catalysts

Review Synthesis of 1,3-Butadiene and Its 2-Substituted Monomers for Synthetic Rubbers

Yanlong Qi 1 , Zaizhi Liu 2, Shijun Liu 1,3, Long Cui 1, Quanquan Dai 1 , Jianyun He 1, Wei Dong 1 and Chenxi Bai 1,*

1 Key Laboratory of High-Performance Synthetic Rubber and Its Composite Materials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Road, Changchun 130022, China; [email protected] (Y.Q.); [email protected] (S.L.); [email protected] (L.C.); [email protected] (Q.D.); [email protected] (J.H.); [email protected] (W.D.) 2 Phytochemistry, College of Life Sciences, Jiangxi Normal University, Nanchang 330022, China; [email protected] 3 Applied Chemistry, University of Science and Technology of China, Jinzhai Road, Hefei 230026, China * Correspondence: [email protected]; Tel.: +86-453-85262304



Received: 17 November 2018; Accepted: 14 January 2019; Published: 17 January 2019


Abstract: Synthetic rubbers fabricated from 1,3-butadiene (BD) and its substituted monomers have been extensively used in tires, toughened plastics, and many other products owing to the easy polymerization/copolymerization of these monomers and the high stability of the resulting material in manufacturing operations and large-scale productions. The need for synthetic rubbers with increased environmental friendliness or endurance in harsh environments has motivated remarkable progress in the synthesis of BD and its substituted monomers in recent years. We review these developments with an emphasis on the reactive routes, the products, and the synthetic strategies with a scaling potential. We present reagents that are primarily from bio-derivatives, including ethanol, C4 alcohols, unsaturated alcohols, and tetrahydrofuran; the major products of BD and isoprene; and the by-products, activities, and selectivity of the reaction. Different catalyst systems are also compared. Further, substituted monomers with rigid, polar, or sterically repulsive groups, the purpose of which is to enhance thermal, mechanical, and interface properties, are also exhaustively reviewed. The synthetic strategies using BD and its substituted monomers have great potential to satisfy the increasing demand for better-performing synthetic rubbers at the laboratory scale; the laboratory-scale results are promising, but a big gap still exists between current progress and large scalability.

Keywords: Synthetic rubber; 1,3-butadiene synthesis; biomass-derived feedstock; sustainability; functional substituents

1. Introduction “Rubber” was coined by a chemist named Joseph Priestley in 1770. It has since come to be regarded as the “wheel” of civilization. Today, the term “rubber” is related to any material characterized by properties similar to those of natural rubber (NR). Attributed to the well-known importance of rubber since World War II, much effort has gone into producing rubbers by various synthetic routes [1]. In 2017, around 15.05 billion kilograms of synthetic rubbers were produced worldwide. The most prevalent synthetic rubber is styrene-butadiene rubber (SBR) which is derived from a copolymer of 1,3-butadiene (BD) and styrene. SBR has good aging stability and abrasion resistance [2], and it occupies about half of the market for car tires. Also widely used in tires is butadiene rubber (BR); it is made solely from the BD monomer and shows excellent wear resistance, good cold resistance, lower

Catalysts 2019, 9, 97; doi:10.3390/catal9010097 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 97 2 of 25 heat generation, and dynamic performance. Unfortunately, BR still falls short of meeting the goal of producing rubbery materials that are more environmentally friendly or robust to harsh environments. For example, BR needs to be blended with SBR and/or NR when it is used for sidewall and tread. A lot of effort has been made to clarify the nature of BR and its blends (such as with SBR and NR), and significant progress has been achieved in the regulation of the structure and performance of the polymer [3–5]. When used in tires, the elastic properties of synthetic rubbers made from 1,3-dienes play a primary role and are enormously affected by the microstructures of the repeating units, especially the presence or absence of a side substituent [6]. For example, cis-1,4-polybutadiene shows flexibility and excellent strong deformation tolerance [7,8] due to its low glass transition temperature (Tg) of −94 ◦C, which is ◦ much lower than that of the cis-1,4-polyisoprene with a –CH3 group at 2-C (from −64 to −70 C) [9]. The latter, known as isoprene rubber (IR, the major component of NR), has excellent overall rubbery properties and is widely used as a synthetic substitute for NR without blending with other polymers. Therefore, IR can be used for carcass and off-the-road tires without blending with other rubbery materials. It can be easily concluded that the side substituent of 1,3-butadienes plays a crucial role in tuning the properties of the resulting materials. However, serious problems are encountered when these polydienes are used for the production of mechanical engineering goods, such as heavy-lorry tires and aircraft tires, due to their inadequate mechanical properties (e.g., tensile strength, elongation at break, and tear resistance) [10]. Therefore, substituted butadienes are used to form polydienes with some BD derivatives, such as 2-phenylbutadiene, making it likely that a solution to these problems will be found. To this end, a functionalized 1,3-diene containing a rigid group is necessary. On the other hand, it is well-known that tires are made by blending synthetic rubbers with inorganic fillers, such as carbon black and silica. Unfortunately, polydienes containing solely carbon and hydrogen always exhibit poor surface properties and low adhesive properties. Therefore, in the manufacturing of tires, fillers such as hydrophilic silica are completely incompatible with synthetic rubbers, as the resulting tires have poor tear resistance, abrasion resistance, and static toughness properties [11]. To solve this problem, considerable attention has been paid to the synthesis of polar-functionalized dienes because it is very difficult to modify inorganic fillers [9]. The incorporation of these kinds of monomers into copolymers and/or homopolymers can significantly improve the existing properties of synthetic rubbers. For example, nitrile butadiene rubber (NBR) can enhance the wear resistance of tires. Additionally, NBR is also widely used for seals, hoses, and gloves. In this respect, the polar-functionalized 1,3-diene attracts a lot of attention. Accordingly, considering the entropy-elasticity of synthetic rubbers, various side substituents in 1,3-butadienes affect the properties of the resulting materials. To attain rubbers with tunable properties for meeting different requirements, introducing designed substituents into 1,3-butadiene is a promising strategy; it is also a great challenge, but this makes the success all the more rewarding. Functionalized monomers are also used to enhance the properties of synthetic rubbers, either by monopolymerization or copolymerization. In the present review, we highlight the following: (1) the production of BD and IP, which are in large demand in industry; (2) the synthesis of 2-substituted 1,3-butadienes with rigid groups, such as adamantly, phenyl, and its derivatives, which can be used to enhance the thermal and mechanical properties of the synthetic rubbers; and (3) the synthesis of polar-functionalized 1,3-butadienes containing O, Si, and N atoms and their unique properties for special applications. BD and IP are the basic monomers of the synthetic rubbers consumed widely in the automotive industry. Both BD and IP are mainly supplied by ethylene plants as by-products of naphtha cracking; taking sustainability into account, the alternative routes are urgently required [12]. The synthetic processes of substituted 1,3-butadienes always suffer from complicated operations under rigorous conditions. Catalysts 2019, 9, 97 3 of 25

Catalysts 2018, 8, x FOR PEER REVIEW 3 of 25 2. Synthesis of 1,3-butadiene At present, more than 95% of the world’s total BD is produced from either naphtha cracking or At present, more than 95% of the world’s total BD is produced from either naphtha cracking or n-butane dehydrogenation [13,14]. Unfortunately, these routes require expensive extractive n-butane dehydrogenation [13,14]. Unfortunately, these routes require expensive extractive distillation distillation with a low selectivity (for BD) of around 4–5%. Besides that, the shale gas revolution has with a low selectivity (for BD) of around 4–5%. Besides that, the shale gas revolution has led to a led to a decrease in the production of naphtha-based ethylene, which could reduce the production of decrease in the production of naphtha-based ethylene, which could reduce the production of BD BD too [15,16]. Additionally, the global supply of BD faces trouble due to the variation in chemical too [15,16]. Additionally, the global supply of BD faces trouble due to the variation in chemical feedstock and the unstable price of petrochemicals. In Japan, 3 out of 15 ethylene production plants feedstock and the unstable price of petrochemicals. In Japan, 3 out of 15 ethylene production plants have been shut down in the 3 years before June 2016. [17] Therefore, the industry urgently calls for have been shut down in the 3 years before June 2016. [17] Therefore, the industry urgently calls for the the development of an alternative process to produce BD without depending on petroleum. development of an alternative process to produce BD without depending on petroleum. The first production of BD from ethanol, carried out by Ipatiev in Russia in 1903, had a very The first production of BD from ethanol, carried out by Ipatiev in Russia in 1903, had a very low low yield of 1.5% [18], and great improvements were achieved by Lebedev’s group using an yield of 1.5% [18], and great improvements were achieved by Lebedev’s group using an undisclosed undisclosed composition [19]. A high yield and a BD selectivity of up to 70% under atmospheric composition [19]. A high yield and a BD selectivity of up to 70% under atmospheric pressure at 350 pressure at 350 °C were reported. Interessen-Gemeinschaft Farbenindustrie AG (IG Farben) ◦C were reported. Interessen-Gemeinschaft Farbenindustrie AG (IG Farben) reported that magnesia reported that magnesia catalysts promoted by Cr or Co exhibited a BD yield of 60% [20]. catalysts promoted by Cr or Co exhibited a BD yield of 60% [20]. Subsequently, the ethanol-to-BD Subsequently, the ethanol-to-BD route was abandoned in the US and most European countries as route was abandoned in the US and most European countries as this technology was deemed poorly this technology was deemed poorly competitive compared with the production of BD from competitive compared with the production of BD from petrochemical sources [21]. Currently, a return petrochemical sources [21]. Currently, a return to the old ethanol-to-BD production route is gaining to the old ethanol-to-BD production route is gaining renewed interest because of its remarkable capacity renewed interest because of its remarkable capacity for producing bio-ethanol and potential for producing bio-ethanol and potential environmental benefits [22]. BD formation from ethanol has environmental benefits [22]. BD formation from ethanol has been studied intensively, and several been studied intensively, and several possible BD formation proposals have been debated [19,23–35]. possible BD formation proposals have been debated [19, 23–35]. A reaction network proposed by A reaction network proposed by Quattlebaum et al. is presented in Figure1[ 30]. Obviously, BD Quattlebaum et al. is presented in Figure 1 [30]. Obviously, BD formation involves formation involves dehydrogenation, aldol condensation, hydrogenation, and a dehydration reaction, dehydrogenation, aldol condensation, hydrogenation, and a dehydration reaction, which implies which implies that various active sites such as basic, acidic, and metallic species are required for an that various active sites such as basic, acidic, and metallic species are required for an efficient efficient catalyst in this reaction. Unfortunately, it seems very difficult to develop these multifunctional catalyst in this reaction. Unfortunately, it seems very difficult to develop these multifunctional catalysts. Benefiting from an understanding of the reaction pathway, a more efficient two-step process catalysts. Benefiting from an understanding of the reaction pathway, a more efficient two-step employing acetaldehyde/ethanol as feedstock was developed (Figure2), but there is still a lot of work process employing acetaldehyde/ethanol as feedstock was developed (Figure 2), but there is still a to do to realize a BD production process with a yield higher than 90%. Although extensive studies have lot of work to do to realize a BD production process with a yield higher than 90%. Although been performed over the past 3 years [36–47] (Table1), as well as several published reviews [ 17,48–53], extensive studies have been performed over the past 3 years [36–47] (Table 1), as well as several the development of efficient processes and robust catalysts still remains a challenge and issues related published reviews [17,48–53], the development of efficient processes and robust catalysts still to carbon efficiency, catalyst cost, toxicity, feedstock tolerance and backend optimization need further remains a challenge and issues related to carbon efficiency, catalyst cost, toxicity, feedstock tolerance investigation [49]. and backend optimization need further investigation [49].

