Carbometallation, catalyst systems

Asymmetric carbometalation has proved less successful with the [(EBTHI)ZrCl2] catalyst this may be due to deactivation of the catalytically active cationic species by the formation of a rather stable p-AlMe3 adduct, which does not occur with the [(NMI)2ZrCl2] catalyst system [75]. 

The C-H/olefin coupling of aryloxazolines proceeds with unusual product selectivity. In this case, alkylation products, i.e., formally dehydrogenation products, are obtained as a major product (Eq. 22) [11]. These types of dehydrogenation compounds are believed to be formed via a carbometalation pathway. The first example of this type of alkenylation of arenes with olefins using palladium(II) complexes via C-H bond cleavage was reported in 1967 [32]. Later, several efforts were made to perform this reaction in a catalytic manner [33]. In 2001, Milstein et al. [34] reported the oxidative alkenylation of arenes with olefins using a Ru/02/C0 catalyst system (Eq. 23). Details of the reaction mechanism have not been elucidated. 

D. CATALYST SYSTEM AND OTHER BASIC CONDITIONS FOR CARBOMETALLATION REACTIONS... 

To improve the poor reactivity of 95, a number of copper-catalyst systems for carbomagnesiations were screened in a subsequent work [27]. While CuBr was an ineffective catalyst for the carbometallation, the addition of PBUj in the presence of a substoichiometric amount of Cul improved the efficiency and led to the syn-adduct 97 in 87% yield at room temperature. The optimal ratio of PBUj/CuI was found to be 4 1 at 30 mol% of catalyst loading. A further increase in selectivity (>96%) for syn-97 was achieved when the reaction was carried out at —78°C, albeit with a lower conversion (80%) after 3 h. The optimized catalytic system was then applied to the synthesis of substituted cyclopropane derivatives containing quaternary all-carbon-substituted stereocenters (Scheme 10.30). 

Carbometallation of l,4-bls(trlmethylsllyl>-l,3-butadlyne with trlmethylalumlnium In the presence of cpaZrCls catalyst selectively produces an alkenylalumlnlum Intermediate which, on treatment with water and various electrophiles, affords 2-methyl-1,4-bls(trlmethylsllyl>-l-buten-3-yne and Its 1-substltuted derivatives (Scheme 8). The TlClA-EtaAlCl catalyst system Induces endo-C2+21 cycloaddltlon of bl8(trlmethylsllyl>acetylene to norbornadlene . 

Both syn and anti carbometalation products were prepared with excellent stereochemistry in the above reactions. The role of solvent was not well studied, however, it is clear that stereocontrol is influenced in these systems by the nature of the solvent. It was suggested that a 16 electron intermediate zirconocene [(Cp)2Zr] species is the active catalyst in such systems. 

Grignard reagents-iron(III) chlorU Carbometallation. Grignard re of an iron catalyst. When the allylic bicyclo[2.2.1]octane system a net Ss2... 

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