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While the formation and breaking of transition metal (TM)-carbon bonds plays a pivotal role in the catalysis of organic compounds, the reactivity of inorganometallic species, that is, those involving the transition metal (TM)-metalloid (E) bond, is of key importance in most conversions of metalloid derivatives catalyzed by TM complexes. This Review presents the background of inorganometallic catalysis and its development over the last 15 years. The results of mechanistic studies presented in the Review are related to the occurrence of TM-E and TM-H compounds as reactive intermediates in the catalytic transformations of selected metalloids (E = B, Si, Ge, Sn, As, Sb, or Te). The Review illustrates the significance of inorganometallics in catalysis of the following processes: addition of metalloid-hydrogen and metalloid-metalloid bonds to unsaturated compounds; activation and functionalization of C-H bonds and C-X bonds with hydrometalloids and bismetalloids; activation and functionalization of C-H bonds with vinylmetalloids, metalloid halides, and sulfonates; and dehydrocoupling of hydrometalloids. This first Review on inorganometallic catalysis sums up the developments in the catalytic methods for the synthesis of organometalloid compounds and their applications in advanced organic synthesis as a part of tandem reactions.

Selenium NMR has become a standard tool for scaling the π-accepting character of carbenes. Herein, we highlight that non-classical hydrogen bonding (NCHB), likely resulting from hyperconjugation, can play a significant role in the carbene-selenium 77 Se NMR chemical shift, thus triggering a non-linear behavior of the Se-Scale.

Cyclic (amino)(aryl)carbenes (cAArCs) based on the isoindoline core were successfully generated in situ by α‐elimination of 3‐alkoxyisoindolines at high temperatures or by deprotonation of isoindol‐2‐ium chlorides with sodium or copper(I) acetates at low temperatures. 3‐Alkoxy‐isoindolines 2 a ,b‐OR (R=Me, Et, i Pr) have been prepared in high yields by the addition of a solution of 2‐aryl‐1,1‐diphenylisoindol‐2‐ium triflate (1 a ,b‐OTf ; a : aryl=Dipp=2,6‐diisopropylphenyl; b : Mesityl‐, Mes=2,4,6‐trimethylphenyl) to the corresponding alcohol (ROH) with NEt3 at room temperature. Furthermore, the reaction of 2 a ,b‐OMe in diethyl ether with a tenfold excess of hydrochloric acid led to the isolation of the isoindol‐2‐ium chlorides 1 a ,b‐Cl in high yields. The thermally generated cAArC reacts with sulfur to form the thioamide 3 a . Without any additional trapping reagent, in situ generation of 1,1‐diphenylisoidolin‐3‐ylidenes does not lead to the isolation of these compounds, but to the reaction products of the insertion of the carbene carbon atom into an ortho C−H bond of a phenyl substituent, followed by ring‐expansion reaction; namely, anthracene derivatives 9‐N(H)aryl‐10‐Ph‐C14H8 4 a ,b (a : Dipp; b : Mes). These compounds are conveniently synthesized by deprotonation of the isoindol‐2‐ium chlorides with sodium acetate in high yields. Deprotonation of 1 a‐Cl with copper(I) acetate at low temperatures afforded a mixture of 4 a and the corresponding cAArC copper(I) chloride 5 a , and allowed the isolation and structural characterization of the first example of a cAArC copper complex of general formula [(cAArC)CuCl].

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A practical and general synthetic approach to a series of 4-aryl-but-3-en-1-ynes is described. In the presence of palladium complexes a variety of aryl bromides (or iodides) undergo coupling with two equivalents of trimethylsilylacetylene with the formation of (E)-4-aryl-1,3-bis(trimethylsilyl)but-3-en-1-ynes. The protocol is simple, efficient, and affords synthesis of regio- and stereoselectively target products in good to high yields.

Palladium complexes of the general formula [PdCl2(tzNHC)(pyridine)] bearing a series 1,2,3-triazolin-5-ylidene (tzNHC) ligands were synthesized and characterized by spectroscopic methods and X-ray analysis. Complexes exhibit catalytic activity in the Suzuki-Miyaura coupling of boronic acids with aryl bromides.

