Organometallic Coupling without the Metal

Carbon-carbon bond formation between sp -hybridized carbons is not easy even with organometallic reagents. John Vederas of the University of Alberta reports (Organic Lett. 5 2963, 2003) that UV irradiation at low temperature of diacyl peroxides such as 4 gives the coupled product 5. The diacyl peroxides can be prepared by the DCC-mediated condensation of acids with peracids, 

The alcohol 10 looks like it might be formed by the addition of a Grignard reagent to an aldehyde. In fact, Patrick Steel of the University of Durham prepared 10 (Tetrahedron Lett. 44 9135, 2003) by Diels-Alder addition of the transient silene derived from 7 to the diene 8. More highly substituted dienes lead to more complex arrays of stereogenic centers. The intermediate silacyclohexenes, exemplified by 9, should also engage in the other reactions of allyl silanes. 

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As mentioned in the previous section, there are good reasons to search for new reaction conditions for Heck and related reactions, which permit catalyst recovery, the use of less toxic solvents, and simpler product recovery. The use of liquid or supercritical (SC) CO2 addresses all of these issues [171]. Until recently, however, the use of supercritical COj had been limited to organometallic Pd complexes functionalized with perfluorinated ligands [172-174], due to the limited solubility of metal colloids in CO2, and often required the use of water as a co-solvent [175]. The work described here shows that dendrimers can be used to solubilize Pd nanoclusters in liquid and SC CO2. This new finding opens the door to the combined benefits of a catalyst that promotes Heck couplings, but without the need for toxic ligands or solvents. 

The stereodefined alkenyl halides are of prime importance due to the recent developments of di- or trisubstituted alkene synthesis by cross-coupling reactions between organometallics and alkenyl halides catalyzed by transition metal compounds 171). These alkenyl halides can be conveniently obtained from alkenylboranes or alkeneboronic acids. B-Alk enylcatecholboranes undergo rapid hydrolysis when stirred with excess water at 25 °C (Eq. 109)102). The alkeneboronic acids are usually crystalline solids of low solubility in water and can be easily isolated and handled in air without significant deterioration. 

Only a small minority of organometallic reactions have cleared the hurdle to become catalytic reality in other words, catalyst reactivation under process conditions is a relatively rare case. As a matter of fact, the famous Wacker/Hoechst ethylene oxidation achieved verification as an industrial process only because the problem of palladium reactivation, Pd° Pd", could be solved (cf. Section 2.4.1). Academic research has payed relatively little attention to this pivotal aspect of catalysis. However, a number of useful metal-mediated reactions wind up in thermodynamically stable bonding situations which are difficult to reactivate. Examples are the early transition metals when they extrude oxygen from ketones to form C-C-coupled products and stable metal oxides cf. the McMurry (Ti) and the Kagan (Sm) coupling reactions. Only co-reactants of similar oxophilicity (and price ) are suitable to establish catalytic cycles (cf. Section 3.2.12). In difficult cases, electrochemical procedures should receive more attention because expensive chemicals could thus be avoided. Without going into details here, it is the basic, often inorganic, chemistry of a catalytic metal, its redox and coordination chemistry, that warrant detailed study to help achieve catalytic versions. 

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