Transmetalation Reactions with other Metals

Transmetalation using tin-lithium exchange is an excellent method — even better than mercury-lithium exchange — for the preparation of organolithium compounds, but it works only for compounds being more stable than n-butyllithium. This is also true for the synthesis of polylithiumorganic compounds. Examples are ( , )-1,5-dilithio-1,4-pentadiene 99the (Z,Z)-l,5-dilithio-l,4-pentadiene derivative 101 — 

Neither /r jj-l,2-dilithioethylene 9 nor 1,1-dilithio-1-alkenes 81 however, could be prepared by tin-lithium exchange, the reaction stops after the replacement of one stannyl group only. 

In our view this means that dilithioethylenes are less stable than n-butyllithium in contrast to theoretical proposals concluding that rfln.s-l,2-dilithioethylene 9 should be more stable than methyllithium 2 and even vinyllithium 8. 

Therefore we used a stronger base than n-butyllithium, Zert-butyllithium, but in this case one has to switch over to mercury-lithium exchange (see Sect. 3.3), because it is not possible to place four terZ-butyl groups around the tin atom due to steric crowding. Thus Kuivila and coworkers recently have found that upon treatment of tetramethylstannane with tert-butyllithium only two methyl groups can be displaced by rerz-butyl groups. 

Transmetalation reactions with several other elements of the periodic table can also be used for the synthesis of lithiumorganic compounds but polylithiated 

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In general, the syntheses of these complexes are achieved through (i) nucleophilic addition/substitution reactions of silver(i) fluoride or (ii) transmetallation reactions with other metal alkyl, alkenyl, and aryl complexes. 

The chemical reactivities of such titanium homoenolates are similar to those of ordinary titanium alkyls (Scheme 2). Oxidation of the metal-carbon bond with bromine or oxygen occurs readily. Transmetalations with other metal halides such as SnCl4, SbClj, TeCl4, and NbCls proceed cleanly. Reaction with benzaldehyde gives a 4-chloroester as the result of carbon-carbon bond formation followed by chlorination [9]. Acetone forms an addition complex. No reaction takes place with acid chloride and tm-alkyl chlorides. 

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. 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

Additions of metallacyclopentadienes to carbon—carbon triple bonds are rare, and only a few examples are known (Eq. 2.45) [37]. The 1,1-addition of zirconacyclopentadienes is quite different from other carbon—carbon bond-forming reactions described in this chapter. This reaction does not require transmetalation of zirconacyclopentadienes to other metals. Thus, in the absence of any added metal halide, zirconacyclopentadienes react with propynoates to give cyclopentadiene derivatives. This reaction requires the use of at least 2 equivalents of the propynoate (Eq. 2.46). 

The other commercially important routes to alkyltin chloride intermediates utilize an indirect method having a tetraalkyltin intermediate. Tetraalkyltins are made by transmetallation of stannic chloride with a metal alkyl where the metal is typically magnesium or aluminum. Subsequent redistribution reactions with additional stannic chloride yield the desired mixture of monoalkyltin trichloride and dialkyltin dichloride. Both -butyltin and /z-octyltin intermediates are manufactured by one of these schemes. 

The transmetallation of lithiated heterocycles has been described as a method to provide NHC complexes, and other Fischer-type carbene complexes [161]. The method proceeds via the alkylation of an alkylimidazole with BuLi to generate the lithiated azole, which can transmetallate to the metal complex. Further reaction with an acid or an alkylating agent would provide the desired NHC - M complex (Scheme 47). 

Having in mind the fact that lithium is the most electropositive metal among those with wide synthetic applications (Li, Mg, Zn, Cd, Cu), the preparation of an organolithium compound formally represents the access to any other of such classes of organometallic compounds by a transmetallation reaction (Scheme 88).240... 

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