NHC–palladium(II) complex [{Pd(μ–OH)Cl(IPr)}2] (IPr = bis(2,6-di­isopropylphenyl)imidazolin-2-ylidene) catalyzes the oxidative coupling of a broad spectrum of aryl- and alkenylboronic acids at loadings down to 5 ppm. At the concentration of 0.05 mol% the catalyst permits an efficient reaction under base-free conditions.

Highly regio- and stereoselective dimerization of terminal acetylenes occurs in the presence of [PdCl2(IPr)(3-chloropyridine)], other members of the family of PEPPSI precatalysts and the structurally related N-heterocyclic carbene palladium dimers, i.e. [{Pd(μ–Cl)Cl(IPr)}2] and [{Pd(μ–OH)Cl(IPr)}2] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). The reaction leads to exclusive formation of E-isomer of head to head dimerization product. [{Pd(μ–OH)Cl(IPr)}2] catalyzes dimerization of a broad spectrum of aryl- and silylacetylenes at room temperature and is able to act in base-free conditions.

New dimeric palladium 1,2,3-triazol-5-ylidene complexes of the general formula [{Pd(μ-Cl)Cl(tzNHC)}2] (tzNHC = 1-mesityl-3-methyl-4-phenyl-1H-1,2,3-triazol-5-ylidene; 1-hexyl-3-methyl-4-phenyl-1H-1,2,3-triazol-5-ylidene and 1-butyl-3-methyl-4-phenyl-1H-1,2,3-triazol-5-ylidene) were synthesized and characterized by spectroscopic methods, X-ray analysis and DFT calculations. For the mesityl substituted complex, the crystallographic structures of both the cisoidal and transoidal isomer were obtained. In the presence of an excess of 4-tolylboronic acid, the loss of the tzNHC ligand from the palladium coordination sphere as a 5-tolyltriazolium salt was observed. The complexes are precursors of palladium species catalytically active in the coupling of arylboronic acids with aryl bromides.

[{Pd(μ-OH)Cl(IPr)}2] [IPr=N,N-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] was found to be a highly efficient precatalyst for Suzuki–Miyaura coupling that is able to act under base-free conditions.

The regioselectivity of metathesis reactions of trisubstituted vinylsilanes H2C═CHSiR3 (SiR3 = SiCl3, SiCl2Me, SiClMe2, SiMe3, Si(OEt)3) with the second-generation ruthenium alkylidene complex has been studied theoretically, by density functional theory (DFT), and experimentally. The DFT results indicate that cycloreversion is the rate-determining step and the formation of a thermodynamically stable ruthenium methylidene complex and PhCH═CHSiR3 is generally preferred. However, the regioselectivity of the process can be also governed by the relative stabilities of the ruthenacyclobutane intermediates, which depend on the electronic and steric properties of the SiR3 substituent. Higher stability of α,β-disubstituted ruthenacyclobutanes in comparison to α,α-disubstituted ruthenacyclobutanes is predicted, in contrast to the corresponding intermediates formed during metathesis of common α-olefins. The stabilizing Ru–Cβ interaction in the ring is strengthened by the electron-donor SiR3 substituent at Cβ. The ex...

Platinum complexes bearing bulky N-heterocyclic carbene (NHC) ligands, i.e., [Pt(IPr*)(dvtms)] (where, IPr* = 1,3-bis{2,6-bis(diphenylmethyl)-4-methylphenyl}imidazol-2-ylidene) and [Pt(IPr*OMe)(dvtms)] (where, IPr*OMe = 1,3-bis{2,6-bis(diphenylmethyl)-4-methoxyphenyl}imidazol-2-ylidene, dvtms = divinyltetramethyldisiloxane) catalyse nearly quantitatively and highly or completely the selective hydrosilylation of terminal olefins as well as terminal or internal acetylenes.

Second generation Grubbs catalyst [RuCl2(H2IMes)(PCy3)( CHPh)] initiates regio- and stereoselective cross-metathesis of silylacetylenes with 1-alkenes and α,ω-dienes leading to the formation of E-2-silyl-1,3-dienes and E-2-silyl-1,3,n-trienes, respectively. Efficient progress of the reaction is achieved by using 5–8 mol% of the benzylidene initiator and twofold excess of olefin (with respect to silylacetylene) as well as ethereal solution of HCl (10–20 mol% with respect to the catalyst) as an activator. The reaction is proposed to be catalysed by alkylidene complex [RuCl2(H2IMes)(PCy3)( CHR)] (where R = alkyl, n-alkenyl) generated from the benzylidene initiator, and proceed via initial interaction of a metal alkylidene with a triple bond.

