Article pubs.acs.org/Organometallics
Reactivity of Cyclic Silenolates Revisited Michael Haas,* Mario Leypold,* Lukas Schuh, Roland Fischer, Ana Torvisco, and Harald Stueger Institute of Inorganic Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria S Supporting Information *
ABSTRACT: The stable exocyclic silenolates 2a−c (2a, R = Mes; 2b, R = o-Tol; 2c, R = 1-Ad) were fully characterized by NMR and UV−vis spectroscopy. According to spectroscopic and structural features, 2a−c are best described as acyl silyl anions (tautomeric structure I) in solution. This behavior is also reflected by the reaction of 2a,c with MeI. Both alkylation reactions take place at the corresponding silicon atom and lead to the formation of the methylated structures 4a,b in nearly quantitative yields. Furthermore, the thermal stability of exocyclic silenolates 2a,c was investigated. In the case of 2a, a thermally induced intramolecular sila-Peterson alkenation was observed at 60 °C. This transformation allowed straightforward access to 2-oxahexasilabicyclo[3.2.1]octan-8-ide 5 as a structurally complex, bicyclic silicon framework. In contrast to that, heating of 2c, as an example of an alkyl-substituted silenolate, led to an unexpected degradation to uncharacterized polymers. However, we were able to isolate the 1-adamantyl-substituted, bicyclic compound 8, which is structurally closely related to 5, by the treatment of 1,4-dipotassium-1,4-bis(trimethylsilyl)cyclohexasilane with 1 equiv of 1-adamantoyl chloride. Again an intramolecular sila-Peterson alkenation is responsible for the formation of 8. The mechanism for this highly selective reaction sequence is outlined and supported by density functional theory (DFT) calculations, which highlight the thermodynamic driving force and the low activation barriers of this multistep transformation.
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INTRODUCTION The synthesis and characterization of silenolates is still a challenging endeavor. As in the case of metal enolates, two tautomeric structures for silenolates can be drawn: in the keto form the negative charge resides predominantly on the silicon atom (I), while in the enol form (II) the negative charge is primarily located on the oxygen atom (Scheme 1).1
thermal stability of Li silenolates prevented them from using this substance class for further derivatizations. Significant improvement was achieved by Ottosson et al. in replacing lithium with potassium as counterion.2g Potassium silenolates were found to be thermodynamically more stable and thus could be stored under an inert atmosphere and ambient temperature over a few months without degradation. These observations allowed Ottosson et al. some structural insights into K silenolates. They found that the keto form is the predominant tautomeric structure in K silenolates, which manifests as a strongly pyramidal central silicon atom with an elongated Si−C single bond.2g Further studies of BravoZhivotovskii and Apeloig et al. disclosed that the polarity of the solvent used influences the stability of silenolates enormously. In addition to this, they reported on the first synthesis, isolation, and X-ray molecular structures of two enol form silenolates, [(tBuMe2Si)2SiC(OLi)Ad and (tBu2MeSi)2Si C(OLi)Ad], by taking advantage of nonpolar solvent systems. These compounds display remarkably short Si−C bonds (between 1.819 and 1.823 Å) and consequently adopt a planar structure around the central silicon atom.2h Later on, our group demonstrated the possibility of synthesizing and characterizing cyclic silenolates. Furthermore, the reaction of 2a−c with different chlorosilanes R3SiCl was investigated. The reaction of iPr3SiCl with aryl-substituted
Scheme 1. Tautomeric Structures for Silenolates
The dominant structure of metal enolates is in general the enol form and preferably exists in the solid state as well as in solution.1 However, silenolates show in particular a significantly different tautomeric behavior. The position of this equilibrium is strongly influenced by the chosen alkali metal and the solvent system as well as the substituent at the carbonyl moiety.2 Lithium silenolates were explored and reported first in the early 1990s. In this context, four scientists made valuable contributions in this field, namely Bravo-Zhivotovskii,3 Apeloig,4 Ishikawa, and Ohshita.2a−e Unfortunately, the low © XXXX American Chemical Society
Received: July 18, 2017
A
