Reactivity of Tungsten-aryloxides with Hydrosilane Cocatalysts in Olefin Metathesis

This reaction allows for new olefins to be synthesized and used as raw materials in polymers, perfumes, drugs and agrochemical industrial products, also serving as the starting point for more complex organic molecules. Catalytic systems are based mainly on the metals W, Mo, Re and Ru. The metal-alkylidenes synthesized by Schrock, [Mo(CHR)(NAr)(OR f ) 2 ], Grubbs, [Ru(CHR)(PCy 3 ) 2 Cl 2 ], Herrmann and Dixneuf are particularly interesting catalysts for olefin metathesis. When a catalyst is not a metal-alkylidene, a cocatalyst is added to generate a metalcarbene, known as the active species for metathesis. The homogeneous system [WCl 6 ]/SnR 4 is extensively studied in the literature and is appropriate for non-functionalized and fuctionalized olefins. Tungsten-aryloxides [WCl 4 (OAr) 2 ] are useful because they can be easily synthesized from [WCl 6 ] and its corresponding phenol and offer good handling stability. Moreover, these compounds are very versatile, because their activity and stereoselectivity can be modified, by changing their ring substituents. Thus, electronegative groups increase the reduction potential of tungsten atoms, enhancing the catalytic activity of these compounds in metathesis. Furthermore, stereoselectivity can be achieved by placing bulky groups in 2,6 positions of the ring. In previous studies we investigated the catalytic activity of a new system based on [WCl 6 ] plus PMHS Reactivity of Tungsten-aryloxides with Hydrosilane Cocatalysts in Olefin Metathesis


Introduction
2][3] The reaction consists of an alkene double bond cleavage, followed by a statistic redistribution of alkylidene fragments.This reaction allows for new olefins to be synthesized and used as raw materials in polymers, perfumes, drugs and agrochemical industrial products, also serving as the starting point for more complex organic molecules.Catalytic systems are based mainly on the metals W, Mo, Re and Ru.The metal-alkylidenes synthesized by Schrock,4 [Mo(CHR)(NAr)(OR f ) 2 ], Grubbs, [Ru(CHR)(PCy 3 ) 2 Cl 2 ], 5 Herrmann 6 and Dixneuf 7 are particularly interesting catalysts for olefin metathesis.When a catalyst is not a metal-alkylidene, a cocatalyst is added to generate a metalcarbene, known as the active species for metathesis.The homogeneous system [WCl 6 ]/SnR 4 is extensively studied in the literature and is appropriate for non-functionalized and fuctionalized 8 olefins.
Tungsten-aryloxides [WCl 4 (OAr) 2 ] are useful because they can be easily synthesized from [WCl 6 ] and its corresponding phenol and offer good handling stability. 9oreover, these compounds are very versatile, because their activity and stereoselectivity can be modified, by changing their ring substituents.Thus, electronegative groups increase the reduction potential of tungsten atoms, enhancing the catalytic activity of these compounds in metathesis. 10,11urthermore, stereoselectivity can be achieved by placing bulky groups in 2,6 positions of the ring.
In previous studies 12,13 we investigated the catalytic activity of a new system based on [WCl 6 ] plus PMHS (polymethylhydrosiloxane) in functionalized olefins.The new system displayed a behavior similar to others reported in the literature. 1 Furthermore, a study of different hydrosilane compounds and promoters, such as phenols, were used in the metathesis of 1-hexene. 11We also reported on olefin metathesis in the presence of [WCl 4 (OAr) 2 ], which showed good selectivity without the need for promoters.

Reactivity of Tungsten-aryloxides with Hydrosilane Cocatalysts in Olefin Metathesis
In order to further investigate these systems, we have studied the catalytic activity of [WCl 4 (OAr) 2 ] (OAr = O-2,6-C 6 H 3 Cl 2 , O-2,6-C 6 H 3 F 2 and O-2,6-C 6 H 3 Me 2 ) in the 1hexene reaction.By changing the electronic effects of the ring, we hoped to see differences in the activity of these compounds.The compounds Ph 2 SiH 2 and PMHS were used as cocatalysts while SnMe 4 served for comparison purposes.Furthermore, we extended the use of these compounds to methyl-10-undecenoate metathesis.Although the Schrock and Grubbs metal alkylidenes are still considered the "pillars of contemporary olefin metathesis", 14 research on simpler systems based on [WCl 6 ] or its derivatives is worthwhile owing to the less expensive nature of the latter compounds, which allows for their large scale application as, for instance, in vegetable oils metathesis. 15Therefore, the study of these systems applied to a functionalized olefin is fully justified.

Experimental
Reagents and solvents ] were synthesized according to procedures reported in the literature. 16

1-Hexene metathesis
The reactions were performed in a 50 mL flask connected to a condenser.The reactants were introduced into the flask with degassed solvent in the following order: 1-hexene, [W] (10 -4 mole) and cocatalyst in a molar ratio of 100 : 1 : 4, respectively.The reactions were carried out at 323 K for 4 h and interrupted by admitting air, followed by filtration through a silica/celite preparative column.Cyclohexane was added to the reaction mixture as an internal standard for GC analysis.The reactions were performed at least three times to ensure reproducibility.
The products were analyzed using an HP-5890 GC instrument equipped with a methyl-silicone megabore column (HP-1) and FID detector.The internal standard method (with cyclohexane) was used to calculate the mass percentage of 1-hexene and products.Peak identification was performed using GC/MS and spiking on authentic samples.
The mass spectra were measured with an HP 5988A mass spectrometer coupled to an HP 5890 GC.The results are presented as yields in 5-decene and byproducts that are formed mainly by dimerization and by traces of isomerization and co-metathesis between the different isomers formed, which are defined as: The term "Σ m products " includes the ethylene mass, taking into account that 1 mole of ethylene is released by each mole of 5-decene that is formed.
The total conversion and selectivities can be calculated by:

Methyl-10-undecenoate metathesis reaction
In a typical reaction, the tungsten compound was added to a flask containing methyl-10-undecenoate followed by the addition of cocatalyst according to a molar ratio of 25:1:4.The reactions were carried out at 348 K for 4 h and interrupted by allowing air into the system, followed by filtration through a silica/celite preparative column and analysis by 1 H NMR spectroscopy.
The NMR spectra were obtained using a VARIAN CXR 200 MHz spectrometer and CDCl 3 as solvent.The conversion factors (C) were estimated from the integration of the olefinic proton signals by the equation C = 2D/(E 2 + 2D), where D = integration of the diester olefinic proton signals and E 2 = integration of the two terminal ester olefinic proton signals.Tests with calibrated solutions ensured a maximum margin of error of 4%. 12

Results and Discussion
Table 1 presents the yields obtained in the metathesis of 1-hexene at 50 °C, using tungsten-aryloxides as catalysts and silicon and tin compound cocatalysts.
The reaction is satisfactory at this temperature, using silicon compounds, but gives poor yields when SnMe 4 is the co-catalyst.This result is surprising since SnMe 4 is a classical cocatalyst in olefin metathesis. 8Furthermore, no product is formed when the tungsten-aryloxide is [WCl 4 (O-C 6 H 3 Me 2 ) 2 ], except when the cocatalyst is PMHS.It has been previously reported 17 that [WCl 4 (O-C 6 H 3 Me 2 ) 2 ] is the only tungsten-aryloxide that presents a negative reduction potential, and is the most difficult complex to be reduced.On the other hand, PMHS is known to be a reducer, 18 hence it increases the activity of the tungsten compound.
The cocatalysts most frequently reported in the literature for [WCl 4 (OAr) 2 ] are aluminumalkyl and leadalkyl compounds.The literature contains few examples 11,19 of [WCl 4 (OAr) 2 ]-SnMe 4 , whose reactions were performed at 85 °C.Vosloo et al. 19 reported that activity increased drastically from 50 to 65 °C in metathesis with the [WCl 4 (OAr) 2 ]-SnBu 4 system, although they found no activity at 50 o C for that system.Therefore, we repeated the reaction for [WCl 4 (O-C 6 H 3 Cl 2 ) 2 ] at 65 °C (Table 2).As can be observed, SnMe 4 proved to be an effective cocatalyst at this temperature, contrary to the results shown in Table 1, when the temperature was 50 °C.
Figure 1 illustrates the yield vs. time curves for 1-hexene metathesis using [WCl 4 (O-C 6 H 3 Cl 2 ) 2 ]-cocatalyst.As can be seen, PMHS and SnMe 4 initially display a similar behavior The by-products contain mainly dodecene, a product of hexene dimerization.The reactions were performed at least three times.(100 -120 h -1 ) and the same final conversion.The system with Ph 2 SiH 2 presents lower initial activity (50 h -1 ) and lower final conversion, and is also the least selective, as shown by the results in Table 2.The activities and selectivities obtained with the [WCl 4 (OAr) 2 ]-cocatalyst are comparable to those obtained previously 13 with [WCl 6 ]cocatalyst-phenol, when phenol was added as a reaction promoter probably forming tungsten aryloxides in situ.However, the absence of phenol in this system leads primarily to dimerization and not to metathesis.Table 3 shows the results of the metathesis reaction of methyl-10-undecenoate using [WCl 4 (O-C 6 H 3 Cl 2 ) 2 ] and [WCl 6 ] from a previous study 12 , for comparison.The results are as good as the earlier ones, with the added advantage that the use of [WCl 4 (O-C 6 H 3 Cl 2 ) 2 ] requires no drastically inert conditions such as glove box handling, which is necessary when handling tungstenhexachloride. 12,20 To summarize, the silicon compounds Ph 2 SiH 2 and PMHS were used as cocatalysts with tungsten phenoxides in the metathesis of a non-functionalized and a functionalized olefin.The results showed activities and selectivities comparable to those obtained by using the known [WCl 6 ]-SnMe 4 -based system.Furthermore, [WCl 4 (OAr) 2 ] plus PMHS or Ph 2 SiH 2 is applicable with both non-functionalized and functionalized olefins.On the other hand, the [WCl 6 ]/silicon compounds, as demonstrated in a previous study, 13 provided good results when an ester was used as an olefin, although the addition of phenol promoters was necessary with 1-hexene.

Conclusions
The systems based on the [WCl 4 (OAr) 2 ]-silicon compound proved to be active and selective for the metathesis reaction of 1-hexene, particularly when the structure of the aryloxide ligands contains electronegative groups.Of all the cocatalysts studied, polymethylhydrosiloxane (PMHS) proved to be very active and selective, even with [WCl 4 (C 6 H 3 Me 2 ) 2 ], which displayed little activity with other cocatalysts.PMHS was the best cocatalyst for both 1-hexene and methyl-10-undecenoate metathesis.It should be noted that this compound is nontoxic, non-volatile, easy to handle and inexpensive, rendering it a good alternative for such reactions.The reactions were performed at least three times.