The combination of catalytic amounts of [Pd(PPh3)4], copper thiophene-2-carboxylate (CuTC) and [Ph2PO2][NBu4] allowed a series of exigent Stille-Migita reactions to be performed with high yields; as the protocol is fluoride free, a variety of O-silyl and C-silyl groups remained intact.

A largely catalysis based approach to optically active haouamine A (-)-1 is presented, which provides the hexacyclic compound previously used to construct this cytotoxic marine alkaloid.

Cinchona alkaloids catalyze the oxa-Michael cyclization of 4-(2-hydroxyphenyl)-2-butenoates to benzo-2,3-dihydrofuran-2-yl acetates and related substrates in up to 99% yield and 91% ee (ee = enantiomeric excess). Catalyst and substrate variation studies reveal an important role of the alkaloid hydroxy group in the reaction mechanism, but not in the sense of a hydrogen-bonding activation of the carbonyl group of the substrate as assumed by the Hiemstra-Wynberg mechanism of bifunctional catalysis. Deuterium labeling at C-2 of the substrate shows that addition of RO-H to the alkenoate occurs with syn diastereoselectivity of ≥99:1, suggesting a mechanism-based specificity. A concerted hydrogen-bond network mechanism is proposed, in which the alkaloid hydroxy group acts as a general acid in the protonation of the α-carbanionic center of the product enolate. The importance of concerted hydrogen-bond network mechanisms in organocatalytic reactions is discussed. The relative stereochemistry of protonation is proposed as analytical tool for detecting concerted addition mechanisms, as opposed to ionic 1,4-additions.

Silsesquioxanes are employed as ligand backbones for the synthesis of novel phosphite compounds with 3,3'-5,5'-tetrakis(tert-butyl)-2,2'-dioxa-1,1'-biphenyl substituents. Both mono-and bidentate phosphites are prepared in good yields. Two types of silsesquioxanes are employed as starting materials. The monophosphinite 1 and the monophosphite 2 are prepared from the thallium silsesquioxide derived from a completely condensed silsesquioxane framework (c-C 5 H 9 ) 7 Si 7 O 12 SiOTl. The diphosphite 3 is synthesized starting with the incompletely condensed monosilylated disilanol (c-C 5 H 9 ) 7 Si 7 O 9 (OSiMePh 2 )(OH) 2 . For monophosphite 2, the corresponding trans-[PtCl 2 (2)] complex 4 is characterized by NMR spectroscopy as well as by Xray crystallography, as the first example of a completely condensed oxo-functionalized silsesquioxane framework. The coordination of the bidentate ligand 3 towards Pd, Mo and Rh is studied, both by NMR spectroscopy as well as by X-ray crystallography.

The incompletely condensed silsesquioxane disilanol (c-C 5 H 9 ) 7 Si 7 O 9 (OH) 2 OSiMePh 2 has been used as a backbone for the synthesis of the diphosphinite ligand (c-C 5 H 9 ) 7 Si 7 O 9 (OPPh 2 ) 2 -OSiMePh 2 (1), based on a silsesquioxane framework. By reaction with black selenium, the corresponding selenide (2) was obtained, showing a J Se-P value of 815 Hz in the 31 P NMR spectrum. DFT calculations established a good insight into the electron density of the P atoms present in the two model compounds CH 3 OPPh 2 (3) and Ph 2 P(HSiOH) 2 OPH 2 (4). The Mulliken charge distributions show a clear electron-withdrawing effect of the siloxy group, which is also present in diphosphinite ligand 1. By reaction of compound 1 with PdCl 2 -(C 6 H 5 CN) 2 , the palladium complex [PdCl 2 R(OPPh 2 ) 2 ] (5) was obtained (R ) (c-C 5 H 9 ) 7 Si 7 O 9 -OSiMePh 2 ). From a similar reaction of 1 with PtCl 2 (cod), the platinum analogue [PtCl 2 R(OPPh 2 ) 2 ] (6) could be isolated. The equimolar reaction of Mo(CO) 4 (pip) 2 (pip ) piperidine) with 1 yielded the molybdenum complex [Mo(CO) 4 R(OPPh 2 ) 2 ] (7). Ligand 1 showed a clear tendency to coordinate in a cis fashion for all complexes 5-7, as was determined by NMR spectroscopy and X-ray crystallography. However, the analogous reaction of 1 with RhCl(CO) 2 dimer yielded the mononuclear trans-[RhCl(CO)(1)] (8). Complexes 5-8 have been structurally characterized as the first examples of transition-metal complexes with a silsesquioxane-based bidentate phosphinite ligand.

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A Versatile Protocol for Stille-Migita Cross Coupling Reactions. -A fluoride--free, mild protocol for the coupling of substrates with fragile functional groups and silyl protecting groups is presented. -(FUERSTNER*, A.; FUNEL, J.-A.; TREMBLAY, M.; BOUCHEZ, L. C.; NEVADO, C.; WASER, M.; ACKERSTAFF, J.; STIMSON, C. C.; Chem. Commun. (Cambridge) 2008, 25, 2873-2875; MPI Kohlenforsch., D-45470 Muelheim/Ruhr, Germany; Eng.) -D. Singer 44-041

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