Direct Dehydrogenative Coupling of Alcohols with

catalysts Article

Direct Dehydrogenative Coupling of Alcohols with Hydrosilanes Promoted by Sodium tri(sec-butyl)borohydride Maciej Skrodzki 1,2 , Maciej Zaranek 1,2, * , Samanta Witomska 1,2 1 2

*

and Piotr Pawluc 1,2, *

Faculty of Chemistry, Adam Mickiewicz University in Poznan, ´ Umultowska 89B, 61-614 Poznan, ´ Poland; [email protected] (M.S.); [email protected] (S.W.) Center for Advanced Technology, Adam Mickiewicz University in Poznan, ´ Umultowska 89C, 61-614 Poznan, ´ Poland Correspondence: [email protected] (M.Z); [email protected] (P.P.); Tel.: +48-61-829-1700 (P.P)

Received: 13 November 2018; Accepted: 29 November 2018; Published: 4 December 2018

Abstract: Alkoxysilanes find application in many areas of chemistry ranging from research-scale organic synthesis to multi-ton production of materials. Classically, they are obtained in stoichiometric reaction of alcoholysis of chlorosilanes, however, recent years brought development in the field of direct dehydrogenative coupling of hydrosilanes with alcohols, which is a more atom-economic and benign alternative to the former process. In this paper, we report the use of sodium tri(sec-butyl)borohydride as a convenient promoter of this reaction. Exemplary syntheses carried out under mild conditions and without additional solvents, followed by very easy work-up procedure, show excellent potential for application of so devised catalytic system. Keywords: alkoxysilanes; dehydrogenative coupling; trialkylborohydrides; solvent-free

1. Introduction Alkoxysilanes constitute a group of compounds widely used in materials science as precursors of polymers [1], silica-based mesoporous materials [2–4], and surface coatings [5,6], in particular as silane coupling agents [7–9]. In organic chemistry, alkoxysilanes are often referred to as silyl ethers and are a widely used class of protecting groups for alcohols [10,11], while silyl ethers of enolates are a set of convenient nucleophilic reagents [12]. Classical method of synthesis of alkoxysilanes relies mostly on the reaction of alcohols with chlorosilanes in the presence of a base and, sometimes, a silyl transfer agent [13,14]. Over the years, direct dehydrogenative coupling of alcohols with hydrosilanes emerged as a green, halogen-free alternative to the former approach. At the moment, it can be catalyzed by a wide range of compounds including both transition metal complexes [15–30] and simple inorganics and organocatalysts [31–35]. As the only byproduct of these reaction is molecular hydrogen, they are also considered chemical storage of this element [36], whose capacity greatly varies with the silane being used and can be as high as over 4 wt % [16]. Our interest in this reaction stems from the observation made during a previous study on trialkylborohydride-catalyzed hydrosilylation, especially as triethylborohydrides are widely used as additives in various other catalytic systems [37]. Then, we noticed that attempts to hydrosilylate 2-allyloxyethanol were unsuccessful, leading only to complete transformation of hydroto alkoxysilanes (Scheme 1).

Catalysts 2018, 8, 618; doi:10.3390/catal8120618

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Scheme 1. 1. The sodium Scheme The reaction reaction of of phenylsilane phenylsilane with with 2-allyloxyethanol 2-allyloxyethanol in in the the presence presence of of sodium Scheme 1. The reaction of phenylsilane with 2-allyloxyethanol in the presence of sodium triethylborohydride. triethylborohydride. triethylborohydride.

An interesting interesting point this side side reaction reaction could could be be further further exploited exploited and and applied a wider wider An point was was if if this applied to to a An interesting point was if this side reaction could be further exploited and applied to a wider range of of silanes silanes and and alcohols alcohols as as possible possible method method in in organic organic synthesis. synthesis. range range of silanes and alcohols as possible method in organic synthesis.

