Transition metal alkyl compounds activity

Attempts to synthesize transition metal alkyl compounds have been continuous since 1952 when Herman and Nelson (1) reported the preparation of the compound C H6>Ti(OPri)3 in which the phenyl group was sigma bonded to the metal. This led to the synthesis by Piper and Wilkinson (2) of (jr-Cpd)2 Ti (CH3)2 in 1956 and a large number of compounds of titanium with a wide variety of ligands such as ir-Cpd, CO, pyridine, halogen, etc., all of which were inactive for polymerization. An important development was the synthesis of methyl titanium halides by Beerman and Bestian (3) and Ti(CH3)4 by Berthold and Groh (4). These compounds show weak activity for ethylene polymerization but are unstable at temperatures above — 70°C. At these temperatures polymerizations are difficult and irreproduceable and consequently the polymerization behavior of these compounds has been studied very little. In 1963 Wilke (5) described a new class of transition metal alkyl compounds—x-allyl complexes,... 

The results of polymerizing ethylene using varying sigma-bonded transition metal alkyl compounds are summarized in Table VII. It is evident that none of the catalysts are very active and are comparable with the simple allyl compounds listed in Table I. 

III. Ligand Replacement in Transition Metal Alkyl Compounds and Polymerization Activity... 

The above substitution effects appear to be independent of the nature of the ligand (16) since the benzyl compounds behave similarly, Table XI. It would appear from these observations that the introduction of anionic ligand would be sufficient to increase activity of transition metal alkyl compounds for polymerization. This, however, is probably an oversimplifica-... 

Comparison of the homogeneous polymerizations of transition metal alkyl compounds with their heterogeneous equivalents shows that the higher activity of the latter is due to ... 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems... 

Ballard, D. G. H., Jones, E., Wyatt, R. J., Murray, R. T., and Robinson, P. A. 1974. Highly active polymerization catalysts of long life derived from a- and Jt-bonded transition metal alkyl compounds. Polymer 15 169-174. 

Transition-Metal Alkji and Related Catalysts There continues to be a lively interest in the use of transition-metal alkyl compounds as polymerization catalysts both in the absence and in the presence of alkylaluminium cocatalysts. Hie more active catalysts of this type are invariably supported sterns and indeed it is of some interest that many non-supported transition metal alkyls do not polymerize propylene at all. Sinc this field has been extensively reviewed in recent years by Ballard and also features in an article by Yermakov only the most recent publications will be mentioned in this Report. 

Wilke s allyl compounds were found to be very poor catalysts indeed, e.g., Ti(2 Me-allyl)4, was found only to have an activity equal to 0.5 gm/m.M Ti/atm/C2H4/hr. For this reason there has been considerable dispute that transition metal alkyls can be the intermediates in Ziegler polymerization. 

The activity of transition metal allyl compounds for the polymerization of vinyl monomers has been studied by Ballard, Janes, and Medinger (10) and their results are summarized in Table II. Monomers that polymerize readily with anionic initiators, such as sodium or lithium alkyls, polymerize vigorously with allyl compounds typical of these are acrylonitrile, methyl methacrylate, and the diene isoprene. Vinyl acetate, vinyl chloride, ethyl acrylate, and allylic monomers do not respond to these initiators under the conditions given in Table II. 

The surfaces of some types of silica and alumina freed from adsorbed water contain acidic -OH groups. Ballard et al. (15) showed that these -OH groups react readily with transition metal alkyls giving stable compounds that are highly active polymerization catalysts for olefins. These systems are best described with reference to silica. 

Although it is clear that alkane activation is possible, the work done so far using soft metals as catalyst has not produced a reaction that converts alkanes into useful products. Nor does it point very clearly in the direction where progress will be made. It may be that studies on dimeric and cluster compounds, on polyhydrides, or on transition metal alkyls will indicate the paths to be explored. The work using hard catalysts is at present very limited and can be expected to be developed much further in future years. 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

The two-component catalytic systems used for olefin polymerization (Ziegler-Natta catalysts) are combinations of a compound of a IV-VIII group transition metal (catalyst) and an organometallic compound of a I-III group non-transition element (cocatalyst) An active center (AC) of polymerization in these systems is a compound (at the surface in the case of solid catalysts) which contains a transition metal-alkyl bond into which monomer insertion occurs during the propagation reaction. In the case of two-component catalysts an AC is formed by alkylation of a transition metal compound with the cocatalyst, for example ... 

Reaction of R Mg with a transition metal compound produces a reduced transition metal composition co-precipitated with an inorganic magnesium compound. In this respect, dialkylmagnesium compounds are functioning in much the same way as aluminum alkyls described in section 4.2.2. As before, additional aluminum alkyl cocatalyst must be introduced in the polymerization reactor to alkylate the transition metal and create active centers. 

The study of the reactivity of transition metal hydride compounds towards unsaturated organic molecules, mainly olefines and alkynes, has been traditionally centered in monohydrides. In general, the reactions lead to the insertion products, alkyl or alkenyl, which have a limited chemistry. Furthermore, the generation of subsequent carbon-carbon or carbon-heteroatom bonds requires the presence of other active ligands, such as halogen or carbon monoxide, in the coordination sphere of the metallic center of the alkyl or alkenyl intermediates. 

Some other transition metal carbonyl compounds were also investigated instead of Ru3(CO)i2, such as Os3(CO)i2, Rh4(CO)i2, Re2(CO)io, but none of them showed any activity in this reaction. The mechanism behind this reaction was most likely the fact that a coordinatively unsaturated metal center of the trinuclear cluster is attacked and coordinated by pyridine, and subsequent ortho-metalation gives the key intermediate. Olefin insertion into a linear and branched alkyl species, followed by CO coordination and insertion, produces the acyl species, which reacts further to the acylated product by reductive elimination. 

Organosulfur Adsorbates on Metal and Semiconductor Surfaces. Sulfur compounds (qv) and selenium compounds (qv) have a strong affinity for transition metal surfaces (206—211). The number of reported surface-active organosulfur compounds that form monolayers on gold includes di- -alkyl sulfide (212,213), di- -alkyl disulfides (108), thiophenols (214,215), mercaptopyridines (216), mercaptoanilines (217), thiophenes (217), cysteines (218,219), xanthates (220), thiocarbaminates (220), thiocarbamates (221), thioureas (222), mercaptoimidazoles (223—225), and alkaneselenoles (226) (Fig. 11). However, the most studied, and probably most understood, SAM is that of alkanethiolates on Au(lll) surfaces. 

Many pharmacologically active compounds have been synthesized using 5-bromoisoquinoline or 5-bromo-8-nitroisoquinoline as building blocks.6 7 8 9 10 11 The haloaromatics participate in transition-metal couplings 81012 and Grignard reactions. The readily reduced nitro group of 5-bromo-8-nitroisoquinoline provides access to an aromatic amine, one of the most versatile functional groups. In addition to N-alkylation, TV-acylation and diazotiation, the amine may be utilized to direct electrophiles into the orthoposition. 

---