Metal-alkyl complexes Preparation

Initial indications are that this oxidative route to cationic early metal alkyl complexes will be fairly general. Cationic Cp2Hf(CH3)(L) complexes, monocyclopentadienyl systems (C5Mej)Zr(R.)2(L)2, and trialkyl complexes Zr(CH2Ph)3(THF)3 have recently been prepared by this method (44,45). 

The second most common method to prepare metal-alkyl complexes is the alkylation of nucleophilic metal complexes. Like protonations, these alkylations are formal oxidative additions, and are discussed in that context in Chapter 7, which covers oxidative addition of polar reagents. The reaction between an anionic metal-alkyl complex and an alkyl halide is most common (Equations 3.6 and 3.7), but examples of such reactions between neutral metal-alkyl complexes and alkyl halides are also known (Equation 3.8). ... 

Vinyl complexes are typically prepared by the same methods used to prepare aryl complexes. Vinyl mercury compounds, like aryl mercury compoimds, are easily prepared (by the mercuration of acetylenes), and are therefore useful for the preparation of vinyl transition metal complexes by transmetallation. The use of vinyl lithium reagents has permitted the s rnthesis of homoleptic vinyl complexes by transmetallation (Equation 3.35). Reactive low-valent transition metal complexes also form vinyl complexes by the oxidative addition of vinyl halides with retention of stereochemistry about the double bond (Equation 3.36). Vinyl complexes have also been formed by the insertion of alkynes into transition metal hydride bonds (Equation 3.37), by sequential electrophilic and nucleophilic addition to alkynyl ligands (Equation 3.38), and by the addition of nucleophiles to alkyne complexes (Equation 3.39). The insertion of alkynes into transition metal alkyl complexes is presented in Chapter 9 and, when rearrangements are slower than insertion, occurs by s)m addition. In contrast, nucleophilic attack on coordinated alkynes, presented in Chapter 11, generates products from anti addition. 

As stated in Chapter 3, carbene complexes can be divided into the five classes illustrated in Figure 13.2. One class of carbene complex encompasses the Fischer carbenes that were first prepared in the laboratory of E. O. Fischer. These complexes were the first transition metal carbene complexes prepared, and they contain a ir-donating group on the carbene carbon. Complexes of these carbenes are typically electrophilic at the carbene carbon. A second class of carbene complex was first prepared by Richard Schrock. These complexes contain alkyl groups or hydrogens on the carbene carbon and are called alkylidene complexes or often "Schrock carbenes." Complexes of these carbenes are typically electrophilic at the metal and nucleophilic at the carbene carbon. 

The structural consequences of bonding for carbene complexes are discussed. Photochemical reactions of metal alkyl complexes result in very reactive intermediates that can be trapped and used preparatively. Matrix isolation studies of organometallic intermediates are considered in detail. Transition metals in organic synthesis for the year 1982 are reviewed extensively and a further article concerns a-(1,n)-alkanediy1 complexes. ... 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

Danopoulos described the preparation of indenyl- and fluorenyl-fimctiona-lized NHC ETM complexes 29 and 30 through a salt-elimination reaction (Figure 6.3). By a dilferent synthetic method Wang et al. reported elosely related complexes [(NHC)M(CH2SiMe3)2] 31-33 bearing Y, Lu or Sc, which represented the first examples of functionalized NHC covalently bonding to rare earth metal alkyl complexes." ... 

Mononuclear metal-alkyl complexes with long-chain alkyl groups have been prepared from sodium carbonylmetallates and alkyl halides, Eq. 5.2 ... 

An important development over the last two years has been the confirmation that cationic Group IV metal alkyl complexes are the catalytically active species in olefin polymerizations with homogeneous Ziegler-Natta catalysts. A number of complexes of the type [Cp2MR(L)] (M = Ti, Zr, Hf) have now been prepared, particularly when stabilized by donor ligands such as L = THF. The chemistry of zirconium complexes of this type has been reviewed. Some cationic benzyl complexes with stereorigid ligands (29)... 

Many other organometaUic compounds also react with carbonyl groups. Lithium alkyls and aryls add to the ester carbonyl group to give either an alcohol or an olefin. Lithium dimethyl cuprate has been used to prepare ketones from esters (41). Tebbe s reagent, Cp2TiCH2AlCl(CH2)2, where Cp = clyclopentadienyl, and other metal carbene complexes can convert the C=0 of esters to C=CR2 (42,43). 

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. 

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,... 

T. Yamamoto, T. Murauyama, Z.-H. Zhou, T. Ito, T. Fukuda, Y. Yoneda, F. Begum, T. Ikeda, S. Sasaki, H. Takezoe, A. Fukuda, and K. Kubota, -ir-Conjugated poly(pyridine-2,5-diyl), poly(2,2 -bipyridine-5,5 -diyl), and their alkyl derivatives. Preparation, linear structure, function as a ligand to form their transition metal complexes, catalytic reactions, //-type electrically conducting properties, optical properties, and alignment on substrates, J. Am. Chem. Soc., 116 4832-4845,... 

The trans influence of a ligand arises principally from its inductive a-donor ability, transmitted to the trans-ligand by the appropriate metal pa-orbital. How far can this statement, made by Mason (18) and cited by Shustorovich (9), be applied to metalloporphyrins There are only a few examples of alkylmetalloporphyrins M(P)R with M = Fe, Co (69), or Rh (70) in the d6 configuration. In metal alkyl systems the strongest tram effects are to be expected hydridorhodium porphyrins are also known (71). Ogoshi (70) has prepared a variety of complexes Rh(OEP)LX ([22] ... 

A further method for preparing acyl complexes consists in the treatment of alkyl complexes containing at least one carbonyl ligand with a strong ligand [44,105,106], Thereby 1,1-insertion of the carbonyl group into the metal-carbon bond can... 

In many of the reported preparations of stable carbene complexes from alkyl complexes, alkyl groups without p-hydrogen (e.g. neopentyl, 2,2,2-trifluoroethyl, trimethylsilylmethyl, methyl, benzyl) were chosen in order to avoid p-elimination. There are, however, also examples of moderately stable, non-heteroatom-substituted alkylidene complexes with hydrogen in the -position to the metal (see, e.g.. Figure 3.8). 

Because a-alkoxyalkyl iron complexes are thermally unstable [467] they cannot be stored for long periods of time. More suitable carbene precursors are the corresponding a-(dimethylsulfonium)alkyl complexes, which can be stored indefinitely under ambient conditions [468-473], These complexes are prepared by S-alkylation of a-(methylthio)alkyl complexes, which can be prepared by alkylation of metallates with a-halothioethers, by addition of C-nucleophiles to (alkylthio)carbene complexes, or by addition of thiols to carbene complexes. 

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