A library of new bifunctional alkenes obtained by a highly regiodivergent silylation of 1,5-hexadiene

An efficient methodology for the synthesis of two groups of silicon-containing alkenes is reported. It includes a highly regioselective functionalization of 1,5-hexadiene through hydrosilylation and dehydrogenative silylation with organofunctional silanes and siloxanes. The established conditions enable selective monofunctionalization of 1,5-hexadiene regardless of the organosilicon modifier used as well as the type of functional group bonded to the silicon-based compound. All products were isolated and fully characterized by NMR spectroscopy and MS techniques.


Introduction
High reactivity of the C]C bonds in a wide gamut of catalytic and stoichiometric transformations makes alkenes an undoubtedly important class of compounds and basic building blocks in organic chemistry. 1 Therefore, functional olens are commonly applied as intermediates in the synthetic routes and can be easily transformed into the desired products or may be used as carriers of physicochemical properties. [2][3][4][5] Moreover, the relatively low cost and widespread availability of functional alkenes make them widely used, inter alia, in the synthesis of novel monomers, 6 functional polymers, 7 and to enhance hydrophobic 8 or ame retardant properties. 9 On the other hand, the presence of a silicon atom in the structure provides many benets such as better solubility in nonpolar solvents, improvement of thermal stability and/or ame retardancy of silicon-containing polymeric materials. 10,11 Therefore, efficient synthetic protocols enabling a high-yield synthesis of unique alkenyl-functionalized organosilicon derivatives are highly desirable.
Considering the available literature data on suitable protocols leading to this type of compounds, the hydrosilylation of multiple bonds is the most popular synthetic approach enabling an efficient preparation of new molecular and macromolecular organosilicon compounds. [12][13][14][15][16] Remarkable tolerance of a wide range of functional groups, low catalyst loading, and high conversions of the reactants made hydrosilylation reaction the fundamental process in both, laboratory and industrial applications. 17,18 The catalytic hydrosilylation can be complicated by side reactions, among which the most disturbing are alkene isomerization and/or dehydrogenative silylation accompanied with hydrogenation of olen. The above-mentioned side transformations have been mainly reported to occur in the modication of molecular olens such as styrene and its derivatives. [19][20][21][22][23][24] Nevertheless, dehydrogenative silylation has also been observed during the silylfunctionalization of organic and organosilicon polymers. 25,26 However, due to the possibility of easy activation of the multiple C-C bonds, there has recently been a signicant increase in interest in alkenylsilanes and their synthesis. Conventional routes to target compounds involve transformations of vinylsilanes via silylative coupling, 27,28 cross metathesis 29,30 and silyl-Heck coupling. 31,32 However, the hydrosilylation is still the most convenient and the most oen used synthetic tool allowing direct access to new organosilicon derivatives. 33 Nevertheless, the direct regioselective dehydrogenative silylation of alkenes seems to be more preferable than the hydrosilylation of alkynes, thanks to their widespread availability and lower cost when compared to alkynes. Therefore, attempts for selective dehydrogenative silylation of alkenes have been undertaken. [34][35][36][37][38][39] However, most of the studies reported so far have concerned selective functionalization of styrene derivatives. Moreover, mainly inert alkyl-, 19,20,22,24,38 alkylaryl-, 19,22,24,36 trialkoxysilanes 19,22 as well as alkylsiloxanes 24,34-36 were applied as silylation agents. Hence, the development of methodologies for the selective functionalization of alkenes with organofunctional silanes and siloxanes is still highly desirable.
Herein, we report studies concerning an efficient synthesis of novel, silicon-containing bifunctional alkenes obtained through highly regioselective mono-hydrosilylation and monodehydrogenative silylation of 1,5-hexadiene. The presented approach enables the formation of functional organosilicon

