Reactions of an Isolable Dialkylsilylene with Aroyl Chlorides. A New Route to Aroylsilanes

The reactions of isolable dialkylsilylene 1 with aromatic acyl chlorides afforded aroylsilanes 3a–3c exclusively. Aroylsilanes 3a–3c were characterized by 1H-, 13C-, and 29Si-NMR spectroscopy, high-resolution mass spectrometry (HRMS), and single-crystal molecular structure analysis. The reaction mechanisms are discussed in comparison with related reaction of 1 with chloroalkanes and chlorosilanes.

During the course of our studies of the reactions of an isolable dialkylsilylene with various functional groups [26][27][28][29][30], we have found that the silylene inserts exclusively into the C-Cl bond of (1) The dithiane route applicable for the synthesis of a wider range of acylsilanes was studied by Brook et al. [24] and Corey et al. [25] at the same time in 1967 (Equation ( 2)).The defect of the method is the use of a toxic mercury compound for the hydrolysis of the silylated dithianes.
During the course of our studies of the reactions of an isolable dialkylsilylene with various functional groups [26][27][28][29][30], we have found that the silylene inserts exclusively into the C-Cl bond of aroyl chlorides providing rather exceptional aroyl(chloro)silanes that cannot be obtained via conventional methods.Very recently, an acyl(halo)silane was utilized to synthesize an isolable silenyllithium (Equation ( 3)) [31].aroyl chlorides providing rather exceptional aroyl(chloro)silanes that cannot be obtained via conventional methods.Very recently, an acyl(halo)silane was utilized to synthesize an isolable silenyllithium (Equation ( 3)) [31]. (3)

Synthesis and Characterization
The 1:1 reactions of dialkylsilylene 1 [32][33][34][35][36][37] with benzoyl and 4-substituted benzoyl chlorides 2a-2c at −30 °C afforded the corresponding benzoyl(chloro)silanes 3a-3c in high yields, indicating that the C(carbonyl)-Cl bond is much more reactive than the carbonyl group (Equation ( 4)) [38].No significant difference was observed in the reactivity among benzoyl chlorides 2a-2c.Even when an excess amount of 1 was used to a benzoyl chloride (2:1 mol ratio), the corresponding benzoyl (chloro)silane was obtained solely as the product.The expected reactions of 3 with silylene 1 would be prohibited due to the steric effects of bulky silylene moiety of 3. The reactions of 1 with alkanoyl chlorides like acetyl chloride and butanoyl chloride afforded complex reaction mixtures.Because simple alkanoyl chlorides are more reactive than aroyl chlorides, the products of the reactions between 1 and the alkanoyl chlorides may react further with 1 to give the unidentified products.

NMR Spectroscopy
In the 1 H-NMR spectra of 3a-3c, two singlet signals due to four trimethylsilyl groups were observed in the region of 0.1-0.3ppm [0.16, 0.30 (3a); 0.17, 0.29 (3b); 0.17, 0.29 (3c)], indicating that there are two types of trimethylsilyl groups because of their Cs symmetry of 3. In accord with the observation, two TMS carbon (δ ca.3.2 and 4.3 ppm) and silicon signals (δ ca.2.8 and 5.5 ppm) were observed in the 13 C-and 29 Si-NMR spectra of 3a-3c.The signals at 225.3 (3a), 224.5 (3b), and 224.8 (3c) ppm in 13 C-NMR spectra are ascribed to the carbonyl carbon signals, which are at higher field relative to the typical acylsilanes (ca.240 ppm) [39,40].However, these chemical shift values are significantly lower than those for typical ketones like benzophene (δ 196.7) and acetophenone (δ 198.2), indicating the unique electronic feature of acylsilanes.The 29 Si-NMR resonances due to the ring silicon of 3a-3c appear at the same chemical shifts of 27.8 ppm.

