chemical shift and by comparison to the chemical shifts of the allenic carbons in 11 and 12 at 213.1 and 210.2 ppm, respectively. This suggests that 8 is a seven-membered cyclic allene similar in structure to 11 and 12. Further evidence for the regiochemistry of the phenyl and methoxy substituents was provided by 29Si−1H HMBC spectroscopy. The signal assigned to the CH−Ph hydrogen at 4.02 ppm correlates to both silicon signals in the 29Si dimension, at −28 and −31 ppm, whereas the signal assigned to the CH−OMe hydrogen at 3.71 ppm correlates to only one silicon signal (−28 ppm). These correlations confirm that 8 has the same regiochemistry as 7 with the phenyl substituent α to the disilane moiety. Furthermore, the signal at −31 ppm in the 29Si dimension correlates to the signals at 7.60 and 8.28 ppm in the 1H dimension, which were assigned to Ph hydrogens, demonstrating that the silicon that gives rise to the signal at −31 ppm is substituted with the phenyl group. The structures of compounds 9 and 10 were also elucidated by 2D NMR spectroscopy and by comparison to the relevant NMR data for disilacyclobutene 13 (Chart 1 and Table S2).12 While the chemical shifts of the 1H signals of 7 and 8 assigned to the benzylic hydrogens appear at 4.93 and 4.02 ppm, respectively, the corresponding signals in 9 and 10 appear at noticeably lower frequencies (2.84 and 2.66 ppm, respectively). These chemical shifts are more consistent with the analogous signal in disilacyclobutene 13. Unequivocal evidence for the disilacyclobutene core of 9 and 10 was provided by 29Si−1H gHMBC spectroscopy (Figure 2). Two 29Si signals were observed for both 9 and 10. In each case, both signals correlated to the signal assigned to the vinylic hydrogen; however, only the low-frequency signal correlated to the signal assigned to an o-Ph hydrogen, and thus, the lowfrequency 29Si signals were assigned to the SiPhTip groups. The magnitude of the coupling constants between the vinylic hydrogen and the two silicon atoms is typical of 2JSi−H (∼7 Hz) and 3JSi−H (∼23−30 Hz) couplings in disilacyclobutene rings.12,13 Furthermore, in both 9 and 10 the vinylic hydrogen is two bonds away from the phenyl-substituted silicon atom, suggesting that 9 and 10 are not regioisomers and therefore must be diastereomers. The coupling constants between the cyclopropyl hydrogens in 9 and 10 are similar in magnitude (4.7 and 5.8 Hz, respectively) and somewhat ambiguous. Typical coupling constants for 2,3-cis hydrogens in trans,trans2-alkoxy-3-phenyl-substituted cyclopropyl rings range from 6 to 8 Hz, whereas the analogous hydrogens in the trans isomers
Figure 2. 29Si−1H gHMBC NMR spectrum of 9 (600 MHz, C6D6).
are generally 2−4 Hz.10a With a coupling constant of 5.8 Hz, 10 likely has a cis arrangement of the phenyl and methoxy groups. With a coupling constant of 4.7 Hz, the stereochemistry of the phenyl and methoxy groups on the cyclopropyl ring in 9 is ambiguous, although we note that the coupling constant of the analogous hydrogens in 13 is exactly the same.10a We propose that the phenyl and methoxy groups in 9 are also cis, as it seems unlikely that the stereochemistry of phenyl and methoxy groups in 13, the only disilacyclobutene formed in the reaction of tetramesityldisilene with alkyne 1, would have completely converted to trans. Therefore, the stereochemistry in the three-membered rings cannot be responsible for the difference in the structures of 9 and 10. We conclude that the two disilacyclobutene stereoisomers are derived from the addition of the alkyne to different faces of the disilene. These two addition pathways would yield diastereomers that differ in stereochemistry only at the phenylsubstituted silicon center (Scheme 5). Scheme 5. Addition of Alkyne 1 to Disilene 4 To Yield Different Stereoisomers of the Disilacyclobutenes
The regiochemistry of the phenyl and methoxy groups in 7 and 8 is fully consistent with the formation of a biradical intermediate. The mechanism for the formation of 7−10 is shown in Scheme 6. The addition of alkyne 1 to disilene 4 forms biradical intermediate 14. Closure of the biradical leads to disilacyclobutenes 9 and 10. Alternatively, the cyclopropyl ring in 14 could open, yielding biradical 15. Ring closure in 15 results in the formation of 8. Since the stereochemistry of the cyclopropyl rings has not changed, the ring-opening rearrangement of biradical 14 is not reversible. The formation of both the seven-membered-ring and the four-membered-ring products indicates that closure of the singlet biradical competes effectively with ring-opening rearrangement. Given the similarity in the frameworks of compounds 7 and 8, we C
DOI: 10.1021/acs.organomet.9b00054 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
precipitate showed no substantial signals in the 1H NMR spectrum and is believed to be a lithium salt that was generated in the synthesis of 4. The 1H NMR spectrum of the crude mixture revealed the presence of compounds 8, 9, and 10 in a ratio of 1.0:0.8:1.5, respectively. During preparation for TLC, the mixture was dissolved in DCM under the ambient atmosphere, which caused a color change from red to colorless. After separation of the sample by preparative TLC (hexanes/CH2Cl2, 70:30), compounds 7−10 were obtained. Approximately 40% of the sample applied to the plate was recovered. Purity was assessed by 1H NMR spectroscopy. While compounds 7− 10 were separated by chromatography, none of the compounds were obtained analytically pure. All of the major signals observed in the 1H NMR spectrum of the crude reaction mixture were accounted for in the isolated products.