DOI: 10.1021/acs.organomet.7b00540 Organometallics XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION UV−vis Spectroscopy and TDDFT-PCM Calculations. The potassium silenolates 2a−c were synthesized according to the same method as previously reported by our group.2i THF and Et2O were used as solvents to determine the charge transfer behavior for the longest wavelength absorption band.6 Figure 2 depicts the measured UV−vis spectra in THF and Et2O together with their calculated frontier Kohn−Sham orbitals. In order to examine the differences between aromatic and saturated substituents at the carbonyl moiety, the mesityland adamantyl-substituted derivatives 2a,c were investigated. The longest wavelength absorption bands were assigned to two conformational minimum structures due to the small energy differences in cyclohexasilane conformers, which is caused by the high flexibility of the polysilane framework.7 All UV−vis calculations were performed via TDDFT-PCM in THF as well as in Et2O at the B3LYP/6-31+G(d,p) level of theory. The obtained experimental and computational data are summarized in Table 1 and show reasonable agreement. Silenolates 2a,c both exhibit an intense absorption maximum in the region between 433 and 448 nm, which is red-shifted in the order 2c → 2a. The HOMOs of 2a,c (Figure 2) correspond mainly to the pz orbital of the silicon atom with little variation in shape and energy. Upon excitation, electron density is displaced into the π* orbital of the carbonyl moiety (LUMO or LUMO+1). In the LUMO of the aryl-substituted species 2a, our calculations additionally showed considerable conjugation of the carbonyl group and the aromatic π systems, which is not possible for the alkyl-substituted silenolate 2c. As a consequence of this, the empty orbitals (LUMO or LUMO +1) are energetically stabilized in the order 2c → 2a. This stabilization results in smaller excitation energies and in the observed bathochromic shift of the corresponding absorption bands. Therefore, silenolates 2a−c are best described as acyl silyl anions (keto form in Scheme 1) in solution irrespective of the nature of the R group attached to the carbonyl moiety. This is of particular interest, because the reaction site of these silenolates 2a−c with chlorosilanes as electrophiles was found to be different. While 2c reacts with an equimolar amount of i Pr3SiCl at 0 °C in THF to give the Si-silylated product 1d, the aromatic compounds 2a,b exclusively afforded the O-silylated silenes 3a,b under the same conditions (Scheme 2). Although calculated charges q(E1) from the natural population analysis (NPA) at the PCM(THF) B3LYP/6-31+G(d,p) level revealed a slightly higher negative charge placed on the central silicon atom Si1 in 2c (−0.16) in comparison to 2a,b (−0.12, −0.13), simple steric effects might be responsible for the observed regioselectivity.2b The same tendency was also found in the case of exocyclic germenolates.5 In addition to this, a different coordination of the potassium ion in the X-ray crystal structure of 2b (see Figure 1) was found, which we attribute to crystal-packing effects. This changed packing mode of the potassium ion in 2b can be explained by the well-known interaction of the potassium ions with the aryl groups.8 Moreover, this different coordination causes an altered ring conformation of the cyclohexasilane ring (twisted boat). Reactivity of 2a,c toward MeI. In the reaction of 2a,c with MeI, same reactivities in terms of reaction sites were found. In both cases, alkylation of the silicon atom was observed in nearly quantitative yields (Scheme 3). This observation can be based on the classical orbital control,
silenolates 2a,b allowed the formation of previously unknown silenes with exocyclic structures 3a,b.2i We have recently reported on a series of stable exocyclic germenolates.5 In contrast to germenolates, the structural features of exocyclic silenolates seem to be strongly influenced by the nature of the substituents attached to the carbonyl moiety (see Figure 1). Exocyclic germenolates do not show
Figure 1. ORTEP diagram for compound 2c (above) and 2b (1:1 adduct with 18-crown-6). Thermal ellipsoids are depicted at the 50% probability level. Hydrogen atoms are omitted for clarity.
similar substituent dependencies. On the basis of the observed structural features (NMR data and DFT calculations), they are best described as acyl germyl anions (keto form) with Ge−C single bonds, CO double bonds, and markedly pyramidal central Ge atoms. Interestingly, the reactivity of these germenolates versus chlorosilanes parallels that observed earlier for silenolates as described by Ohshita, Ishikawa, and our group.2b,i Due to the fact that our cyclic silenolates exhibit the same reactivity in comparison to the published germenolates, we investigated their electronic nature and chemical behavior in more detail. Furthermore, the thermal stability of 2a,c was explored and an anionic rearrangement cascade, specifically an intramolecular sila-Peterson alkenation followed by the trapping of the formed silene with an anionic oxygen nucleophile, was found to occur under the given reaction conditions. This allowed direct access to 2oxahexasilabicyclo[3.2.1]octan-8-ides as structurally complex silicon frameworks. Moreover, we report on the UV−vis spectroscopy of the cyclic silenolates 2a,c and their assignment according to TDDFT-PCM calculations in THF and Et2O at the B3LYP/6-31+G(d,p) level of theory. Finally, we focus on the reactivity of 2a,c toward MeI. B
DOI: 10.1021/acs.organomet.7b00540 Organometallics XXXX, XXX, XXX−XXX
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Figure 2. Measured UV−vis spectra and the calculated frontier Kohn−Sham orbitals in THF and Et2O (1 × 10−3 mol/L) at the TDDFT-PCM B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory.