Figure 1. The general reaction mechanism and major by-products of 1,3-butadiene (BD) production Figure 1. The general reaction mechanism and major by-products of 1,3-butadiene (BD) production from ethanol [19,23–35]. from ethanol [19,23–35].

Catalysts 2019, 9, 97 4 of 25 Catalysts 2018, 8, x FOR PEER REVIEW 4 of 25

FigureFigure 2. 2. SynthesisSynthesis of of BD BD from from ethanol. ethanol. Route Route (a (a) )was was proposed proposed by by Ipatiev Ipatiev at at al. al. [18]; [18]; Route Route (b (b) )was was proposedproposed by by Ostromyslensky Ostromyslensky at at al. al. [54]. [54]. Table 1. Synthesis of BD from ethanol over various catalyst systems recently. Table 1. Synthesis of BD from ethanol over various catalyst systems recently. Reaction BD Selectivity BD Formation Catalyst Reaction◦ Conversion%Conversio BD Ref. BD Formation rate/mmol−1 Catalyst Temperature C % Ref. rate/mmol (gcat.h) temperature °C n% selectivity % (gcat.h)−1 ZnO–MgO/H–β280 350 43.6 63.4 [36] 0.92 ZnO–MgO/H–β280 350 43.6 63.4 [36] 0.92 NaZn1Zr10Oz–H 350 54.4 28 [37] 9.07 NaZn1Zr10Oz–H 350 54.4 28 [37] 9.07 Cu/MCF–Zr/MCF 235, 400 96 37 [38] 25.92 Cu/MCF–Zr/MCFa 235, 400 96 37 [38] 25.92 MgO–SiO2–500 500 Total 29.7 80.7 [39] 0.96 a MgO–SiOTalc/Zn2–500 400500 Total 48.4 29.7 61.080.7 [39] [40 ]0.96 8.53 Talc/Zn 400 48.4 61.0 [40] 8.53 MgO–SiO2 (65:35) 450 95 77 [41] 25 3%Au/MgO–SiOMgO–SiO2 (65:35)2 300 450 >45 95 ~60 77 [41] [42 ] 25 2.4 Ag/Zr(3.3)BEA(38)3%Au/MgO–SiO2 320 300 15 >45 ~60 ~60 [42] [43 ] 2.4 10.3 Cu/SiOAg/Zr(3.3)BEA(38)2 MgO/H–β 280 100,320 300 -15 33%~60 yield [43] [44 ]-10.3 Ta3.0SiBEA–EtOH/AA=3.2Cu/SiO2 MgO/H–β280 100,350 300 58.9 - 33% 73.1 yield [44] [45 ]- - a Ta3.0SiBEA–EtOH/AA=3.22%ZrO2/NanoSiO2–500 320350 58.52 58.9 93.1873.1 [45] [46 ]- - 2%ZrO5 % Ag/MgO–SiO2/NanoSiO2-5002 a 275320 >5058.52 >2893.18 yield [46] [47 ]- - 2.5% Cu-2.5%wt 5 % Ag/MgO–SiO2 300 275 60 >50 >28 >40 yield yield [47] [47 ]- - Ag/MgO–SiO2 2.5% Cu-2.5%wt Ag/MgO–SiO2 300 60 >40 yield [47] - a aAcetaldehydeAcetaldehyde was was co-fed co-fed withwith ethanol. ethanol.

NextNext to to ethanol, ethanol, alternative alternative bio-derived bio-derived C4 C4 alcohols, alcohols, such such as as 1,3-butanediol 1,3-butanediol (1,3-BDO), (1,3-BDO), 2,3-butanediol2,3-butanediol (2,3-BDO), (2,3-BDO), and and 1,4-butanediol 1,4-butanediol (1,4-BDO), (1,4-BDO), are are regarded regarded as as promising promising feedstocks feedstocks to to produceproduce BD, BD, and and their their use use in in BD BD synthesis synthesis has has been been demonstrated demonstrated (Figure (Figure 33).). Makshina et et al. al. [49] [ 49 ] summarizedsummarized this this part part based based on on many many research research efforts efforts as as well well as reviews. They They have have mainly mainly concentratedconcentrated on on the the dehydration and deoxydehydration ofof thesethese C4 C4 alcohols alcohols to to BD BD from from the the aspects aspects of ofmechanistic mechanistic and and catalytic catalytic chemistry. chemistry. Very Very recently, recent Jingly, etJing al. [et55 ]al. studied [55] studied the dehydration the dehydration of 1,3-BDO of 1,3-BDOto BD over to BD aluminosilicates. over aluminosilicates. The best The result best was result achieved was achieved over H–ZSM–5 over H–ZSM–5 (silica/alumina (silica/alumina = 260) with = ◦ 260)a BD with yield a ofBD 60% yield at 300of 60%C. at They 300 pointed °C. They out poin thatted the out good that catalytic the good performance catalytic performance of the catalysts of the is a catalystsresult of is the a presence result of of the Brønsted presence acid of sites Brønst withed weak acid and sites medium with weak strength. and Accompanied medium strength. by the Accompaniedunavoidable formationby the unavoidable of carbon formation deposits, theof carb conversionon deposits, decreases the conversi from 100%on decreases to 80% afterfrom a 100% 102 h torun. 80% Subsequently, after a 102 h Al–dopedrun. Subsequently, SBA–15 catalysts Al–doped were SBA–15 applied catalysts for this were reaction applied [56]. for The this highest reaction BD [56].yield, The 59%, highest was achieved BD yield, over 59%, Al–SBA–15 was achieved (silica/alumina over Al–SBA–15 = 76), with (silica/alumina stable activity = in76), long-term with stable runs. activityNotably, in thelong-term operation runs. is performed Notably, the at aoperation relatively is low performed temperature at a (200relatively◦C). low temperature (200 °C). Zeng et al. [57] studied the direct conversion of 2,3-BDO to BD over alumina catalysts with a wide rangeZeng of temperatures et al. [57] studied from 240 the to direct 450 ◦ C.conversion They clarified of 2,3-BD that 3-buten-2-olO to BD over (3B2OL) alumina is a catalysts key intermediate with a widein the range dehydration of temperatures of 2,3-BDO from to BD 240 by to DFT, 450 and °C. 3B2OL They clarified can be easily that converted3-buten-2-ol to methyl (3B2OL) ethyl is ketonea key intermediate(MVK) rather in than the BDdehydration in the presence of 2,3-BDO of sodium-modified to BD by DFT, aluminaand 3B2OL catalysts can be because easily ofconverted the natural to methylbasicity ethyl of the ketone catalysts. (MVK) The rather experimental than BD results in the suggestpresence that of asodium-modified higher selectivity alumina to BD is catalysts obtained becauseunder a of higher the natural reaction basicity temperature. of the Fangcatalysts. et al. [The58] conductedexperimental the results dehydration suggest of 1,3-BDOthat a higher to BD selectivityover a series to ofBD Ce@MOR is obtained catalysts under with a higher a flower-like reaction morphology temperature. at Fang 350 ◦ C.et Ce@MORal. [58] conducted with Si/Ce the at dehydrationan atomic ratio of 1,3-BDO of 50 presents to BD aover high a 1,3-BDOseries of conversionCe@MOR catalysts of 100% butwith a a poor flower-like selectivity morphology of 46% to BD.at 350The °C. high Ce@MOR activity with of Ce@MOR Si/Ce at an is atomic attributed ratio to of the 50 highpresents density a high of Brønsted1,3-BDO conversion acid sites with of 100% medium but a poor selectivity of 46% to BD. The high activity of Ce@MOR is attributed to the high density of Brønsted acid sites with medium strength. Nguyena et al. [59] reported the dehydration of 2,3-BDO

Catalysts 2018, 8, x FOR PEER REVIEW 5 of 25 to BD over a GdPO4 catalyst which resulted in the complete conversion of 2,3-BDO and a selectivity of 58% to BD at 300 °C. Both the fresh and regenerated GdPO4 displayed relatively stable catalytic activity in a 50 h run. Kim et al. [60] carried out the direct dehydration of 2,3-BDO over silica-supportedCatalysts 2019, 9, 97 sodium phosphates catalysts at 400 °C. These findings show that the conversion5 of of 25 2,3-BDO can be easily tuned up to 100%, while the selectivity to BD never exceeds 70%. All of these studies confirm that Brønsted acid sites with weak and medium strength are a key factor in the strength. Nguyena et al. [59] reported the dehydration of 2,3-BDO to BD over a GdPO catalyst which conversion of butanediols (BDOs) to BD. A great improvement was observed by Tsukamoto4 [61], resulted in the complete conversion of 2,3-BDO and a selectivity of 58% to BD at 300 ◦C. Both the who investigated the dehydration of 2,3-BDO to BD over SiO2-supported CsH2PO4 catalysts at fresh and regenerated GdPO displayed relatively stable catalytic activity in a 50 h run. Kim et al. [60] 350 °C. The highest conversion4 of 2,3-BDO (>99.9%) and an excellent selectivity to BD (91.9%) were carried out the direct dehydration of 2,3-BDO over silica-supported sodium phosphates catalysts at obtained over 10% CsH2PO4, with a slight decrease over an 8 h run. This impressive catalytic activity 400 ◦C. These findings show that the conversion of 2,3-BDO can be easily tuned up to 100%, while is ascribed to a combination of the proper acid–base sites of Cs phosphate and the large ionic radius the selectivity to BD never exceeds 70%. All of these studies confirm that Brønsted acid sites with of Cs+, which is significantly different from Na+ [57]. weak and medium strength are a key factor in the conversion of butanediols (BDOs) to BD. A great However, the direct dehydration of C4 alcohols into BD always needs a high reaction improvement was observed by Tsukamoto [61], who investigated the dehydration of 2,3-BDO to BD temperature. As BD can be readily obtained via the dehydration of unsaturated C4 alcohols (UOLs) over SiO -supported CsH PO catalysts at 350 ◦C. The highest conversion of 2,3-BDO (>99.9%) and an over solid2 acid catalysts, a 2novel4 path way to produce BD from C4 alcohols via the dehydration of C4 excellent selectivity to BD (91.9%) were obtained over 10% CsH PO , with a slight decrease over an alcohols to their corresponding UOLs was developed, and it 2is performed4 at a relatively low 8 h run. This impressive catalytic activity is ascribed to a combination of the proper acid–base sites of temperature. Sato’s group has done a lot of work in this field. Duan et al. have reviewed the Cs phosphate and the large ionic radius of Cs+, which is significantly different from Na+ [57]. dehydration of BDOs to UOLs in connection with their further dehydration to BD [17].