Amidobenzylidene ruthenium chelates – latent catalysts of olefin metathesis can be easily activated by the addition of Bronsted or Lewis acids. Their activation in the presence of hydrogen chloride involves the formation of catalytically active trans-dichloro carbamatobenzylidene ruthenium chelates.

First generation Grubbs catalyst [RuCl2(PCy3)2( CHPh)] and related complexes of a general formula [RuCl2(PCy3)2( CHR)], where R is an aromatic or methyl group, undergo efficient reaction with trimethylsilylacetylene and related silylacetylenes to form vinyl carbene complexes [RuCl2(PCy3)2( CHCH CHR)] and disiloxane. The reaction requires the presence of water in the reaction medium. It is a convenient method for the high-yield synthesis of ruthenium vinyl carbene complexes. For benzylidene complexes, the reaction gives exclusively E-geometry of vinyl carbene ligand.

New aryloxybenzylidene ruthenium chelates behave like a promising latent catalyst of olefin metathesis. The catalysts are characterised by substantial stability and catalytic inactivity in their dormant forms and a dramatic increase in activity after addition of a solution of HCl in ether. The mechanism of activation involves protonation of the phenoxide and the formation of a highly catalytically active hydroxybenzylidene ruthenium chelate.

A series of ruthenium complexes (RuCl 3 × 3H 2 O, [C 5 H 5 Ru(CH 3 CN) 3 ] + [PF 6 ] − , [RuCl 2 (PPh 3 ) 3 ], [RuHCl(CO)(PPh 3 ) 3 ], [RuHCl(CO)(PCy 3 ) 2 ]) immobilized in [bmim][Tf 2 N] were tested in homocoupling of vinylsilanes (H 2 C CHSi(OEt) 3 , H 2 C CHSi(O i Pr) 3 , H 2 C CHSi(OSiMe 3 ) 3 , H 2 C CHSiMe(OSiMe 3 ) 2 , and H 2 C CHSiPh(OSiMe 3 ) 2 ) performed in a biphasic system. The highest catalytic activity and selectivity of homocoupling product was observed for [RuHCl(CO)(PCy 3 ) 2 ]. The complex [RuHCl(CO)(PCy 3 ) 2 ] immobilized in a variety ionic liquids ([bmim][PF 6 ], [bmim][BF 4 ], [bmim][Tf 2 N], [bmim][TfO], [bmim][HSO 4 ], [bmim][Cl], [trimim][MeSO 4 ], [NBu 3 Me][MeSO 4 ], [PBu 4 ][Cl], [bpy][PF 6 ], [bpy][BF 4 ], [bpy][Cl]) exhibits high catalytic activity in homocoupling of H 2 C CHSiMe 2 Ph, H 2 C CHSi(OEt) 3 and H 2 C CHSiMe(OSiMe 3 ) 2 and enables high-yield stereoselective synthesis of the corresponding E -1,2-bis(silyl)ethenes. The immobilized complex can be easily recycled up to 12 times in the homocoupling of H 2 C CHSiMe 2 Ph and up to 10 times in the homocoupling of H 2 C CHSiMe(OSiMe 3 ) 2 without a significant drop in activity and selectivity. Results of a split test indicate that the source of catalytic activity is the catalyst immobilized in ionic liquid phase.

Selected β-diketones bearing unsaturated derivatives have been demonstrated to undergo homo-metathesis and cross-metathesis with selected olefins in the presence of Grubbs catalysts. The reactions led to respective homo- and cross-metathesis products mainly with good yields and selectivities.

A series of functionalised cubic monovinylheptaisobutylsilsesquioxanes and octavinylsilsesquioxanes were obtained via efficient and highly stereoselective cross-metathesis and silylative coupling with substituted styrenes.

