Iron-catalyzed Carbometallation Reactions

Since an excess of n-BuLi has been inevitable, iron-ate complexes were proposed as catalytic relevant species, which generate a vinyllithium intermediate by carbometallation. The latter was verified by a quench with aldehydes and ketones with generation of the corresponding allylic alcohol. It is important to note that these reactions took place in low yields with alkyl-Grignard reagents. 

Note that no product formation could be obtained in absence of the iron or copper catalyst. Without copper, the iron catalyst generates only stoichiometric amounts (with respect to iron) of an alkenyliron species, which is not reactive towards further transformations such as transmetallation or polymerization. A cuprate, generated by CuBr and ArMgBr, did not undergo carbometallation with the alkyne. These results indicate that the main role of the copper catalyst is most likely to promote the metal 

In contrast to the carbometallation with aryl-Grignard reagents to unfunctionalized alkynes, this reaction does not require a co-catalyst such as copper. Only in one example, applying PhMgBr, was CuBr added in accord with the protocol reported by Hayashi s group. Although the nature of the catalytically active species remains unclear, an alkoxide-directed carbometallation which yields a (vinyl)iron intermediate is proposed. 

Since the discovery of transistion metal-catalyzed cross-coupling reactions by the groups of Kochi, Kumada and Corriu in the early 1970s, impressive progress has been achieved in the development of these methods into a synthetically indispensable 

1 (a) N. Miyaura (ed.), Topics in Current Chemistry, Vol. 219, Springer, New York, 2002 (b) A. de Meijere, F. Diederich (eds.), Metal-catalyzed Cross-coupling Reactions, Wiley-VCH, Weinheim, 2004 (c) J. Tsuji (ed.), Palladium Reagents and Catalysis, Innovations in Organic Synthesis, Wiley, New York, 1995 (d) M. Beller, A. Zapf, 

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Another example of the iron-catalyzed carbometallation reaction of unactivated alkynes was reported for the aryknagnesiation of alkyl(aryl)acetylenes [64b]. The addition of arylmagnesium bromide to the disubstituted aryl(alkyl)alkynes 234 was found to proceed efficiently in the presence of Fe(acac)j as a catalyst and a substoichiometric amount of an N-heterocyclic carbene (NHC) ligand (IPr) as an additive, which significantly increased the yield of this carbometallation process (Scheme 10.79). 

Reaction of the resulting cyclopropylmetal derivatives with different electrophiles provided diastereomerically enriched products 123 in good yields [34]. Catalytic processes promoted by transition metals are powerful synthetic tools for the selective formation of carbon-carbon bonds including carbometallation processes. Thus, a highly diastereoselective iron-catalyzed carbometallation of oxa- and azabicycloalkenes with arylzinc reagents was reported recently by Nakamura et al. 

Iron-Catalyzed Addition Reactions 3.4.1 Carbometalation of Aikynes... 

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