Figure 3. Synthesis of BD from butanediols (BDOs) or C4 unsaturated alcohols (UOLs) [17,62–65]. Figure 3. Synthesis of BD from butanediols (BDOs) or C4 unsaturated alcohols (UOLs) [17,62–65]. Evolution from reagents and intermediates to the target BD is labeled by arrows, and the by-products Evolution from reagents and intermediates to the target BD is labeled by arrows, and the are produced during Dehydration 1. by-products are produced during Dehydration 1. However, the direct dehydration of C4 alcohols into BD always needs a high reaction temperature. As BDNot can long be ago, readily Duan obtained et al. [62] via theinvestigated dehydration the ofdehydration unsaturated ofC4 2,3-BDO alcohols to (UOLs)3B2OL over over modified solid acid monocliniccatalysts, a ZrO novel2 catalysts, path way suggesting to produce that BD from BaO/ZrO C4 alcohols2, SrO/ZrO via the2, and dehydration CaO/ZrO2of have C4 alcoholsgood catalytic to their capacities.corresponding The optimized UOLs was developed,data were observed and it is performed over BaO–0.0452–800 at a relatively with low a temperature. 2,3-BDO conversion Sato’s group of 72.4%has done and aa lot 3B2OL of work selectivity in this field. of 74.4% Duan during et al. havethe early reviewed stage the of dehydration5 h at 350 °C. of Unfortunately, BDOs to UOLs ina connection with their further dehydration to BD [17]. Catalysts 2019, 9, 97 6 of 25

Not long ago, Duan et al. [62] investigated the dehydration of 2,3-BDO to 3B2OL over modified Catalysts 2018, 8, x FOR PEER REVIEW 6 of 25 monoclinic ZrO2 catalysts, suggesting that BaO/ZrO2, SrO/ZrO2, and CaO/ZrO2 have good catalytic capacities. The optimized data were observed over BaO–0.0452–800 with a 2,3-BDO conversion of decrease in catalytic activity occurred over a long-term operation of 25 h. This is due to the 72.4% and a 3B2OL selectivity of 74.4% during the early stage of 5 h at 350 ◦C. Unfortunately, a formation of Perovskite compounds, such as BaZrO3, which are inactive for the reaction. The decrease in catalytic activity occurred over a long-term operation of 25 h. This is due to the formation modified monoclinic ZrO2 also presents high catalytic activity for the dehydration of 1,4-BDO to of Perovskite compounds, such as BaZrO3, which are inactive for the reaction. The modified monoclinic 3-buten-1-ol (3B1OL), especially 7–CaO+2–ZrO2/m–ZrO2–800, which produces an excellent 1,4-BDO ZrO also presents high catalytic activity for the dehydration of 1,4-BDO to 3-buten-1-ol (3B1OL), conversion2 of 95.2% and a high 3B1OL selectivity of 89.3% at 350 °C [63]. Then, the dehydration of especially 7–CaO+2–ZrO2/m–ZrO2–800, which produces an excellent 1,4-BDO conversion of 95.2% UOLs to BD was conducted by Sun et al. [64] over solid acid catalysts, like SiO2–Al2O3, Al2O3, TiO2, and a high 3B1OL selectivity of 89.3% at 350 ◦C[63]. Then, the dehydration of UOLs to BD was and ZrO2, and basic rare earth metal oxides, including Yb2O3 and CeO2. This suggests that acid conducted by Sun et al. [64] over solid acid catalysts, like SiO –Al O , Al O , TiO , and ZrO , and catalysts have a higher activity for the dehydration of 2B1OL and2 3B2OL2 3 than2 3 the rare2 earth metal2 basic rare earth metal oxides, including Yb2O3 and CeO2. This suggests that acid catalysts have a higher oxides. The 10SiO2/Al2O3 catalyst shows the highest 2-buten-1-ol (2B1OL) conversion (92.5%) and activity for the dehydration of 2B1OL and 3B2OL than the rare earth metal oxides. The 10SiO2/Al2O3 selectivity (75.0%) at 260 °C, but there is a rapid deactivation. Modified 10SiO2/Al2O3 with Ag can catalyst shows the highest 2-buten-1-ol (2B1OL) conversion (92.5%) and selectivity (75.0%) at 260 enhance◦ the catalytic activity and inhibit the formation of carbon deposits. The best result of the C, but there is a rapid deactivation. Modified 10SiO2/Al2O3 with Ag can enhance the catalytic dehydration of 2B1OL was achieved over 5Ag–SiO2/Al2O3, displaying a BD selectivity of 94.6% at a activity and inhibit the formation of carbon deposits. The best result of the dehydration of 2B1OL was conversion of 99.1% at 260 °C. Also, 5Ag–SiO2/Al2O3 efficiently inhibits catalytic deactivation, with a achieved over 5Ag–SiO /Al O , displaying a BD selectivity of 94.6% at a conversion of 99.1% at 260 ◦C. slight decrease in the 2B1OL2 2 conversion3 (from 99.4% to 96.2%) and BD selectivity (from 95.0% to Also, 5Ag–SiO2/Al2O3 efficiently inhibits catalytic deactivation, with a slight decrease in the 2B1OL 87.1%) after 10 h. In contrast, the rare earth metal oxides, such as CeO2, show poor activity for the conversion (from 99.4% to 96.2%) and BD selectivity (from 95.0% to 87.1%) after 10 h. In contrast, the dehydration of 2B1OL and 3B2OL to BD, but they have a remarkable activity in the dehydration of rare earth metal oxides, such as CeO2, show poor activity for the dehydration of 2B1OL and 3B2OL 3B1OL to BD, which is attributed to the weak basicity and the redox nature of CeO2 via Ce4+–Ce3+. to BD, but they have a remarkable activity in the dehydration of 3B1OL to BD, which is attributed The synthesis of BD from 3B1OL over the acid catalysts shows a much lower BD yield resulting from to the weak basicity and the redox nature of CeO via Ce4+–Ce3+. The synthesis of BD from 3B1OL the C–C cleavage of 3B1OL to propylene. Thus, it 2is concluded that the dehydration mechanism of over the acid catalysts shows a much lower BD yield resulting from the C–C cleavage of 3B1OL to 3B1OL to BD is quite different from the dehydration of 2B1OL and 3B2OL. propylene. Thus, it is concluded that the dehydration mechanism of 3B1OL to BD is quite different Therefore, Wang et al. [65] studied the production of BD by the dehydration of 3B1OL and from the dehydration of 2B1OL and 3B2OL. 1,4-butanediol over rare earth oxides, such as Lu2O3 and Yb2O3. The optimal result of the Therefore, Wang et al. [65] studied the production of BD by the dehydration of 3B1OL and dehydration of 3B1OL was obtained with a high conversion of 99.6% and a good BD selectivity of 1,4-butanediol over rare earth oxides, such as Lu2O3 and Yb2O3. The optimal result of the dehydration 96.7% over the Yb2O3 catalyst at 340 °C. Accordingly, the direct dehydration of 1,4-BDO to BD over of 3B1OL was obtained with a high conversion of 99.6% and a good BD selectivity of 96.7% over Yb2O3 also exhibited excellent a catalytic performance with a yield of 96.6% at 360 °C. Thus, the Yb O catalyst at 340 ◦C. Accordingly, the direct dehydration of 1,4-BDO to BD over Yb O also producing2 3 BD from bio-derived C4 alcohols and especially unsaturated C4 alcohols via2 3 the exhibited excellent a catalytic performance with a yield of 96.6% at 360 ◦C. Thus, producing BD dehydration process is a promising route, although a large amount of work still needs to be done, especiallyfrom bio-derived for improving C4 alcohols catalyst and life especially for industry unsaturated usage. C4 alcohols via the dehydration process is a promisingTetrahydrofuran route, although (THF) can a large also amounteasily generate of work 1,4-BDO, still needs which to be comes done, from especially five-carbon for improving sugars suchcatalyst as xylose life for or industry furfural usage. [66–69]. Therefore, Abdelrahman et al. [70] developed a novel way to synthesizeTetrahydrofuran BD via the (THF) catalytic can alsoring-opening easily generate dehydration 1,4-BDO, of which THF, comes which from is conducted five-carbon in sugars the presencesuch as xyloseof a solid or acid furfural catalyst [66– with69]. Therefore,a high selectivity Abdelrahman of 85–99% et to al. BD [70 at] developeda high THF a conversion novel way of to synthesize BD via the catalytic ring-opening dehydration of THF, which is conducted in the presence of 89% (Figure 4). These encouraging data were obtained at 400 °C with a space velocity of 0.2 s–1. a solid acid catalyst with a high selectivity of 85–99% to BD at a high THF conversion of 89% (Figure4). Unfortunately, the conversion of THF significantly decreases to 8.9% with a turning space velocity These encouraging data were obtained at 400 ◦C with a space velocity of 0.2 s−1. Unfortunately, of up to 9.7 s–1. Therefore, a large change in this route must be implemented to improve the BD −1 formationthe conversion capacity. of THF significantly decreases to 8.9% with a turning space velocity of up to 9.7 s . Therefore, a large change in this route must be implemented to improve the BD formation capacity.

Figure 4. Synthesis of BD from THF through the intermediate 3-buten-1-ol (3B1OL), and the Figure 4. Synthesis of BD from THF through the intermediate 3-buten-1-ol (3B1OL), and the by-products in the decyclization of THF [70]. by-products in the decyclization of THF [70].