Molecular Structure Analysis
Molecular structures of compounds 3a-3c were determined by X-ray single-crystal diffraction analysis.Yellow single crystals of 3a-3c suitable for X-ray crystallography were obtained by slowly evaporating the solvent from their hexane solutions.The ORTEP drawing of compound 3a is depicted in Figure 1.Compound 3a was crystallized in space group P-1 with two crystallographically independent molecules in an asymmetric unit.The structural parameters of the two molecules in a

Synthesis and Characterization
The 1:1 reactions of dialkylsilylene 1 [32][33][34][35][36][37] with benzoyl and 4-substituted benzoyl chlorides 2a-2c at −30 • C afforded the corresponding benzoyl(chloro)silanes 3a-3c in high yields, indicating that the C(carbonyl)-Cl bond is much more reactive than the carbonyl group (Equation ( 4)) [38].No significant difference was observed in the reactivity among benzoyl chlorides 2a-2c.Even when an excess amount of 1 was used to a benzoyl chloride (2:1 mol ratio), the corresponding benzoyl(chloro)silane was obtained solely as the product.The expected reactions of 3 with silylene 1 would be prohibited due to the steric effects of bulky silylene moiety of 3. The reactions of 1 with alkanoyl chlorides like acetyl chloride and butanoyl chloride afforded complex reaction mixtures.Because simple alkanoyl chlorides are more reactive than aroyl chlorides, the products of the reactions between 1 and the alkanoyl chlorides may react further with 1 to give the unidentified products.aroyl chlorides providing rather exceptional aroyl(chloro)silanes that cannot be obtained via conventional methods.Very recently, an acyl(halo)silane was utilized to synthesize an isolable silenyllithium (Equation ( 3)) [31]. (3)

Synthesis and Characterization
The 1:1 reactions of dialkylsilylene 1 [32][33][34][35][36][37] with benzoyl and 4-substituted benzoyl chlorides 2a-2c at −30 °C afforded the corresponding benzoyl(chloro)silanes 3a-3c in high yields, indicating that the C(carbonyl)-Cl bond is much more reactive than the carbonyl group (Equation ( 4)) [38].No significant difference was observed in the reactivity among benzoyl chlorides 2a-2c.Even when an excess amount of 1 was used to a benzoyl chloride (2:1 mol ratio), the corresponding benzoyl (chloro)silane was obtained solely as the product.The expected reactions of 3 with silylene 1 would be prohibited due to the steric effects of bulky silylene moiety of 3. The reactions of 1 with alkanoyl chlorides like acetyl chloride and butanoyl chloride afforded complex reaction mixtures.Because simple alkanoyl chlorides are more reactive than aroyl chlorides, the products of the reactions between 1 and the alkanoyl chlorides may react further with 1 to give the unidentified products.

NMR Spectroscopy
In the 1 H-NMR spectra of 3a-3c, two singlet signals due to four trimethylsilyl groups were observed in the region of 0.1-0.3ppm [0.16, 0.30 (3a); 0.17, 0.29 (3b); 0.17, 0.29 (3c)], indicating that there are two types of trimethylsilyl groups because of their Cs symmetry of 3. In accord with the observation, two TMS carbon (δ ca.3.2 and 4.3 ppm) and silicon signals (δ ca.2.8 and 5.5 ppm) were observed in the 13 C-and 29 Si-NMR spectra of 3a-3c.The signals at 225.3 (3a), 224.5 (3b), and 224.8 (3c) ppm in 13 C-NMR spectra are ascribed to the carbonyl carbon signals, which are at higher field relative to the typical acylsilanes (ca.240 ppm) [39,40].However, these chemical shift values are significantly lower than those for typical ketones like benzophene (δ 196.7) and acetophenone (δ 198.2), indicating the unique electronic feature of acylsilanes.The 29 Si-NMR resonances due to the ring silicon of 3a-3c appear at the same chemical shifts of 27.8 ppm.

