inorganics Article Stepwise Introduction of Different Substituents to α-Chloro-!-hydrooligosilanes: Convenient Synthesis of Unsymmetrically Substituted Oligosilanes Ken-ichiro Kanno *, Yumi Aikawa, Yuka Niwayama, Misaki Ino, Kento Kawamura and Soichiro Kyushin * Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu, Gunma 376-8515, Japan; [email protected] (Y.A.); [email protected] (Y.N.); [email protected] (M.I.); [email protected] (K.K.) * Correspondence: [email protected] (K.-i.K.); [email protected] (S.K.); Tel.: +81-277-30-1292 (K.-i.K. & S.K.) Received: 12 August 2018; Accepted: 14 September 2018; Published: 18 September 2018 Abstract: A series of unsymmetrically substituted oligosilanes were synthesized via stepwise introduction of different substituents to α-chloro-!-hydrooligosilanes. The reactions of α-chloro-!- 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. Keywords: α-chloro-!-hydrooligosilane; titanium; ruthenium; dehydrogenative alkoxylation 1. Introduction Recently, various functionalized oligosilanes have attracted growing interests due to their unique photophysical and electronic properties [1–13]. In the synthesis of such compounds, introduction of functionality to oligosilanes is an important process. Oligosilanes having both chlorosilane and hydrosilane moieties are especially interesting because two different functionalities could be introduced to the oligosilanes. For example, α-chloro-!-hydrooligosilanes are potential precursors for such modification. A problem to be overcome is limitation of facile functionalization of the hydrosilane moiety in the oligosilanes. Thus far, halogenation [14–16], Lewis acid-catalyzed reactions [17–19] and radical-promoted reactions [20–24] have been used for the Si–H modification of hydrooligosilanes. Transition metal-catalyzed reactions of hydrosilanes are well known as Si–H conversion methods, including hydrosilylation [25–27], dehydrogenative silylation of OH and NH groups [28–31], silylation of aromatic rings [32–36] and so on. however, most of them are applicable only to hydromonosilanes. When hydrooligosilanes are subjected to these transition metal-catalyzed reactions, major products come from the cleavage of Si–Si bonds. Although the transition metal-catalyzed reactions are utilized as valuable organic synthesis methodologies [37–40], it is not the case for the transformation of hydrooligosilanes. As a rare example, Yamanoi and Nishihara have reported the palladium-catalyzed arylation of hydrooligosilanes with aryl iodides to afford the corresponding arylated oligosilanes with preservation of the Si–Si bonds [41,42]. Previously, we reported titanium-catalyzed synthesis of the hydrogen-terminated oligosilanes from α,!-dichlorooligosilanes [43]. This method enabled us to synthesize α-chloro-!-hydrooligo- silanes with high selectivity. The α-chloro-!-hydrooligosilanes are good precursors of variously substituted hydrooligosilanes via nucleophilic substitution of the chlorosilane moiety. Furthermore, we have recently found ruthenium-catalyzed dehydrogenative alkoxylation of a hydrodisilane Inorganics 2018, 6, 99; doi:10.3390/inorganics6030099 www.mdpi.com/journal/inorganics Inorganics 2018, 6, x FOR PEER REVIEW 2 of 14 Inorganics 2018, 6, x FOR PEER REVIEW 2 of 14 chlorosilane moiety. Furthermore, we have recently found ruthenium-catalyzed dehydrogenative chlorosilanealkoxylation moiety.of a hydrodisilane Furthermore, withwe have alcohols recently [44]. found Interestingly, ruthenium-catalyzed the reactions dehydrogenative proceed with alkoxylationpreserving the of Si–Si a hydrodisilane bond despite itswith susceptible alcohols nature [44]. toInterestingly, transition metals the reactions[37–40]. Combination proceed with of Inorganics 2018, 6, 99 2 of 14 preservingthese two methodsthe Si–Si bondleads despiteto convenient its susceptible synthesis nature of unsymmetrically to transition metals substi [37–40].tuted Combinationoligosilanes. ofIn thesethis twopaper, methods we leadsreport to theconvenient synthesis synthesis of variousof unsymmetrically substituted substihydrooligosilanestuted oligosilanes. from In thisα-chloro- paper,withω-hydrooligosilanes alcohols we [44report]. Interestingly, the and synthesis thetransformation reactions of proceed various of withthus-obtained preservingsubstituted the hydrooligosilanes Si–Sihydrooligosilanes bond despite its tofrom the susceptible nature to transition metals [37–40]. Combination of these two methods leads to convenient αcorresponding-chloro-ω-hydrooligosilanes alkoxy oligosilanes. and transformation of thus-obtained hydrooligosilanes to the synthesis of unsymmetrically substituted oligosilanes. In this paper, we report the synthesis of correspondingvarious substituted alkoxy oligosilanes. hydrooligosilanes from α-chloro-!