Table 1. Experimental and TDDFT-PCM B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d,p) Calculated Absorption Bands λ in THF and Et2O, Extinction Coefficients ε, and Oscillator Strengths f for the K Silenolates 2a,c assignment 2a
2c
solvent
λmax,exp (nm) (ϵ (L mol−1 cm−1))
conformation
THF
448 (2576)
Et2O
445 (2393)
THF
438 (2571)
chair (60%) boat (40%) chair (60%) boat (40%) chair (61%)
429 440 434 445 418
Et2O
433 (2447)
boat (39%) chair (61%)
430 (0.0875) 422 (0.0622)
boat (39%)
435 (0.0869)
λmax,calc (nm) ( f) (0.1210) (0.1369) (0.1124) (0.1264) (0.0640)
pz → π*(CO/aryl)
HOMO → LUMO
pz → π*(CO/aryl)
HOMO → LUMO
pz → π*(CO)
HOMO → LUMO HOMO → LUMO+1
pz → π*(CO)
HOMO → LUMO HOMO → LUMO+1
silenolates in more detail. We immediately discovered that a new set of additional, clearly resolved signals grew in the 29Si NMR spectra when toluene solutions of 2a were stirred at 22 °C for several hours. Prolonged reaction times (usually more than 48 h) led to the formation of increasing amounts of further degradation products of unknown structure. Raising the temperature to 60 °C, however, not only resulted in an accelerated product formation but also enhanced the selectivity of this reaction. Thus, after a toluene solution of 2a was stirred for 5 h at 60 °C, the 29Si NMR spectrum of the resulting solution (compare Figure 3) was consistent with the formation of one major rearrangement product, namely the structurally
given the fact that the HOMOs of 2a,c are localized on the silicon atom of the corresponding silenolate. The same tendency was published in the case of acyclic silenolates by the groups of Ohshita and Ottosson earlier.2b,g The methylated silicon atom undergoes a significant low-field shift from −70 to −45 ppm, which is caused by the lower shielding of the methyl group in comparison to the trimethylsilyl group (see the Experimental Section). Thermal Behavior and Rearrangement Cascade of Silenolates 2a,c. Interestingly, the yield of silene 3a was drastically decreased when solutions of 2a were stored at room temperature for several hours prior to quenching. This observation encouraged us to investigate the stability of these C
DOI: 10.1021/acs.organomet.7b00540 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 2. Synthesis of Cyclic Silenolates 2a−c and Their Reactivity toward iPr3SiCl
Scheme 4. Rearrangement Cascade Leading to the Mesityl Substituted Carbanion 5 and Its Subsequent Conversion with MeI to 6
will be trapped by the present oxygen nucleophile intramolecularly. It is worth noting that this general type of silaPeterson alkenation was simultaneously introduced by Oehme, Apeloig, and Ishikawa.9 Kira and Scheschkewitz likewise exploited this method in order to obtain silenes in a stepeconomical way.10 A closely related pattern (first sila-aldol reaction) was previously found by us when 1,4-bis(mesitoyl) cyclohexasilane was reacted with equimolar amounts of KOtBu. Again this silenolate underwent a rearrangement to the same bicyclic structure.11 The 29Si NMR spectrum of the resulting solution together with the spectrum of 2a is depicted in Figure 3. In addition to the resonance lines arising from residual starting material and minor byproducts, this spectrum showed eight signals consistent with the presence of eight magnetically nonequivalent silicon atoms. Previous experiences with structurally related compounds allowed a qualitative assignment of the spectrum. The signal at −71.1 ppm is assigned to the bridgehead Si2 atom. The measured chemical shift value compares reasonably well with those observed for structurally related bicyclo[2.2.2]octasilanes.12 Furthermore, the incorporation of the oxygen atom into the cyclopolysilane framework breaks the symmetry of the ring. Therefore, 5 displays three SiMe2 signals in the typical range between −47 and −53 ppm as well as two resonance lines for the SiMe3 groups at −12.2 and −17.3 ppm. The signal of the second bridgehead silicon (Si5) and the endocyclic OSiMe2 group appear at a significantly lower field (δ(29Si) −21.1 and 2.5 ppm, respectively) due to the effect of the attached oxygen atom.13 Unfortunately, all attempts to crystallize 5 failed. Moreover, stored reaction solutions of 5 afforded increasing amounts of unidentified decomposition products. Concentration of the reaction solution in vacuo to facilitate crystallization also caused extensive product degradation. However, a definite proof for the proposed structure of 5 was finally obtained after derivatization. Upon the addition of MeI to a freshly prepared toluene solution of 5, the corresponding methylated bicyclic adduct 6 was formed in the diastereospecific ratio endo:exo = 2:1. The resulting isomeric mixture was subsequently isolated by preparative thin-layer chromatography. Analytical and spectroscopic data (for details consult the Experimental Section) clearly support the bicyclic structure of 6 and 5. The endo diastereomer of 6, furthermore, could be crystallized from acetone and was subjected to X-ray diffraction analysis.
Scheme 3. Reactivity of 2a,c toward MeI
highly interesting carbanion 2-oxahexasilabicyclo[3.2.1]octan-8ide 5. This observation obviously implicated that the silenolate 2a was only the kinetic product, which thermodynamically rearranged via a mild, selective silyl-migration cascade to the bicyclic carbanion 5 (Scheme 4). Representing the first example of an intramolecular sila-Peterson alkenation, the formed silene
Figure 3. 29Si NMR spectroscopy (INEPT, bb decoupled) of a toluene solution of 2a before (top) and after thermolysis for 5 h at 60 °C (bottom). D
DOI: 10.1021/acs.organomet.7b00540 Organometallics XXXX, XXX, XXX−XXX
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Organometallics This resulting molecular structure is depicted in Figure 4 along with selected bond distances and bond and dihedral angles.
