Abstract
The reaction of several dialkylzinc reagents with aromatic aldehydes 1 in the presence of a catalytic amount of nickel(II) acetylacetonate (5 mol%) at 0 oC gave the expected addition products 2 in 1 h in very good yields. Aliphatic aldehydes 1 required stirring at room temperature for 24 h to afford the addition products in good yields. In comparison with non-catalyzed reactions this process represents a great improvement in reaction rate and selectivity for the formation of addition products. The enantioselective addition of diethylzinc to benzaldehyde 1a in the presence of the nickel catalyst and chiral aziridino alcohols 3 results in an interesting switch of the sense of the asymmetric induction in comparison with the nickel-free process.
Highlights
The formation of carbon-carbon bonds is one of the fundamental operations in organic synthesis
Activation of the aldehyde by Lewis acids[6] or by activation of the organozinc compound with Lewis bases.6a,7 The use of a catalytic amount of a chiral β-amino alcohol as an additive has rendered the addition reaction into an enantioselective process, and addition products with excellent optical purities can be obtained nowadays.3b,8 Titanium(IV) complexes have been shown to be very efficient catalysts for the enantioselective addition to both aromatic and aliphatic aldehydes.8d,9,10 Cobalt(II) and palladium(II) complexes are able to catalyze the addition of diethylzinc to aromatic aldehydes with moderate enantioselectivities.[11]
Continuing our studies on the use of organometallic reagents in organic synthesis, we decided to explore the utility of nickel catalysts for the addition of dialkylzinc reagents to aldehydes, because, to the best of our knowledge, there was only one previous report in the literature using nickel complexes derived from α-amino amides and Ni(OAc)2.12 According to the success reported on conjugate addition reactions,[13] we considered that Ni(acac)[2] would be a good candidate to promote the addition of dialkylzinc reagents to aldehydes.[14,15]
Summary
The formation of carbon-carbon bonds is one of the fundamental operations in organic synthesis. Abstract The reaction of several dialkylzinc reagents with aromatic aldehydes 1 in the presence of a catalytic amount of nickel(II) acetylacetonate (5 mol%) at 0 oC gave the expected addition products 2 in 1 h in very good yields.
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