Communication pubs.acs.org/Organometallics
Influence of the Silyl Group on the Reactivity of Some OrthoLithiated Aryl Alkyl Sulfides Krzysztof Durka,† Tomasz Kliś,*,† Janusz Serwatowski,† and Krzysztof Woźniak‡ †
Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland Warsaw University, Faculty of Chemistry, Pasteura 1, 02-093 Warsaw, Poland
‡
S Supporting Information *
ABSTRACT: The lithiation of brominated aryl (α-dimethylsilyl)alkyl sulfides in diethyl ether produces stable heterocyclic silanes, which were characterized by 1H, 13C, and 29Si NMR spectroscopy and by X-ray crystallography. The reaction involves the intramolecular attack of the phenyl carbanion on the silicon atom with the formation of a pentacoordinated silicon intermediate. The stability of the formed intermediate depends on the solvent. It decomposes easily in THF at −78 °C with Si−C bond cleavage; however, it is stable in diethyl ether at room temperature. Addition of water results in the Si− H bond cleavage, while the heterocyclic ring containing the silicon atom is conserved.
D
Scheme 1. Influence of the Solvent on the Lithiation of 1
irected lithiation of substituted arenes is widely used for functionalization of aromatic systems; however, with the appropriate substrates, competition between possible reaction pathways could lead to the formation of a mixture of products.1−5 We have already demonstrated that the selectivity of the lithiation of aryl benzyl ethers or sulfides is strongly influenced by the competitive deprotonation at the benzylic position.6−8 Benzylic deprotonation in these compounds occurs easily in THF at −78 °C, and the substituents bonded to the phenyl ring strongly influence the rate of the reaction. The effect of the substituent on benzylic deprotonation has been determined using quantum calculations of proton transfer reactions as well as kinetic studies of benzylic deprotonation of sulfides.9,10 The thermodynamic stability of the benzylic anion may result in the isomerization of the ortho-lithiated aryl benzyl sulfides to the respective benzyllithium derivatives.11 In this work we report on the generation of aryllithiums from aryl alkyl sulfides containing a −SiMe3 or −SiHMe2 group bonded to an aliphatic carbon atom. As the C−Si bond is polarized toward the carbon, the silyl group simultaneously exhibits two properties: the ability to stabilize carbanions on the α carbon and the susceptibility to be attacked by nucleophiles. We hypothesize that the presence of the silyl group will significantly influence the reactivity of the ortho-lithiated sulfides. We were interested in studying the selectivity of the lithiation of 1 depending on the solvent. The reaction in diethyl ether was accomplished using a stepwise protocol which involved lithiation with t-BuLi at −78 °C followed by quenching of the reaction mixture with Me2HSiCl (Scheme 1). The reaction afforded exclusively 2a. Its structure was confirmed by a 1H NMR spectrum, which showed a septet at 4.63 ppm (Si−H, J = 3.6 Hz) and a singlet at 3.77 ppm (Cbenz− H) (1:1) as well as two doublets at 0.46 and 0.45 ppm (SiH(CH3)2, J = 3.6 Hz) and a singlet at 0.13 ppm (Si(CH3)3). This result indicates that 1-Li, formed in the first step, is stable © 2013 American Chemical Society
