Stepwise Introduction of Different Substituents to α -Chloro- ω -hydrooligosilanes: Convenient Synthesis of Unsymmetrically Substituted Oligosilanes

: A series of unsymmetrically substituted oligosilanes were synthesized via stepwise introduction of different substituents to α -chloro- ω -hydrooligosilanes. The reactions of α -chloro- ω hydrooligosilanes with organolithium or Grignard reagents gave hydrooligosilanes having various alkyl, alkenyl, alkynyl and aryl groups. Thus-obtained hydrooligosilanes were converted into alkoxyoligosilanes by ruthenium-catalyzed dehydrogenative alkoxylation with alcohols.

In the cases of polymethylated oligosilanes 2 and 3, Grignard reagents are more suitable for the substitution of the chlorosilane moiety. The Grignard reagents were prepared in the presence of lithium chloride and DIBAL-H to activate magnesium metal [45]. As shown in Scheme 3, various alkyl, alkenyl, alkynyl and aryl groups were introduced, and substituted hydrooligosilanes were successfully synthesized. As alkyl group installation, 5-hexenyl-and 2-phenylethylmagnesium reagents were used to afford hydrotrisilanes 6 and 7. Mono-and disubstituted alkenyl groups such as styryl and 2-buten-2-yl ones were also introduced to the trisilane skeleton successfully. The stereochemistry of the alkene moieties of trisilanes 8 and 9 was determined by 1 H NMR spectroscopy. The E geometry of the styryl group in 8 was confirmed by the J value of the vinylic coupling (19 Hz). The major Z isomer of 9 was confirmed by NOE experiment. Three types of alkynyl Grignard reagents with aliphatic, aryl and silyl substituents were subjected to the reaction to afford alkynyltrisilanes 10-12. Various aryl groups were also introduced to 2 to afford phenyl-, 4-methoxyphenyl-, 4-dimethylaminophenyl-, and 2-thienyltrisilanes 13-16. The similar arylation was also applicable to 3, and 2-thienyltetrasilane 17 was obtained. All of these reactions proceed without loss of the hydrosilane moieties, which can be used for further modification of the hydrooligosilanes. The reason for low yields in some cases is attributed to loss during the isolation As shown in Scheme 2, the chlorosilane moiety of 1 was smoothly substituted by organolithium reagents. When 1 was treated with 2-thienyllithium, 2-thienyldisilane 4 was obtained in high yield. 4-Methoxyphenyl group was also introduced to afford 4-methoxyphenyldisilane 5. chlorosilane moiety. Furthermore, we have recently found ruthenium-catalyzed dehydrogenative alkoxylation of a hydrodisilane with alcohols [44]. Interestingly, the reactions proceed with preserving the Si-Si bond despite its susceptible nature to transition metals [37][38][39][40]. Combination of these two methods leads to convenient synthesis of unsymmetrically substituted oligosilanes. In this paper, we report the synthesis of various substituted hydrooligosilanes from α-chloro-ω-hydrooligosilanes and transformation of thus-obtained hydrooligosilanes to the corresponding alkoxy oligosilanes.