Molecular Structure Analysis
Molecular structures of compounds 3a-3c were determined by X-ray single-crystal diffraction analysis.Yellow single crystals of 3a-3c suitable for X-ray crystallography were obtained by slowly evaporating the solvent from their hexane solutions.The ORTEP drawing of compound 3a is depicted Benzoylsilanes 3a-3c, which are stable thermally with definite melting points and under moist air, were characterized by 1 H-, 13 C-, and 29 Si-NMR spectroscopy, high-resolution mass spectrometry (HRMS), and X-ray structure analyses.

NMR Spectroscopy
In the 1 H-NMR spectra of 3a-3c, two singlet signals due to four trimethylsilyl groups were observed in the region of 0.1-0.3ppm [0.16, 0.30 (3a); 0.17, 0.29 (3b); 0.17, 0.29 (3c)], indicating that there are two types of trimethylsilyl groups because of their Cs symmetry of 3. In accord with the observation, two TMS carbon (δ ca.3.2 and 4.3 ppm) and silicon signals (δ ca.2.8 and 5.5 ppm) were observed in the 13 C-and 29 Si-NMR spectra of 3a-3c.The signals at 225.3 (3a), 224.5 (3b), and 224.8 (3c) ppm in 13 C-NMR spectra are ascribed to the carbonyl carbon signals, which are at higher field relative to the typical acylsilanes (ca.240 ppm) [39,40].However, these chemical shift values are significantly lower than those for typical ketones like benzophene (δ 196.7) and acetophenone (δ 198.2), indicating the unique electronic feature of acylsilanes.The 29 Si-NMR resonances due to the ring silicon of 3a-3c appear at the same chemical shifts of 27.8 ppm.

