Organometallic compounds metal alkyls

The catalyst component consists of halides of IV-VIII group elements having transition valence and the cocatalysts are organometallic compounds like alkyls, aryls and hydrides of group I-IV metals. Although there are hundreds of such catalyst cocatalyst systems listed in table below. Systems based on the organoaluminium compounds such as triethyl aluminium (AlEt3) or diethyl aluminium chloride... 

The volatile metal-containing precursors which satisfy the ALD criteria fall into four main categories (i) halides, (ii) y0-diketonate complexes, (iii) alkoxides, and (iv) true organometallics, viz. metal alkyls and cyclopentadienyl-type compounds (Fig. 3). Also amido complexes have recently gained attention as possible ALD precursors. Occasionally other compounds have been used as ALD precursors for thin films, for instance, metal nitrates, carboxy-lates and isocyanates [17,18]. 

In this article the term organometallic compound includes alkyl and aryl derivatives of the rare earths—the transition metals of group III, scandium, yttrium, lanthanum and the lanthanides cerium to liitetium with covalent metal-to-carbon a-bonds, as well as the so-called 77-complexes with more than monohapto metal-to-carbon bonds, for example cyclopentadienyl and olefin complexes, metal acetylides, but not carbonyls, cyanides and isocyanide complexes. Derivatives of scandium, yttrium and lanthanum are included and discussed together with the compounds of the lanthanides, because of many similarities in the synthesis and the chemistry of these organometallic derivatives of the rare earths. 

Mercury has three stable oxidation states 0, 1 +, 2+. It also forms relatively stable organometallic compounds with alkyl or aryl residues. Elemental mercury dissolves numerous other metals to alloys, called amalgams. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Organometallic compounds are named as substituted derivatives of metals The metal is the parent and the attached alkyl groups are identified by the appropriate prefix... 

Organometallic compounds contain a carbon-metal bond They are named as alkyl (or aryl) derivatives of metals... 

From a synthetic point of view, direct alkylation of lithium and magnesium organometallic compounds has largely been supplanted by transition-metal-catalyzed processes. We will discuss these reactions in Chapter 8 of Part B. 

Before we describe the applications of organometallic reagents to organic synthesis, let us examine their preparation. Organolithium compounds and other Group I organometallic compounds are prepared by the reaction of an alkyl halide with the appropriate metal. 

Grignard reagents are a very important class of organometallic compounds. For their preparation an alkyl halide or aryl halide 5 is reacted with magnesium metal. The formation of the organometallic species takes place at the metal surface by transfer of an electron from magnesium to a halide molecule, an alkyl or aryl radical species 6 respectively is formed. Whether the intermediate radical species stays adsorbed at the metal surface (the A-modelf, or desorbs into solution (the D-model), still is in debate ... 

Many other kinds of organometallic compounds can be prepared in a manner similar to that of Grignard reagents. For instance, alkyllithium reagents, RLi, can be prepared by the reaction of an alkyl halide with lithium metal. Alkyllithiums are both nucleophiles and strong bases, and their chemistry is similar in many respects to that of alkylmagnesium halides. 

Ziegler-Natta catalysts-—there are many different formulations—are organometallic transition-metal complexes prepared by treatment of an alkyl-aluminum with a titanium compound. Triethylaluminum and titanium tetrachloride form a typical preparation. 

A quite surprising development, even to experienced workers in elemental-fluorine chemistry, has been the synthesis of trifluoromethyl organometallic compounds by direct fluorination of metal alkyls (25). Even more surprising is the fact that, for certain metal and metalloid systems, such as the reaction of elemental fluorine with tetramethyl-germane, this t5rpe of low-temperature synthesis is a practical method 26) for the laboratory preparation of the perfluoro analog. 

It seems likely that the mechanism of the Wurtz reaction consists of two basic steps. The first is halogen-metal exchange to give an organometallic compound (RX -(- M —+ RM), which in many cases can be isolated (12-36). Following this, the organometallic compound reacts with a second molecule of alkyl halide (RX + RM —> RR). This reaction and its mechanism are considered in the next section (10-94). 

Many organometallic compounds are best prepared by this reaction, which involves replacement of a metal in an organometallic compound by another metal. The compound RM can be successfully prepared only when M is above M in the electromotive series, unless some other way is found to shift the equilibrium. That is, RM is usually an unreactive compound and M is a metal more active than M. Most often, RM is R2Hg, since mercury alkyls are easy to prepare and mercury is far down in the electromotive series." Alkyls of Li, Na, K, Be, Mg, Al, Ga, Zn, Cd, Te, Sn, and so on have been prepared this way. An important advantage of this method over 12-36 is that it ensures that the organometallic compound will be prepared free of any possible halide. This method can be used for the isolation of solid sodium and potassium alkyls." If the metals lie too close together in the series, it may not be... 

Ge, P, As, Sb, and Bi, can also be prepared in this manner." Except for alkali metal alkyls and Grignard reagents, the reaction between RM and M X is the most conunon method for the preparation of organometallic compounds." ... 

Metallation of alkyl or aryl halides with organometallic compounds... 

There are distinct structural types of organic compounds containing metals and metalloids. The first contain covalent carbon-metal bonds and are strictly organometallic compounds, for example, the alkylated compounds of Hg, Sn and Pb, and of Li, Mg, and A1 (and formerly Hg), which have been extensively used in laboratory organic synthesis, and A1(C2H5)3 that is a component of the... 

In organometallic compounds of the form RR R"C—M, pretty well the whole spectrum of bonding is known from the essentially covalent, via the polar-covalent, RR R"C, —M4+, to the essentially ionic, RR R"CeM . In their reactions, predominant retention, racemisation, and inversion of configuration have all been observed the outcome in a particular case depending not only on the alkyl residue, but also on the metal, and particularly on the solvent. Even with the most ionic examples it seems unlikely that we are dealing with a simple carbanion thus in the reaction of EtI with [PhCOCHMe]6 M , the relative rates under analogous conditions are found to differ over a range of 104 for M = Li, Na and K. 

It occurs with the alkyls, aryls or acetylides of metals more electropositive than magnesium, but including Grignard reagents, and is often carried out by adding a solution of the organometallic compound in an inert solvent to a large excess of powdered, solid C02 it is a particularly useful method for the preparation of acetylenic acids. The Kolbe-Schmidt reaction (p. 291) is another example of carbanion carbonation. 

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