Synthesis of 1-Hydrooligosilanes
As reported previously, α-chloro-ω-hydrooligosilanes 1-3 are synthesized by the selective monoreduction of α,ω-dichlorooligosilanes with alkylmagnesium chlorides in the presence of a catalytic amount of TiCl4 (Scheme 1) [43]. The remaining chlorosilane moiety is possible functionality for introduction of various substituents. Scheme 1. Monoreduction of α,ω-dichlorooligosilanes with i-PrMgCl in the presence of a catalytic amount of TiCl4.
In the cases of polymethylated oligosilanes 2 and 3, Grignard reagents are more suitable for the substitution of the chlorosilane moiety. The Grignard reagents were prepared in the presence of lithium chloride and DIBAL-H to activate magnesium metal [45]. As shown in Scheme 3, various alkyl, alkenyl, alkynyl and aryl groups were introduced, and substituted hydrooligosilanes were successfully synthesized. As alkyl group installation, 5-hexenyl-and 2-phenylethylmagnesium reagents were used to afford hydrotrisilanes 6 and 7. Mono-and disubstituted alkenyl groups such as styryl and 2-buten-2-yl ones were also introduced to the trisilane skeleton successfully. The stereochemistry of the alkene moieties of trisilanes 8 and 9 was determined by 1 H NMR spectroscopy. The E geometry of the styryl group in 8 was confirmed by the J value of the vinylic coupling (19 Hz). The major Z isomer of 9 was confirmed by NOE experiment. Three types of alkynyl Grignard reagents with aliphatic, aryl and silyl substituents were subjected to the reaction to afford alkynyltrisilanes 10-12. Various aryl groups were also introduced to 2 to afford phenyl-, 4-methoxyphenyl-, 4-dimethylaminophenyl-, and 2-thienyltrisilanes 13-16. The similar arylation was also applicable to 3, and 2-thienyltetrasilane 17 was obtained. All of these reactions proceed without loss of the hydrosilane moieties, which can be used for further modification of the hydrooligosilanes. The reason for low yields in some cases is attributed to loss during the isolation procedure by column chromatography over silica gel. Scheme 2. Synthesis of substituted hydrodisilanes from 1-chloro-2-hydrodisilane.
In the cases of polymethylated oligosilanes 2 and 3, Grignard reagents are more suitable for the substitution of the chlorosilane moiety. The Grignard reagents were prepared in the presence of lithium chloride and DIBAL-H to activate magnesium metal [45]. As shown in Scheme 3, various alkyl, alkenyl, alkynyl and aryl groups were introduced, and substituted hydrooligosilanes were successfully synthesized. As alkyl group installation, 5-hexenyl-and 2-phenylethylmagnesium reagents were used to afford hydrotrisilanes 6 and 7. Mono-and disubstituted alkenyl groups such as styryl and 2-buten-2-yl ones were also introduced to the trisilane skeleton successfully. The stereochemistry of the alkene moieties of trisilanes 8 and 9 was determined by 1 H NMR spectroscopy. The E geometry of the styryl group in 8 was confirmed by the J value of the vinylic coupling (19 Hz). The major Z isomer of 9 was confirmed by NOE experiment. Three types of alkynyl Grignard reagents with aliphatic, aryl and silyl substituents were subjected to the reaction to afford alkynyltrisilanes 10-12. Various aryl groups were also introduced to 2 to afford phenyl-, 4-methoxyphenyl-, 4-dimethylaminophenyl-, and 2-thienyltrisilanes 13-16. The similar arylation was also applicable to 3, and 2-thienyltetrasilane 17 was obtained. All of these reactions proceed without loss of the hydrosilane moieties, which can be used for further modification of the hydrooligosilanes. The reason for low yields in some cases is attributed to loss during the isolation procedure by column chromatography over silica gel.

Dehydrogenative Alkoxylation of Hydrooligosilanes with Alcohols
As mentioned above, we have reported ruthenium-catalyzed dehydrogenative alkoxylation of a hydrodisilane with alcohols without Si-Si bond cleavage [44]. The reactions are also applicable to various hydrotrisilanes. As shown in Scheme 4, some of the hydrotrisilanes synthesized above were subjected to the ruthenium-catalyzed dehydrogenative alkoxylation with methanol. All reactions proceeded smoothly in toluene at room temperature in the presence of 2.5 mol % of [RuCl2(p-cymene)]2 to afford methoxytrisilanes 18-21 in good yields. It is worth noting that the alkenyl and alkynyl moieties tolerate the reactions. Neither hydrosilylation nor bis-silylation occurred under these reaction conditions. To gain further insight into the substituent effects in the ruthenium-catalyzed alkoxylation, ethyl-substituted hydrodisilane 22 and 2-hydrotrisilane 26 were used for the reactions with methanol. The results are summarized in Tables 1 and 2. Even though an excess amount of methanol (10 equiv) was used in the reaction of 22, the reaction rate is much slower than that of PhMe2SiSiMe2H, which finished within 2 h under the same reaction conditions [44]. When bulkier alcohols are used, the reaction rate becomes slower. The reaction of 22 with ethanol needed heating at 50 °C to be completed within one day. The reaction with a large excess of 2-propanol is much Scheme 3. Synthesis of substituted hydrooligosilanes from α-chloro-ω-hydrooligosilanes.

