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A new method for the synthesis of conjugated polyenes containing up to eight double bonds with all-E configuration is reported. The procedure is based upon a homocoupling reaction of dienyl-, trienyl-, or tetraenylsilanes, promoted by PdCl(2) in methanol and in the presence of LiCl and CuCl(2). Configurational and conformational assignments were rigorously made on the basis of NMR spectra. The compounds obtained represent a novel interesting class of symmetrically substituted polyenes with potential optical and electrooptical properties. By changing the solvent, the homocoupling process can be switched to the halogenation of the silyl-substituted terminal double bond, thus leading to polyenyl halides.

The coordination of two novel, sterically constrained diphosphonite ligands towards different palladium, platinum and rhodium precursors has been studied. Complexes of this hitherto unexplored class of diphosphonites, based on rigid xanthene backbones, have been characterized by X-ray crystallography as well as by NMR and IR spectroscopy. A short and concise overview is given on the scarce literature on phosphonite related chemistry. Structural differences in the complexes obtained due to steric influences of the ligands are discussed. Ligand 1, bearing 2-tert-butyl-phenolate groups, led to orthometallated complexes cis-[MCl{1-κ 3 C,P,P}] (M = Pd, Pt) and to trans-[RhCl(CO)(1)]. These complexes were compared with those formed with ligand 2, bearing 2,2-dioxo-3,3-di-tert-butyl-5,5-dimethoxy-1,1-biphenyl groups, preventing orthometallation. The complexes trans-{PdCl 2 (2)], cis-[PtCl 2 (2)] and trans-[RhCl(CO)(2)] were structurally characterized. Valuable data were collected on chemical shifts, Pt-P coupling constants, as well as CO stretch frequencies of Rh-CO complexes as important tools for structural characterization of such complexes. Introduction Progress and development of homogeneous catalysis is very much related and dependent on the synthesis and availability of ligands. In recent years a number of powerful new ligand classes have emerged, most of which are chelating phosphorus-based compounds 1 such as phosphines 2 or phosphites. 3 Phos-phonites however have been widely neglected and only very recently a few examples were reported. Reetz et al. used several backbones, amongst others ferrocene, to prepare chelating diphosphonites and used them in asymmetric Rh-catalyzed hydrogenations of the typical benchmark substrates 4 as well as in conjugate addition of aryl boronic acids. 5 Zanotti-Gerosa et al. also described diphosphonites but they used the para-cyclophane backbone and performed hydrogenation reactions. 6 Pringle andco-workers used binaphthol-derived mono-and bidentate phosphonites for conjugate additions of diethyl zinc to enones. 7 The same monophosphonites were used by Claver and Pringle in asymmetric hydrogenations. 8 Scharf and co-workers used the monodentate phosphonites based on TADDOL as the chiral auxiliary for the rhodum-catalyzed hydrosilylation of ketones. 9 Börner and co-workers have published on the use of both monodentate and bidentate phosphonite ligands in the hydroformylation of alkenes, with varying results. 10 Finally the Reetz group, in line with their earlier work, prepared phosphite-phosphonite bidentate ligands but no applications in catalysis were reported so far. 11 During recent years xanthene-based chelating ligands have been introduced and extensively used in transition metal catalyzed CC bond forming reactions. 12 These backbones provide a unique combination of rigidity and P-P distance in such a way, that specific, large bite-angle coordination modes are supported. Additionally interaction of the central oxygen atom in the backbone with a metal atom coordinated is po...

Abstract1H-, 13C -NMR , IR, UV-Vis, and MS spectra of p-hydroxyphenylpyruvic acid (pH PPA) have been recorded and fully interpreted.pHPPA exists in solution as a mixture of interconverting forms: keto, hydrated keto and only one enol tautomer to which the Z configuration has been assigned according to the value (3.7 Hz) of the vicinal IH-C = C-13COOH coupling constant.In organic solvents the enol isomer is far more stable whereas in aqueous solutions the keto form predominates. The tautomeric equilibrium is pH-dependent and the anion is present as keto form not only in aqueous solution but also in H2O-DMSO mixtures with a high content of DMSO.The Z enol tautomer does form a coloured complex (λmax = 680 nm) with Fe+3 ions. The complex decomposes rapidly in all considered solvents except DMSO.In view of a possible use of H2O-DMSO mixtures for clinical analysis purpose, the enol fraction of pHPPA and the stability of the pH PPA -FeCl3 com plex in H2O-DMSÒ mixtures have been examined. Our results suggest that a solvent com position containing at least 80 vol.% in DM SO could be an appropriate solvent for pH PPA determination by FeCl3 method.

We sought a thiocarbonyl reagent that could be introduced by simple acylation. A second consideration was that the intermediate radical species resulting from attack of tin at thione sulfur would not be stabilized at the a-carbon. Such a stabilization could allow abstraction of hydrogen from the trialkylstannane to compete effectively with alkyl carbon-oxygen bond homolysis. Dethiation (thiobenzoyl esterbenzyl ether) and collapse (dithiocarbonate esteralcohol starting material) byproducts have been observed by using the a-benzylic-and a-thiol-stabilized ~p e c i e s .~~*~~~ Treatment of thiophosgene with phenol gave phenyl chloro-thionocarb~nate.'~ Pyridine effectively catalyzed reactions of this thioacyl chloride with relatively unhindered alcohols, but 4-(dimethy1amino)pyridine was requiredl6 for smooth conversion of nucleosides to their 2'-O-phenoxythiocarbonyl derivatives.Reductive cleavage of these compounds occurred readily when tri-n-butylstannane in toluene at 75 OC with a,a'-azobisisobutyronitrile as initiator was used.17 No dethiation or alcohol byproducts were detected in cases we have examined. As seen in Table I, thioacylation (generally quantitative) and reductive cleavage (proceeds to completion in 3 h) give good overall yields of deoxygenation of isolated secondary alcohols (entries 4, 5 ) . An epoxide function is tolerated (entry 5b).Selective 3' and 5' protection was required for specific 2'deoxygenation of ribonucleosides. Multistep procedures have been required previously,' but a hindered bifunctional disiloxane reagent became available recently.18 Treatment of ribofuranosyl compounds (1) with 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane in pyridine gave the cyclic 3',5'-trioxadisila derivatives (2, R = H) in over 90% yields (Scheme I). Thioacylation of 2 (R = H) gave the 2'-O-phenoxythiocarbonyl esters (2, R = CSOC6Hs). Re-

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