Molecular Structure Analysis
Molecular structures of compounds 3a-3c were determined by X-ray single-crystal diffraction analysis.Yellow single crystals of 3a-3c suitable for X-ray crystallography were obtained by slowly evaporating the solvent from their hexane solutions.The ORTEP drawing of compound 3a is depicted in Figure 1.Compound 3a was crystallized in space group P -1 with two crystallographically independent molecules in an asymmetric unit.The structural parameters of the two molecules in a unit cell are similar but different in the torsion angles of C( 1)Si( 1)C( 17)O(1) and its equivalent, C( 24)Si( 6)C( 40  Similarly, compounds 3b and 3c were crystallized in space group P21/n and P-1 and their molecular structures are shown in Figures 2 and 3. A single crystal of 3b has two crystallographically independent molecules in the asymmetric unit, while that of 3c has one independent molecule.Their structural parameters are similar to those of 3a.Similarly, compounds 3b and 3c were crystallized in space group P21/n and P-1 and their molecular structures are shown in Figures 2 and 3 Similarly, compounds 3b and 3c were crystallized in space group P21/n and P-1 and their molecular structures are shown in Figures 2 and 3. A single crystal of 3b has two crystallographically independent molecules in the asymmetric unit, while that of 3c has one independent molecule.Their structural parameters are similar to those of 3a.
(5) (6) The diverse modes of the reactions between 1 with chloroalkanes [43] suggest a complex nature of the mechanisms.The reactions may be understood uniformly starting from initially formed Lewis acid-base complexes as shown in Scheme 1. From the complex, ionic cleavage of the C-Cl bond followed by recombination would yield an alkylchlorosilane such as 4 [43].The ionic mechanism is also applicable for the reaction of 1 with cyclopropylmethyl chloride, in which the intermediary cyclopropylmethyl cation or its equivalent 3-butenyl cation reacts with an extra silylene 1 forming 3butenylsilyl cation and then finally 6; the 3-butenylsilyl cation would be stabilized by the coordination of the terminal π bond.Chloroalkanes with less electron donating substituents like CHCl3 and CCl4 destabilize the carbocation intermediates and instead yield 5 after the homolysis of the C-Cl bond [54].
The aroylation of 1 may occur concertedly from the acylsilane-silylene complex as shown in Equation (7).Alternatively, the facile heterolysis of the C(carbonyl)-Cl bond from the complex followed by the coupling in cage may occur exclusively; the silylene serves as a Lewis acid to activate the C(carbonyl)-Cl bond (Equation ( 7)).The former concerted mechanism is preferred to the latter because of the similarity of the reactions with those of chlorislanes with 1 [50,51]. (5)
(5) (6) The diverse modes of the reactions between 1 with chloroalkanes [43] suggest a complex nature of the mechanisms.The reactions may be understood uniformly starting from initially formed Lewis acid-base complexes as shown in Scheme 1. From the complex, ionic cleavage of the C-Cl bond followed by recombination would yield an alkylchlorosilane such as 4 [43].The ionic mechanism is also applicable for the reaction of 1 with cyclopropylmethyl chloride, in which the intermediary cyclopropylmethyl cation or its equivalent 3-butenyl cation reacts with an extra silylene 1 forming 3butenylsilyl cation and then finally 6; the 3-butenylsilyl cation would be stabilized by the coordination of the terminal π bond.Chloroalkanes with less electron donating substituents like CHCl3 and CCl4 destabilize the carbocation intermediates and instead yield 5 after the homolysis of the C-Cl bond [54].
The aroylation of 1 may occur concertedly from the acylsilane-silylene complex as shown in Equation (7).Alternatively, the facile heterolysis of the C(carbonyl)-Cl bond from the complex followed by the coupling in cage may occur exclusively; the silylene serves as a Lewis acid to activate the C(carbonyl)-Cl bond (Equation ( 7)).The former concerted mechanism is preferred to the latter because of the similarity of the reactions with those of chlorislanes with 1 [50,51].
The diverse modes of the reactions between 1 with chloroalkanes [43] suggest a complex nature of the mechanisms.The reactions may be understood uniformly starting from initially formed Lewis acid-base complexes as shown in Scheme 1. From the complex, ionic cleavage of the C-Cl bond followed by recombination would yield an alkylchlorosilane such as 4 [43].The ionic mechanism is also applicable for the reaction of 1 with cyclopropylmethyl chloride, in which the intermediary cyclopropylmethyl cation or its equivalent 3-butenyl cation reacts with an extra silylene 1 forming 3-butenylsilyl cation and then finally 6; the 3-butenylsilyl cation would be stabilized by the coordination of the terminal π bond.Chloroalkanes with less electron donating substituents like CHCl 3 and CCl 4 destabilize the carbocation intermediates and instead yield 5 after the homolysis of the C-Cl bond [54].The insertion reactions of silylene 1 into the Si-Cl bonds of chlorosilanes have been found to occur cleanly [49,50]; hence, the concerted mechanism via three-membered cyclic transition states has been proposed.The mechanism has been supported by the detailed DFT calculations [55][56][57]. (8)

General Procedures
Manipulation of air-sensitive compounds was performed under a controlled dry argon atmosphere using standard Schlenk techniques.Tetrahydrofuran (THF), hexane, and toluene were distilled from sodium-benzophenone.All the other reagents were obtained from commercial suppliers and used without further purification.Dialkylsilylene 1 was prepared according to literature procedures [32]. 1 H-(400 MHz), 13 C-(100.6MHz), and 29 Si-(79.5 MHz) NMR spectra were recorded on a Bruker AV-400 spectrometer at room temperature (Bruker, Rheinstetten, Germany), using CDCl3 as the solvent.Melting points are uncorrected.High-resolution mass spectra (HRMS) were recorded on a Bruker The aroylation of 1 may occur concertedly from the acylsilane-silylene complex as shown in Equation (7).Alternatively, the facile heterolysis of the C(carbonyl)-Cl bond from the complex followed by the coupling in cage may occur exclusively; the silylene serves as a Lewis acid to activate the C(carbonyl)-Cl bond (Equation ( 7)).The former concerted mechanism is preferred to the latter because of the similarity of the reactions with those of chlorislanes with 1 [50,51].The insertion reactions of silylene 1 into the Si-Cl bonds of chlorosilanes have been found to occur cleanly [49,50]; hence, the concerted mechanism via three-membered cyclic transition states has been proposed.The mechanism has been supported by the detailed DFT calculations [55][56][57].
Scheme 1. Mechanisms of the reactions of 1 with chloroalkanes.
The insertion reactions of silylene 1 into the Si-Cl bonds of chlorosilanes have been found to occur cleanly [49,50]; hence, the concerted mechanism via three-membered cyclic transition states has been proposed.The mechanism has been supported by the detailed DFT calculations [55][56][57]. (8)