in diethyl ether at −78 °C. The reaction in THF was carried out similarly to that in diethyl ether; however, we used MeI as the electrophile. After treatment of 1 with t-BuLi (2 equiv) in THF, we expected facile intramolecular benzylic deprotonation of the initially formed 1-Li to give 1-Li′. Unexpectedly, the analysis of the 1H NMR spectrum of the isolated product revealed the formation of a mixture of 2b and 2c (molar ratio 3.5:1). Whereas the formation of 2b resulted from a hydrogen [1-4] migration affording 1-Li′ from 1-Li, the formation of 2c may be attributed to a silicon [1-4] migration with the formation of the intermediate 1-Li″. We speculate that, similarly to the Brook rearrangement, the isomerization of 1Li to 1-Li″ may occur via a pentacoordinate silicon-containing intermediate.12,13 However, in contrast to that process, the driving force in our system is the formation of a stable benzylic carbanion. We next turned our attention to a study of the lithiation of 3, which contains a −SiHMe2 group. In contrast to the lithiation of 1, the treatment of 3 with t-BuLi (2 equiv) in THF resulted in a smooth silicon [1-4] migration, affording 2a after addition Received: March 20, 2013 Published: May 28, 2013 3145
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Organometallics
Communication
should be noted that the formation of 5 from 3 can become an alternative route for the synthesis of cyclic silanes. Earlier papers have described the synthesis of this class of compounds by the reaction of the respective dilithium compounds with Me2SiCl2.17−21 Considering the possibility of the synthesis of various cyclosilanes, we decided to evaluate the reactivity of other sulfides containing the −SiHMe2 group. In contrast to the reactivity of 3, the lithiation of 6 followed by hydrolysis with water afforded 7, regardless of the solvent used (Scheme 3). The formation of 7 was confirmed by its 1H
of Me3SiCl as the electrophile (Scheme 2). On the basis of the previous results, we initially expected that lithiation of 3 in Scheme 2. Influence of the Solvent on the Lithiation of 3
Scheme 3. Proposed Mechanism of the Formation of 7 diethyl ether, followed by treatment of the reaction mixture with Me3SiCl, will result in the replacement of the bromine atom by the −SiMe3 group. Unexpectedly, after hydrolysis of the reaction mixture with water, we isolated 5 as a crystalline solid. The 1H NMR spectrum of 5 showed two singlets at 0.04 and 0.45 ppm (Si(CH3)2) and a singlet at 3.92 ppm (Cbenz−H) (3:3:1, respectively). The formation of 5 was confirmed by single-crystal X-ray diffraction (Figure 1). It can be assumed
NMR spectrum, which showed a singlet at 4.28 ppm (Cbenz− H2) as well as a septet at 4.65 ppm (Si−H, J = 4.0 Hz). We postulate that the [1-3] migration of the silyl group observed here is preceded by the formation of the intermediate 6-Li, containing a strained four-membered ring. The decomposition of 6-Li to 7-Li is dictated by the lowering of the energy barrier resulting from ring opening and formation of a stable benzylic anion. To obtain more information on the ability of the −SiHMe2 group to undergo intramolecular nucleophilic attack by the phenyl carbanion, we extended our study to the lithiation of two more elaborated sulfides, 8 and 10, containing two 2bromophenyl groups connected with various sulfur−carbon linkers with the −SiHMe2 group bonded to the aliphatic carbon atom. The reactions were carried out in diethyl ether at −78 °C using 4 equiv of t-BuLi followed by treatment of the reaction mixtures with Me3SiCl as the electrophile. Interestingly, we did not observe any incorporation of the −SiMe3 group into the molecules of the reactants. Lithiation of 8 afforded the eightmembered heterocyclic compound 9, whose structure was confirmed by single-crystal X-ray diffraction (Figure 2). Considering the mechanism of the formation of 9, we assumed that the dilithiation of 8 should afford 8a followed by intramolecular five-membered ring closure to give 8b (Scheme 4). A second nucleophilic attack at the silicon atom affords 8c;
Figure 1. Labeling of atoms and estimation of their atomic thermal motion as anisotropic displacement parameters (50% probability level) for 5. Selected bond lengths (Å) and angles (deg): Si1−C2 = 1.86(10), Si1−C13 = 1.91(11), S1−C13 = 1.82(10), C1−S1 = 1.77(10); C2−Si1−C13 = 97.2(5), Si1−C13−S1 = 105.4(5), C1− C2−Si1−C13 = 16.2(9).