2. Results and and Discussion Discussion 2. Results 2. Results and Discussion At in aa model At first, first, the the usability usability of of different different alkali alkali metal metal trialkylborohydrides trialkylborohydrides was was examined examined in model At first, the usability of different alkali metal trialkylborohydrides was examined in a model reaction with dimethylphenylsilane dimethylphenylsilane 22 (Table one possible possible reaction of of benzyl benzyl alcohol alcohol 11 with (Table 1). 1). This This reaction reaction leads leads to to one reaction of benzyl alcohol 1 with dimethylphenylsilane 2 (Table 1). This reaction leads to one possible alkoxysilane Table 11 summarizes alkoxysilane 3. 3. Table summarizes the the results results of of optimization optimization ran ran on on 1-mmol 1-mmol scale. scale. alkoxysilane 3. Table 1 summarizes the results of optimization ran on 1-mmol scale. Table Results ofof optimization optimization of of dehydrogenative dehydrogenative coupling coupling of of 11 with with 22 using Table 1.1. Results using various various Table 1. Results of optimization of1 dehydrogenative coupling of 1 with 2 using various trialkylborohydride trialkylborohydride salts salts as as promoters promoters 11.. trialkylborohydride salts as promoters .

2 Entry [MHBR3][MHBR3 ] Solvent, [°C] Conv. of 2 2 Entry Solvent,cccof of 2 T [◦ C]T Entry [MHBR3] Solvent, of 22 T [°C] Conv. of 2Conv. of 2 2 1 3 Neat 17 1 NaHBEt Neat RT RT 17 1 NaHBEt3 NaHBEt3 Neat RT 17 2 3 Neat 26 2 LiHBEt Neat RT RT 26 2 LiHBEt3 LiHBEt3 Neat RT 26 3 KHBEt3 KHBEt3 Neat RT RT 31 3 Neat 31 3 Neat 31 4 KHBEt3 Neat 40 RT 66 4 Neat 40 66 4 Neat 40 66 5 NaHB(s-Bu)3 Neat RT 57 5 NaHB(s-Bu) 3 Neat RT 57 6 Neat 40 RT 100 5 NaHB(s-Bu) 3 Neat 57 6 Neat1M 40 100 7 THF, RT 11 6 Neat 40 100 8 Toluene, RT RT 55 7 THF, 1M1M 11 7 THF, 1M RT 11 9 Neat1M RT RT 693 8 Toluene, 55 8 Toluene, 1M RT 55 4 10 Neat 40 RT 100 9 Neat 6933 9 Neat RT 69 1 2 Notes: reaction conditions: [1]:[2]:[MHBR ] = 1:1:0.1, neat, RT, 20 h; determined by GC using 100 µL of decane as 4 3 10 Neat 40 100 3 using 5% of borohydride; 4 conversion after 10 Neat 40 1004 reference; 1 h.

Notes: 11 reaction conditions: [1]:[2]:[MHBR3] = 1:1:0.1, neat, RT, 20 h; 22 determined by GC using 100 µL Notes: reaction conditions: [1]:[2]:[MHBR3] = 1:1:0.1, neat, RT, 20 h; determined by GC using 100 µL of decane as reference; 33 using 5% of borohydride; 44 conversion after 1 h. of decane reference; using 5% of borohydride; conversion after 1 h.out to be not as successful as the The initialastrial with sodium triethylborohydride (entry 1) turned