3. Synthesis of Isoprene Isoprene (IP), also called 2-methyl-1,3-butadiene, is well-known as an irreplaceable monomer of synthetic natural rubber. It is a colorless liquid with a characteristic odor, and it is highly

Catalysts 2019, 9, 97 7 of 25

3. Synthesis of Isoprene Isoprene (IP), also called 2-methyl-1,3-butadiene, is well-known as an irreplaceable monomer of synthetic natural rubber. It is a colorless liquid with a characteristic odor, and it is highly significant for producing stereoregular polyisoprene because of the ability to purposefully manipulate its properties. The huge demand for this kind of material inspires the rapid development of the synthesis of isoprene (Figure5). The main methods used to produce isoprene in the industry have been reported elsewhere [71]. In this review, the focus is on a promising route to produce isoprene using isobutene and formaldehyde as starting materials via one step; this has attracted great interest both in the industry and in academia. This process is conducted in the presence of solid acid catalysts, such as zeolites, silver, oxide catalysts, phosphates, and heteropolyacid catalysts. Moreover, the recent development of the isoprene synthesis using another method is also presented. Catalysts 2019, 9, 97 8 of 25 Catalysts 2018, 8, x FOR PEER REVIEW 8 of 25

Figure 5. Overview of isoprene (IP) production routes [71–74]. Catalysts 2019, 9, 97 9 of 25

Dumitriu et al. studied the reaction over zeolite catalysts [75,76]. The strength of the acid sites is suggested to play a crucial role in isoprene synthesis. The weak Brønsted acid sites on zeolites are proved to be the most efficient sites. In this respect, the highest selectivity, 99–100%, was obtained over boralites and ferrisilicates. Nonetheless, these results were obtained in a pulse catalytic reactor, which cannot be compared with those obtained in continuous flow systems. Ag–Sb catalysts have been employed as catalysts in bed reactors and resulted in good activity, with a conversion of around 81% (for formaldehyde) and selectivity of about 70% (for isoprene). However, the activity of Ag–Sb catalysts decreases quickly and presents poor stability [77]. CuSO4/SiO2 catalysts were developed which show a formaldehyde conversion of about 63% and selectivity of about 65% to isoprene. It has been suggested that doping CuSO4/SiO2 with basic oxides to decrease the acidity can greatly improve their activity. A formaldehyde conversion of 87% and selectivity of about 65% to isoprene were obtained over CuSO4/SiO2 doped with MgO. However, the short lifetime of the CuSO4/SiO2 catalysts impedes their application in the industry, and this deficiency can be attributed to the decreasing number of acid sites resulting from the rapid formation of carbon deposits and the destruction of the CuSO4 structure [78,79]. Krzywicki et al. [80] first reported the reaction of formaldehyde and isobutene over Al2O3–H3PO4 catalysts. A high H3PO4 content favors isoprene production owing to the increased number of acid sites. However, the phosphate catalysts still displayed low activity, with an isoprene yield of 22%. Ai et al. [81] tried to use the oxides of transition metals to improve the activity of the phosphate catalysts. MoO3-, WO3-, and V2O5–based phosphate catalysts were employed for this reaction, using tert-butyl alcohol as a raw material instead of isobutene. V2O5–P2O5 (P/V = 1.06) exhibited outstanding activity, with an isoprene yield of about 60%. The author pointed out that the basic sites also play a certain role in the synthesis of isoprene. Although the phosphate catalysts show a rather excellent activity in both conversion and selectivity, they are characterized by rapid deactivation due to the formation of large carbon deposits. Therefore, catalysts with longer lifetimes and an understanding of the influence of carbon deposition are urgently needed. Recently, Sushkevich et al. [82] conducted the condensation of formaldehyde with isobutene over a Nb2O5–P2O5 catalyst. This catalyst is characterized by a much longer lifetime and stable formaldehyde conversion, producing a steady isoprene yield of 57% within more than 30 h of stream. Nb2O5–P2O5 shows high resistance to deactivation, which is ascribed to the in situ restoration of Brønsted acid sites in the presence of water during the reaction. Moreover, spent ◦ Nb2O5–P2O5 can be completely regenerated by calcination at 500 C in an air flow. It was verified that Lewis acid sites are responsible for the side reaction, which led to a low selectivity of 62.6%. Therefore, supported Keggin-type heteropolyacids catalysts with weak Brønsted sites are being developed [83,84]. Most importantly, Ivanova et al. clarified that unsaturated branched carbonaceous species in carbon deposits can generate active sites for the condensation of formaldehyde with isobutene to produce isoprene [85]. To further identify the effect of carbon deposition on the synthesis of isoprene, our group conducted the reaction over MoO3–P2O5 [86]. 4,4-Dimethyldioxane-1,3 (DMD) has been demonstrated to be an important intermediate which can readily form on acid sites over a catalyst and further be converted into isoprene in the presence of carbonaceous species. In this regard, the formation of carbon deposits can give rise to a synergetic effect between acid species and carbonaceous species for isoprene synthesis. Slight carbon deposition is able to shorten the induction period of the reaction, while a large carbon deposition leads to catalyst deactivation because it attaches to Brønsted acid sites. For the above reasons, a simple way to restore the reactivity of deactivated phosphate catalysts was developed with the assistance of the carbon deposition [87]. Moreover, additional studies on the synthesis of isoprene by alternative routes have been reported. Songsiri et al. [72,73] synthesized isoprene using heteropolyacids as catalysts in a liquid-phase system under moderate conditions in which carbon deposits can be inhibited. Methyl tertiary-butyl ether (MTBE) was used as the source of isobutene. MTBE is more available because its use as an additive in gasoline has been prohibited in developed countries, such as the United States, Japan, and Western Catalysts 20182019, 89, 97x FOR PEER REVIEW 1010 of of 25

Japan, and Western Europe. High formaldehyde conversion (>95.8%) and isoprene yield (68.3–77.5%)Europe. High were formaldehyde achieved over conversion a Cs-exchanged (>95.8%) andsilico isoprenetungsticyield acid catalyst. (68.3–77.5%) Unfortunately, were achieved it is overstill a Cs-exchangedchallenge to completely silicotungstic separate acid catalyst. the catalysts Unfortunately, from the it reaction is still a challengesystem, and to completelya large amount separate of organicthe catalysts solvent from is employed the reaction in this system, process, and which a large could amount result of in organic environmental solvent isproblems. employed in this process,Considering which could environmental result in environmental concerns and problems. sustained availability, the biosynthesis of isoprene was developedConsidering based environmental on metabolic concerns engineering. and sustained However, availability, the necessary the productivity biosynthesis has of not isoprene been achievedwas developed with microbes based on [88]. metabolic It was engineering.necessary to However,develop an the alte necessaryrnative method productivity for the has sustainable not been productionachieved with of isoprene microbes at [ 88a large]. It was scale. necessary Therefore, to developit is a promisin an alternativeg and competitive method for route the sustainable to convert biomass-derivedproduction of isoprene feedstocks at a large into scale.isoprene Therefore, via chemical it is a promisingprocesses. andItaconic competitive acid from route glucose to convert [89] wasbiomass-derived employed as feedstocks raw material into by isoprene Abdelrahman via chemical [74]. processes.Firstly, itaconic Itaconic acid acid was from converted glucose into [89] 3-methyl-tetrahydrofuranwas employed as raw material (3-MTHF) by Abdelrahmanwith a yield of [about74]. Firstly,80% in a itaconic liquid phase acid wasusing converted Pd–Re/C intoas a catalyst.3-methyl-tetrahydrofuran Then the dehydration (3-MTHF) of 3-MTHF with a was yield performed of about 80%in a ingas a phase liquid over phase a usingP-SPP Pd–Re/C (self-pillared as a pentasil)catalyst. Thencatalyst the with dehydration an isoprene of 3-MTHFselectivity was of performed70% at a 20–25% in a gas conversion. phase over For a P-SPP this process, (self-pillared much workpentasil) is needed catalyst withto improve an isoprene the yield selectivity of isoprene of 70%, atto a understand 20–25% conversion. the catalytic For this pathway, process, and much to developwork is needed a more to efficient improve catalysts. the yield Extensive of isoprene, research to understand on IP synthesis the catalytic is urgently pathway, required, and to develop and the a explorationmore efficient of catalysts.novel platform Extensive chemicals research faces on IPch synthesisallenges, but is urgently success required,would be and greatly the explorationrewarding. of novel1,3-Pentadiene platform chemicals (piperylene) faces challenges, has similar but properties success would to isoprene be greatly and rewarding.is a large-scale by-product accompanying1,3-Pentadiene the production (piperylene) of has isoprene. similar propertiesIt is widely to used isoprene in the and produc is a large-scaletion of sticky by-product tapes, adhesives,accompanying and theplastics. production In Russia, of isoprene. 1,3-pentadiene It is widely is usedalso inused the productionto produce ofliquid sticky rubber, tapes, adhesives, which is intensivelyand plastics. used In Russia, in paint 1,3-pentadiene and varnish isindustry also used [90–92]. to produce Moreover, liquid rubber,many studies which ishave intensively focused usedon the in potentialpaint and application varnish industry of poly(1,3-pentadiene) [90–92]. Moreover, in many the domain studies of have elastomers. focused on All the of potentialthese factors application inspire wideof poly(1,3-pentadiene) interest to study inthe the production domain of elastomers.of 1,3-pentadiene All of these [93–95]. factors From inspire the wideview interest of sustainable to study development,the production many of 1,3-pentadiene efforts have [93 been–95]. Frommade the to viewdevelop of sustainable alternative development, technologies many for effortsproducing have 1,3-pentadienebeen made to develop based on alternative biomass-derived technologies feedstocks for producing [96–99]. 1,3-pentadiene Recently, Sun based et al. on [100] biomass-derived developed a novelfeedstocks two-step [96– 99synthetic]. Recently, approach Sun et al.using [100 xylitol] developed as a resource a novel two-step(Figure synthetic6). Xylitol approach is industrially using producedxylitol as aby resource extracting (Figure hardwoods6). Xylitol or is corncobs. industrially In the produced first step, by extractingthe deoxydehydration hardwoods or (DODH) corncobs. of ◦ xylitolIn the firstwas step,conducted the deoxydehydration at 235 °C in the (DODH)presence ofof xylitolformic was acid conducted to give 2,4-pentadien-1-ol, at 235 C in the presence 1-formate of (2E),formic with acid an to optimized give 2,4-pentadien-1-ol, yield of 62.9%, 1-formate followed (2E), by deoxygenation with an optimized to obtain yield 1,3-pentadiene of 62.9%, followed over by a ◦ Pd/Cdeoxygenation catalyst at to 100 obtain °C. 1,3-pentadiene The total yield over of a1,3-pentadiene Pd/C catalyst atreached 100 C. 51.8%. The total Kumbhalkar yield of 1,3-pentadiene et al. [101] employedreached 51.8%. 2-methyltetrahydrofuran Kumbhalkar et al. [101 (2-MTHF)] employed which 2-methyltetrahydrofuran can be easily obtained (2-MTHF) from biomass-derived which can be intermediateseasily obtained (levulinic from biomass-derived acid and furfural, intermediates etc.), as the (levulinic starting material acid and to furfural, produce etc.), 1,3-pentadiene as the starting in ◦ amaterial continuous to produce system. 1,3-pentadiene This reaction inis aconducted continuous at system.350 °C, Thisand the reaction catalyst is conducted undergoes at deactivation 350 C, and fromthe catalyst the formation undergoes of deactivation carbon deposits from thein the formation presence of carbonof dienes. deposits Therefore, in the presencea decrease of dienes.in the conversionTherefore, aof decrease 2-MTHF in (from the conversion 100% to 77%) of 2-MTHF and yield (from of 1,3-pentadiene 100% to 77%) and(from yield 67.8% of 1,3-pentadieneto 51.8%) was observed(from 67.8% over to a 51.8%) period was of 58 observed h. over a period of 58 h.

FigureFigure 6. 6.Synthesis Synthesis of 1,3-pentadieneof 1,3-pentadiene from from 2-methyltetrahydrofuran 2-methyltetrahydrofuran (2-MTHF) (2-MTHF) (a) and ( Xylitola) and (Xylitolb)[100 ,(101b) ]. [100,101]. 4. Rigid-Group Functionalized 1,3-butadienes 4. Rigid-Group Functionalized 1,3-butadienes 4.1. Synthesis of 2-(1-adamantyl)-1,3-butadiene 4.1. SynthesisDue to its of highly2-(1-adamantyl)-1,3-butadiene symmetrical and rigid tricyclic structure, adamantane features chemical and thermal stability, hydrophobic character, and UV transparency. It has been introduced into polymers Due to its highly symmetrical and rigid tricyclic structure, adamantane features chemical and thermal stability, hydrophobic character, and UV transparency. It has been introduced into

Catalysts 2019, 9, 97 11 of 25 Catalysts 2018, 8, x FOR PEER REVIEW 11 of 25 polymersto improve to their improve thermal their and thermal mechanical and mechanical properties properties [102–111]. Therefore,[102–111]. introductionTherefore, introduction of adamantyl of adamantylinto 1,3-dienes into is1,3-dienes explored is as explored presented as in presented Figure7[112 in Figure,113]. 7 [112,113].