Results and discussion
Considering the structure of 1,5-hexadiene, its selective functionalization may be problematic because of the presence of two active, terminal C]C groups. Moreover, the addition of silicon hydrides to alkenes can lead to the formation of two regioisomers, known as Markovnikov and anti-Markovnikov addition products, respectively. As mentioned in the previous paragraph, transition metal complex-catalyzed reactions of silylfunctionalization of olens may also be accompanied by some undesirable side processes. This means that during the silylation of 1,5-hexadiene, even a dozen of various products may be formed. The four main products, without breakdown by isomerism are presented in Scheme 1. At the beginning of our studies, special attention was paid to the reaction conditions so that to optimize them towards the selective formation of one type of product. To nd the best conditions enabling the monofunctionalization of 1,5-hexadiene, a model reaction of a diene with pentamethyldisiloxane (1) in the presence of four commercially available catalysts was performed ( Table 1).
The results presented in Table 1 clearly demonstrate that the selected Pt-based catalysts proved to be highly active hydrosilylation promotors and enabled high conversion of 1 under mild conditions. Unfortunately, the target product 2A was always accompanied by the unwanted, bissilylated derivative 2B. Moreover, no dehydrogenative silylation products were observed, regardless of the stoichiometric ratios, catalyst loading, and temperature set in the catalytic tests. As 1,5-hexadiene is a cheap reagent when compared to organosilicon compounds and can be easily removed from the postreaction mixture by distillation, thanks to its low boiling point (70 C), we decided to signicantly increase the excess of this alkene, which led to the formation of the desired product 2A with good selectivity under mild conditions (  13) and 2A was found as the main product in the postreaction mixture. However, an increase in the reaction temperature resulted in the formation of a signicant amount of the dehydrogenative silylation products (2C-2D), even if a slight excess of the olen was used. It should be strongly underlined that further increase in the olen amount in the reaction system caused a shi of equilibrium towards the formation of unsaturated silyl products, thereby ensuring the complete conversion of 1 as well as the exclusive formation of the mono-dehydrogenative silylated product 2C. Moreover, detailed NMR analysis of the isolated compounds revealed that the established conditions led to the formation of target molecules with very high regioselectivities. It is noteworthy that the Pt-based complex enabled the selective formation of bisomeric product. However, the distribution of resonance lines in the olen region in the 1 H NMR spectrum of the isolated 2C compound clearly indicated the existence of two isomers. Nevertheless, the vast predominance of the b-E product was observed (b-E/b-Z ¼ 95/5). Besides, the complete transformation of 1 into the target compounds was also conrmed by the 29 Si NMR analysis. The NMR experiments showed the complete disappearance of the peak located at À6.78 ppm, originating from H-SiMe 2 -moiety in 1, and the appearance of new signals shied to lower eld. However, the shi for 2A was more signicant than for 2C. Exemplary 1 H NMR spectra are presented in Fig. 1, while the 29 Si spectra are given in Fig. 2.
Having optimized the conditions, the silylation of 1,5-hexadiene with a wide range of functional silanes and disiloxanes Scheme 1 Main products distribution during functionalization of 1,5hexadiene with pentamethyldisiloxane. was carried out in two different manners, namely in the presence of the Pt-Karstedt's complex or the rhodium catalyst. It resulted in the formation of a family of monosubstituted 1,5hexadiene derivatives containing sp 3 -or sp 2 -hybridized carbon atoms directly bonded to the silicon atom (Scheme 2). The established conditions seem to be versatile because the products distribution was similar to that observed for the model reaction, regardless of the type of functional group present in the silicon modiers (a-m). However, only for dimethyl(tertbutyl)silane (3e, 4e) the reactions did not run as expected. Considering the structure of this compound, it can be explained by the steric hindrance resulting from the presence of the tertbutyl substituent. In this specic case, Karstedt's complex enabled only 36% conversion of the silane, so the second portion of the catalyst was added and the reaction was continued for another 18 hours, which resulted in the increase