Monovinylheptaisobutylsilsesquioxane undergoes efficient cross-metathesis and silylative coupling with styrenes. Allyl derivatives were successfully tested in cross-metathesis in the presence of first generation Grubbs' catalyst, while heteroatom-substituted vinyl derivatives (vinyl ethers, 9-vinylcarbazole) efficiently undergo silylative coupling catalysed by ruthenium hydride complexes. Both reactions proceed highly stereoselectively and lead to nearly quantitative formation of E isomers. Only when vinyl ethers are used does the reaction lead to a mixture of stereoisomers. Atmospheric pressure photoionisation has been successfully used for recording mass spectra of the functionalised silsesquioxanes.

Efficient silylative coupling of linear vinyl-substituted oligo- and polysiloxanes with styrene in the presence of [RuHCl(CO)(PPh 3 ) 3 ] ( 1 ) and particularly [RuHCl(CO)(PCy 3 ) 2 ] ( 2 ) combined with copper(I) chloride is described. Treatment of styrene, with terminal or side vinyl group at siloxane skeleton catalyzed by [RuHCl(CO)(PCy 3 ) 2 ]/CuCl results in the quantitative and selective formation of respective silylative coupling products.

Neutral Fischer-type ruthenium(II) aminocarbene complexes bearing the bis(3,5-dimethylpyrazol-1-yl)acetato (bdmpza) ligand have been synthesized by addition of ammonia or methylamine to the α-carbon atom of vinylidene and allenylidene complexes. Reaction with dimethylamine was not successful. These differences in the chemical behavior can be explained by the intermediate formation of intramolecular hydrogen bridges that have been indicated by DFT calculations. Furthermore, octahedral ruthenium(II) benzylidene complexes with the bis(pyrazol-1-yl)acetato (bpza) and the bdmpza ligand were obtained by reacting [RuCl2(═CHPh)(PR3)2] (R = Cy, Ph) with K[bpza] or K[bdmpza], respectively. The latter complexes have successfully been tested for RCM reactions.

Efficient cross-metathesis (CM) of linear and cyclic vinyl-substituted oligosiloxanes with selected olefins in the presence of Grubbs type ruthenium alkylidene complexes is described. Treatment of styrene or 4-substituted styrenes with vinyl group at siloxane skeleton results in the formation of respective cross-metathesis products with good yields and selectivities.

2-Silyl-1,3-dienes (III) and 2-silyl-1,3,n-trienes (V) are efficiently obtained with complete E-selectivity.

Efficient cross-metathesis of divinylsilanes and divinyldisiloxanes, carrying different electron-withdrawing substituents at silicon, with selected olefins in the presence of the first and second generation Grubbs catalyst and Hoveyda–Grubbs catalyst is described. The reaction was proved to be a valuable method for synthesis of unsaturated organosilicon derivatives and a model for the study of synthesis of oligo- and polymeric products via ADMET copolymerization of divinylsubstituted silanes and disiloxanes with dienes.

A general and mostly highly stereoselective method for effective synthesis of 1-boryl-3-(silyl)propenes via one-step catalytic processes, i.e. silylative coupling of vinylsilanes with allyl boronates, borylative coupling of vinyl boronates with allylsilanes catalysed by [RUHCl(CO)(PCy 3 ) 2 ] and cross-metathesis of respective derivatives in the presence of Grubbs catalysts, is described.

Cross-metathesis of 4-chlorostyrene with vinylsilanes in the presence of second generation of Grubbs catalyst [Cl2(PCy3)(IMesH2)Ru( CHPh)] or silylative coupling in the presence of [RuH(Cl)(CO)(PPh3)3] followed by palladium-catalysed Hiyama coupling have been proved convenient and effective methods for stereoselective synthesis of unsymmetrical stilbenoids.

A series of neutral ruthenium(II) carbonyl, carbene, vinylidene, and allenylidene complexes [Ru(bdmpza)(Cl)(L)(PPh3)] (L = C(OR‘)R, CCHR, CCCR2, CO) containing the bis(3,5-dimethylpyrazol-1-yl)acetato (bdmpza) ligand, an N,N,O heteroscorpionate ligand, have been prepared. Treatment of [Ru(bdmpza)(Cl)(PPh3)2] (1a) with a variety of alkynes HC⋮CR (R = Ph, Tol, Pr, Bu) afforded the vinylidene complexes [Ru(bdmpza)(Cl)(CCHR)(PPh3)] (2a−d). The carbonyl complex [Ru(bdmpza)(Cl)(CO)(PPh3)] (3) is formed via oxidative or acid-induced degradation pathways from the vinylidene complexes. Reaction of 1a with the hydroxy-functionalized alkynes HC⋮C(CH2)nOH (n = 2, 3) yielded the cyclic Fischer type carbene complexes (4a) and (4b). The ruthenium(II) allenylidene complexes [Ru(bdmpza)(Cl)(CC CR2)(PPh3)] (5a, R = Ph; 5b, R = Tol) were prepared by the reaction of 1a with propargyl alcohols HC⋮CC(R)2OH via the vinylidene intermediates [Ru(bdmpza)(Cl)(CCHCR2OH)(PPh3)]. X-ray crystal structures of one structural isomer of th...