Figure 7. Synthesis of 2-adamantyl-1,3-butadiene [112,113]. [112,113].

Firstly, methylmagnesium iodide is prepared by adding methyl iodide and magnesium into diethyl ether.ether. 1-Adamantanecarboxylic1-Adamantanecarboxylic acid acid (Figure (Figure7a) and7a) and thionyl thionyl chloride chloride are charged are charged into a vessel,into a ◦ vessel,and the and mixture the mixture is heated is toheated 80 Cto and 80 °C then and left then for 2left h withfor 2 stirring.h with stirring. After the After reaction, the reaction, thionyl thionylchloride chloride is removed is removed to afford to aafford white a solid white of so 1-adamantanecarbonyllid of 1-adamantanecarbonyl chloride chloride (Figure (Figure7b) under 7b) underreduced reduced pressure. pressure. Then, diethylThen, diethyl ether and ether CuCl and are CuCl added are toadded 1-adamantanecarbonyl to 1-adamantanecarbonyl chloride chloride under a ◦ underN2 atmosphere, a N2 atmosphere, followed followed by the dropwise by the additiondropwise of addition methylmagnesium of methylmagnesium iodide at 0 iodideC under at a0 N°C2 underatmosphere. a N2 atmosphere. After stirring After at roomstirring temperature at room temperatur for 30 min,e for the 30 reaction min, the is reaction quenched is quenched with 2 N with HCl. The2 N HCl. mixture The is mixture extracted is extracted with diethyl with ether. diethyl Then, ethe ther. Then, organic the layerorganic is driedlayer overis dried anhydrous over anhydrous MgSO4. AfterMgSO the4. After removal the ofremoval the solvent, of the the solvent, product the (Figure product7c) (Figure is obtained. 7c) is In obtained. an ice/water In an bath ice/water under abath N 2 underatmosphere, a N2 atmosphere, the THF solution the THF of vinylsolution bromide of vinyl is addedbromide dropwise is added to dropwise magnesium, to magnesium, which is activated which withis activated 1,2-dibromoethane with 1,2-dibromoethane and stirred atand this stirred temperature at this temperature for another 1for h, another then at room1 h, then temperature at room temperaturefor 1 h. Prior for to the1 h. additionPrior to the of theaddition THF solutionof the THF of thesolution product of the in Figureproduct7c, in the Figure reaction 7c, the mixture reaction is ◦ mixturecooled down is cooled to 0 downC. Finally, to 0 °C. stirring Finally, at roomstirring temperature at room temperature overnight isovernight maintained. is maintained. The reaction The is quenchedreaction is withquenched 2 N HCl, with and 2 N diethyl HCl, and ether diethyl is used ethe to extractr is used the to reaction extract the system. reaction After system. the evaporation After the evaporationof the solution, of the productsolution, (Figure the product7d) is obtained.(Figure 7d) The is solutionobtained. of The the solution product (Figureof the product7d) in benzene (Figure 7d)refluxes in benzene for 2 h inrefluxes the presence for 2 h ofinp the-toluenesulfonic presence of p acid.-toluenesulfonic After cooling, acid. the After organic cooling, layer isthe separated organic layerand dried. is separated The solvent and dried. is removed The solvent to afford is removed the final productto afford (Figure the final7e). product (Figure 7e).

4.2. Synthesis of 2-phenyl-1,3-butadienes 2-Phenyl-1,3-butadiene (2-PB) (2-PB) can can be regarded as either an α-vinyl-substituted styrene or 2-phenyl-substituted 1,3-butadiene. Therefore, 2-PB is a unique monomer because of the importance of styrene and 1,3-butadiene in synthetic rubbers. Moreover, Moreover, with with a a phenyl group replacing methyl, Poly(2-PB) exhibits distinctive properties comparedcompared with polyisoprene.polyisoprene. Moreover, Poly(2-PB) can ◦ also be used to produce high-temperaturehigh-temperature thermoplastics with a Tg as high as 325 °CC[ [114].114]. Since the successful synthesis of 2-PB by Marvel, [[115115] it received muchmuch attentionattention [[116,117].116,117]. As displayed in Figure8, vinyl bromide and magnesium are used to prepare the THF solution of vinylmagnesium bromide, followed by the addition of acetophenone (Figure8a). The mixture must be maintained under gentle reflux and then maintained with stirring for 1 h. A saturated aqueous solution of ammonium chloride is applied for the hydrolysis process to obtain the crude product. After the distillation and removal of the solvent, methylphenylvinylcarbinol (Figure8b) is afforded with a yield of 75%. Methylphenylvinylcarbinol, aniline hydrobromide, and hydroquinone are charged into a round-bottomed flask equipped with a Vigreux column. The mixture is heated

Catalysts 2019, 9, 97 12 of 25 at 100 ◦C in an oil bath, followed by gentle distillation with the temperature increasing to 150 ◦C. The fraction with a b.p. of 57–63 ◦C is collected and dried by anhydrous calcium chloride to obtain 2-PB (Figure8c). Recently, Yao et al. [118] used acetophenone as the starting material to prepare 2-PB via elaborate manipulations, which are performed under a dried and oxygen-free atmosphere, and the solvents involved in the reaction must be purified. In the typical procedure, acetophenone is first converted into 2-phenylbut-3-en-2-ol in the presence of vinylmagnesium bromide at 0 ◦C, followed by a dehydration reaction to afford 2-PB in the presence of pyridinium p-toluenesulfonate at 80 ◦C; THF and toluene are employed as solvents, respectively. Similar to 2-PB, 1-phenyl-1,3-butadiene (1-PB) and

1-(4-methypenyl)-1,3-butadieneCatalysts 2018, 8, x FOR PEER REVIEW (1-MPB) have also gained much attention [120–125]. 12 of 25

Figure 8. SynthesisSynthesis of 2-phenyl-1,3-butadiene [115–119]. [115–119].

AsMoreover, displayed Yao in etFigure al. synthesized 8, vinyl bromide 2-(4-methoxyphenyl)-1,3-butadiene and magnesium are used to prepare (2-MOPB) the THF through solution the ofdehydration vinylmagnesium of 2-(4-methoxyphenyl) bromide, followed but-3-en-2-ol by the addi intion the of presence acetophenone of pyridinium (Figure p-toluenesulfonate 8a). The mixture mustwhich be is maintained used as a dehydrating under gentle agent. reflux The and manipulations then maintained were with conducted stirring under for 1 mild h. A conditions, saturated aqueouswith a moderate solution yieldof ammonium of 46% [119 chloride]. Phenyl-1,3-butadiene is applied for the derivatives hydrolysis provide process a promisingto obtain the platform crude product.for the synthesis After the ofdistillation polar-group and removal functionalized of the solvent, 1,3-butadienes methylphenylvinylcarbinol by introducing substituents (Figure 8b) into is affordedphenyl [ 126with–128 a yield]. In of our 75%. previous Methylphenylvinylc work, 4-methyl-4-phenyl-1,3-dioxanearbinol, aniline hydrobromide, (MPD) and was hydroquinone synthesized arevia Prinscharged condensation into a round-bottomed of alpha-methylstyrene flask equipped with formaldehydewith a Vigreux and column. MPD couldThe mixture be easily is convertedheated at ◦ 100into °C 2-PB in an over oil solid bath, acid followed catalysts by gentle at above distillation 250 C[129 with]. Notably, the temperature 2-PB can increasing hardly be modifiedto 150 °C. withThe fractionfunctional with groups a b.p. in of the 57–63 presence °C is of collected a conjugated and dried bond. by However, anhydrous it is calcium possible chloride to introduce to obtain functional 2-PB (Figuregroups into8c). Recently, MPD, and Yao then etthe al. functionalized[118] used acetophenone MPD can beas easilythe starting converted material into functionalizedto prepare 2-PB 2-PB via elaborateand 1-PB manipulations, (Figure9). Therefore, which thisare performed is a promising under candidate a dried and route oxygen-free for the synthesis atmosphere, of 2-PB and with the solventsfunctional involved groups onin athe large reaction scale. must be purified. In the typical procedure, acetophenone is first converted into 2-phenylbut-3-en-2-ol in the presence of vinylmagnesium bromide at 0 °C, followed by a dehydration reaction to afford 2-PB in the presence of pyridinium p-toluenesulfonate at 80 °C; THF and toluene are employed as solvents, respectively. Similar to 2-PB, 1-phenyl-1,3-butadiene (1-PB) and 1-(4-methypenyl)-1,3-butadiene (1-MPB) have also gained much attention [120–125]. Moreover, Yao et al. synthesized 2-(4-methoxyphenyl)-1,3-butadiene (2-MOPB) through the dehydration of 2-(4-methoxyphenyl) but-3-en-2-ol in the presence of pyridinium p-toluenesulfonate which is used as a dehydrating agent. The manipulations were conducted under mild conditions, with a moderate yield of 46% [119]. Phenyl-1,3-butadiene derivatives provide a promising platform for the synthesis of polar-group functionalized 1,3-butadienes by introducing substituents into phenyl [126–128]. In our previous work, 4-methyl-4-phenyl-1,3-dioxane (MPD) was synthesized via Prins condensation of alpha-methylstyrene with formaldehyde and MPD could be easily converted into 2-PB over solid acid catalysts at above 250 °C [129]. Notably, 2-PB can hardly be modified with functional groups in the presence of a conjugated bond. However, it is possible to introduce functional groups into MPD, and then the functionalized MPD can be easily converted into functionalized 2-PB and 1-PB (Figure 9). Therefore, this is a promising candidate route for the synthesis of 2-PB with functional groups on a large scale.

Catalysts 2019, 9, 97 13 of 25 CatalystsCatalysts 2018 2018, 8, ,8 x, xFOR FOR PEER PEER REVIEW REVIEW 1313 of of 25 25

FigureFigure 9. 9. A A straightforward straightforward way way to to synthesize synthesize 2-phenyl-1,3-butadiene 2-phenyl-1,3-butadiene [129]. [[129].129].

