Neopentylidene complexes metathesis

The exact nature of the alkylidenes formed on various oxide surfaces is still uncertain, as is the nature of the alkylidenes responsible for the often observed metathesis activity. Mo(N)(CH2CMe3)3 also has been employed as a precursor to a surface-bound species believed to be of the type Mo(NH)(CHCMe3)(CH2CMe3) (Osurf) [115]. Although the alkylidene carbon atom could not be observed in solid state NMR spectra, which is typical of surface supported alkylidenes, reaction with acetone to give 2,4,4-trimethylpent-2-ene quantitatively confirmed the presence of the reactive neopentylidene complex. Such species would initiate various metathesis reactions when prepared on partially dehydroxylated silica. 

The surface species is an active catalyst for propene metathesis (780 eq., TOI min = 0-89 molpropene hioImo s" for the catalyst obtained by the benzene impregnation route) and ethyl oleate metathesis (lOOeq. TOI mm =0.11 moloieate hioImo s for the catalyst obtained by the benzene impregnation route) [136]. The high catalytic activity of the surface neopentylidene complex has not been modeled by the silsesquioxane analogue [136]. 

As regards the metathesis polymerisation of cyclic trienes, it has been carried out in an attempt to find alternative routes for preparing soluble and meltable precursors of polyacetylene [149, 150], Hence, several substituted or unsubstituted tricyclic or other polycyclic trienes were subjected to polymerisation in the presence of metathesis catalysts such as WCl6-SnMe4 [151-154] and the tungsten neopentylidene complex [Me(F3C)2CO]2W(=NAr)(=CHCMe3) [155]. A successful solution of the problem is outlined below [125,150] ... 

Carbene complexes for which metathesis is the preferred synthetic method include the W alkylidene complexes, W(0)(PEt3)2Cl2CHR (R = CH3, Ph, C2H5, H), prepared from the corresponding neopentylidene complex ... 

Application of cyclometalated neopentylidene complexes to the metathesis of olefinic esters. Very promising results were obtained in the metathesis of an olefin bearing an ester group such as ethyl oleate (ethyl-9-octadecenoate) (Scheme 5)... 

Supported Ta-Neopentylidene Complexes A silica-supported Ta-alkylidene [(=SiO) Ta(=CHCMe3)(CH2CMe3)p j] catalyst has been synthesized and well characterized [76, 77]. This Ta-alkylidene catalyst possesses a neopentyl ligand and an alkylidene group, and was found to be active for propane metathesis (TON=33). Since the precursor is not a hydride and a proposed carbene hydride is known to be the intermediate, this implies that Ta-neopentyl and neopentylidene organometaUic species are initially transformed into this key species (Scheme 2.10) [78-83]. Different initiation steps have been suggested. One involves the initial addition of the alkane into the alkylidene moiety to form multiple Ta-alkyl species, which can then decompose via an a-H abstraction to produce a Ta-alkylidene (Scheme 2.10a) [84, 85]. 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

The aesthetically pleasing silica-supported alkyl, alkylidene, alkylidyne Re complex =SiORe[=C u][=CH Bu][CH2 Bu] (Scheme 3, Structures 15-17), prepared from the molecular precursor, has also been found to be highly reactive for the metathesis of propene [13]. Moreover, the evolution of roughly one equivalent of a 1 3 mixture of 3,3-dimethylbutene and 4,4-dimethyl-2-pentene is consistent with a cross-metathesis of the neopentylidene ligand of 1 and propene. The turnover obtained also exceeds those obtained with classical heterogeneous catalysts such as W03/Si02 used industrially for decades (Lummus process). 

The same neopentylidene-alkoxo complexes react with cis-2-pentene to give the two initial metathesis products (4,4-dimethyl-2-pentene and 5,5-dimethyl-3-hexene) and catalyze the metathesis of cis-2-pentene to 2-butenes and 3-hexenes. Furthermore, propylene and ethylene appear in the reaction medium as the catalyst deactivates. These latter olefins are formed by rearrangement of the ethylidene and propylidene intermediates, providing the mechanism for the metathesis chain termination step ... 

