Metal Alkyl Initiators

For clarifying the factors influencing the ease of CO insertion and its reverse process, it is desirable to know the metal-carbon bond energies in the initial metal alkyl and the product metal acyl species. However, the presently available thermochemical data for the bond dissociation energies in acyl-transition metal complexes are not sufficient to allow us to advance a reasonable argument for the thermodynamic feasibilities of insertion and deinsertion processes [22-24],... 

Polyacetaldehyde, a mbbery polymer with an acetal stmcture, was first discovered in 1936 (49,50). More recentiy, it has been shown that a white, nontacky, and highly elastic polymer can be formed by cationic polymerization using BF in Hquid ethylene (51). At temperatures below —75° C using anionic initiators, such as metal alkyls in a hydrocarbon solvent, a crystalline, isotactic polymer is obtained (52). This polymer also has an acetal [poly(oxymethylene)] stmcture. Molecular weights in the range of 800,000—3,000,000 have been reported. Polyacetaldehyde is unstable and depolymerizes in a few days to acetaldehyde. The methods used for stabilizing polyformaldehyde have not been successful with poly acetaldehyde and the polymer has no practical significance (see Acetalresins). 

The mechanism of the asymmetric alkylation of chiral oxazolines is believed to occur through initial metalation of the oxazoline to afford a rapidly interconverting mixture of 12 and 13 with the methoxy group forming a chelate with the lithium cation." Alkylation of the lithiooxazoline occurs on the less hindered face of the oxazoline 13 (opposite the bulky phenyl substituent) to provide 14 the alkylation may proceed via complexation of the halide to the lithium cation. The fact that decreased enantioselectivity is observed with chiral oxazoline derivatives bearing substituents smaller than the phenyl group of 3 is consistent with this hypothesis. Intermediate 13 is believed to react faster than 12 because the approach of the electrophile is impeded by the alkyl group in 12. 

Initiation presumably involves metal alkyls as the primary source of carbanions. These are immediately available from the Grignard reagents, organosodium compounds, or sodium amide used as catalysts when the alkali metal itself or its solution in liquid ammonia is used, addition to the monomer may precede actual initiation. ... 

Reaction of the same neutral borabenzene-ligand adduct, C5H5B-PMe3, with a transition, rather than an alkali, metal alkyl or amide can furnish r 6-boratabenzene complexes in a single step (Scheme 8).17 This efficient transformation presumably proceeds through initial ir-coordination of CsHsB-PMes to the transition metal, followedby an intramolecular substitution reaction. In contrast to other approaches to the synthesis of T 6-boratabenzene complexes, this synthetic route does not have a parallel in if-cyclopentadienyl chemistry. 

It has been suggested that polymerization initiated by transition metal alkyls is preceded by coordination of the monomer to the metal atom. 

If coordination were not important, the polymerization process would presumably resemble those initiated by alkali metal alkyls. The latter are very effective initiators for the polymerization of such monomers as styrene, butadiene, etc., and it is generally considered that propagation proceeds through intermediates of the type (43)... 

These initiators cannot, however, produce high molecular weight poly-ethylenes except at high pressures ( 1000 atm). The difference is presumably that (XXVI) cannot coordinate ethylene, whereas the propagating centers derived from transition metal alkyls have this characteristic. 

Polymerizations. The polymerizations were carried out in an argon atmosphere in capped glass bottles fitted with a neoprene rubber gasket inner liner. In charging the polymerizations, the order of addition of materials was solvent first, then metal alkyls, next the barium salt, and finally the monomer(s). The amount of metal alkyl charged was sufficient to titrate the acidic impurities present in the solvent and polymerization bottle, plus the calculated amount for initiation of polymerizations. The mole ratio of barium to metal alkyl(s) was based on the moles of total alkalinity of barium to the moles of carbon-metal assayed. Unless otherwise stated,... 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

It must be emphasized that the duodectet rule (4.6) initially has no structural connotation, but is based on composition only. Indeed, the compositional regularity expressed by (4.6) encompasses both molecular species (such as the metal alkyls) and extended lattices (such as the oxides and halides) and therefore appears to transcend important structural classifications. Nevertheless, we expect (following Lewis) that such a rule of 12 may be associated with specific electronic configurations, bond connectivities, and geometrical propensities (perhaps quite different from those of octet-rule-conforming main-group atoms) that provide a useful qualitative model of the chemical and structural properties of transition metals. 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

Many of these catalysts are derived from metal complexes which, initially, do not contain metal hydride bonds, but can give rise to intermediate MH2 (al-kene) species. These species, after migratory insertion of the hydride to the coordinated alkene and subsequent hydrogenolysis of the metal alkyl species, yield the saturated alkane. At first glance there are two possibilities to reach MH2 (alkene) intermediates which are related to the order of entry of the two reaction partners in the coordination sphere of the metal (Scheme 1.2). 

It would be desirable to be able to use data such as that given in Table 12 to predict Dt values for other methyl metallic alkyls and to set a pattern for ethyl and possibly higher alkyls. These dissociation energies should be approximately equal to the kinetic activation energy for the first stage of dissociation in a nonchain decomposition or to the activation energy of the initiation step in a chain decomposition. 

It would be inappropriate, and indeed superfluous, to review here the whole of this very complicated field, since this has been done adequately by several authors [2, 4-7]. For our present purposes we need to pick out only one other aspect, that of co-initiation by alkyl halides. It was proved that with certain olefins and metal halides the addition of an alkyl halide to a mixture of metal halide and olefin would initiate polymerisation [8], and this was interpreted by an extension of the theory of co-initiation, in terms of the reaction scheme (2) ... 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

In these reaetions, the most reliable mechanism is considered to involve the initial metal-coordination at the nitrogen atom of the pyridine ring and the subsequent attack of an alkyl or aryl anion at the most probable cationic sites on the ring, namely, the 2- and/or 4-position of the ring. If a 2-halogen substituted pyridine is used, the nucleophilic anion attacks the 6-position. Thus, the addition is a more prefered reaction than the ipso-substitution as shown in reaction (29). The substitution of amide or phenyl-... 

Alkyllithium compounds are probably the most useful of these initiators, employed com-merically in the polymerizations of 1,3-butadiene and isoprene. Initiation proceeds by addition of the metal alkyl to monomer... 

Isocyanates are polymerized through the carbon-nitrogen double bond to 1-nylons by anionic initiators such as metal alkyls, sodium naphthalene, and sodium cyanide [Bur and Fetters,... 

The anionic polymerization of epoxides such as ethylene and propylene oxides can be initiated by metal hydroxides, alkoxides, oxides, and amides as well as metal alkyls and aryls, including radical-anion species such as sodium naphthalene [Boileau, 1989 Dreyfuss and Drefyfuss, 1976 Inoue and Aida, 1984 Ishii and Sakai, 1969]. Thus the polymerization of ethylene oxide by M+A involves initiation... 

The well-known alkylation of ferrocyanide ion to form isocyanide iron complexes (48) can be explained by an insertion mechanism if the metal is alkylated initially, and then metal alkyl adds across a cyanide group. This mechanism also explains how external radioactive cyanide ion can enter the isocyanide ligands (48). 

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