5.5. Polar-Group Polar-Group Functionalized Functionalized 1,3-butadienes 1,3-butadienes AA large large number numbernumber of ofof functionalized functionalizedfunctionalized dienes dienes with with po polarpolarlar groups groups containing containing O, O, N, N, and and Si, Si,Si, such such as as vinyloxy,vinyloxy, ethoxymethyl, ethoxymethyl,ethoxymethyl, cyanomethyl, cyanomethyl,cyanomethyl, amine amine derivatives, derivatives, alkyloxysilyl, alkyloxysilyl, alkylsilyl, alkylsilyl, alkyloxymethyl, alkyloxymethyl, N,N-dialkylaminoN,N-dialkylamino dimethylsilyl, dimethylsilyl, etc., etc.,etc., have havehave been beenbeen reported. reported.reported. 2-Vinyloxy-1,3-butadiene2-Vinyloxy-1,3-butadiene is is a a potentially potentially valuable valuable building building block block and and monomer, monomer, as as it itcan can be be taken as either a functionalized 1,3-butadiene or vinyl ether (CH =CHOR). It has been synthesized takentaken as as either either a afunctionalized functionalized 1,3-butadiene 1,3-butadiene or or vinyl vinyl ether ether (CH (CH2=CHOR).22=CHOR). It It has has been been synthesized synthesized ◦ byby a a ahydrative hydrative hydrative trimerization trimerization trimerization reaction reaction reaction using using using acetylene acetylene acetylene and and water and water water as as raw raw as materials rawmaterials materials at at 80–115 80–115 at 80–115°C °C in in the theC in the KOH·H O/DMSO system (Figure 10)[130,131]. It is suggested that one molecule of KOH·HKOH·H2O/DMSO2O/DMSO2 system system (Figure (Figure 10) 10) [130,131]. [130,131]. It It is is suggested suggested that that one one molecule molecule of of acetylene acetylene reacts reacts withacetylenewith one one molecule molecule reacts with of of water water one moleculeto to form form acetaldehyde acetaldehyde of water to (Figure form (Figure acetaldehyde 10c), 10c), which which underg (Figure undergoes 10oesc), ethynylation ethynylation which undergoes in in the the presenceethynylationpresence of of another another in the presencemolecule molecule ofof of anotheracetylene acetylene molecule to to give give ofacetylenic acetylenic acetylene alcohol toalcohol give (Figure acetylenic (Figure 10d), 10d), alcohol followed followed (Figure by by 10the thed), vinylationfollowedvinylation by to theto vinylationaffordafford (Figure(Figure to afford 10e). (Figure10e). After After10e). Afterisomerizatioisomerizatio isomerizationn n ofof ofacetylene-allene-diene, acetylene-allene-diene, 2-vinyloxy-1,3-butadiene2-vinyloxy-1,3-butadiene is isis obtained obtained obtained [[132]. 132[132].]. Recently,Recently, Recently, VitkovskayaVitkovskaya Vitkovskaya et et al.et al. studiedal. studied studied the the the mechanism mechanism mechanism of thisof of thisreactionthis reaction reaction by by using by using using the the MP2/6-311++G**//B3LYP/6-31+G*the MP2/6-311++G**//B3LYP/6 MP2/6-311++G**//B3LYP/6-31+G*-31+G* quantum quantumquantum chemical chemical framework framework [133,134]. [133,134]. [133,134 ]. TheyThey clarify clarifyclarify that that the thethe formation formationformation of ofof acetaldehyde acetaldehydeacetaldehyde and and its itsits ethynylation ethynylationethynylation occur occur in in complexes complexescomplexes of of 5DMSO·KOH,5DMSO5DMSO·KOH,·KOH, andand the thethe addition additionaddition of the ofof hydroxide thethe hydroxhydrox ionide toide the ionion acetylene toto thethe molecule acetyleneacetylene is the moleculemolecule rate-determining isis thethe rate-determiningsteprate-determining of this reaction. step step of of this this reaction. reaction.

FigureFigure 10. 10. The The pathway pathway of of synthesis synthesis of of 2-vinyloxy-1,3-butadiene 2-vinyloxy-1,3-butadiene [130–132]. [[130–132].130–132].

2-Ethoxymethyl-1,3-butadiene2-Ethoxymethyl-1,3-butadiene is is synthesized synthesized as as shown shown in in Figure Figure 11 1111 [135–137].[ [135–137].135–137]. The The prepared preparedprepared 2-bromomethyl-1,3-butadiene2-bromomethyl-1,3-butadiene (Figure (Figure (Figure 11a) 11a)11a) is is added added dropwise dropwise to to sodium sodium ethoxide ethoxide solution solution at at 0 0 °C, ◦°C,C, followedfollowed by by by stirring stirring stirring overnight overnight overnight at at 40 at40 °C; 40°C; 2-ethoxymethyl-1,3-butadiene◦ C;2-ethoxymethyl-1,3-butadiene 2-ethoxymethyl-1,3-butadiene (Figure (Figure (Figure 11b) 11b) is11 is obtainedb) obtained is obtained after after distillationafterdistillation distillation under under reduced under reduced reduced pressure pressure pressure at at a ayield yield at of aof 63%. yield 63%. Dryof Dry 63%. THF THF is Dry is employed employed THF is employedas as the the solvent. solvent. as the The The solvent. used used 2-bromomethyl-1,3-butadieneThe2-bromomethyl-1,3-butadiene used 2-bromomethyl-1,3-butadiene can can be be synthesized synthesized can be synthesizedas as follows: follows: firstly, asfirstly, follows: bromine bromine firstly, is is added added bromine dropwise dropwise is added to to isoprenedropwiseisoprene (Figure(Figure to isoprene 11h)11h) (Figure inin a a dry11dryh) ice/acetone inice/acetone a dry ice/acetone bathbath (maintained(maintained bath (maintained belowbelow –20–20 below °C)°C) –20toto obtain◦obtainC) to 1,4-dibromo-2-methyl-2-buteneobtain1,4-dibromo-2-methyl-2-butene 1,4-dibromo-2-methyl-2-butene (Figure(Figure (Figure 11i)11i) withwith 11 i) a a with yieldyield a yield ofof 100%.100%. of 100%. FollowingFollowing Following thethe addition theaddition addition ofof 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinoneof1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU),(DMPU), (DMPU), 1,4-dibromo-2-methyl-2-butene1,4-dibromo-2-methyl-2-butene 1,4-dibromo-2-methyl-2-butene is is convertedconverted into into 2-bromomethyl-1,3-butadiene 2-bromomethyl-1,3-butadiene which whichwhich is isis collected collectedcollected by byby applying applyingapplying a a avacuum. vacuum.vacuum.

Catalysts 2019, 9, 97 14 of 25 Catalysts 2018, 8, x FOR PEER REVIEW 14 of 25

Figure 11. SynthesisSynthesis of functionalized 1,3-butadien 1,3-butadienee using 2-bromomethyl-1,3-butadiene [[135–144].135–144].

2-Cyanomethyl-1,3-butadiene can can be be used used for for th thee production of synthetic rubbers that show excellent oiloil andand solvent solvent resistance, resistance, as as well well as adhesiveas adhesive properties properties owing owing to the to introduction the introduction of cyano of cyanointo butadiene into butadiene [138]. As[138]. is known,As is known, a higher a higher acrylonitrile acrylonitrile content content in NBR in can NBR give can rise give to arise higher to a higherresistance resistance to hydrocarbons to hydrocarbons and impermeability and impermeability to gases, to gases, but it but also it leadsalso leads to lower to lower resilience resilience and andtemperature temperature flexibility flexibility [145,146 [145,146],], which which indicates indicates that oil resistancethat oil resistance and low-temperature and low-temperature flexibility flexibilityare mutually are mutually incompatible. incompatible. Cyano-substituted Cyano-substi monomerstuted monomers could provide could provide an ideal an solution ideal solution to this problemto this problem [139]. Jing [139]. et al.Jing [138 et, 139al. ][138,139] synthesized synthesized 2-cyanomethyl-1,3-butadiene 2-cyanomethyl-1,3-butadien in threee steps,in three with steps, the withsynthesis the synthesis of 2-bromomethyl-1,3-butadiene of 2-bromomethyl-1,3-butadiene from isoprene from andisoprene Br2 via and the Br first2 via two the steps first (Figure two steps 11). (FigureIn the third 11). step,In the sodium third cyanide,step, sodium acetonitrile, cyanide, and acetonitrile, tetrabutylammonium and tetrabutylammonium chloride are added chloride into theare addedreaction into mixture. the reaction After stirring mixture. at roomAfter temperaturestirring at room for 48 temperature h, the mixture for is 48 quenched, h, the mixture extracted, is washed,quenched, and extracted, distillated washed, to afford and 2-cyanomethyl-1,3-butadienedistillated to afford 2-cyanomethyl-1,3-butadiene (Figure 11c). The total(Figure yield 11c). is Theabout total 35%. yield is about 35%. Quite different from 2-cyanomethyl-1,3-butadiene,2-cyanomethyl-1,3-butadiene, the the introduc introductiontion of amine groups into dienes could remarkably change the properties of their finalfinal polymers, such as adhesion and solubility. It It has has been been verified verified that that the the solubility solubility of this of thiskind kind of polymer of polymer in polar in polarsolvents solvents is better is thanbetter that than of that a ofpolymer a polymer without without polar-side polar-side chains, chains, such such as as polybuta polybutadienediene and and polyisoprene. polyisoprene. Moreover, the polymer containing the amine always presents presents chemical chemical and and structural structural versatility. versatility. Therefore, a a lot lot of of effort effort has has been been put putinto intothe sy thenthesis synthesis of 1,3-butadiene of 1,3-butadiene containing containing amines (Figure amines (Figure11) [140,141]. 11)[140 2-((N,N-Dimethylamino)methyl)-1,3-bu,141]. 2-((N,N-Dimethylamino)methyl)-1,3-butadienetadiene (Figure 11d) (Figure is synthesized 11d) is synthesized by adding 2-bromomethyl-1,3-butadieneby adding 2-bromomethyl-1,3-butadiene dropwise into dropwise the mixture into the of mixture diethyl of amine diethyl and amine ether and at ether 0 °C, at followed0 ◦C, followed by stirring by stirring overnight overnight at room attemperature. room temperature. In this process, In this an process, aqueousan solution aqueous of dimethyl solution amineof dimethyl (40%) amineis used. (40%) After isdistillation, used. After a colorless distillation, liquid a product colorless is liquidobtained product with a is 65% obtained yield with[142–144]. a 65% By yield a similar [142–144 method,]. By a2-((N,N-diethylamino similar method, 2-((N,N-diethylamino)) methyl)-1,3-butadiene methyl)-1,3-butadiene (Figure 11e) and (Figure2-((N,N-di-n-propylamino)methyl)-1,3-butad 11e) and 2-((N,N-di-n-propylamino)methyl)-1,3-butadieneiene (Figure 11g) (Figure are 11 g) aresynthesized synthesized using 2-bromomethyl-1,3-butadiene andand thethe correspondingcorresponding amines, amines, and and both both of theirof their yields yields can reachcan reach 80%. An80%. alternative An alternative route route is displayed is displayed in Figure in Figure 12. 12. The pre-prepared 2-bromomethyl-1,3-butadiene can also be synthesized as shown in Figure 13[ 144]. Prior to charging condensed SO2 as a liquid into a stainless-steel reactor, an acetone/dry ice bath is employed to maintain the mixture temperature below –10 ◦C. After the introduction of isoprene, methanol, and hydroquinone, the reactor is sealed and heated to 85 ◦C for 4 h with stirring. After cooling the mixture to room temperature, deionized water is added to allow for recrystallization of the product in Figure 13a, which is obtained with a yield of 90%. Then, the product (Figure 13a), together with N-bromosuccinimide, benzyl peroxide, and chloroform, is charged into the round-bottom flask equipped with a condenser. After refluxing for 20 h, evaporation of the solvent and recrystallization of the product are carried out to afford the product in Figure 13b with a yield of 25%. A trace amount of hydroquinone and this product (Figure 13b) are charged into a preheated flask Figure 12. Alternative route for the synthesis of amine groups functionalized 1,3-butadiene [144].