Scheme 5. Reaction Cascade Leading to the Formation of the Adamantyl-Substituted Carbanion 7 and its HAbstraction Product 8
Figure 4. ORTEP diagram for compound (1R,5R,8R)-endo-6 (endo configuration). Thermal ellipsoids are depicted at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg) with estimated standard deviations: Si(1)− O(1) 1.661(1), Si(2)−O(1) 1.651(1), Si(1)−C(1) 1.953(1), Si(4)− C(1) 1.980(1), Si−Si (mean) 2.372; Si(1)−O(1)−Si(2) 141.0(1), Si(1)−Si(6)−Si(7) 98.1(1), Si(6)−Si(7)−Si(4) 99.8(1), Si(7)− Si(4)−Si(3) 107.7(1), Si(4)−Si(3)−Si(2) 104.8(1).
endo-6 crystallized in the monoclinic space group P21/c with unexceptional bond lengths and angles. The unit cell contains four molecules as a racemic mixture (see the Supporting Information). After a toluene solution of 2c (R = Ad), as an example of an alkyl-substituted silenolate, was stirred for 3 h at 60 °C, significant polymerization of the starting material was detected. Prolonged warming of the reaction solution led to complete degradation of 2c to uncharacterized material. Therefore, we assumed that, in the case of alkyl-substituted systems, the negative charge could not be distributed in the same way as in compound 5 and the primarily formed carbanion 7 reacted further under the applied reaction conditions (compare NPA charges in Computational Studies for aromatic and aliphatic carbanions: NPA charge (Bz-J) = −1.33 vs NPA charge (Ac-J) = −1.40). It is quite noteworthy that the 1-adamantyl-substituted bicyclic compound 8, which is structurally closely related to 5, was obtained as the final product upon treating 1,4dipotassium-1,4-bis(trimethylsilyl)cyclohexasilane with 1 equiv of 1-adamantoyl chloride (AdCOCl) and stirring the resulting solution for 30 min at 22 °C (Scheme 5). In accordance with our computational studies presented below, the formation of 8 can be rationalized by assuming the intermediate formation of the silanide 9, which subsequently rearranges to give the bicyclic carbanion 7. Apparently, 7 is very unstable, losing its intense red color within minutes presumably by the abstraction of one proton from the surrounding media to give 8 as the final product. 8 was isolated from the reaction mixture after extractive workup and preparative thin-layer chromatography (prep-TLC) as a colorless solid in 69% yield. The structure proposed for 8 is well established by the analytical data summarized in the Experimental Section and by the results of single-crystal X-ray diffraction studies (see the Supporting Information). It is worth mentioning that the same outcome was observed when 2 equiv of 1-adamantoyl chloride was reacted with the dianion. In this case, the primary aim was to synthesize the corresponding bisacylsilane.
Density functional theory (DFT) calculations were performed for the observed rearrangement reactions of monoacylcyclohexasilanyl anions to elucidate the reaction mechanism responsible for the highly selective formation of the bicyclic structures 5 and 7. To reduce the complexity of the simulated systems, we chose acetyl and benzoyl groups instead of the adamantoyl and the mesitoyl moieties together with a hydrosilane skeletal framework. The resulting model systems are depicted in Chart 1. Computational details can be found in the Experimental Section. Chart 1. Model Substances for DFT Calculations
The computed reaction profiles for the rearrangement of BzA and Ac-E are depicted in Figure 5. Our calculations demonstrate that the rearrangement of the alkyl- and arylsubstituted monoacyl anions follow identical reaction pathways indicated by similar activation energies for equal transformations with values consistently lower than ΔΔG° = 20.0 kcal mol−1. However, as shown in Scheme 5, alkyl-substituted cyclic silenolates are synthetically not accessible. The reaction cascade, therefore, starts directly from the anion Ac-E in that case. Scheme 6 presents the resulting overall reaction steps leading to the formation of the phenyl- and methyl-substituted bicyclic final products Bz-J and Ac-J, respectively, representing structures 5 and 7. For Bz-J, the most stable conformer Bz-A of the silenolate needs to rearrange into the boat conformation Bz-B for the initial 1,4-silyl migration with a calculated E
DOI: 10.1021/acs.organomet.7b00540 Organometallics XXXX, XXX, XXX−XXX
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Figure 5. Computed reaction profile for the rearrangement cascade of Bz-A and Ac-E calculated at the PCM(THF) B3LYP/6-311+G(2df,p)// B3LYP/6-31+G(d) level of theory. Relative free energy values ΔΔG° are given in relation to Bz-A and Ac-E, respectively, as starting points for the rearrangement cascade. Hydrogen atoms are omitted for better clarity in the depicted structures.