General Procedures
Manipulation of air-sensitive compounds was performed under a controlled dry argon atmosphere using standard Schlenk techniques.Tetrahydrofuran (THF), hexane, and toluene were distilled from sodium-benzophenone.All the other reagents were obtained from commercial suppliers and used without further purification.Dialkylsilylene 1 was prepared according to literature procedures [32]. 1 H-(400 MHz), 13 C-(100.6MHz), and 29 Si-(79.5 MHz) NMR spectra were recorded on a Bruker AV-400 spectrometer at room temperature (Bruker, Rheinstetten, Germany), using CDCl3 as the solvent.Melting points are uncorrected.High-resolution mass spectra (HRMS) were recorded on a Bruker Daltonics Apex-III spectrometer (Bruker, Rheinstetten, Germany).

General Procedures
Manipulation of air-sensitive compounds was performed under a controlled dry argon atmosphere using standard Schlenk techniques.Tetrahydrofuran (THF), hexane, and toluene were distilled from sodium-benzophenone.All the other reagents were obtained from commercial suppliers and used without further purification.Dialkylsilylene 1 was prepared according to literature procedures [32]. 1 H-(400 MHz), 13 C-(100.6MHz), and 29 Si-(79.5 MHz) NMR spectra were recorded on a Bruker AV-400 spectrometer at room temperature (Bruker, Rheinstetten, Germany), using CDCl 3 as the solvent.Melting points are uncorrected.High-resolution mass spectra (HRMS) were recorded on a Bruker Daltonics Apex-III spectrometer (Bruker, Rheinstetten, Germany).

X-ray Crystallography
The diffraction data of 3a-3c were collected on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).All of the data were collected at ambient temperatures, and the structures were solved via the direct method and subsequently refined on F 2 using full-matrix least-squares techniques (SHELXTL) [58].Absorption corrections were applied empirically using the SADABS program [59].The non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located at calculated positions.A summary of the crystallographic data and selected experimental information is given in Table S1.

Conclusions
Isolable dialkylsilylene 1 was found to react with the C(carbonyl)-Cl bonds in aroyl chlorides 2 at low temperatures highly chemoselectively to give aroyl(chloro)silanes 3; the carbonyl groups in neither 2 nor 3 react with silylene 1.The structural analysis using NMR and X-ray crystallography indicate the lower field 13 C-NMR resonance of the carbonyl carbon and longer Si-C(carbonyl) bond distance than the standard values.The facile and highly selective nature of the reactions suggests that the insertion occurs concertedly from the initial Lewis acid-base complexes, similarly to that of 1 into the Si-Cl bonds in chlorosilanes.We are hoping the present synthetic methodology is applicable in general for a wide variety of silylenes.The silylenes should be however relatively long-lived and their reactions with the aroyl chlorides should be fast enough to prevent their oligomerization.Further works on the acylsilanes with unique electronic properties are under progress in our laboratory.

Scheme 1 .
Scheme 1. Mechanisms of the reactions of 1 with chloroalkanes.

Scheme 1 .
Scheme 1. Mechanisms of the reactions of 1 with chloroalkanes.