Catalysts 2018, 8, x FOR PEER REVIEW 14 of 25

Figure 11. Synthesis of functionalized 1,3-butadiene using 2-bromomethyl-1,3-butadiene [135–144].

2-Cyanomethyl-1,3-butadiene can be used for the production of synthetic rubbers that show excellent oil and solvent resistance, as well as adhesive properties owing to the introduction of cyano into butadiene [138]. As is known, a higher acrylonitrile content in NBR can give rise to a higher resistance to hydrocarbons and impermeability to gases, but it also leads to lower resilience and temperature flexibility [145,146], which indicates that oil resistance and low-temperature flexibility are mutually incompatible. Cyano-substituted monomers could provide an ideal solution to this problem [139]. Jing et al. [138,139] synthesized 2-cyanomethyl-1,3-butadiene in three steps, with the synthesis of 2-bromomethyl-1,3-butadiene from isoprene and Br2 via the first two steps (Figure 11). In the third step, sodium cyanide, acetonitrile, and tetrabutylammonium chloride are added into the reaction mixture. After stirring at room temperature for 48 h, the mixture is quenched, extracted, washed, and distillated to afford 2-cyanomethyl-1,3-butadiene (Figure 11c). The total yield is about 35%. Quite different from 2-cyanomethyl-1,3-butadiene, the introduction of amine groups into dienes could remarkably change the properties of their final polymers, such as adhesion and solubility. It has been verified that the solubility of this kind of polymer in polar solvents is better than that of a polymer without polar-side chains, such as polybutadiene and polyisoprene. Moreover, the polymer containing the amine always presents chemical and structural versatility. Therefore, a lot of effort has been put into the synthesis of 1,3-butadiene containing amines (Figure 11) [140,141]. 2-((N,N-Dimethylamino)methyl)-1,3-butadiene (Figure 11d) is synthesized by adding 2-bromomethyl-1,3-butadiene dropwise into the mixture of diethyl amine and ether at 0 °C, followed by stirring overnight at room temperature. In this process, an aqueous solution of dimethyl

amineCatalysts (40%)2019, 9, 97is used. After distillation, a colorless liquid product is obtained with a 65% 15yield of 25 [142–144]. By a similar method, 2-((N,N-diethylamino) methyl)-1,3-butadiene (Figure 11e) and 2-((N,N-di-n-propylamino)methyl)-1,3-butadiene (Figure 11g) are synthesized using Catalysts◦ 2018, 8, x FOR PEER REVIEW 15 of 25 2-bromomethyl-1,3-butadiene(170 C) using an oil bath. After and melting the corresponding the solid, a vacuum amines, is applied and both and of the their greenish-brown yields can liquidreach 80%.productThe An (Figurealternativepre-prepared 13c) route is 2-bromomethyl-1,3-butadiene gained is displayed with a yield in Figure of 80% 12. (2-bromomethyl-1,3-butadiene). can also be synthesized as shown in Figure 13 [144]. Prior to charging condensed SO2 as a liquid into a stainless-steel reactor, an acetone/dry ice bath is employed to maintain the mixture temperature below –10 °C. After the introduction of isoprene, methanol, and hydroquinone, the reactor is sealed and heated to 85 °C for 4 h with stirring. After cooling the mixture to room temperature, deionized water is added to allow for recrystallization of the product in Figure 13a, which is obtained with a yield of 90%. Then, the product (Figure 13a), together with N-bromosuccinimide, benzyl peroxide, and chloroform, is charged into the round-bottom flask equipped with a condenser. After refluxing for 20 h, evaporation of the solvent and recrystallization of the product are carried out to afford the product in Figure 13b with a yield of 25%. A trace amount of hydroquinone and this product (Figure 13b) are charged into a preheated flask (170 °C) using an oil bath. After melting the solid, a vacuum is applied and the greenish-brown liquid product (Figure 13c) is gained with a yield of 80% FigureFigure 12. 12. AlternativeAlternative route route for for the the synthesis synthesis of amin aminee groups functionalized 1,3-butadiene [144]. [144]. (2-bromomethyl-1,3-butadiene).

FigureFigure 13. 13. SynthesisSynthesis of of 2-bromomethyl-1,3-butadiene 2-bromomethyl-1,3-butadiene [144]. [144].

SatoSato et et al. al.[147] [147 reported] reported a synthetic a synthetic route of route2-(triisopropoxysilyl)-l,3-butadiene of 2-(triisopropoxysilyl)-l,3-butadiene in 1984; since in then,1984; sincethis then,kind thisof kindfunctional of functional diene dienehas attracted has attracted wide wide interest. interest. In Inthis this route, route, 2-(triisopropoxysilyl)-1,3-butadiene2-(triisopropoxysilyl)-1,3-butadiene is is prepared prepared using 1,4-dichloro-2-(trichlorosilyl)-2-butene and isopropylisopropyl alcohol asas startingstarting materialsmaterials (Figure (Figure 14 14))[ 148[148,149].,149]. Firstly, Firstly, the the solution solution of of isopropyl isopropyl alcohol alcohol in intriethylamine triethylamine is is added added dropwise dropwise to to a a THFTHF solutionsolution ofof l,4-dichloro-2-(trichlorosilyl)-2-butenel,4-dichloro-2-(trichlorosilyl)-2-butene (Figure(Figure 14a)14a) atat 0 °C. 0 ◦AfterC. After stirring stirring overnight overnight at 20 °C, at the 20 mixture◦C, the is mixture heated to is 60 heated °C and to kept 60 for◦C severaland kept hours, for followed several hours,by cooling followed to 0 °C. by Hexane cooling is applied to 0 ◦ C.to precipitate Hexane is the applied inorganic to salt, precipitate which isthe inorganicremoved salt, whichusing is removedHyflo-Super-Ce using Hyflo-Super-Cel”l" (Johns-Manville (Johns-Manville Co.). Co.). Then, Then, l,4-dichloro-2-(triisopropoxysilyl)-2-butenel,4-dichloro-2-(triisopropoxysilyl)-2-butene (Figure (Figure 14b)14b) is is obtained by distillation. In In the the second step,step, the the THF THF solution solution of of l,4-dichloro-2-(triisopropoxysilyl)-2-butene l,4-dichloro-2-(triisopropoxysilyl)-2-butene (Figure (Figure 14b)14b) is is dropwise dropwise added toto a a mixture of zinc powder in THF under refluxing refluxing condition. After After refluxing refluxing for for 2 2 h, h, the the reaction mixturemixture is is cooled cooled down down to to 0 °C.◦C. In this stage, pentane pentane is is used used to to precipitate precipitate the the inorganic inorganic salt. salt. 2-(Triisopropoxysilyl)-l,3-butadiene2-(Triisopropoxysilyl)-l,3-butadiene (Figure (Figure 14c)14c) is is afforded afforded with with a a yield yield of of 64% after removing the saltsalt and and solvent. solvent. By By a a similar similar method, method, 2-triethoxysil 2-triethoxysilyl-1,3-butadieneyl-1,3-butadiene (Figure (Figure 14d)14d) is is synthesized synthesized from from 1,4-dichloro-2-(trichlorosilyl)-2-butene and and ethano ethanol,l, and 2-(diisopropoxymethylsilyl)-l,3-butadiene2-(diisopropoxymethylsilyl)-l,3-butadiene (Figure(Figure 14e)14e) and and 2-(dimethylisopropoxysilyl)-l,3-butadiene 2-(dimethylisopropoxysilyl)-l,3-butadiene (Figure (Figure 14f) are14f) synthesized are synthesized via a reactionvia a reactionof isopropyl of isopropyl alcohol alcohol with with 1,4-di 1,4-dichloro-2-chloro-2- (dichloromethylsilyl)-2-butene (dichloromethylsilyl)-2-butene or or l,4-dichloro-2-(dimethylchlorosilyl)-2-butene,l,4-dichloro-2-(dimethylchlorosilyl)-2-butene, respectively. 2-Triethoxymethyl-1,3-butadiene is synthesized by a coupling reaction of tetraethyl orthocarbonate and 2-(1,3-butadienyl) magnesium chloride (Figure 14)[150]. A THF solution of tetraethyl orthocarbonate is charged into a flask equipped with a reflux condenser. A THF solution ◦ of 2-(1,3-butadienyl) magnesium chloride is added dropwise under a N2 atmosphere at 55–65 C. Then the reaction mixture is stirred for 48 h. After the reaction, the mixture is concentrated and poured into a NH4Cl solution containing ice, then extracted three times with ether. After removing the solvent, distillation is employed to attain 2-triethoxymethyl-1,3-butadiene (Figure 14j).

Catalysts 2019, 9, 97 16 of 25 Catalysts 2018, 8, x FOR PEER REVIEW 16 of 25

Figure 14. Synthesis of 2-substituted 1,3-butadienes [135,147–151]. [135,147–151].