ones The withinitial the other among which(entry sodium has proven trialtrialkylborohydrides with sodium triethylborohydride 1) tri(sec-butyl)borohydride turned out to be not as successful as The initialgiving trial with sodium triethylborohydride (entry 1) turned out to be not as successful as to be the best, complete conversion of 2 even in 1 h (entry 10). Further, we decided to usehas no the ones with the other trialkylborohydrides among which sodium tri(sec-butyl)borohydride the onesaswith the other trialkylborohydrides among which sodium has solvent, at the room temperature the reaction carried in solution wastri(sec-butyl)borohydride slower (entrieswe 5 vs. 7 & 8). proven to be best, giving complete conversion ofout 2 even in 1 h (entry 10). Further, decided to proven to be the best, giving complete conversion of 2 even in 1 h (entry 10). Further, we decided to In solvent, the above-optimized conditions, investigated of this reaction, using various use no as at room temperature thewe reaction carried the out scope in solution was slower (entries 5 vs. 7 use no solvent, as at room temperature the reaction carried out in solution was slower (entries 5 vs. aliphatic and aromatic alcohols as well as selected primary, secondary and tertiary silanes. The results7 & 8). & 8). are presented in Table 2. In the above-optimized conditions, we investigated the scope of this reaction, using various In the above-optimized conditions, we investigated the scope of this reaction, using various aliphatic and aromatic alcohols as well as selected primary, secondary and tertiary silanes. The results aliphatic and aromatic alcohols as well as selected primary, secondary and tertiary silanes. The results are presented in Table 2. are presented in Table 2.

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Table 2. Substrate scope of NaHB(s-Bu) 3—promoted dehydrogenative coupling of silanes with 1. 2. of 3— dehydrogenative with Table Substrate scope of NaHB(s-Bu) NaHB(s-Bu) 3—promoted promoted dehydrogenative coupling of silanes silanes with Table 2.2. Substrate Substrate scope of NaHB(s-Bu) 3— promoted dehydrogenative coupling of silanes with 3— TableTable 2. Substrate scope ofscope NaHB(s-Bu) dehydrogenative coupling coupling of silanes of with alcohols 3— 3— promoted Table 2. scope of 3— dehydrogenative coupling of with 3— Table 2.11.Substrate Substrate scope of NaHB(s-Bu) NaHB(s-Bu) 3—promoted promoted dehydrogenative coupling of silanes silanes with alcohols alcohols alcohols1111111... alcohols alcohols alcohols . . ## Silane Alcohol Product Isol. Isol. YieldYield t/h 2t/h 2 Silane Alcohol Product ### Silane Alcohol Product t/h Isol. Yield Silane Alcohol Product t/h2222 Isol.Yield Yield Silane Alcohol Product t/h Isol. Silane Alcohol Product t/h2222 Isol.Yield Yield ## Silane Alcohol Product t/h Isol. 1 Me 2 PhSiH Benzyl alcohol 1 h 98% 111 Me PhSiH Benzyl alcohol 1 h11hh 98% 98% Me Benzyl Me222PhSiH PhSiH Benzylalcohol alcohol 98% Me PhSiH Benzyl alcohol hh 98% Me22222PhSiH PhSiH Benzylalcohol alcohol 98% 111 Me Benzyl 111h 98% 22222 22

Ethanol Ethanol Ethanol Ethanol Ethanol Ethanol Ethanol

min 3 min 3333min min min min 33min

93% 93% 93% 93% 93% 93% 93%

33333 33

Methanol Methanol Methanol Methanol Methanol Methanol Methanol

10 min 10 min 10 10min min 10 min 10 10min min

86% 86% 86% 86% 86% 86% 86%

44444 44

2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol

48 h 48 48h 48 h 48 hh 48 48hh

86% 86% 86% 86% 86% 86% 86%

5 33 55553553333333 5

4-Bromobenzyl alcohol 4-Bromobenzyl 4-Bromobenzylalcohol alcohol 4-Bromobenzyl alcohol 4-Bromobenzyl alcohol 4-Bromobenzyl alcohol 4-Bromobenzylalcohol alcohol 4-Bromobenzyl

24 hh 24 24h 24 h 24 h 24 hh 24h 24

46% 46% 46% 46% 46% 46% 46% 46%

6 333 6666366333333

4-Methylbenzyl alcohol 4-Methylbenzyl 4-Methylbenzylalcohol alcohol 4-Methylbenzyl alcohol 4-Methylbenzyl alcohol 4-Methylbenzyl 4-Methylbenzylalcohol alcohol