Efficient cross-metathesis of vinylsilanes, carrying a large spectrum of different substituents at silicon, with various olefins in the presence of the first and second generation Grubbs catalyst and Hoveyda–Grubbs catalyst is described. On the basis of the results of equimolar reactions of vinylsilanes with ruthenium alkylidene complexes and experiments with deuterium-labelled reagents, a general, metallacarbene mechanism for the cross-metathesis of trisubstituted vinylsilanes with olefins has been suggested. Reaction was proved to be a valuable method for synthesis of unsaturated organosilicon derivatives.

An efficient silylative coupling of trisubstituted vinylsilanes with dialkoxy(vinyl)boranes is described. Under optimum conditions the reaction offers a selective route to 1-(silyl)-1-(boryl)ethenes, which are potentially attractive intermediates in organic synthesis

The efficient cross-metathesis of vinylsilanes carrying electron-withdrawing substituents with various olefins is described. High yields and selectivities were obtained when styrene, 1-hexene, and selective functional allyl derivatives were used as the olefins.

The acyclic diene metathesis (ADMET) copolymerization of divinyltetraethoxydisiloxane with 1,9-decadiene, catalyzed by RuCl 2 (=CHPh)(PCy 3 ) 2 , is the first example of a siloxylene-vinylene-alkenylene copolymer reported in the literature. This copolymer was isolated and characterized by 1 H, and 13 C NMR spectroscopy and GPC analysis (M n = 18000, PDI = 2.9).

The cross-metathesis of vinyltrialkoxy- and vinyltrisiloxysilanes as well as divinyltetraethoxydisiloxane with various allyl alkyl ethers catalyzed by Cl2(PCy3)2Ru(CHPh) (I) is presented. The reaction is accompanied by self-metathesis of allyl alkyl ethers. The catalytic examination presented in this paper has allowed us to develop optimum conditions for selective synthesis of 1-silyl-3-alkoxy-propenes (R‘O)3SiCHCHOR (where R = Et, n-Bu, Cy, Ph, PhCH2, Gly, Me3Si, R‘ = Me, Et, SiMe3) with high preference of the E-isomers (E/Z > 7:1). This allowed the selective isolation of the E-isomer. A stoichiometric study of I with substrates has been carried out. The results are discussed on the basis of a metallacarbene mechanism.

The first ruthenium-silsesquioxyl complexes have been synthesised and characterized via spectroscopic and X-ray methods. Mechanistic studies were performed and the complexes obtained were proved to be intermediates in the catalytic cycle of silylative coupling of olefins with vinylsilsesquioxane. Moreover, a mechanism for silylative coupling of styrene with vinylsilsesquioxanes was proposed.

The benzylidene complexes [Cl2(PCy3)2RuC(C6H4R-p)H] [R = H (1a), Cl (1b), OMe (1c)] react with (E)-bis(trimethylsilyl)ethene (2) to give 3,3-bis(trimethylsilyl)-1-arylprop-1-ene, H[H(SiMe3)2C]CC(C6...

A highly stereo- and regioselective silylative coupling of styrene and p-substituted styrenes with trisubstituted vinylsilanes, siloxanes, and divinyl-substituted silicon compounds catalyzed by [{(...