Scheme 6. Computed Rearrangement Cascade for the Formation of Aryl-Substituted 2-Oxahexasilabicyclo[3.2.1]octane-8-idesa
a Relative free activation barriers ΔΔGAE° are shown for the calculated benzoyl derivative Bz-A at the PCM(THF) B3LYP/6-311+G(2df,p)// B3LYP/6-31+G(d) level of theory.
activation energy of ΔΔG° = 25.8 kcal mol−1. Under formal Walden inversion of the configuration at the migrating silicon atom, this transformation is comparable to the well-studied Brook-type rearrangements in the literature and presumably proceeds intramolecularly.14 The calculated value of ΔΔG° = 13.8 kcal mol−1 for the subsequent pyramidal silyl anion inversion of Bz-C to Bz-D through the planar transition state TSBz‑C‑D is consistent with calculated inversion barriers of silyl anions, extensively studied by Flock and Marschner in 2002.15
In this context, three silyl substituents in position α to the negatively charged silicon atom of compound Bz-C are known to stabilize the negative charge via hyperconjugation and lower the conformational stability of this intermediate.15 After conformational isomerization of Bz-D to the rotamer Bz-E, intramolecular, nucleophilic attack of the silyl anion on the carbonyl moiety in Bz-E occurs with a relatively low activation barrier of ΔΔG° = 12.2 kcal mol−1. This step provides a straightforward access to the highly symmetric bicycle Bz-F, in F
DOI: 10.1021/acs.organomet.7b00540 Organometallics XXXX, XXX, XXX−XXX
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spectrometer in C6D6 or CDCl3 solution and referenced versus TMS using the internal 2H-lock signal of the solvent. The starting materials 1,1,4,4-tertakis(trimethylsilyl)octamethylcyclohexasilane and 1,4-dipotassium-1,4-bis(trimethylsilyl)octamethylcyclohexasilane were synthesized according to published procedures.17 HRMS spectra were run on a Kratos Profile mass spectrometer equipped with a solid probe inlet. Infrared spectra were obtained on a Bruker Alpha-P Diamond ATR Spectrometer from the solid sample. Melting points were determined using a Buechi 535 apparatus and are uncorrected. Elemental analyses were carried out on a Hanau Vario Elementar EL apparatus. UV absorption spectra were recorded on a PerkinElmer Lambda 5 spectrometer. Synthesis of 4a. A 300 mg portion (0.458 mmol, 1.00 equiv) of 1a was dissolved in 4 mL of THF and cooled to −78 °C, and 53.9 mg (0.481 mmol, 1.05 equiv) of KOtBu was added. After it was stirred for an additional 30 min, the mixture was warmed to room temperature and finally stirred for an additional 60 min (the reaction mixture turned dark red). At this time reaction control by NMR spectroscopy showed clean conversion of the starting material to the desired silenolate 2a, Me3SiOtBu, and Me3SiOSiMe3. Now the resulting red solution was cooled to 0 °C and 0.2 mL of MeI was added dropwise. The solution immediately turned yellow. After aqueous workup with 10 mL of 3% sulfuric acid the organic layer was separated and dried over Na2SO4 and the solvents were stripped off on a rotary evaporator. Drying under vacuum (0.001 mbar) and crystallization from acetone solution at −30 °C afforded 190 mg (70%) of the analytically pure acylcyclohexasilane 4a as yellow crystals. Mp: 120−121 °C. Anal. Found: C, 50.30; H, 9.25. Calcd for C25H56OSi8: C, 50.26; H, 9.45. 29Si NMR (CDCl3, TMS, ppm): −7.27, −8.82 (SiMe3); −37.33, −40.86 (SiMe2); −45.73 (SiCO); −131.32 (Si(SiMe3)2). 13C NMR (CDCl3, TMS, ppm): 251.45 (C O); 144.20, 137.75, 131.55, 128.70 (Mes-C); 21.03, 19.31 (Mes-CH3); 3.70, 3.61 (Si(CH3)3); −0.78, −1.72, −4.70, −5.27 (Si(CH3)2); −6.27 (SiCOMe). 1H NMR (CDCl3, TMS, ppm): 6.78 (s, 2H, Mes-H); 2.26 (s, 6H, Mes-CH3); 2.15 (s, 3H, Mes-CH3); 0.34−0.21 (s, 45H, Si(CH3)2, Si(CH3)3 and SiCOCH3). IR (neat): ν(CO) 1610 (m) cm−1. HRMS: calcd for [C25H56OSi8]•+ (M+), 596.2485; found, 596.2469. Synthesis of 4b. The same procedure as for 4a was followed with 1.00 g (1.50 mmol, 1.00 equiv) of 1c and 176 mg (1.56 mmol, 1.04 equiv) of KOtBu. Yield: 568 mg (62%) of 4b as colorless crystals. Mp: 166−169 °C. Anal. Found: C, 50.95; H, 9.83. Calcd for C26H60OSi8: C, 50.91; H, 9.86. 29Si NMR (C6D6, TMS, ppm): −7.79, −8.40 (SiMe3); −37.84, −41.62 (SiMe2); −45.45 (SiCO); −131.32 (Si(SiMe3)2). 13C NMR (C6D6, TMS, ppm): 245.74 (CO); 51.68 (Ad-C−CO); 36.68, 36.54 (Ad-CH2); 28.04 (Ad-CH); 3.54, 3.46 (Si(CH3)3); 1.03, 1.48, −4.21, −5.26 (Si(CH3)2); −3.24 (SiCOMe). 