Opening the silole ring: Efficient and specific cleavage of the endo-C(sp²)-Si bond with AcOH/ROH system

Abstract

An efficient and specific cleavage of the endo-C(sp²)-Si bond of silole rings was developed. The stable silole ring was effectively opened in an unexpected regioselectivity by using an AcOH/ROH system, in which the AcOH behaved as a catalyst and the ROH as a nucleophile. Depending on the nature of the substituents at the 2- and 5-positions of the silole ring, the cooperative effect of the AcOH/ROH system exclusively cleaved one of the two endo-C-Si bonds to afford silylbutadiene derivatives.

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Introduction

Carbon-silicon bonds are key structural features of organic molecules, and thus selective cleavage of C–Si bonds has become one of the central issues in recent organic synthesis.¹ Several excellent methods have been developed for the cleavage of C–Si bonds, including the C(sp³)-Si bond^2–4 and the C(sp²)-Si bond,⁵ by applying either the transition-metal catalyzed protocol or the stoichiometric reaction with strong bases or acids. As our continuous interest in activation and synthetic applications of C–Si bonds,⁴ we commenced an investigation into possible methods to cleave the endo-C(sp²)-Si bonds of siloles, aiming at challenging the following facts: (1) A silole ring of the general type contains two kinds of C–Si bonds, the endo-C(sp²)-Si bond and the exo-C(sp³)-Si bond. Furthermore, if the silole ring is unsymmetrically substituted, two different endo-C(sp²)-Si bonds will be available; However, research on the cleavage of such C–Si bonds has not been reported in the literature, except a single case of 1,1-dimethoxysilole.⁶ (2) No method has been reported for the efficient opening of the well-known silole rings,⁷ which are much more stable than the linear non-conjugated C–Si bonds.^1,2 Here, we present a selective cleavage of the endo-C(sp²)-Si bond of silole using an AcOH/ROH catalysis system^8,9 in which the AcOH behaved as a catalyst and the ROH as a nucleophile.

This protocol lead to unexpected specific cleavage of the C(sp²)-Si bond of siloles substituted with different groups at the α and α′ positions (Scheme 1). A bifunctional mechanism of this AcOH/ROH system was theoretically elucidated as well.

Scheme 1 Specific cleavage of the endo-C(sp²)-Si bond of silole rings with AcOH/ROH bifunctional system.

Results and discussion

In our initial attempts to open the silole ring of 1a, we found that those known methods used for the cleavage of C–Si bonds in the literature do not work well on siloles.^1,4a–b,10 Surprisingly, however, we found that when a mixture of AcOH and CF₃CH₂OH was used, the silole ring could be opened via selective cleavage of one of the endo-C(sp²)-Si bonds. After careful evaluation of their ratios, we found that treating silole 1a with 15 mol% of AcOH in neat CF₃CH₂OH was the best reaction condition, affording the silylbutadiene 2aa as the sole product in 75% isolated yield (Entry 1, Table 1).

Table 1 Ring-opening of symmetrically substituted silole 1a

Entry	ROH (alcohol or phenol)	pKa	Diene 2; (iso. yield)
1	CF3CH2OH	12.5	2aa (75%)
2	CCl3CH2OH	12.8	2ab (48%)
3	CH3CH2OH	16.0	—
4	PhOH	9.95	2ac (72%)
5	p-MeO-PhOH	10.2	2ad (64%)
6	p-CF3-PhOH	8.56	2ae (82%)
7	p-NO2-PhOH	7.16	—
8	p-Ac-PhOH	8.05	—

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The pK_a values of the components involved were also critical for this transformation. CCl₃CH₂OH, which has a similar pK_a to CF₃CH₂OH, could generate the product 2ab, albeit in a lower yield (Entry 2), while alcohols with higher pK_a values, such as EtOH (Entry 3), were ineffective. Similarly, the reaction proceeded with various phenols, including PhOH, p-MeO-PhOH, and p-CF₃-PhOH, producing silylbutadienes 2ac–e in good yields (Entries 4–6). However, when phenols with stronger acidity, such as p-Ac- or p-NO₂-PhOH, were used, no reaction took place. When acetate salt such as Cu(OAc)₂ was used instead of AcOH, the same product 2a was obtained in similar yields.

Scheme 2 shows representative results of ring-opening reaction of symmetrically substituted siloles 1via cleavage of one of the two endo-C-Si bonds. All the ring-opening products, silylbutadienes 2b with all alkyl substituents, 2c with a fused ring, and phenyl-substituted 2d without substituents at the α and α′ positions of its butadienyl skeleton, were obtained in excellent yields from their corresponding siloles 1b–d. However, siloles 1e and 1f did not undergo ring-opening reactions even under severe reaction conditions, probably due to their chemically more stable conjugated structures and bulkiness.

Scheme 2 Ring-opening of symmetrically substituted silole derivatives 1 with CF₃CH₂OH.

