The Preparation of Metal Alkyls

The chief methods for the synthesis of alkyls involve (1) an R reagent, (2) an reagent, (3) oxidative addition, and (4) insertion. Typical examples of these are shown in Eqs. 3.6-3.15  

NbClj NbMe2Cl3 + ZnCl2 2. From an R+ reagent (electrophilic attack on the metal)  

OxidatIvB Addition With the third general method of making alkyls, we encounter a new and very important reaction in organometallic chemistry, called oxidative addition, which we study in detail in Chapter 6. This term is used any time we find that an X—Y bond has been broken by the insertion of a metal fragment L M into the X—Y bond. X and Y can be any one of a large number of groups, some of which are shown in Eq. 3.16. 

Certain L M fragments are often considered carbene-like because there is an analogy between their insertion into X—Y bonds and the insertion of an organic carbene, such as CH2, into a C—H, Si—H, or O—H bond (Eq. 3.17). In Section 13.2, we will see how the isolobal principle allows us to understand the orbital analogy between the two systems. There are several mechanisms 

METAL ALKYLS. ARYLS, AND HYDRIDES AND RELATED a-BONDED LIGANDS 

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Reactivity and yields are greatly enhanced by the presence of 0.5-1% Na in the Li. The reaction is also generally available for the preparation of metal alkyls of the heavier Group 1 metals. Lithium aryls are best prepared by metal-halogen exchange using LiBu" and an aryl iodide, and transmetalation is the most convenient route to vinyl, allyl and other unsaturated derivatives ... 

Heating the adduct of ethylene oxide and sulfur dioxide with primary alcohols in the presence of alkaH hydhdes or a transition-metal haHde yields dialkyl sulfites (107). Another method for the preparation of methyl alkyl sulfites consists of the reaction of diazomethane with alcohoHc solutions of sulfur dioxide (108). 

In the skeleton of many chelating diphosphines, the phosphorus atoms bear two aryl substituents, not least because the traditional route to this class of compounds involves the nucleophilic substitution with alkali metal diarylphosphides of enantiopure ditosylates derived from optically active natural precursors, approach which is inapplicable to the preparation of P-alkylated analogs. The correct orientation of these aryl substituents in the coordination sphere has been identified as a stereo chemically important feature contributing to the recognition ability of the metal complex [11,18-20]. 

Methods for the preparation of tris(0-ethyl dithiocarbonato) complexes of chromium(III), indium(III), and cobalt(III) are presented and serve to illustrate procedures applicable to the preparation of O-alkyl dithiocarbonato, alkyl trithiocarbonato, iV,A7-dialkyldithiocarbamato, and 0,0 -dialkyl dithiophosphato complexes of several metals. 

Excellent enantioselectivities up to complete asymmetric induction are achieved in the preparation of a-alkylated aldehydes, acyclic and cyclic ketones via (-)-(S)- and (+ )-(7 )-1 -amino-2-methoxymethylpyrrolidine (SAMP/RAMP-hydrazones) (see Section 1.1.1.4.2.). Due to the unique mechanism of metalation and alkylation, the absolute configuration of the final products can be predicted. Since both antipodes of the auxiliary are available, either enantiomer of the desired alkylated carbonyl compound can be prepared... 

The use of metal alkyls, metal aryls, metal hydrides, and metal carbides for preparing alcoholates of carbohydrates in inert, aprotic solvents has not yet been reported. 

The ligands that have been used in the preparation of metal nitride complexes are quite varied. They include halides, carbogenic groups (alkyl moiety and Cp ), pnic-togens (amine, amide, imide, phosphine, and arsine), and chalcogens (ether, alkox-ide, oxo, thioalkoxide, and selenides). Chart 1 provides a listing of the various ligands and the associated nitrido metal complexes. 