1 H NMR (CDCl3, TMS, ppm): 2.05, 1.77, 1.70, 1.65 (15H, Ad-H); 0.54 (s, 3H, Si(CH3)); 0.24, 0.23 (s, 18H, Si(CH3)3); 0.36, 0.26, 0.18, 0.13 (s, 6H each, Si(CH3)2). IR (neat): ν(CO) 1612 (m) cm−1. HRMS: calcd for [C26H60OSi8]•+ (M+), 612.2798; found, 612.2828. Thermolysis of 2a. A 300 mg (0.458 mmol, 1.00 equiv) portion of 1a and 127 mg (0.481 mmol, 1.05 equiv) of 18-crown-6 were dissolved in 4 mL of toluene and cooled to −78 °C. Subsequently, 53.9 mg (0.481 mmol, 1.05 equiv) of KOtBu was added and the resulting mixture was stirred at this temperature for an additional 30 min. After this time, the solution was warmed to room temperature and finally stirred until the starting material had been totally consumed (monitored by 29Si NMR spectroscopy). Now the resulting solution of 2a was stirred at 60 °C for 17 h, after which time complete conversion to the carbanionic species 5 was observed by 29Si NMR. Due to its high reactivity the product could not be crystallized from the reaction solution. Upon the attempted removal of the volatile components in vacuo, 1a decomposed to uncharacterized degradation products. 29 Si NMR (D2O, TMS, ppm): −2.49, −12.16 (SiMe3); −17.30, −47.91, −48.24, −52.03 (SiMe2); −21.14 (Si-OSiMe2); −71.03 (Siq). Synthesis of 6. A 500 mg (0.763 mmol, 1.00 equiv) portion of 1a and 212 mg (0.801 mmol, 1.05 equiv) of 18-crown-6 were dissolved in 10 mL of toluene and cooled to −78 °C. Subsequently, 89.9 mg (0.801 mmol, 1.05 equiv) of KOtBu was added and the resulting mixture was
which the negative charge becomes punctually localized on the oxygen atom (NPA charge −0.92). This fact subsequently leads to the breakup of the silicon−silicon framework of Bz-F and the former carbonyl oxygen becomes incorporated into the ring system, forming intermediate Bz-G (TSBz‑F‑G: ΔΔG° = 2.6 kcal mol−1). A second pyramidal silyl anion inversion of Bz-G with an inversion barrier of ΔΔG° = 14.1 kcal mol−1 affords substructure Bz-H. In the next step stereospecific elimination to the corresponding silene Bz-I again generates a highly silicophilic alkoxide as a potential nucleophile (NPA charge −1.33) with an activation barrier of ΔΔG° = 13.1 kcal mol−1. This step completes the intramolecular sila-Peterson alkenation. The final cyclization of the free alkoxide within silene Bz-I terminates the proposed reaction sequence and gives the exceptionally stable product Bz-J with an calculated overall reaction energy of ΔΔG° = −41.4 kcal mol−1. Moreover, this simulated reaction pathway emphasizes the need for warming in the case of aromatic substituents for a more selective reaction, because of the initial 1,4-silyl migration. This transformation is not required in compounds with aliphatic residues.
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CONCLUSION In summary, we investigated the UV−vis spectroscopic behavior of the cyclic silenolates 2a−c and could assign the longest wavelength transitions according to TDDFT-PCM calculations in THF and Et2O at the B3LYP/6-31+G(d,p) level of theory. We can state that silenolates 2a−c are best described as acyl silyl anions (keto form in Scheme 1) in solution, irrespective of the residue attached to the carbonyl function. This is also reflected in the reactivity of 2a,c toward MeI and the thermal rearrangement of 2a, which represents to the best of our knowledge the first example of an intramolecular silaPeterson alkenation followed by an intramolecular trapping reaction of the bound alcoholate. These rearrangement reactions gave rise to a highly selective route for the formation of 2-oxahexasilabicyclo[3.2.1]octan-8-ides 5 and 7 as a structurally complex silicon framework with aromatic as well as aliphatic residues. Hereby, silyl anions with cyclic structures were identified as global intermediates of this rearrangement cascade that happens to occur under given reaction conditions. These anionic species were fully characterized by NMR spectroscopy and X-ray crystallography after derivatization with MeI. Methylation of the monomesitylated anion 5 preferentially resulted in the regiospecific formation of the corresponding endo stereoisomer endo-6 in a ratio of 2:1. Density functional theory (DFT) calculations highlight the thermodynamic driving force and the low activation barriers during this rearrangement cascade with an overall reaction energy of about ΔΔG° = −40.0 kcal mol−1. Since most of the individual activation barriers are fairly small, these simulations could perfectly explain why the reaction proceeds smoothly with aliphatic substituents and why warming is essential for compounds with aromatic moieties caused by an initial, energetically unfavorable 1,4-silyl migration.