Dehydrogenative Alkoxylation of hydrooligosilanes with Alcohols
As mentioned above, we have reported ruthenium-catalyzed dehydrogenative alkoxylation of a hydrodisilane with alcohols without Si-Si bond cleavage [44]. The reactions are also applicable to various hydrotrisilanes. As shown in Scheme 4, some of the hydrotrisilanes synthesized above were subjected to the ruthenium-catalyzed dehydrogenative alkoxylation with methanol. All reactions proceeded smoothly in toluene at room temperature in the presence of 2.5 mol % of [RuCl 2 (p-cymene)] 2 to afford methoxytrisilanes 18-21 in good yields. It is worth noting that the alkenyl and alkynyl moieties tolerate the reactions. Neither hydrosilylation nor bis-silylation occurred under these reaction conditions.

Dehydrogenative Alkoxylation of Hydrooligosilanes with Alcohols
As mentioned above, we have reported ruthenium-catalyzed dehydrogenative alkoxylation of a hydrodisilane with alcohols without Si-Si bond cleavage [44]. The reactions are also applicable to various hydrotrisilanes. As shown in Scheme 4, some of the hydrotrisilanes synthesized above were subjected to the ruthenium-catalyzed dehydrogenative alkoxylation with methanol. All reactions proceeded smoothly in toluene at room temperature in the presence of 2.5 mol % of [RuCl2(p-cymene)]2 to afford methoxytrisilanes 18-21 in good yields. It is worth noting that the alkenyl and alkynyl moieties tolerate the reactions. Neither hydrosilylation nor bis-silylation occurred under these reaction conditions. To gain further insight into the substituent effects in the ruthenium-catalyzed alkoxylation, ethyl-substituted hydrodisilane 22 and 2-hydrotrisilane 26 were used for the reactions with methanol. The results are summarized in Tables 1 and 2. Even though an excess amount of methanol (10 equiv) was used in the reaction of 22, the reaction rate is much slower than that of PhMe2SiSiMe2H, which finished within 2 h under the same reaction conditions [44]. When bulkier alcohols are used, the reaction rate becomes slower. The reaction of 22 with ethanol needed heating at 50 °C to be completed within one day. The reaction with a large excess of 2-propanol is much more sluggish. Even though the reaction was carried out on heating, more than 40 h were needed for To gain further insight into the substituent effects in the ruthenium-catalyzed alkoxylation, ethyl-substituted hydrodisilane 22 and 2-hydrotrisilane 26 were used for the reactions with methanol. The results are summarized in Tables 1 and 2. Even though an excess amount of methanol (10 equiv) was used in the reaction of 22, the reaction rate is much slower than that of PhMe 2 SiSiMe 2 H, which finished within 2 h under the same reaction conditions [44]. When bulkier alcohols are used, the reaction rate becomes slower. The reaction of 22 with ethanol needed heating at 50 • C to be completed within one day. The reaction with a large excess of 2-propanol is much more sluggish. Even though the reaction was carried out on heating, more than 40 h were needed for complete consumption of 22. For the alkoxylation of 26, optimization of the ruthenium catalyst was necessary, as shown in Table 2. The reaction of 26 with methanol in the presence of the (p-cymene)ruthenium catalyst gave the desired 2-methoxytrisilane 27 in low yield along with monosilane 28, which was produced by Si-Si bond cleavage (Entry 1). Changing the aromatic ligand to mesitylene slightly improved the formation of 27, but the Si-Si bond cleavage still occurred significantly (Entry 2). In contrast, the (benzene)ruthenium catalyst gave 27 more selectively and suppressed the formation of 28 (Entry 3). Compared with the (arene)ruthenium complexes, RuHCl(CO)(PPh3)3 showed little catalytic activity, and no alkoxylation product was detected in the reaction mixture (Entry 4).
The superior performance of the (benzene)ruthenium catalyst over the (p-cymene)ruthenium catalyst might be attributed to the less steric hindrance around the coordinated arenes. The intermediate of the reaction might be the hydrosilane-bound ruthenium complex via σ-coordination or oxidative addition of the Si-H bond to the ruthenium atom. Nucleophilic attack of methanol to the silicon atom having the Si-H bond produces 27. At this step, more crowded p-cymene prevents methanol from attacking the central silicon atom. As a result, methanol attacks the terminal silicon atom of 26 to afford 28 via Si-Si bond cleavage.

