Grignard reagents transmetallation

Organolithium and -magnesium compounds. Compared with extensive studies carried out on the Ni-catalyzed transmetallation reaction of Grignard reagents[43I,432], few examples of the Pd-catalyzed reactions of Mg are... 

On August 7, 1951, Kealy and Pauson reported the formation of Cp2pe starting from CpMgBr and FeCls (3.5 g from 9.05 g FeCls, Fig. 2) [2]. Initially, it was planned to form fulvalene via generation of a cyclopentadienyl radical by SET from Cp to Fe(lll), radical combination and subsequent oxidation of Cp2 by Fe(lll). Instead, Fe(ll) formed by reduction with the Grignard reagent reacted further by transmetallation. 

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... 

An alternative transmetallation procedure employs Grignard reagents, such as allenylmagnesium bromide, which reacts with Bu2SnCl2 in ether to afford dibutyl-... 

The mechanism is very similar to the stoichiometric one, except that the key dialkylcuprate reagent is made from transmetallation of the Grignard reagent. 

Figure 3.52). Based on the catalytic cycle described in Figure 3.53, it was proposed that the stereochemical outcome of this coupling reaction is determined at or after the transmetallation step, since the ee of the product depends on the nature of the Grignard reagent (e.g., entry 1 vs. entry 4). 

The ortholithiated products 19 and 22 will then react with a wide range of electrophiles the only reported important exceptions are enolisable aldehydes and allylic halides. Products requiring these electrophiles are best made by first transmetallating the organolithium to a Grignard reagent with MgCl2 or copper salts . 

The alkyl substituent meta to the methoxy substituent was easily introduced into the symmetrical diamide 72 by yet another ortholithiation. Allyl electrophiles react poorly with aryllithiums, so the ortholithiated amide 73 was first transmetallated to the Grignard reagent before allylation with allyl bromide to give 74. 

A similar type of transmetalation was also seen with l,2-dimethyl-5-trimethylstannylimidazole, which gave the 5-lithio derivative at -100°C, but rearranged to the 2-lithiomethyl derivative at higher temperatures [83JCS(P1)271]. However, transmetalation does not occur with Grignard reagents and 5-substituted imidazoles can be successfully prepared via this route (Scheme 49) (81 Mil 82OPP409). 

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