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EXPERIMENTAL SECTION
General Considerations. All experiments were performed under a nitrogen atmosphere using standard Schlenk techniques. Solvents were dried using a column solvent purification system.16 Commercial KOtBu (97%), MesCOCl (99%), and AdCOCl (98%), were used as purchased. 1H (299.95 MHz), 13C (75.43 MHz), and 29Si (59.59 MHz) NMR spectra were recorded on a Varian INOVA 300 G
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Organometallics
imaginary frequency, respectively. Single-point energy calculations were carried out on the B3LYP/6-31+G(d) optimized geometries at the B3LYP/6-311+G(2df,p) level of theory. Solvent effects were considered by using the self-consistent reaction field (SCRF) method based on the polarizable continuum model (PCM) for tetrahydrofurane (THF) and diethyl ether (Et2O) with dielectric constants ε(THF) = 7.4257 and ε(Et2O) = 4.2400, respectively.23 Vertical excitations of the conformational minimum structure of silenolates 2a,c were calculated via time-dependent DFT (TDDFT) calculations at the PCM(THF) or PCM(Et2O) TDDFT B3LYP/6-31+G(d,p)//B3LYP/ 6-31+G(d,p) level of theory.20,21,23,24 The relative Gibbs free energies (ΔΔG°) at 298.15 K and 1 atm are reported in kcal mol−1. Atomic charges were obtained by natural population analyses (NPA) calculations as implemented in the Gaussian09 program. All computed structures were visualized using the Gabedit software package.25 For more details see the Supporting Information. X-ray Crystallography. All crystals suitable for single-crystal X-ray diffractometry were removed from a vial and immediately covered with a layer of silicone oil. A single crystal was selected, mounted on a glass rod on a copper pin, and placed in the cold N2 stream provided by an Oxford Cryosystems cryostream. XRD data collection was performed for all compounds on a Bruker APEX II diffractometer with use of Mo Kα radiation (λ = 0.71073 Å) and a CCD area detector. Empirical absorption corrections were applied using SADABS.26,27 The structures were solved with use of the intrinsic phasing option in SHELXT and refined by the full-matrix least-squares procedures in SHELXL.28−30 The space group assignments and structural solutions were evaluated using PLATON.31,32 Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were located in calculated positions corresponding to standard bond lengths and angles. Whole molecule disorder, as observed in compound 8, was handled by modeling the occupancies of the individual orientations using free variables to refine the respective occupancy of the affected fragments (PART).33 Disordered positions were refined using 60/40 split positions. The distances between arbitrary atom pairs were restrained to possess the same value using the distance restraints (DFIX) to certain target values. Anisotropic Uij values of some atoms were restrained (ISOR) to behave more isotropically. Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC1561727 (4a), CCDC-1561726 (4b), CCDC-1529184 (6), and CCDC-1529182 (8).
stirred at this temperature for an additional 30 min. After this time, the solution was warmed to room temperature and finally stirred until the starting material had been completely consumed (monitored by 29Si NMR spectroscopy). After complete conversion the reaction solution was heated to 60 °C. After the silenolate 2a was completely converted to the carbanionic species 5 (monitored by 29Si NMR spectroscopy), an excess of MeI was added to the reaction solution. The reaction solution immediately turned colorless. After aqueous workup with 10 mL of 3% sulfuric acid, the organic layer was separated and dried over Na2SO4. After removal of the volatile components on a rotary evaporator, the product was chromatographed on a precoated TLC plate SIL G-200 UV254, with heptane as eluent, to give 427 mg (94%) of endo-6 and exo-6. Crystallization from acetone at −30 °C afforded 260 mg (57%) of the analytically pure endo-6 as colorless crystals. Data for endo-6 are as follows. Mp: 177−179 °C. Anal. Found: C, 50.45; H, 9.21. Calcd for C25H56OSi8: C, 50.26; H, 9.45. 29Si NMR (CDCl3, TMS, ppm): −7.54, −12.53 (SiMe3); −14.89 (Siq-OSiMe2); 7.31, −40.04, −42.07, −44.77 (SiMe2); −53.82 (Siq). 