Li, Zn transmetallation

SCHEME 10. The benzyhc organozinc reagent syn-41 can be prepared in a near diastereomerically pure form syn anti 47>98 >2) by Li-Zn transmetallation occurring with net inversion of the configuration. 5y>i-47 inverts spontaneously and mostly completely to the thermodynamically more stable anti-41... 

SCHEME 22. Preparation and uses of a zinc reagent via a Li/Zn transmetalation sequence... 

Alkali metals form porphyrin complexes of the type M2(Por) when their alkoxides are heated with porphyrin in dry pyridine, but are readily hydrolyzed.17 The transmetallation experiments indicate the stability of M2(Por) in the order K < Na < Li (Using radioactive sodium. Na2(TPP) is shown to undergo rapid metal exchange with Nal in pyridine (equation 2). 

The DoM-cross-coupling connections that can be favorably established using li for B, Mg, Sn, and Zn transmetallation starting points in the context of the various carbon- and heteroatom-based DMGs in both aromatic and heteroaromatic... 

With Mel or dimethylsulfate, the methyl derivatives are prepared with acetyl chloride-Cnl or the Weinreb amides, the ketones with methyl chloroformate, the methyl esters and with dialkyl oxalate, the a-oxo esters. Alkyl long-chain derivatives are better prepared by lateral (benzylic) metalation (see Section 26.7). Besides, aryllithiums may undergo transmetalation to afford a large panel of organometaUic species (Li Zn, Cu, Sn, Si, B, etc.), key reactants for transition metal-catalyzed reactions. The cross-conpling of aryllithium reagents with alkenyl triflates in the presence of the commercially avaUable catalytic system [Pdjfdbaljl/DavePhos] was recently reported [40]. 

Formation of Zincate Complexes. Zincate compounds such as K(ZnCH2SiMe3)3 are formed in a transmetalation reaction of bis[(trimethylsilyl)methyl]zinc with elemental potassium, or by reaction with potassium salts. Lithium zincates such as Li[Zn(CH2SiMe3)2R] [R = Me, Ph, N(SiMe3)2] can be prepared by reacting the title compound with the corresponding organo-lithium compounds.Transmetalation with alkaline earth metals such as strontium, barium, and calcium affords zincates of the type illustrated in eq... 

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. 

Yasuda S, Yorimitsu H, Oshima K (2008) Synthesis of aryliron complexes by palladium-catalyzed transmetalation between [CpFe(CO)2l] and aryl Grignard reagents and their chemistry directed toward organic synthesis. Organometallics 27 4025 027 Jonas K, Schieferstein L (1979) Simple route to Li- or Zn-metalated r -cyclopentadien-yliron-olefin complexes. Angew Chem Int Ed Engl 18 549-550... 

For a long time, zirconacyclopentadiene was considered to be inert towards carbon-carbon bond formation. However, through transmetalation to Cu, Ni, Zn, Li, and Al, various kinds of carbon—carbon bond-formation methodologies have been widely developed. One major advantage of this chemistry is that zirconacyclopentadienes can be conveniently prepared in situ, in high yields and with excellent selectivities, from two different alkynes. Combination of the selective formation of zirconacyclopentadienes and the chemistry of Cu, Ni, Zn, Li, or Al provides useful tools in organic synthesis. In the near future, it can be expected that more metals will be used for the transmetalation reactions of zirconacycle derivatives and some catalytic reactions will hopefully be developed. 

Watt and Drummond, were considered to be inert for C-C bond forming reactions. However, by the expedient of transmetallation to Cu, Ni, Zn, Li, and Al, methodologies for the stereoselective synthesis of olefins and dienes, as well as unusual heterocycles, aromatics and their ring-annulated products are now available which are beginning to make impact on material science, e. g. synthesis of pentacenes and polyphenylenes. Takahashi and Li provide evidence that, with further developments in transmetallation and handling the zirconacycles outside of the Schlenk tube techniques, synthetic utility will increase and new catalytic reactions will be developed. 

The relative stabilities of metalloporphyrins can be compared by transmetallation reactions.18 As expected from the Zjyi ratio, monovalent metals are easily displaced by other metal ions. The order of stability is indicated as follows Cu > Zn > Cd > Hg > Pb (> Ba) > Li > Na > K. In most cases the transmetallation rate is first order both in metal ion and in metalloporphyrin, and the intermediate is considered to be the dinuclear complex [M(Por)M ] For Cu/Zn(2,4-disubstituted DPIX DME) in boiling pyridine, the substitution rate increases as the porphyrin basicity decreases, i.e. substituent = Br > CH—CH2 > CH2CH3(> Etio). 

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... 

In contrast to the polar nature of C-Li and C-MgX bonds, the C-Zn bond is highly covalent and hence less reactive, allowing the preparation of functionalized derivatives. Utilization of organozinc reagents in organic synthesis has mainly centered around the preparation and utilization of functional organozinc compounds in organic syntheses (Reformatsky reaction), cyclopropanation (Simmons-Smith reaction), and transmetalations with transition metals. 

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