[reaction: see text] Ligand-free Pd(OAc)(2) can be used as a catalyst in the Heck reaction of aryl bromides as long as the amount of catalyst is kept between 0.01 and 0.1 mol %. At higher concentrations palladium black forms and the reaction stops. The actual catalyst is monomeric. Palladacycles merely serve as a source of ligand-free palladium in Heck reactions of aryl bromides.

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Enantioselective catalytic conjugate addition of dialkylzinc reagents using copperphosphoramidite complexes; Ligand variation and non-linear effects Arnold, L.A.; Imbos, R.; Mandoli, A.; de Vries, A.H.M.; Naasz, R.; Feringa, B.L.

COMMUNICATIONSX-ray crystal structure analysis of [ (Li+)2(Zz-)(OEt,),(thf)2]: C44H76Li201r Pi, a = 9.782(1), b=14.594(1), c=15.640(1) A, a=74.999(2), p=81.977 (2), y = 83.417(2)", V=2128.34 A3, Z=2, pcalcd = 1.07 gem-), p =0.060 mm-I, T = 150 K, crystal dimensions ca. 0.25 x 0.50 x 0.50 mm3, 90 frames, 26428 measured (8101 unique) reflections, R = 0.0472 and R, = 0.0475 for 5823 reflections with t > 6o(l), maximum and minimum peak in final Fourier difference synthesis 0.58 and -0.42 e k 3 . Unit weights were used in the refinements. The asymmetric unit consists of half of a molecule of [(Li'),(ZZ-)(OEt2),] and half a molecule [(Li+)2(22-)(thf)4]. each located about an inversion center. X-ray crystal structure analysis of [ (K+)2(22-)( [18]crown-6),1: C26H88KZ0,2, P2.,ln, u = 13.959(1), b = 10.403(1), C = 19.733(1) A, b= 107.376(1)", V= 2740.31 A3, Z = 2, pca,d = 1.19 gcm-', # =0.22 mm-', T = 150 K, crystal dimensions ca. 0.20 x 0.25 x 0.40 mm', 90 frames, 29555 measured (6266 unique) reflections, R = 0.0422 and R,=0.0464 for 5403 reflections with t>3o(I), maximum and minimum peak in final Fourier difference synthesis 0.36 and -0.23 e k 3 . A weighting scheme based on an optimized three-coefficient Chebyshev polynomial was used in the refinements [31]. General crystallographic information: Data were collected on an Enraf-Nonius DIP 2000 image plate diffractometer using graphite-monochromated Mo,, radiation (step of 2" between frames, B, , = 26"). Corrections were made for Lorentz and polarization effects [32]. The structures were solved by direct methods using SIR92[33] and refined by using full-matrix least-squares. Hydrogen atoms were fixed in geometrically idealized positions and allowed to ride on their attached carbon atoms. Corrections for the effects of anomalous dispersion and isotropic extinction (through an overall extinction coefficient) [34] but not for absorption, were made in the final stages of refinement. All crystallographic calculations were performed by using the Oxford CRYSTALS system[35] running on a Silicon Graphics Indigo R4000 computer. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-100693. Copies of the data can he obtained free of charge on application to The Director, CCDC, 12 Union Road, Cambridge CB21 lEZ, UK

Asymmetric hydrogenations are increasingly being used to introduce stereogenic centres into products used in the life sciences industries. There are a number of potential pitfalls when moving from a laboratory reaction to a manufacturing process, not least of which is safety. Time-to-market pressure leads to short development times, which in the past could be a large barrier for the implementation of catalytic steps; now there are new ways to minimise this problem. The potential problems associated with impurities and other methods that can shut down the hydrogenation reactions are highlighted in this critical review (353 references).

Enantioselective conjugate addition of dialkylzinc reagents to cyclic and acyclic enones catalyzed by chiral copper complexes of new phosphorus amidites Vries, André H.M. de; Meetsma, Auke; Feringa, Bernard

Abstract:We have previously shown that the use of ligand-free palladium employing Pd(OAc) 2 as catalyst precursor in the Heck reaction of aryl bromides is possible if low catalyst loadings, typically between 0.01 -0.1 mol % are used. We have now tested this phenomenon, which we have dubbed "homeopathic" palladium, in biaryl formation using the Suzuki, the Negishi and the Kumada cross-coupling reactions. The Suzuki reaction of aryl bromides, both activated and deactivated, is possible using 0.02 -0.05 mol % of Pd(OAc) 2 . In this reaction turnover frequencies up to 30,000 have been reached with activated substrates. Even aryl chlorides could be reacted if strongly electron-withdrawing substituents were present. The Negishi coupling with a variety of arylzinc halides was possible on aryl bromides containing electronwithdrawing substituents. The Kumada reaction only gave low yields of products under "homeopathic conditions.