13C NMR (CDCl3, TMS, ppm): 144.65, 138.14, 134.77, 133.11, 131.57, 129.83 (aryl-C); 28.54, 28.47, 26.64 (aryl-CH3); 24.64 (C-CH3); 20.20 (−CH3); 4.51 (Si(CH3)2); 4.10 (Si(CH3)3); 2.67 (Si(CH3)2); 0.36 (Si(CH3)3; −0.48 (Si(CH3)2; −1.33 (Si(CH3)2); −2.04 (Si(CH3)2); −2.09 (Si(CH3)2); −3.08 (Si(CH3)2); −5.07 (Si(CH3)2). 1H NMR (CDCl3, TMS, ppm): 6.75 (s, 1H, aryl-H); 6.64 (s, 1H, aryl-H); 2.58 (s, 3H, aryl-CH3); 2.48 (s, 3H, aryl-CH3); 2.18 (s, 3H, aryl-CH3); 1.38 (s, 3H, C−CH3); 0.43 (s, 3H, Si(CH3)2); 0.41 (s, 3H, Si(CH3)2); 0.32 (s, 6H, Si(CH3)2); 0.29 (s, 3H, Si(CH3)2); 0.26 (s, 3H, Si(CH3)2); 0.15 (s, 12H, Si(CH3)2 and Si(CH3)3); 0.02 (s, 9H, Si(CH3)3); −0.06 (s, 3H, Si(CH3)2). HRMS: calcd for [C25H56OSi8]•+ (M+), 596.2485; found, 596.2516. Data for exo-6 are as follows. 29Si NMR (CDCl3, TMS, ppm): −7.50, −15.74 (SiMe3); 5.25 (Siq-OSiMe2); 13.18, −30.54, −32.98, −42.99 (SiMe2); −57.00 (Siq). Synthesis of 8. A solution of 1,4-dipotassium-1,4-bis(trimethylsilyl)octamethylcyclohexasilane in 10 mL of DME was freshly prepared from 500 mg (0.860 mmol, 1.00 equiv) of (Me3Si)2Si6Me8(SiMe3)2 and 203 mg (1.81 mmol, 2.10 equiv) of KOtBu. The solution was stirred until the starting material was completely consumed (monitored by NMR spectroscopy). Afterward the solvent was removed under reduced pressure and the resulting brownish oil was redissolved in 10 mL of toluene and slowly added to a solution of 179 mg (0.900 mmol, 1.05 equiv) of 1-adamantoyl chloride in 20 mL of diethyl ether at −78 °C. Subsequently, the red solution was stirred for another 30 min at −78 °C, warmed to room temperature, and finally stirred for an additional 30 min. After decolorization an aqueous workup with 100 mL of 3% sulfuric acid was performed. The organic layer was separated and dried over Na2SO4 and the solvents were stripped off on a rotary evaporator. Finally, the crude material was chromatographed on a precoated TLC-plate SIL G200 UV254, with heptane as eluent, to give 357 mg (69%) of 8. Mp: 218−222 °C. Anal. Found: C, 50.17; H, 9.68% Calcd for C25H58OSi8: C, 50.09; H, 9.75. 29Si NMR (C6D6, TMS, ppm): 8.57 (OSiMe2); 6.81 (Siq-OSiMe2); −7.36, −16.85 (SiMe3); −39.43, −43.33, −48.13 (SiMe2); −79.34 (Siq). 13C NMR (C6D6, TMS, ppm): 47.01, 37.53, 36.50, 29.49 (Ad-C); 35.34 (CH-Ad); 2.91, 0.32 (Si(CH3)3); 3.15, 2.55, −1.25, −2.27, −2.34, −2.70, −5.52, −6.55 (Si(CH3)2). 1H NMR (CDCl3, TMS, ppm): 1.97−1.58 (m, 16H, AdH and CH-Ad); 0.49, 0.42, 0.39, 0.36, 0.28, 0.26, 0.25, 0.17 (s, 3H each, Si(CH3)2); 0.34, 0.32 (s, 9H each, Si(CH3)3). HRMS: calcd for [C25H58OSi8]•+ (M+), 598.2642; found, 598.2646. Computational Studies. All computational studies were executed on a computing cluster with blade architecture using the Gaussian09 software package.18 Geometry optimizations of all structures (local minima (LMs) and transition states (TSs)) were performed in the gas phase with B3LYP as the hybrid density functional19,20 together with the 6-31+G(d) or 6-31+G(d,p) split-valence basis set of Pople and coworkers for all atoms.21 The connectivity of TS structures was confirmed by intrinsic reaction coordinate (IRC) analyses at the same computational level.22 All stationary points (LMs and TSs) were characterized by harmonic frequency calculations, yielding none or one
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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/acs.organomet.7b00540. NMR spectra, X-ray data, and details of computations (PDF) Cartesian coordinates of calculated structures (XYZ) Accession Codes
CCDC 1529182, 1529184, and 1561726−1561727 contain 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 emailing
[email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.H.:
[email protected]. *E-mail for M.L.:
[email protected]. ORCID
Michael Haas: 0000-0002-9213-940X H
DOI: 10.1021/acs.organomet.7b00540 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
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Harald Stueger: 0000-0001-8531-1964 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge support from NAWI Graz.
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DOI: 10.1021/acs.organomet.7b00540 Organometallics XXXX, XXX, XXX−XXX