in the silane conversion up to 76%. NMR and GC-MS analyses revealed the formation of the hydrosilylation product only, nevertheless, a signicant amount of by-product (9%) was also formed as a result of olen isomerization (Fig. S55 in ESI †). On the other hand, analysis of the postreaction mixture obtained aer completion of the Rh-catalyzed process revealed the lack of the expected product, which should be formed under predetermined conditions. Moreover, the introduction of another portion the catalyst to the reaction system caused no signicant change in the conversion of silane. Ultimately, an increase in temperature up to 80 C resulted in 86% consumption of organosilicon derivative. It should be strongly emphasized that the analysis of the postreaction mixture revealed the presence of a signicant amount of undesired disubstituted dehydrogenative silylation product (10%) as well as hydrosilylation products (8%). Thus, the selectivity of 3e formation was lower than that observed for the other products, which means that a high temperature has a very unfavorable inuence on the regio-and chemoselectivity of the reaction. Apart from the abovedescribed example, the optimized reaction conditions ensured regioselective and high yield synthesis of a wide range of novel alkenylsilanes equipped with inert or reactive functional groups, which may enable further modication or provide specic physical properties. It is worth noting that the functionalization of terminal dienes with hydrosilanes may also lead to hydrosilylation and subsequent cyclization, which results in the formation of functionalized carbocycles. This unusual pathway has been reported for the transformation of 1,6-and 1,7-dienes and allowed the synthesis of ve and six-membered substituted carbocycles, respectively. 40,41 With regard to the above, the use of 1,5-hexadiene should lead to formation of the substituted cyclobutane derivatives, which is not Fig. 1  thermodynamically privileged. Therefore, no cycloalkane-based products were found in the postreaction mixtures. Mild conditions, low metal loading, and simple isolation procedure make the presented approach highly efficient. It should be also emphasized that the advantage of the described synthetic method is no need for the use of inert gas, special purication of the reagents, and the use of sophisticated procedures for preparation and/or activation of catalysts. The catalytic systems employed in this work are well-dened, effective and commercially available, which signicantly simplies the whole synthetic procedure.
In view of the literature data, the number of reports describing the attempts for the selective silylation of dienes is scarce. 42, 43 Saiki studied Pt-catalysed hydrosilylation of dienes and discovered that the presence of polar additives (ethers) signicantly inuence the regioselectivity of the reaction. However, the substrate scope was only limited to chlorosilanes. 42 On the other hand, Murai and coworkers have reported RhCl(PPh 3 ) 3 -catalysed dehydrogenative silylation of 1,5-dienes. Nevertheless, only trialkylsilanes (HSiMe 3 , HSiEt 2 Me) and triethoxysilane were applied as silylation agents. Moreover, the authors also observed the formation of by-products (up to 9%) as a result of isomerization reaction, 43 which can be explained by high metal loading (25-fold higher Rh loading than presented in this work). It should be emphasized, that the proposed by us methodology enables direct access to new alkenylsilanes under milder conditions. Furthermore, scope of functional groups used for functionalization of 1,5-hexadiene has been signicantly increased. Therefore, the synthesized compounds can be considered as useful building blocks in the preparation of silicon-containing hybrid materials recently showing raised attention. [44][45][46] The detailed experimental procedures as well as NMR spectra of all products are provided in the ESI. †

Conclusions
In brief, we have reported here a synthetic protocol that allows efficient preparation of unique unsymmetrical organofunctional alkenylsilanes and siloxanes through a highly regioselective and chemoselective hydrosilylation and dehydrogenative silylation of 1,5-hexadiene. The employment of Pt(0) and Rh(I) catalysts enabled the synthesis of new organosilicon derivatives with good isolation yields (69-98%). Moreover, all synthesized compounds contain the most common organic groups as well as the terminal alkene moiety, which can be used in subsequent stoichiometric and catalytic transformations to create further silicon derivatives tailored to specic applications.

Conflicts of interest
There are no conicts to declare.