6 hh h 6 h66666h h h

94% 94% 94% 94% 94% 94% 94%

3 77 333 77737733333

4-Fluorobenzyl alcohol 4-Fluorobenzyl 4-Fluorobenzylalcohol alcohol 4-Fluorobenzyl alcohol 4-Fluorobenzyl alcohol 4-Fluorobenzyl alcohol

hh 66 h hh 6 h6666h

86% 86% 86% 86% 86% 86% 86%

3 8883333 8888383333

1-Cyclopropylethanol 1-Cyclopropylethanol 1-Cyclopropylethanol 1-Cyclopropylethanol 1-Cyclopropylethanol 1-Cyclopropylethanol 1-Cyclopropylethanol 1-Cyclopropylethanol

hh 444h h hh 4 h4444h

98% 98% 98% 98% 98% 98% 98% 98%

999333333 99939333

3-Butyn-1-ol 3-Butyn-1-ol 3-Butyn-1-ol 3-Butyn-1-ol 3-Butyn-1-ol 3-Butyn-1-ol 3-Butyn-1-ol

24 hh 24 h 24h 24 24h h 24 h24

99% 99% 99% 99% 99% 99% 99%


min 3333min min min min 3 min 33min

93% 93% 93% 93% 93% 93% 93%

4,6 11 4,6 4,6 4,6 11 4,6 11 11 4,6 4,6 4,6 4,6 4,6 11 11 1111

Benzyl alcohol Benzyl Benzylalcohol alcohol Benzyl alcohol Benzyl alcohol Benzylalcohol alcohol Benzyl alcohol Benzyl

24 hh 24 24h 24 h 24 hh 24h 24 h24

97% 97% 97% 97% 97% 97% 97% 97%

4,6 12 4,6 4,6 4,6 12 4,6 12 4,6 12 4,6 4,6 4,6 4,6 12 1212

Hexan-1-ol Hexan-1-ol Hexan-1-ol Hexan-1-ol Hexan-1-ol Hexan-1-ol Hexan-1-ol

24 hh 24 24h 24 hh 24 h24 24h

99% 99% 99% 99% 99% 99% 99%

4 13 444 13 13 13 4 44444 13 13 13

Cyclohexanol Cyclohexanol Cyclohexanol Cyclohexanol Cyclohexanol Cyclohexanol Cyclohexanol

h 1111h hh 1 h11hh

88% 88% 88% 88% 88% 88% 88%


48 hh 48 48h 48 hh 48h 48 h48

96% 96% 96% 96% 96% 96% 96%

15 15 15 15 15 15 15

Benzyl alcohol Benzyl Benzylalcohol alcohol Benzyl alcohol Benzylalcohol alcohol Benzyl Benzyl alcohol

48 hh 48 48h 48 hh 48h 48 h48

93% 93% 93% 93% 93% 93% 93%

16 16 16 16 16 16 16

2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol

48 hh 48 48h 48 hh 48h 48 h48

98% 98% 98% 98% 98% 98% 98%

Cyclohexanol Cyclohexanol Cyclohexanol Cyclohexanol Cyclohexanol Cyclohexanol Cyclohexanol

hh 111h hh 1 h111h

89% 89% 89% 89% 89% 89% 89%

4,5 10 4,5 4,5 10 4,5 10 4,5 10 4,5 4,5 4,5 4,5 10 1010 4,5

14 14 14 14 14 14 14

4 17 444 17 4 17 17 4 4444 17 17 17

Ph SiH Ph Ph2222SiH SiH2222 Ph SiH Ph222222SiH SiH222222 Ph SiH Ph

MePh SiH MePh MePh2222SiH SiH MePh SiH MePh222222SiH SiH MePh MePh SiH

PhSiH PhSiH PhSiH3333 PhSiH PhSiH333333 PhSiH PhSiH

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18 4

19 3

Silane

Alcohol

2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol 2-Allyloxyethanol

18 18 184 44

i-Pr3SiH i-Pr i-Pr333SiH SiH i-Pr SiH

19 193 33 19

Table 2. Cont.

Methanol Methanol Methanol Methanol

Product

t/h 2

Isol. Yield

1h

1 h 11 h h

7272h,h,67% 67%conv. 72 72 h, h, 67% 67% conv. conv. conv.

