Metal hydrides alkyl halides

Although the tin hydride + alkyl halide method is probably the most important way of making alkyl radicals, we should mention some other methods that are useful. We said at the beginning of the chapter that carbon-metal bonds, particularly carbon-transition metal bonds, are weak and can homolyse to form radicals. Alkyl mercuries are useful sources of alkyl radicals for this reason. They can be made by a number of routes, for example, from Grignard reagents by transmetailation. 

As we have seen, oxidative addition is the inverse of reductive elimination and vice versa. In principle, each reaction is reversible, but in practice the reactions tend to go in the oxidative or reductive direction only. The position of equilibrium in any particular case is governed by the overall thermodynamics this in turn depends on the relative stabilities of the two oxidation states and the balance of the A—B versus the M—A and M—B bond strengths. Alkyl hydride complexes commonly eliminate alkane, but only rarely do alkanes oxidatively add to a metal. Conversely, alkyl halides commonly add to metal complexes, but the adducts rarely reductively eliminate the alkyl halide. Third-row elements, which tend to have stronger metal-ligand bonds, tend to give more stable adducts. Occasionally, an equilibrium is established in which both the forward and back reactions are observed. 

Darensbourg et al. have conducted extensive studies of the nucleophilic reactivity of a series of anionic metal carbonyl hydrides [24], which have been used for the reduction of alkyl halides [25], acyl chlorides [26], and ketones [27]. The... 

Abstraction of one of the metal-bound hydrides from complex 5a provides the cationic iridium(lll) complex 28, which is an efficient precatalyst for alkyl halide reduction in the presence of EtsSiH (Equation 12.11) [31]. 

A more familiar example is Sn2 addition of an anionic nucleophile to an alkyl halide. In the gas phase, this occurs without activation energy, and the known barrier for the process in solution is a solvent effect (see discussion in Chapter 6). Finally, reactions of electron-deficient species, including transition-metal complexes, often occur with little or no energy barrier. Processes as hydroboration and 3-hydride elimination are likely candidates. 

Lithium aluminium hydride (LiAlH4), a strong reducing agent, reduces alkyl halides to alkanes. Essentially, a hydride ion (H ) acts as a nucleophile displacing the halide. A combination of metal and acid, usually Zn with acetic acid (AcOH), can also he used to reduce alkyl halides to alkanes. 

From the practical point of view //-hydride elimination might also be an obstacle. In reactions that involve metal-alkyl complexes as early intermediates one has to block //-elimination to increase the lifetime of the intermediate and enable subsequent transformations on the complex. A reaction, which proved elusive partially for this very reason, is the coupling of alkyl halides. A set of conditions, which allowed for the Negishi coupling of primary alkyl halides and even tosylates with alkyzinc halides is shown in Equation 1.5.20 The recent work of Fu and others showed that the careful... 

ZIEGLER CATALYST. A type of stereospedfre catalyst, usually a chemical complex derived from a transition metal halide and a metal hydride oi a metal alkyl. The transition metal may be any of those in gioups IV to VIII of the periodic table the hydride or alkyl metals are those of groups I, II. and III. Typical, titanium chloride is added to aluminum alkyl in a hydrocarbon solvent to form a dispersion or precipitate of ilie catalyst complex. These catalysts usually operate at atmospheric pressure and are... 

Boron Hydride Derivatives. Alkyl boranes can be prepd by alkylation of key intermediates, of diborane or of higher boranes. Alkyiation of a borane will proceed more readily if a functional group, such as a halogen or an active merai arom, is attached to the borane. For example, a halo-borane may react with a metal aikyl to produce an alkyl borane, or a metal polyhydropolyborate may react with an alkyl halide to produce an alkyl borane(Ref 35). Boranes may be alkylated or ary-iated with an unsaturated hydrocarbon(Ref 13). Alkoxy derivs of boranes can be prepd by allowing a borane to react with an appropriate alccho (Ref... 

Three transmetallation reactions are known. The reaction starts by the oxidative addition of halides to transition metal complexes to form 206. (In this scheme, all ligands are omitted.) (i) The C—C bonds 208 are formed by transmetallation of 206 with 207 and reductive elimination. Mainly Pd and Ni complexes are used as efficient catalysts. Aryl aryl, aryl alkenyl, alkenyl-alkenyl bonds, and some alkenyl alkyl and aryl-alkyl bonds, are formed by the cross-coupling, (ii) Metal hydrides 209 are another partner of the transmetallation, and hydrogenolysis of halides occurs to give 210. This reaction is discussed in Section 3.8. (iii) C—M bonds 212 are formed by the reaction of dimetallic compounds 211 with 206. These reactions are summarized in Schemes 3.3-3.6. 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

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