Scheme 6. Proposed Mechanism for the Formation of 7−10
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believe that 8 rearranges to 7 via a 1,3-hydrogen shift during chromatography. A zwitterionic mechanism is not consistent with the structures elucidated for 7 and 8. If the reaction were to proceed via a zwitterion derived from nucleophilic attack by the alkyne on the disilene,17 then the cyclopropyl ring would regioselectively open toward the methoxy substituent, and the regiochemistry of the phenyl and methoxy substituents of 7 and 8 would be reversed. Although we cannot formally exclude the possibility that 9 and 10 are formed via a concerted [2 + 2] cycloaddition or from nucleophilic attack by the disilene on the alkyne, there is no evidence that supports or suggests such reaction pathways.10b,17
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00054. NMR chemical shift comparison tables, listing of NMR data, NMR spectra, and X-ray data (PDF) Accession Codes
CCDC 1893620 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
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CONCLUSION The addition of cyclopropyl alkyne 1 to disilene 4 yielded compounds 7−10. The regiochemistry of the phenyl and methoxy substituents, originating from 1, provides strong support for the formation of a biradical intermediate. This result is consistent with the findings from the study of the addition of 1 to tetraaryldisilene 2 and tetrasilyldisilene 3 and contradicts the earlier proposal of a zwitterionic intermediate.5 Evidently, the difference between a phenyl substituent and a Tip substituent is not sufficient to polarize the disilene bond to alter the mechanism of alkyne addition. To proceed through a zwitterionic intermediate, we propose that a non-aryl substituent, such as a fluoro or an amino substituent, would have to be utilized to sufficiently polarize the SiSi bond and change the nature of the intermediate.18
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ASSOCIATED CONTENT
S Supporting Information *
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
David Scheschkewitz: 0000-0001-5600-8034 Kim M. Baines: 0000-0001-6515-5749 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of Western Ontario for financial support. We also thank Nada Tashkandi for the synthesis of alkyne 1.
EXPERIMENTAL SECTION
General Methods and Instrumentation. All reactions were conducted under a nitrogen atmosphere using an MBraun Labmaster 130 glovebox. Precoated silica glass plates suitable for preparative thin-layer chromatography were purchased from Millipore Sigma. Solvents and reagents were purified by standard methods.19 Disilene 4 was prepared in the Scheschkewitz lab using the published procedure.20 NMR data were obtained on a 600 MHz INOVA NMR spectrometer. The standards used were as follows: residual C6D5H (7.15 ppm) for 1H NMR spectra; C6D6 (128.00 ppm) for 13C NMR spectra; external TMS (0.0 ppm) for 29Si NMR spectra. J values are reported in hertz. ESI-MS was performed on a Bruker microTOF II mass spectrometer with an electrospray interface in positive ion mode (reported as mass-to-charge ratios, m/z). Addition of Probe 1 to Asymmetric Disilene 4. Disilene 4 (180 mg, 0.24 mmol) and alkyne 1 (44 mg, 0.24 mmol) were dissolved in hexanes (∼10 mL) to give a deep-red-orange solution. The mixture was allowed to stir for 18 h. During this time, the deepred-orange color of the solution faded to orange with a pale-yellow precipitate. The precipitate was removed by centrifugation before the solvent was removed in vacuo (mass recovery = 96.5%). The
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