2-Triethoxymethyl-1,3-butadiene2-(Trimethylsilyl)-1,3-butadiene isis synthesized synthesized as givenby a in Figurecouplin 14g .reaction Trimethylsilyl of tetraethyl chloride orthocarbonateis added to a solutionand 2-(1,3-butadienyl) of pre-prepared magnesium 2-(1,3-butadienyl) chloride magnesium(Figure 14) chloride[150]. A inTHF THF solution at room of tetraethyltemperature. orthocarbonate Then the mixture is charged is heated into and a flask refluxed equipped for 3 h. with After a reflux the reaction, condenser. the mixture A THF is solution poured ofinto 2-(1,3-butadienyl) 2 N HCl and extracted magnesium with pentane. chloride After is added removing dropwise the solvent, under 2-(trimethylsilyl)-1,3-butadienea N2 atmosphere at 55–65 °C. Thenis obtained the reaction (Figure mixture 14k). The is synthesisstirred for of 48 pre-prepared h. After the 2-(1,3-butadienyl) reaction, the mixture magnesium is concentrated chloride and has pouredbeen described into a NH elsewhere4Cl solution [151 ].containing ice, then extracted three times with ether. After removing the solvent,2-(N,N-Dialkylamino)dimethylsilyl-1,3-butadiens distillation is employed to attain 2-triethoxymethyl-1,3- are synthesizedbutadiene by (Figure the 14j). reactions of 1,3-butadien-2-ylmagnesium2-(Trimethylsilyl)-1,3-butadiene chloride is synthesized with the corresponding as given in Figure (N,N-dialkylamino)dimethylsilyl 14. Trimethylsilyl chloride is addedchloride to (Figure a solution 14)[135 of]. pre-prepared In typical procedures, 2-(1,3-but 2-((N,N-diethylamino)adienyl) magnesium dimethylsilyl)-1,3-butadienechloride in THF at room (Figuretemperature. 14n) isThen synthesized the mixture by dropwiseis heated addingand refl auxed solution for 3 ofh. (N,N-diethylamino)After the reaction, the dimethylsilyl mixture is pouredchloride (Figureinto 2 14 m)N toHCl 1,3-butadien-2-ylmagnesium and extracted with pentane. chloride (FigureAfter 14removingh) under anthe atmosphere solvent, 2-(trimethylsilyl)-1,3-butadieneof nitrogen at 0 ◦C. THF is used is asobtained the solvent. (Figure Followed 14k). by The refluxing synthesis for 12 of h, thepre-prepared mixture is 2-(1,3-butadienyl)cooled, filtered, and magnesium washed withchloride a solution has been of described dry pentane/THF elsewhere under[151]. a nitrogen atmosphere. After2-(N,N-Dialkylamino)dimethylsilyl-1,3-butadiens evaporation and distillation over LiAlH4, 2-((N,N-diethylamino) are synthesized dimethylsilyl)-1,3-butadiene by the reactions of is 1,3-butadien-2-ylmagnesiumafforded as a colorless liquid with chloride a yield with of 40%. the According corresponding to similar (N,N-dialkylamino)dimethylsilyl procedures, (N,N-dibutylamino) chloridedimethylsilyl (Figure chloride 14) [135]. (Figure In typical 14 o),procedures, (1-pyrrolidinyl)-dimethylsilyl 2-((N,N-diethylamino) chloridedimethylsilyl)-1,3-butadiene (Figure 14q), and (N-(2(Figure0-(N 14n)0,N0 -dimethylamino)is synthesized by ethyl)-N-methylamino)dimethylsilyldropwise adding a solution of (N,N-diethylamino) chloride (Figure dimethylsilyl 14s) are chlorideused to (Figure synthesize 14m) to 2-((N,N-dibutylamino) 1,3-butadien-2-ylmagnesium dimethylsilyl)-1,3-butadiene chloride (Figure 14h) under (Figure an atmosphere 14p) at of a 13%nitrogen yield, at 0 2-((1-Pyrrolidinyl)°C. THF is used as dimethylsilyl)-1,3-butadienethe solvent. Followed by refluxing (Figure for 1412r) h, atthe a mixture 68% yield, is cooled, and filtered,2-((N-(20 -(Nand0,N washed0-Dimethylamino) with a solution ethyl)-N-methylamino)-dimethylsilyl)-1,3-butadiene of dry pentane/THF under a nitrogen atmosphere. (Figure 14 t)After at a evaporation71% yield. Moreover, and distillation 1,3-butadienes over LiAlH with4, abundant 2-((N,N-diethylamino) functional groups dimethylsilyl)-1,3-butadiene at the 2-position have been is affordedreported (Figureas a 15colorless)[152,153 liquid]. with a yield of 40%. According to similar procedures, (N,N-dibutylamino) dimethylsilyl chloride (Figure 14o), (1-pyrrolidinyl)-dimethylsilyl chloride (Figure 14q), and (N-(2’-(N’,N’-dimethylamino) ethyl)-N-methylamino)dimethylsilyl chloride (Figure 14s) are used to synthesize 2-((N,N-dibutylamino) dimethylsilyl)-1,3-butadiene (Figure 14p) at a 13% yield, 2-((1-Pyrrolidinyl) dimethylsilyl)-1,3-butadiene (Figure 14r) at a 68% yield, and

Catalysts 2018, 8, x FOR PEER REVIEW 17 of 25

2-((N-(2’-(N’,N’-Dimethylamino) ethyl)-N-methylamino)-dimethylsilyl)-1,3-butadiene (Figure 14t) atCatalysts a 71%2019 yield., 9, 97 Moreover, 1,3-butadienes with abundant functional groups at the 2-position 17have of 25 been reported (Figure 15) [152,153].

FigureFigure 15. 15. ReportedReported 2-substituted 2-substituted 1,3-butadienes 1,3-butadienes [152,153]. [152,153].

6.6. Conclusion Conclusions and and Outlook Outlook 1,3-Butadienes1,3-Butadienes (especially (especially IB IBand and IP) IP)are arethe thebasi basicc building building blocks blocks for the for synthetic the synthetic rubbers rubbers used inused the intire the fabrication tire fabrication industry, industry, and it and is needless it is needless to mention to mention that their thattheir production production still stilldepends depends on petrochemicalson petrochemicals in industry. in industry. Taking Taking into consideration into consideration the depletion the depletion of oil reserves, of oil reserves, the volatile the volatile price ofprice petrochemicals, of petrochemicals, and the and issues the issues related related to the to theenvironment, environment, it is it urgent is urgent to toexplore explore renewable renewable resourcesresources for for the the sustainable sustainable production production of of 1,3-buta 1,3-butadienes,dienes, especially especially for for those those in in large large demand demand in in thethe industry industry [88,154,155]. [88,154,155]. The The production production of of 1,3- 1,3-butadienesbutadienes from from biomass-derived biomass-derived feedstocks feedstocks is is a a prospectiveprospective route, route, which which is is increasingly increasingly competitive competitive in in polymer polymer production production [156]. [156]. With With regards regards to to sustainability,sustainability, great great achievements achievements have have been been obtained obtained in in the the production production of of IB IB from from ethanol ethanol in in both both academiaacademia and and the the industry. industry. Unfortunately, Unfortunately, it it is is stil stilll a a great great challenge challenge to to improve improve the the BD BD yield yield and and to to developdevelop robust robust catalysts catalysts because because the the aldol aldol coupling coupling and and the the dehydrogenation dehydrogenation steps steps involved involved in in this this technologytechnology cannot cannot be be efficiently efficiently performed performed on on simple simple metal metal oxide oxide catalysts catalysts [157]. [157]. Bio-derivedBio-derived C4 C4 alcohols alcohols and and especially especially unsatura unsaturatedted C4 C4 alcohols alcohols have have been been declared declared promising promising feedstocksfeedstocks for for the the production production of of BD BD via via the the dehydr dehydrationation process; process; even even the the two-step two-step synthesis synthesis of of BD BD ◦ fromfrom C4 C4 alcohols alcohols via via UOLs UOLs displays displays a atotal total BD BD yield yield of of96.6% 96.6% over over Yb Yb2O32 Oat3 360at 360°C. AC. great A great deal deal of workof work still stillneeds needs to be to bedone, done, especially especially for for the the improving improving catalyst catalyst life life for for industry industry usage. usage. For For the the sustainablesustainable production ofof IP, IP, a lota oflot effortsof efforts must must be made. be Althoughmade. Although some biomass-derived some biomass-derived chemicals, chemicals,such as itaconic such as acid, itaconic have acid, been exploredhave been in explored the last 2 in years, the last it is 2 still years, difficult it is still to meet difficult the requirements to meet the requirementsfor industrial-scale for industrial production.-scale Extensiveproduction. research Extensive studies research on IP studies synthesis on areIP synthesis urgently required,are urgently and required,the exploration and the of novelexploration platform of chemicals novel platform faces challenges chemicals but faces will bechallenges highly rewarding. but will Fortunately,be highly rewarding.the US Department Fortunately, of Energy the US (DOE) Department “Top 10” of report Energy (the (DOE) DOE outlined“Top 10” research report (the needs DOE for bio-basedoutlined researchproducts needs in 2004) for bio-based and its revision products provide in 2004) guidance and its torevision choose provide potential guidance candidate to choose chemicals potential for IP candidatesynthesis, chemicals as well as for BD IP [158 synthesis,–160]. as well as BD [158–160]. InIn contrast, contrast, functionalized functionalized 1, 1,3-butadienes,3-butadienes, without without any any research research on on their their synthesis synthesis from from biomass-derivedbiomass-derived chemicals, chemicals, are are always always synthesized synthesized at at the the lab lab scale scale by by complicated complicated operations operations under under rigorousrigorous conditions, conditions, e.g., e.g., without without water water or or oxygen oxygen,, such such as as the the synthesis synthesis of of these these monomers monomers with with phenyl,phenyl, silicon, silicon, cyano, cyano, and and amino amino groups. groups. However, However, they they play play an essential an essential role rolein enhancing in enhancing the existingthe existing properties properties of synthetic of synthetic rubber, rubber, particularly particularly for for certain certain special special applications. applications. Therefore, Therefore, explorationexploration of of a a simple catalytic routeroute toto synthesize synthesize functionalized-1,3-butadienes functionalized-1,3-butadienes at at a largea large scale scale is still is stillneeded. needed. In China, In China, research research work is ongoingwork is inongoing this direction in this at thedirection Key Laboratory at the Key of High-Performance Laboratory of High-PerformanceSynthetic Rubber andSynthetic its Composite Rubber Materials,and its Composite Changchun Materials, Institute Changchun of Applied Chemistry,Institute of CAS Applied and is Chemistry,expected to CAS continue and is in expected the next to few continue years. in the next few years.

AuthorAuthor Contributions: Contributions: Y.Y.Q. Q. performed thethe literatureliterature search search and and drafted drafted the the manuscript; manuscript; Z.L. Z. andL. and C.B. C. made B. made great greatefforts efforts to revise to revise the manuscript; the manuscript; S.L., L.C., S. L., Q.D., L. C., J.H., Q. andD., J. W.D. H., participatedand W. D. participated in the checking in the of thischecking work. of this work.Funding: This work was supported by the National Natural Science Foundation of China (No. 51873203); National Natural Science Foundation of China (No. 51473156); Key Projects of Jilin Province Science and Technology Catalysts 2019, 9, 97 18 of 25

Development Plan (No. 20160204028GX); and Key Projects of Jilin Province Science and Technology Development Plan (No. 20180201087GX). Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

1,3-BDO 1,3-Butanediol 1,4-BDO 1,4-Butanediol 1-MPB 1-(4-Methypenyl)-1,3-butadiene 1-PB 1-Phenyl-1,3-butadiene 2,3-BDO 2,3-Butanediol 2B1OL 2-Buten-1-ol 2-MOPB 2-(4-Methoxyphenyl)-1,3-butadiene 2-MTHF 2-Methyltetrahydrofuran 2-PB 2-Phenyl-1,3-butadiene 3B1OL 3-Buten-1-ol 3B2OL 3-Buten-2-ol 3-MTHF 3-Methyltetrahydrofuran BD 1,3-Butadiene BDOs Butanediols BR Butadiene rubber CR Chloroprene rubber DMD 4,4-Dimethyldioxane-1,3 IP Isoprene IR Isoprene rubber MPD 4-Methyl-4-phenyl-1,3-dioxane MTBE Methyl tertiarybutyl ether MVK Methyl ethyl ketone NBR Nitrile butadiene rubber NB Natural rubber SBR Styrene butadiene rubber Tg Glass transition temperature THF Tetrahydrofuran UOLs Unsaturated C4 alcohols

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