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99%

99% 99% 99%

-

--

1 general conditions: [SiHn]:[OH]:[NaHB(s-Bu)3] = 1:n:0.1, no additional solvent, 40 °C; 2 time Notes: Notes: 11 general conditions: [SiHn]:[OH]:[NaHB(s-Bu)3] = 1:n:0.1, no additional solvent, 40 °C; 22 time

1 general general conditions: [SiHn]:[OH]:[NaHB(s-Bu) 3] = 1:n:0.1, no additional °C;needed time Notes:Notes: conditions: [SiHn ]:[OH]:[NaHB(s-Bu) solvent,solvent, 40 ◦ C; 2 40 time 3 ] = 1:n:0.1, no additional 3 reaction 5 needed for complete conversion of substrates; at at 65 °C;°C;4 reaction temperature; 44 reactionatatroom 4 reaction 5 using needed for conversion substrates; reaction temperature; for complete of substrates; reaction at 65 33◦ C; at °C; room temperature; 2 mol% of55 neededconversion for complete complete conversion3 of of substrates; reaction at 65 65 reaction at room room temperature; 6 usingusing 2 mol%6 using of NaHB(s-Bu) 3; using 5 3mol% of NaHB(s-Bu) 3. NaHB(s-Bu) 5 mol% of NaHB(s-Bu) .5 NaHB(s-Bu) 33;; 66 using using 322; mol% mol% of of NaHB(s-Bu) using 5 mol% mol% of of NaHB(s-Bu) NaHB(s-Bu)33..