Ruthenium complexes such as RuHCl(CO)(PPh3)3 (I), RuCl(SiMe3)(CO)(PPh3)2 (II), RuCl2(PPh3)3 (III), and Ru(CO)3(PPh3)2 (IV) appear to be very effective catalysts of cross-disproportionation of vinyl-trisubstituted silanes CH2CHSiR3 (where R3 = Me3, PhMe2, (OEt)3) with vinyl alkyl ethers CH2CHOR‘ (where R‘ = ethyl, n-propyl, n-butyl, tert-butyl, tert-pentyl, 2-ethylhexyl, cyclohexyl, trimethylsilyl) to yield a mixture (E+Z) of 1-silyl-2-alkoxyethenes. Stoichiometric insertion of vinyl n-propyl ether into Ru−Si bonds of II and MS study of the products of the deuterated vinyltrimethylsilane with vinyl ethyl ether indicate that the mechanism of cross-disproportionation involves a migratory insertion of the vinyl ether into the Ru−Si bond (and vinylsilane into the Ru−H bond) followed by β-H (and β-Si) transfer to eliminate 1-triorganosilyl-2-alkoxyethenes (and ethene).

The cross-metathesis of styrene with various vinylsilanes, H2CC(H)SiR3, catalyzed by [Cl2(PCy3)2RuCHPh] (1) to give (E)-silylstyrene, (E)-Ph(H)CC(H)SiR3, and ethylene is reported. The reaction proceeds even at room temperature and is highly selective. Very high conversions are observed when R = OEt, OSiMe3 (≥95%, 6 h, 2 mol % of 1). The conversion significantly decreases with increasing substitution of Me for OR‘. The metathesis is reversible. Therefore, removal of ethylene is critical for achieving high conversions. From the study of stoichiometric reactions of 1 with vinylsilanes it follows that in the series SiR3 = Si(OEt)3, SiMe(OEt)2, SiMe2OEt, SiMe3 and SiR3 = Si(OSiMe3)3, SiMe(OSiMe3)2, SiMe2(OSiMe3), SiMe3 the conversion rate increases, but simultaneously the selectivity of the metathesis decreases. The decreasing selectivity readily accounts for the decreasing efficiency in the catalytic metathesis. The product distribution of reactions of styrene-d8 with H2CC(H)SiR3 (R = OEt, OSiMe3) in the pres...

Grubbs complex, (PCy3)2Cl2RuCHPh (I) is a very effective catalyst of the cross-disproportionation of vinyl-trisubstituted silanes H2CCHSiR3 [where R3=Me3, PhMe2, (OEt)3] with vinyl alkyl ethers H2CCHOR′ [where R′=ethyl, propyl, butyl, t-butyl, t-pentyl, 2-(ethyl)hexyl, cyclohexyl, trimethylsilyl] to yield a mixture of (E+Z) 1-silyl-2-alkoxyethenes. The reaction occurs quantitatively under milder conditions (60°C) than the analogous one catalyzed by Ru–H and/or Ru–Si complexes reported previously (80°C). The stoichiometric reaction of (I) and (PCy3)2Cl2RuCH2 (III) with vinyl ethyl ether leads to the formation of (PCy3)2Cl2RuCH(OEt) (II), inactive in the stoichiometric reaction with vinylsilanes but very active in the catalytic process. Experiments with the use of deuterated vinylsilanes indicate the non-metallacarbene mechanism of the reaction and provide evidence for the initiation of Ru–H bond formation ia the hydrovinylation with vinylsilanes.

Effective disproportionation of vinylsiloxanes (vinylmethylbis(trimethylsiloxy)silane ( I ) and vinyltris(trimethylsiloxy)silane ( II )) proceeds in the presence of ruthenium(II) complexes, e.g., RuHCl(CO)(PPh 3 ) 3 and RuCl 2 (PPh 3 ) 3 in air to yield new E -bis(siloxy)ethenes, (except the RuHCl(CO)(PPh 3 ) 3 catalyzed reaction of ( I ) giving 1:1 mixture of E -1,2-bis(siloxy)ethene and 1,1-bis(siloxy)ethene). The reaction can be regarded as a molecular model of a new method of poly(methyl, vinyl)siloxanes cross-linking. Cross-coupling of the two vinylsiloxanes ( I ) and ( II ) with styrene also effectively catalyzed by Ru-complexes allows a synthesizing of new E -1-phenyl-2-(siloxy)ethenes. Mechanistic implications following from the previous works assume the involvement of Ru–H and Ru–Si intermediates responsible for catalytic reactions. β -Si transfer to metal from a β -siloxyethyl ligand bonded to Ru atom is a key step in the homocoupling of vinylsiloxane and its heterocoupling with olefin (styrene).