-hydrooligosilanes and transformation of 2. Resultsthus-obtained and Discussion hydrooligosilanes to the corresponding alkoxy oligosilanes. 2. Results and Discussion 2.1. Synthesis2. Results of 1-Hydrooligosilanes and Discussion 2.1. Synthesis of 1-Hydrooligosilanes As2.1. reported Synthesis ofpreviously, 1-Hydrooligosilanes α-chloro-ω-hydrooligosilanes 1–3 are synthesized by the selective monoreductionAs reportedAs reported of previously, α,ω previously,-dichlorooligosilanes α-chloro-α-chloro-ω!-hydrooligosilanes-hydrooligosilanes with alkylmagnes1 –13–ium3are are synthesizedchlorides synthesized in by the theby selectivepresencethe selective of a monoreductioncatalyticmonoreduction amount of αof, ofω TiCl-dichlorooligosilanesα,!-dichlorooligosilanes4 (Scheme 1) [43]. with with The alkylmagnes alkylmagnesium remainingium chlorosilane chlorides chlorides in the inmoiety presencethe presence is of possible a of a catalyticfunctionalitycatalytic amount for amount introduction of ofTiCl TiCl4 (Scheme(Scheme of various1)[ 431) substituents.]. The[43]. remaining The remaining chlorosilane chlorosilane moiety is possible moiety functionality is possible functionalityfor introduction for introduction of various of substituents. various substituents. Scheme 1. Monoreduction of α,ω-dichlorooligosilanes with i-PrMgCl in the presence of a catalytic Scheme 1. Monoreduction of α,!-dichlorooligosilanes with i-PrMgCl in the presence of a catalytic amount of TiCl4. Schemeamount 1. Monoreduction of TiCl4. of α,ω-dichlorooligosilanes with i-PrMgCl in the presence of a catalytic amount of TiCl4. As shownAs shown in Scheme in Scheme 2, the2, the chlorosilane chlorosilane moiety of of1 1was was smoothly smoothly substituted substituted by organolithium by organolithium reagents.Asreagents. shown When Whenin 1 Schemewas1 wastreated 2, treated the with chlorosilane with 2-thienyllithium, 2-thienyllithium, moiety of 2-thienyldisilane2-thienyldisilane 1 was smoothly4 was substituted4 was obtained obtained inby high organolithium in yield.high yield. 4-Methoxyphenyl group was also introduced to afford 4-methoxyphenyldisilane 5. reagents.4-Methoxyphenyl When 1 wasgroup treated was also with introduc 2-thienyllithium,ed to afford 2-thienyldisilane 4-methoxyphenyldisilane 4 was obtained 5. in high yield. 4-Methoxyphenyl group was also introduced to afford 4-methoxyphenyldisilane 5. Scheme 2. Synthesis of substituted hydrodisilanes from 1-chloro-2-hydrodisilane. Scheme 2. Synthesis of substituted hydrodisilanes from 1-chloro-2-hydrodisilane. In theScheme cases 2. of Synthesis polymethylated of substituted oligosilanes hydrodisil2 andanes3, Grignard from 1-chloro-2-hydrodisilane. reagents are more suitable for Inthe the substitution cases of polymethylated of the chlorosilane oligosilanes moiety. The Grignard2 and 3, Grignard reagents were reagents prepared are in more the presence suitable of for the substitutionInlithium the cases of chloride the of polymethylatedchlorosilane and DIBAL-H moiety. to activateoligosilanes The magnesium Grigna 2 and metalrd 3, reagentsGrignard [45]. As shownwere reagents inprepared Scheme are more3 ,in various thesuitable presence alkyl, for the of alkenyl, alkynyl and aryl groups were introduced, and substituted hydrooligosilanes were successfully substitutionlithium chloride of the and chlorosilane DIBAL-H tomoiety. activate The magnes Grignaiumrd metalreagents [45]. were As shownprepared in inScheme the presence 3, various of synthesized. As alkyl group installation, 5-hexenyl- and 2-phenylethylmagnesium reagents were alkyl, alkenyl, alkynyl and aryl groups were introduced, and substituted hydrooligosilanes were lithiumused chloride to afford and hydrotrisilanes DIBAL-H to6 andactivate7. Mono- magnes andium disubstituted metal [45]. alkenyl As groupsshown such in Scheme as styryl 3, and various alkyl,successfully alkenyl,2-buten-2-yl synthesized. alkynyl ones were and As also aryl alkyl introduced groups group were to theinstallation, trisilaneintroduced, skeleton 5-hexenyl- and successfully. substituted and The2-phenylethylmagnesium hydrooligosilanes stereochemistry of were successfullyreagentsthe were alkene synthesized. used moieties to afford of trisilanesAs hydrotrisilanesalkyl8 and group9 was installation, determined 6 and 7. Mono- by 5-hexenyl-1H NMR and spectroscopy.disubstitute and 2-phenylethylmagnesium Thed alkenylE geometry groups of such reagentsas styrylthe were styryland used2-buten-2-yl