Entry
Alcohol (  For the alkoxylation of 26, optimization of the ruthenium catalyst was necessary, as shown in Table 2. The reaction of 26 with methanol in the presence of the (p-cymene)ruthenium catalyst gave the desired 2-methoxytrisilane 27 in low yield along with monosilane 28, which was produced by Si-Si bond cleavage (Entry 1). Changing the aromatic ligand to mesitylene slightly improved the formation of 27, but the Si-Si bond cleavage still occurred significantly (Entry 2). In contrast, the (benzene)ruthenium catalyst gave 27 more selectively and suppressed the formation of 28 (Entry 3). Compared with the (arene)ruthenium complexes, RuHCl(CO)(PPh3)3 showed little catalytic activity, and no alkoxylation product was detected in the reaction mixture (Entry 4).
The superior performance of the (benzene)ruthenium catalyst over the (p-cymene)ruthenium catalyst might be attributed to the less steric hindrance around the coordinated arenes. The intermediate of the reaction might be the hydrosilane-bound ruthenium complex via σ-coordination or oxidative addition of the Si-H bond to the ruthenium atom. Nucleophilic attack of methanol to the silicon atom having the Si-H bond produces 27. At this step, more crowded p-cymene prevents methanol from attacking the central silicon atom. As a result, methanol attacks the terminal silicon atom of 26 to afford 28 via Si-Si bond cleavage.

Materials and Methods
All reactions were carried out under an argon atmosphere using standard Schlenk techniques unless otherwise noted. THF and diethyl ether were distilled from sodium benzophenone ketyl under a nitrogen atmosphere. Toluene was distilled from sodium under a nitrogen atmosphere. For the alkoxylation of 26, optimization of the ruthenium catalyst was necessary, as shown in Table 2. The reaction of 26 with methanol in the presence of the (p-cymene)ruthenium catalyst gave the desired 2-methoxytrisilane 27 in low yield along with monosilane 28, which was produced by Si-Si bond cleavage (Entry 1). Changing the aromatic ligand to mesitylene slightly improved the formation of 27, but the Si-Si bond cleavage still occurred significantly (Entry 2). In contrast, the (benzene)ruthenium catalyst gave 27 more selectively and suppressed the formation of 28 (Entry 3). Compared with the (arene)ruthenium complexes, RuHCl(CO)(PPh 3 ) 3 showed little catalytic activity, and no alkoxylation product was detected in the reaction mixture (Entry 4).
The superior performance of the (benzene)ruthenium catalyst over the (p-cymene)ruthenium catalyst might be attributed to the less steric hindrance around the coordinated arenes. The intermediate of the reaction might be the hydrosilane-bound ruthenium complex via σ-coordination or oxidative addition of the Si-H bond to the ruthenium atom. Nucleophilic attack of methanol to the silicon atom having the Si-H bond produces 27. At this step, more crowded p-cymene prevents methanol from attacking the central silicon atom. As a result, methanol attacks the terminal silicon atom of 26 to afford 28 via Si-Si bond cleavage.