The stereospecific insertion of 2-monosubstituted alkenyl carbenoids was successfully employed in the preparation of 1-alkyl-1-zircono-dienes. The Z and E carbenoids of 1-chloro-l-lithio-l,3-butadiene (69 and 70, respectively) are generated in situ fromE- andZ-l,4-dichloro-2-butene [53] (Scheme 25). Inversion of configuration at the carbenoid carbon during the 1,2-metalate rearrangement stereospecifically yields terminal dienyl zirconocenes 71 and 72 [54] (Scheme 25). As the carbenoid-derived double bond is formed in 9 1=Z E for 69 and >20 1=E Z isomeric mixtures for 70, the metalated dienes 71 and 72 are expected to be formed with the same isomeric ratio. Carbon-carbon bond formation was achieved by palladium-catalyzed cross-coupling with allyl or vinyl halides to give the functionalized products with >95 5 stereopurity [55-57]. 

The reaction can be performed in different organic solvents. Currently ethanol seems to be the most common solvent. This method has been employed almost exclusively for the preparation of sodium alkyl-, alkenyl-, aryl-, and heteroaryl tellnrolates. Metallic lithium and sodium can also be used for the reduction of diorganyl ditellurides. 

Table 6 Preparation of Metal Alkyls by the Aluminum-Metal Exchange Between Aluminum Alkyls...

The preparation of 2-alkyl- and 2-acyl-l-methylimidazole-4-carboxylates from (Z)-y8-dimethylamino-o -isocyanoacrylate (21) with an alkyl or acyl halide bears similarities to the foregoing DAMN procedures (Scheme 2.1.8) 152, 53]. ITie isocyanoacrylates can be made quite readily by reaction of DMF diethylacetal with the carbanion generated when the isocyanide (22) is metallated f53J. 

The trityl cation (EbjC" ") is a commonly used reagent to effect the abstraction of a hydride from eoordinated ligands. For example, the preparation of metal alkylidene and alkene complexes by a- and jS-hydride abstraction from metal alkyls using trityl salts is a well-established synthetic method, Eqs 35 and 36. 

Preparation of Phosphines from Metallated Phosphines. This route has continued to find extensive application for the preparation of a wide range of new phosphines. The reaction of lithium diarylphosphides with tosylate esters has been employed in the synthesis of the chiral phosphines (19) and (20) . In the preparation of fully alkylated chiral diphosphines in the DIOP series (21), the reactions of lithium dialkylphosphides with ditosylate precursors surprisingly give intractable products. Similar problems are encountered in the related reactions of chloromethyl, bromomethyl, and iodomethyl precursors, the main... 

The reaction of metal alkyl compounds with dihydrogen, leading to hydride species with elimination of alkanes, is commonly observed for Lewis acidic metal centers. This reaction has been used to prepare hydride complexes of the lanthanides, and of titanium(III), see equation 8 [39]. 

A stu on the reaction of metal caihon with naked enolates to produce metallocarbene-enolates and alkyl-carbonylmetallates has been released" and the preparation of metal formates via die oxidation of metal carbonyl halides is also noted and the steiic and electronic tuning of Michael Additian reactions induced by pentacarbonyl metal fiagments has been studied. ... 

The radicals produced may react with molecules of solvent, hydrocarbon, hydrocarbon anion or may dimerize. Reaction with methyl iodide and subsequent determination of iodide anion in the aqueous extract is a recommended method for analyzing compounds like sodium-naphthalene. The etching of polytetrafluorethylene is an interesting example of the reduction of halides an active surface is produced which is then able to form strong bonds to an epoxy resin. The reducing action of hydrocarbon anions has been used in the preparation of metal carbonyls. In this it is commonly convenient to start from the salt of a metal in a +2 or +3 oxidation state, which must be reduced to the zero oxidation state both sodium-naphthalene and aluminium alkyls have been used in this connection. 

Bromocrotonates have featured in three closely similar designs to cyclopropane ring synthesis published this year. Thus, cyclopropane-1,1-dicarboxyl-ates have been prepared by straightforward reduction of the alkylidenemalonates (11) with excess sodium borohydride, " and Kristensen etaC have used the same precursor (11) in a synthesis of cyclopropane monocarboxylates (12) as outlined in Scheme 3. The addition of metallated alkyl mercaptans to 4-bromocrotonates, like that of hydiide-ion addition to (11), leads to cyclopropanes in one step, e.g. (13)->(14). ... 

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