As follows follows fromfrom the results results presented in 2, all allall used silanes and alcohols can be efficiently As follows the results presented in Table 2, used silanesand andalcohols alcoholscan canbe be efficiently efficiently As from the presented in Table Table 2, used silanes coupled with the aid of sodium tri(sec-butyl)borohydride, however, some substrates require either coupled with the aid of sodium tri(sec-butyl)borohydride, however, some substrates require either coupled with the aid of sodium tri(sec-butyl)borohydride, however, some substrates require either higher reaction temperatures or prolonged reaction time, which indicates a visible steric effect mostly higher reaction temperatures or prolonged reaction time, which indicates a visible steric effect higher reaction temperatures or prolonged reaction time, which indicates a visible stericmostly effect of silane. thethe silane. Simple aliphatic alcohols such as ethanol andmethanol methanol reacted efficiently of the Simple aliphatic alcohols such assuch ethanol and reacted efficiently with with mostly of silane. Simple aliphatic alcohols as ethanol and methanol reacted efficiently diphenylsilane and dimethylphenylsilane to give the corresponding alkoxysilanes in good yields diphenylsilane and dimethylphenylsilane to give the corresponding alkoxysilanes in good yields (86– with diphenylsilane and dimethylphenylsilane to give the corresponding alkoxysilanes in good 96%) in a minutes few minutes at room temperature (Table 2, entry 2–3, 10),even evenwith withreduced reduced NaHB(s-Bu) NaHB(s-Bu)33 96%) in a few at room temperature (Table 2, entry 2–3, 10), yields (86–96%) in a few minutes at room temperature (Table 2, entry 2–3, 10), even with reduced loading (Table 2, entry however,silylation silylationof ofethanol ethanol byby more more sterically sterically hindered hindered loading (Table 2, entry 10),10), however, NaHB(s-Bu) 3 loading (Table 2, entry 10), however, silylation of ethanol by more sterically hindered methyldiphenylsilane required a longer reaction time (48 h). Similarly, diphenylmethylsilane seems methyldiphenylsilane required reaction time (48 (48 h). Similarly, diphenylmethylsilane seems methyldiphenylsilane requireda alonger longer reaction time h). Similarly, diphenylmethylsilane to be less reactive in the reactions with benzyl alcohol derivatives than dimethyphenylsilane. Allylto be less reactive in the reactions with benzyl alcoholalcohol derivatives than dimethyphenylsilane. Allylseems to be less reactive in the reactions with benzyl derivatives than dimethyphenylsilane. (Table 2, entries 4, 18) 16, 18) and propargyl alcohol derivatives (Table2,2,entry entry9)9)can canbe be efficiently O(Table 2, entries 4, 16, alcohol derivatives (Table OAllyl(Table 2, entries 4, 16,and 18)propargyl and propargyl alcohol derivatives (Table 2, entry 9) can efficiently be efficiently silylated by primary and tertiary aromatic silanes with excellent chemoselectivity, which is surprising silylated by primary and tertiary aromatic silanes with excellent chemoselectivity, which is surprising O-silylated andreports tertiary silanes chemoselectivity, is surprising in viewby ofprimary our recent onaromatic the reactivity ofwith allylexcellent glycidyl ether and terminalwhich alkynes in sodium in view ofofour recent reports on on the the reactivity of allyl glycidyl ether and terminal alkynesalkynes in sodium in view our recent reports reactivity of allyl glycidyl ether and terminal in triethylborohydride-mediated hydrosilylation [37] or dehydrogenative silylation reactions [38]. All triethylborohydride-mediated hydrosilylation [37] or dehydrogenative silylation reactions [38]. All sodium triethylborohydride-mediated hydrosilylation [37] oremploying dehydrogenative silylation reactions [38]. compounds were isolated by extremely simple protocol precipitation of the remainders compounds were isolated byby extremely simple protocol employing precipitation of the remainders All compounds were isolated extremely simple protocol employing precipitation of the remainders of catalyst with hexane, filtration through a 0.45 µm syringe filter and evaporation of the solvent to of catalyst catalyst with with hexane, hexane, filtration filtration through through aa 0.45 0.45 µm syringe syringe filter filter and and evaporation evaporation of of the the solvent solvent to to of give very high yields (up to 99%, average 91%).µm The structures of the products were confirmed by the give very high yields (up to 99%, average 91%). The structures of the products were confirmed by the 13 give 1very high (up to of 99%, average 91%). The ofInformation). the productsThe were confirmed by 1H and 13C yields NMR spectra isolated compounds (seestructures Supporting limitation of this 1H and 13C NMR spectra of isolated compounds Supporting Information). The The limitation this 1 H and 13 C NMR the spectra of compounds (see Supporting Information). limitation of reaction system concerns itsisolated inapplicability to(see trialkylsilanes, such as triisopropylsilane, in of whose reaction system concerns its inapplicability to trialkylsilanes, such as triisopropylsilane, in whose presencesystem a conversion of 67% was observed only in the reactionsuch withasmethanol after 72 h of this reaction concerns its inapplicability to trialkylsilanes, triisopropylsilane, inheating whose presence conversion of67% 67% was observed onlyininthe reactionwith with after h of heating at 65aa°C (Table 2, entry 19). Other trialkylsilanes e.the g.reaction triethylsilane ormethanol (t-butyl)dimethylsilane appear presence conversion of was observed only methanol after 7272 h of heating at ◦ Cto at 65 °C (Table 2, entry 19). Other trialkylsilanes e.and g. triethylsilane (t-butyl)dimethylsilane appear be unreactive in the reactions with aliphatic aromatic alcohols in the conditions applied. 65 (Table 2, entry 19). Other trialkylsilanes e. g. triethylsilane or or (t-butyl)dimethylsilane appear to to be unreactive in the reactions with aliphatic and aromatic alcohols in the conditions applied. In general, this protocol provides very good chemoselectivity towards dehydrogenative be unreactive in the reactions with aliphatic and aromatic alcohols in the conditions applied. In general, this protocol provides very good chemoselectivity towards dehydrogenative coupling of alcohol with silane and no other processes were observed. Halogen substituents were In general, this protocol provides very good chemoselectivity towards dehydrogenative coupling also preserved, is other important theprocesses point of view ofobserved. possible subsequent functionalization. coupling of alcohol with silane and nofrom other were Halogenwere substituents were of alcohol with silanewhich and no processes were observed. Halogen substituents also preserved, What is also remarkable, the nature of generated active catalyst must be milder than an alkoxide, as also preserved, which is important from the point of view of possible subsequent functionalization. which is important from the point of view of possible subsequent functionalization. What is also potentially dangerousthe redistribution of phenylsilane with evolution of gaseous pyrophoric SiH What is also remarkable, nature of generated active catalyst must be milder than an alkoxide, as44 remarkable, the nature of generated active catalyst must be milder than an alkoxide, as potentially (Scheme 2) was not observed. This reaction was reported to occur in basic conditions [39,40], potentially redistribution dangerous redistribution of with phenylsilane evolution of gaseous dangerous of phenylsilane evolutionwith of gaseous pyrophoric SiH4pyrophoric (Scheme 2) SiH was4 especially in the presence of alkoxides. (Scheme 2) was not observed. This reaction was reported to occur in basic conditions [39,40], not observed. This reaction was reported to occur in basic conditions [39,40], especially in the presence especially in the presence of alkoxides. of alkoxides.