Metathesis is the best route to these complexes. The preparation of W alkylidehes by a-hydride abstraction has had only limited success , and the neopentylidene substrate is prepared by an unusual heterometallic exchange reaction ... 

Addition of 100 equiv of cw-2-pentene to 5yw-Re(C-f-Bu)(CH-r-Bu)(ORF6)2 benzene yielded a 1 2 1 mixture of 2-butenes, 2-pentenes, and 3-hexenes in 2.5 h at 25 °C. When an additional 100 equiv of cw-2-pentene was added to this mixture, equilibrium was restored in less than 30 minutes. Observation of an induction period and an increase in the rate of metathesis with time is consistent with a large increase in the rate of metathesis by complexes containing less bulky propylidene and ethylidene ligands (versus a neopentylidene ligand), a finding that qualitatively has been observed in several other circumstances involving well-characterized metathesis catalysts of the type M(CHR)(NAr)(OR )2. Tiiis catalyst system is stable indefinitely at low concentrations (< 10 mM), and its absolute activity at room temperature is estimated to be approximately 250 equiv h for metathesis of d.y-2-pentene, a rate that is perhaps as much as two orders... 

ABSTRACT. Cyclometalated aryloxy(chloro)neopentylidene-tungsten complexes can be synthesized starting from WCl4(OAr)2 (OAr = 2,6-disubstituted phenoxide), but also starting from the neopentylidyne complex W(CCMe3)Cl3(dme) (by reaction with LiOAr). Some of these cyclometalated neopentylidenes are probably among the most active and stereoselective one-component metathesis catalysts. In particular, they are fairly active in the metathesis of an olefinic ester such as ethyl oleate and they have been successfully used in the metathesis of olefinic sulfides. 

Interestingly, this type of cyclometalated complex can be obtained via an other route starting from the Schrock s neopentylidyne complex W(CCMe3)Cl3(dme) (dme = dimethoxyethane) [25, 26]. The purpose of this paper is to report the main features of this new way of synAesis and to give some examples of the catalytic properties of these cyclometalated aryloxy(chloro)neopentylidene-tungsten complexes in olefin metathesis. 

CATALYTIC PROPERTIES OF CYCLOMETALATED ARYLOXY(CHLORO)-NEOPENTYLIDENE-TUNGSTEN COMPLEXES IN OLEFIN METATHESIS... 

We have found that metathesis of olefmic sulfides can be successfully achieved by using, as homogeneous catalyst, the aryloxy(chloro)neopentylidene-tungsten complex 1. Thus, the self-metathesis of allyl methyl sulfide (4) and of 5-dkylthiocycloooctenes (5) (or their cometathesis with various acyclic or cyclic olefins without functional groups) can lead to a family of new olefins, dienes, or unsaturated polymers containing one or more thioether groups. 

The use of a silica-supported, tantalum alkyhdene as a precursor for alkane metathesis was found to result in a stoichiometric, alkane cross metathesis in which an initial pendant alkyl-alkylidene group was transformed to produce the desired, active species [76, 90]. This reaction was later observed to work with additional, well-defined systems supported on alumina [58] and sihca-alumma [53]. As mentioned previously, this transformation does not occur when the surface organometallic precursor bears no alkyl group. Exposing these supported, metal neopentyl, neopentylidene, and neopentyhdyne complexes to alkane at 150 °C produced alkanes containing a neopentyl fragment (CH jiBu) via cross metathesis. Propane metathesis with these alkyl-alkyhdene surface complexes typically generates stoichiometric amounts of dimethylpropane, 2,2-dimethylbutane, and 2,2-dimethylpentane (Scheme 2.11). 

Basset and his group have observed that propane and propene metathesis give similar Cn+i/Cn+2 ratios of cross-metathesis products on silica-supported tantalum-neopentylidene catalyst at 150°C. The olefin-metathesis activity of these Schrock-type supported complexes results from the presence of the silyloxy ligand (vide infra) - Organometallic complexes are bound to silica or alumina by reaction of soluble complexes and involve die formation of one or several bonds between the central metal and the oxygen atom of the oxide support. 

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