Alkyl halides with metal hydrides

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

By themselves symmetrical organometallic compounds of the RMR type and their solutions in polar solvents possess insignificant electrical conductivity [52, 53] and cannot therefore be subjected to electrolysis. However, a mixture of such compounds with certain salts of the MX type, alkyl halides RX, metal hydrides MH, and finally other organometallic compounds sometimes gives electrically conducting solutions. This effect is explained by the formation of dissociating complexes. 

Sodium/ammonia" treatment also causes disruption of the ring in thiophene and simple thiophenes, however thiophene-2-carboxylic acid and 2-acyl-thiophenes can be converted into the 2,5-dihydro derivatives using lithium in ammonia, followed by protonation or trapping with an alkyl halide."" Side-chain reductions can be carried out with metal hydrides, which do not affect the ring. 

Hydride Transfer Reactions of Metal Formyl Complexes. We have found that metal formyl complexes can act as hydride donors to electrophiles such as ketones, alkyl halides, and metal carbonyls. EUNHrans-[ (CeHsO) 3P] (CO) 3FeCHO" reacts with 2-butanone overnight at ambient temperature to give a 95% yield of 2-butanol. The possibility that 2-butanone is reduced by (CO)4FeH formed in situ from decomposition of the metal formyl complex is excluded since the metal formyl complex reacts with 2-butanone much faster than it decomposes to (CO)4FeH and since no reaction between (CO)4FeH and 2-butanone was observed by IR spectroscopy. 

More familiar reactions but ones whose mechanism has been a controversial subject of interest for a long time have also been studied by this method. This is the case of the reductive demercuration of alkylmercuric halides by metal hydrides. In a careful study, Quirk was able to conclude that alkyl radicals are intermediates and that the process is a radical chain mechanism. This conclusion resulted from the knowledge that the cyclization was a relatively slow process (see Section XII.2) compared with the recombination process in the solvent cage, a conclusion recently confirmed by cidnp experiments. Furthermore, the use of metal deuterides and the measurement... 

Dialkylamino derivatives of elements located in the periodic table to the left or below those listed above cannot be prepared by the above method due to either the ionic character of some of the inorganic halides or the formation of stable metal halide-amine addition products. Therefore, other methods must be applied. Dialkylamino derivatives of tin7 and antimony8 are conveniently obtained by reaction of the corresponding halides with lithium dialkylamides. Others, such as the dialkylamino derivatives of aluminum,9 are made by the interaction of the hydride with dialkylamines. Dialkylamino derivatives of beryllium10 or lithium11 result from the reaction of the respective alkyl derivative with a dialkylamine. 

Main-group elements X such as monovalent F, divalent O, and trivalent N are expected to form families of transition-metal compounds MX (M—F fluorides, M=0 oxides, M=N nitrides) that are analogous to the corresponding p-block compounds. In this section we wish to compare the geometries and NBO descriptors of transition-metal halides, oxides, and nitrides briefly with the isovalent hydrocarbon species (that is, we compare fluorides with hydrides or alkyls, oxides with alkylidenes, and nitrides with alkylidynes). However, these substitutions also bring in other important electronic variations whose effects will now be considered. 

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... 

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... 

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