In the cases of siloles with different substituents at the α and α′ positions, the above cleavage of the endo-C(sp²)-Si bonds might raise regioselectivity issues. However, only one regioisomer was obtained for all such unsymmetrically substituted siloles, as illustrated in Scheme 3. This method was even successfully applied for the ring-opening of structurally more complicated silole 3e to afford the product 4e in 65% isolated yield, also as the only isomer.

Scheme 3 Ring-opening of unsymmetrically substituted siloles 3.

The selectivity is in sharp contrast to the expected orientation according to the traditional manners for acid-catalyzed C(sp²)-Si bond cleavage,^4a–b,10 as shown in Scheme 4. Taking 3a as an example, the path through intermediate 5a should be disfavoured while the one through 5a′ should be favored, if considering the following facts that 1) β-effect from the SiMe₃ group in 5a′ is a potent stabilizer of the cationic 3-position; 2) the crowded gem-position bearing two silyl group visibly lowers the stability of 4a.

Scheme 4 Mismatch between experimentally observed and theoretically expected products.

DFT calculations at the b3lyp/6-31+g* level clearly support this view (Scheme 5).¹¹ In the NBO analysis of intermediate 5A and 5A′, energy of the donor–acceptor interaction indicates high efficiency of the β-effect from the silyl group in stabilizing the neighbouring cation (Part I);¹² this is probably the pivotal reason for the lower energy of 5A′ than that of 5A (Part II). Moreover, computed product analysis, i.e., comparison of the stability of the two possible products 4A and 4A′, shows the same trend (Part III). HOMO orbital analysis and charge calculation of silole also support the classical acid-catalyzed mechanism to afford 4′ (see ESI†). These results indicated that, thermodynamically and kinetically, the endo-Si-C(TMS) bond may be more reactive than the Si–C(H) one. Yet the experimental results are clearly opposite. This situation is somewhat similar to the fact that ipso-substitution of silyl arene through cleaving of C–Si bond was occasionally prevented by replacement of electrophilic substitutions in the para-position of silyl groups.¹³

Scheme 5 Second order perturbative estimates for donor–acceptor interaction by NBO analysis (Part I) and comparison of stability (Part II–III). (Energy data in Part I are stabilization energy from donor–acceptor interaction; energy data in Part II–III are total HF energy and those of 5A and 4A are set at zero, respectively).

This led us to the idea of an acid–base cooperative mechanism of catalysis (Scheme 6).^8,9 In this route, the acid–base complexation of AcOH with CF₃CH₂OH occurs at first.^14a Several plausible structures for AcOH-CF₃CH₂OH complexes are computed to be energetically advantageous. Indeed, the ¹H-NMR spectrum of a mixture of AcOH-CF₃CH₂OH (at the reaction concentrations) showed only a broad signal, and no signals corresponding to the OH group of AcOH or CF₃CH₂OH were observed, implying the existence of complexation and a very fast proton exchange equilibrium.^14b

Scheme 6 Proposed mechanism for the specific cleavage of the endo-C(sp²)-Si bond of siloles with the AcOH/ROH bifunctional system. (Energy of free AcOH and CF₃CH₂OH are set as zero).

The AcOH-CF₃CH₂OH complex would then react with siloles, acting as an acid–base bifunctional system (Scheme 7). The access of the complex to the silole ring is assumed to be controlled mainly by steric effect. In cases where the α and α′ substituents are sterically highly unsymmetrical, such as 3a, the AcOH-CF₃CH₂OH complex plane would approach the silole ring perpendicularly to the side of less steric repulsion (A vs. B), which should lead to the observed regio-specificity. On the other hand, if the α and α′ positions were less sterically different, the direction of the ring opening would be electronically controlled, as the reaction of 3f, which gave 4f (with strong stabilization from SiMe₃) instead of 4f′ (with weak stabilization from Me) as the sole product (Scheme 7).

Scheme 7 Electronically controlled ring-opening of silole 3f.

Conclusions

In this work, we have developed the first method for the specific cleavage of the endo-C(sp²)-Si bond of silole ring under mild conditions, utilizing a simple combination of catalytic AcOH in CF₃CH₂OH. This reaction also provides a novel synthetic method for silylbutadiene-containing conjugated compounds, which are attractive candidates in many aspects.^15,16 Further work to expand the scope of the reaction and to elucidate the reaction pathway with the help of theoretical and spectroscopic studies are in progress.

Acknowledgements

This work was supported by the Natural Science Foundation of China and the Major State Basic Research Development Program (2011CB808705), and a JSPS Research Fellowship (to C.W.). Mr Fei Zhao did some experimental work. Helpful discussions with Prof. Xiaoqing Zhu of Nankai University and Prof. Ryo Takita in the University of Tokyo are appreciated. A generous allotment of computer time from RICC (RIKEN) is gratefully acknowledged.

References

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Footnote

† Electronic supplementary information (ESI) available: Materials including experimental procedures, NMR spectra of all new products and computation details. See DOI: 10.1039/c1sc00356a

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