Scheme Scheme 2. 2. Possible Possible redistribution redistribution of of phenylsilane phenylsilane reported reported under under basic basic conditions. conditions.

Scheme redistribution phenylsilane reported under basicinitially conditions. On the basis2.ofPossible this observation, we of postulate that tri(sec-butyl)borane, generated in the reaction of sodium tri(sec-butyl)borohydride alcohol, is formedinitially as adduct and aids the On the basis of this observation, we postulatewith that an tri(sec-butyl)borane, generated in the On the basis of this observation, we postulate that tri(sec-butyl)borane, initially generated in the catalytic reaction analogous to the one proposed by Schowen [41] in the form presented as example reaction of sodium tri(sec-butyl)borohydride with an alcohol, is formed as adduct and aids the catalytic reaction of sodium tri(sec-butyl)borohydride with an lack alcohol, is formed as which adductcould and lower aids the in Scheme 3. Another advantage of this system is the of external alkoxide the catalytic reaction to the one proposed by other Schowen [41] in theresulting form presented as example effective yieldanalogous by contamination of products with alkoxysilanes from its presence. Scheme 2. Possible redistribution of phenylsilane reported under basic conditions.

in Scheme 3. Another advantage of this system is the lack of external alkoxide which could lower the effective yield by contamination of products with other alkoxysilanes resulting from its presence.


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reaction analogous to the one proposed by Schowen [41] in the form presented as example in Scheme 3. Another advantage of this system is the lack of external alkoxide which could lower the effective yield Catalysts 2018, 8, x FORofPEER REVIEW 5 of 7 by contamination products with other alkoxysilanes resulting from its presence.

Scheme 3. Proposed mechanism mechanism of tri(sec-butyl)borohydride-catalyzed 3. Proposed tri(sec-butyl)borohydride-catalyzed direct dehydrogenative dehydrogenative coupling of dimethylphenylsilane with methanol. methanol.

