Organometallics.

Stable ate complexes

Alkyl- and arylboronic acids are unstable with respect to cyclotrimerisation; sometimes they must be converted to ate complexes to become reactive enough for transmetalation reactions. Yamamoto, Miyaura and co-workers therefore decided it was worth designing air and water stable ate complexes that could be used directly in catalysis; Reaction I shows one of the many examples they devised for this (Angew Chem, 2008, 47, 928). The products, mostly aryl-, were used in ligandless Suzuki couplings with aryl bromides and with aryl chlorides in the presence of 1; both reactions were performed at room temperature.

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Coupling reactions

Palladium-mediated carbonylation of aryl halides tends to compete with deactivation of the catalyst and this is more of an issue for less reactive substrates. On the other hand, reactions of mesylate can be less environmentally destructive than those of chlorides, bromides and iodides, but the latter tend to be more reactive in cross coupling reactions. In J Am Chem Soc (2008, 130, 2754) Buchwald et al teach that ligand 2, under basic conditions, is highly effective in this reaction (Reaction 2).

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A problem with asymmetric reactions involving organozinc reagents is that 'witches brews' of constituents are often used to make these organometallics. This means inorganic salt byproducts are often carried into the reaction and these often have deleterious effects on the conversions and enantioselectivities obtained. Cote and Charette have explored methods to alleviate this problem (Ibid, 2771). They have discovered that chloro-magnesium Grignards reacted with zinc methoxide in ether give soluble organozincs and most of the inorganic impurities precipitate out of solution and can be removed via centrifugation (Reaction 3). It is difficult to analyse the resulting solutions, but they were shown to give enhanced yields and enantiomeric excesses.

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Monodentate ligands in reductive aldol reactions

Krische points out that less progress has been made towards reductive aldol reactions involving [alpha], [beta]-unsaturated ketones relative to [alpha], [beta]-unsaturated esters. To address this his group used monodentate ligands with three sites for diversification and evolved ligand 3 and closely related structures and demonstrated their applicability in Reaction 4 (Ibid, 2008, 130, 2746). More optimisation may be necessary to increase the enantioselectivities to practical levels and to decrease the catalyst loadings, but the results are highly encouraging.

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Ligand effects and Crabtree's catalyst

Finally, Brown and co-workers used combinations of proton NMR and ESI-MS, including some interesting crossover experiments, to investigate the stability of Crabtree's catalyst analogues (Chem Comm, 2008, 199). Their conclusions are that N-heterocycles larger than pyridine and the phosphine P[(4-[FC.sub.6][H.sub.4]).sub.3] are highly desirable. Catalyst 4 was shown to be a faster and more robust catalyst for the hydrogenation of [beta]-pinene (Reaction 5).

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Kevin Burgess Texas A&M University