that, regardless of the solvent used, the lithiation of 3 involves Br−Li exchange followed by intramolecular attack of the carbanion on the silicon atom with the formation of the hypercoordinated intermediate 3-Li. The formation of stable pentacoordinated silicon compounds with five carbon ligands is well-documented, and the structure was confirmed by X-ray crystallography.14,15 The reactivity of 3-Li depends on the solvent. Interestingly, the in situ lithiation/silylation of 3 with tBuLi/Me3SiCl in THF at −78 °C afforded exclusively 2a, containing a −SiMe3 group in a benzylic position. This suggests the rapid formation and decomposition of 3-Li in THF at −78 °C with cleavage of the Si−Cbenzylic bond to give 4-Li. In order to check the stability of 3-Li, a solution of 3 in diethyl ether was treated with t-BuLi (2 equiv) at −78 °C. The reaction mixture was next slowly warmed to room temperature. The obtained reddish slurry was analyzed by 29Si{1H} NMR spectroscopy (room temperature, CDCl3 as the external standard). The 29 Si{1H} NMR spectrum revealed one peak at −59.9 ppm. This result was compared with the 29Si{1H} NMR spectrum of the isolated 5, which showed a resonance at 24.8 ppm. It is generally accepted that an increase of the silicon coordination number leads to a strong low-frequency shift.16 This suggests that 3-Li is stable in diethyl ether solution at room temperature and the decomposition to 5 occurs after addition of water. It
Figure 2. Labeling of atoms and estimation of their atomic thermal motion as anisotropy displacement parameters (50% probability level) for 9. Selected bond lengths (Å) and angles (deg): Si1−C12 = 1.89(27), Si1−C2 = 1.89(29), C12−C7 = 1.42(42), C2−C1 = 1.40(42), C7−S2 = 1.79(30), C1−S1 = 1.79(30), S2−C13 = 1.81(31), S1−C13 = 1.83(34); C12−Si1−C2 = 116.5(1), C12−C7−S2 = 118.6(2), C2−C1−S1 = 122.2(2), C7−S2−C13 = 98.2(1), C1−S1− C13 = 101.8(1), C2−Si1−C12−C7 = 65.4(3). 3146
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Communication
We hope that the cyclization process presented in this work is general and can be extended to the synthesis of a broad range of cyclosilanes. Efforts to expand this new methodology are in progress.
Scheme 4. Proposed Mechanism of the Formation of 9
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ASSOCIATED CONTENT
S Supporting Information *
Text, a table, figures, and CIF files giving experimental procedures for all synthesized compounds, crystal data, data collection, and refinement parameters for 5, 9, and 11, 1H and 13 C NMR spectra, and crystallographic data for 5, 9, and 11. This material is available free of charge via the Internet at http://pubs.acs.org.
however, this intermediate is not reactive toward Me3SiCl. Addition of water to the reaction mixture results in decomposition of 8c to 9. Interestingly, lithiation/silylation of 10 afforded 11 after hydrolysis of the reaction mixture with water (Scheme 5). The
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AUTHOR INFORMATION
Corresponding Author
Scheme 5. Proposed Mechanism of the Formation of 11
*T.K.: tel, +48 22 2347575; fax, +48 22 6282741; e-mail,
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Warsaw University of Technology. The X-ray measurements of compounds 5, 9, and 11 were undertaken at the Crystallographic Unit of the Physical Chemistry Laboratory, Chemistry Department, University of Warsaw. Support from Aldrich Chemical Co., Milwaukee, WI, USA, through the donation of chemicals and equipment is gratefully acknowledged.
structure of 11 was confirmed by single-crystal X-ray diffraction (Figure 3). To explain this result, we postulate that the initially
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REFERENCES
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Figure 3. Labeling of atoms and estimation of their atomic thermal motion as anisotropic displacement parameters (50% probability level) for 11 (two independent molecules in the asymmetric part of the unit cell). Selected bond lengths (Å) and angles (deg): S2−C29 = 1.83(33), Si2−C29 = 1.89(34); C29−Si2−C17 = 137.7(1), S2−C30− C22 = 116.3(2), C30−C22−C17−Si2 = 0.4(4), S2−C30−C22−C17 = 39.1(4), C22−C17−Si2−C29 = 0.1(3).
formed 10a may prefer a six- over a four-membered-ring transition state, leading to the intermediate 10b. The thermodynamic stability of the six-membered 10b prevents the consecutive lithiation. We believe that steric hindrance may be responsible for the lack of reactivity of 10b toward Me3SiCl; however, the addition of water to the reaction mixture results in the protonation of 10b to give 11. In summary, the electrophilic properties of the Me2HSi− group can be utilized in the synthesis of heterocyclic silanes 5, 9, and 11. We believe that the cyclization occurs with the formation of stable pentacoordinated silicate anions. The stability of 3-Li in diethyl ether solution was confirmed by 29 Si NMR. However, these preliminary results require further studies concerning the influence of the solvent on the reaction course and the stability of the hypercoordinated intermediates. 3147
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