It It seems seems very very unlikely unlikely that that any any kind kind of of hydride hydride is is effectively effectively engaged engaged in in this this catalysis catalysis in in the the presence of great excess of alcohol, contrary to what was proposed by Panda [32]. presence of great excess of alcohol, contrary to what was proposed by Panda [32]. 3. Materials and Methods 3. Materials and Methods 3.1. General Remarks 3.1. General Remarks All reactions were performed under inert atmosphere. Oxygen does not seem to influence the All reactions were performed under inert atmosphere. Oxygen does not seem to influence the performance, however, moisture is decreasing reactivity. Solvents were purified by distillation over performance, however, moisture is decreasing reactivity. Solvents were purified by distillation over sodium/benzophenone. Reagents (Sigma-Aldrich/Merck) were used as supplied, however, methanol, sodium/benzophenone. Reagents (Sigma-Aldrich/Merck) were used as supplied, however, methanol, ethanol and benzyl alcohol were stored over molecular sieves. ethanol and benzyl alcohol were stored over molecular sieves. Gas chromatography was performed on a Bruker Scion 436-GC with a 30 m Agilent VF5-ms Gas chromatography was performed on a Bruker Scion 436-GC with a 30 m Agilent VF5-ms 0.53 mm Megabore column and a thermal conductivity (TCD) detector. The temperature program was 0.53 mm Megabore column and a thermal conductivity (TCD) detector. The temperature program as follows: 60 ◦ C (3 min), 20 ◦ C/min, 280 ◦ C (20 min). NMR spectra were recorded on a Bruker Fourier was as follows: 60 °C (3 min), 20 °C/min, 280 °C (20 min). NMR spectra were recorded on a Bruker 300 spectrometer and referenced to the solvent residual peak. Fourier 300 spectrometer and referenced to the solvent residual peak. 3.2. Dehydrogenative Coupling of Alcohols with Silanes 3.2. Dehydrogenative Coupling of Alcohols with Silanes In a typical reaction, 1 mmol of silane, and 1 mmol of alcohol were placed in a Schlenk bomb a typical reaction, mmol Next, of silane, andof1 1M mmol of alcohol were placed in a Schlenk bomb flaskIn dried and filled with1argon. 0.1 mL solution of sodium tri(sec-butyl)borohydride flask dried filled with argon. Next,vessel 0.1 mLwas of 1M solution of sodium in in THF wasand carefully added. Reaction closed and placed in atri(sec-butyl)borohydride preheated oil bath at given THF was carefully added. Reaction vessel was closed and placed in a preheated oil bath at given temperature and stirred. Samples were taken at time intervals and analyzed using gas chromatography. temperature stirred. conversion Samples were taken at time mixture intervalswas and analyzed using with gas After detectionand of complete of substrates, reaction cooled down, dosed chromatography. After detection ofleft complete conversion of substrates, reaction mixture wasand cooled approximately 3 mL of hexane, and for 15 min to precipitate. The suspension was filtered the down, dosed with approximately 3 mL of hexane, and left for 15 min to precipitate. The suspension resulting clear solution was evaporated to yield pure alkoxysilane. was filtered and the resulting clear solution was evaporated to yield pure alkoxysilane. 4. Conclusions 4. Conclusions Direct dehydrogenative coupling of alcohols with hydrosilanes can be efficiently promoted by Direct dehydrogenative coupling of alcohols with hydrosilanes canalmost be efficiently promoted by sodium tri(sec-butyl)borohydride in solvent-free conditions, which enable quantitative isolation sodium in solvent-free conditions, which enable almost quantitative of desiredtri(sec-butyl)borohydride alkoxysilane product. isolation of desired alkoxysilane product. Supplementary Materials: Spectral data of products are available online at http://www.mdpi.com/2073-4344/ 8/12/618/s1. 1. Analytical data of isolated products, 2. Spectra of products. Supplementary Materials: Spectral data of products are available online at www.mdpi.com/xxx/s1. 1. Analytical Author Contributions: Conceptualization, M.Z. and M.S.; methodology, M.S.; validation, M.Z.; formal analysis, data of isolated products, 2. Spectra of products. M.Z.; investigation, M.S., M.Z. and S.W.; writing—original draft preparation, M.Z.; writing—review and editing, Author Conceptualization, M.Z. and M.S.; methodology, M.S.; validation, M.S., SWContributions: and P.P.; supervision, P.P.; project administration, P.P.; funding acquisition, P.P. M.Z.; formal analysis, M.Z.; investigation, M.S., M.Z. and S.W.; writing—original draft preparation, M.Z.; writing—review and editing, Funding: This research and APC were funded by The National Science Centre (Poland), Grant No. UMO-2016/ M.S., SW and P.P.; supervision, P.P.; project administration, P.P.; funding acquisition, P.P. 23/B/ST5/00177.

Funding: This research and APC were funded by The National Science Centre (Poland), Grant No. UMO2016/23/B/ST5/00177. Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.


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Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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