Norbornadiene complexes with palladium

Better catalytic behavior was observed in the copolymerization of CO and norbornadiene with palladium complexes containing a hemilabile pyridyl-carbene ligand [23]. The choice of a pyridine-functionalized carbene can be explained by the fact that the hemilabile arm in such a ligand is capable of reversible dissociation from the metal center, leading to vacant coordination sites for the complexation of the substrates while the strong donor moiety remains bound to the metal. 

Treatment of (//, // -norbornadiene)PdCl2 complex with Ph2Hg or vinyl mercuric chloride derivatives (R R C=CH)HgCl (R ,R = H, Me, r-Bu, Cl) in THF or methanol furnished the corresponding endo,endO 3-phQny - and t/o, /6>-3-vinyl-5-nortricyclyl palladium complexes, respectively (equation 59)102,123-126 approach, employing a thio-... 

Chiral methyl chiral lactic acid (5). This labeled molecule, useful for study of stereospecificity of enzymic reactions, has been prepared in a way that allows for synthesis of all 12 possible isomers. One key step is the stereospecific debromination of 1, accomplished by conversion to the vinyl-palladium cr-complex 2 followed by cleavage with CF3COOT to give the tritium-labeled 3. The next step is the catalytic deuteration of 3, accomplished with a rhodium(I) catalyst complexed with the ligands norbornadiene and (R)-l,2-bis(diphenylphosphino)propane. This reaction gives 4 with an optical purity of 81%. The product is hydolyzed to 5, which is obtained optically pure by cr3rstallization. 

The chloro[exo-5-acetoxytricyclo[2.2.1.0 ]hept-enifo-3-yl]dipyridinepalladium complex 17 [prepared from norbornadiene and palladium(II) chloride bisbenzonitrile complex with subsequent substitution and ligand exchange ] underwent exchange with acetoxy(phenyl)mer-cury, analogous to the known exchange reactions between palladium(II) chloride or acetate and phenylmercury salts, to afford a mixture of products, including biphenyl (54%), exo,exo-3,5-diacetoxytricyclo[2.2.1.0 ]heptane (19,20%) and chloro[exo-5-acetoxytricyclo[2.2.1.0 ]-hept-e Jo-3-yl]mercury (18, 57%). 

Norbornadiene complexes of platinum and palladium 14 reacted with silver carboxylates to form the carboxymetalation products 15 which, on treatment with triphenylphosphane, underwent intramolecular alkene insertion to form the nortricyclene skeleton 16. Reductive decom-plexation gave the acylated tricyclo[2.2.1.0 ]heptan-ex n-3-ol 17. ... 

Norbornadiene reacts with aryl or benzyl halides in the presence of palladium complexes, zinc powder and water to give the corresponding substituted nortricyclenes 45 in good yields. 

The zero-valent palladium complex Pd(nbd)(ma), nbd = norbornadiene, ma = maleic anhydride, is a useful precursor to novel Pd° complexes with nitrogen a-donor ligands. In THF, pyridine, diethyl amine, aniline, and even NH3 displaced the nbd ligand to form L2Pd(ma) complexes. ... 

The latter precursors fail, however, in the preparation of palladium(O) complexes with monodentate ligands of strong donor character such as pyridine or an amine. These can be stabilized only when associated with an acceptor olefin such as ma. Starting with Pd(nbd)(ma), complexes of the type PdL2( -ma) (L = pyridyl (py), EtNH2, PhNH2, NH3, SMc2 L2= 1,6-heptadiene, TMEDA) can be prepared (nbd = norbornadiene). The structure of the complex with L = py has been determined by X-ray diffraction methods. 

Similar studies have been reported for the complexes of norbornadiene with cuprous chloride (5) and palladium chloride 4). A complex of 1,3- and 1,4-cyclooctadiene with palladium chloride has been reported to be a useful catalyst for the polymerization of olefins (5). 

The ligand ap reacts with (norbornadiene) M (CO) 4 (M = Cr, Mo, W) (vide infra), to give complexes (phosphine) M (CO)4. The phosphine is not coordinated as ap but as ortho-cis-propenyl-phenyldiphenylphosphine (pp) the production of this species is discussed below. There is no evidence that this isomerisation takes place on coordination to platinum (II) or palladium(II). The chelate complex PtBr2 (pp) was prepared by Interrante (20) and shown by its spectral properties to be distinct from PtBr2 (ap). 

Under the catalytic action of palladium(O) complexes, (2-siloxyallyl) acetates and the vinylogous [4-(trimethylsiloxy)penta-2,4-dienyl] acetate (Table 18), (2-oxo-3-silylpropyl) acetates (1-acetoxy-3-silylpropan-2-ones), ° and (2-oxoalkyl) carbonates react with the strained, but otherwise nonactivated double bonds in norbornene, norbornadiene, and dicyclopentadiene to form polycyclic cyclopropyl ketones, see also formation of 1, 2 °and 3. In contrast, the (2-siloxyallyl) acetates failed to react with simple alkenes such as dec-1-ene and cyclohexene. " With the substrates mentioned, the exo anti) diastereomers were obtained exclusively. 

Oxatrimethylenemethanepalladium complexes can also be generated by oxidative addition of palladium(O) to 5-methylene-l,3-dioxolan-2-ones and subsequent decarboxylation. Again, reaction with norbornene, norbornadiene and dicyclopentadiene yields polycyclic cyclopropyl ketones in medium to high yield (Table 19). In this case, tetrakis(triphenylphosphane)pal-ladium(O) was the best catalyst found, whereas tris(dibenzylideneacetone)palladium(0)-chloro-form/triphenylphosphane (see above) and bis(cycloocta-l,5-diene)nickel/triphenylphosphane (used in stoichiometric amounts) proved less efficient. 

A mixture of the dipalladium complex (derived from MeOH addition to dichloro(norbornadiene)palladium 5.4 g, 10.2 mmol) and abs MeOH (150mL) was placed in an 500-mL, glass-lined autoclave, which was pressurized with 45 atm of CO and stirred at rt for 72 h. The precipitated palladium was removed by filtration and washed with MeOH. The combined MeOH filtrates were concentrated and the remaining residue extracted with pentane. The pentane was removed to give the product yield 2.7 g (73%). Final purification was achieved by short-path distillation bp 60°C/0.3 Torr. 

Conversion of norbornadiene to nortricyclenes can also be achieved with organomercurials. This reaction proceeds via transmetalation to form a c-bonded organopalladium system which inserts norbornadiene. Thus an enrfo-cw-addition is observed. Conversions of this type were observed in the phenylation of dichloro(t -norbornadiene)palladium either with diphenyl-mercury or, more conveniently, with sodium tetraphenylborate to give di-/r-chlorobis(2 5,6-t -c ffo-3-phenylbicyclo[2.2.1]hept-e /o-2-yl)palladium (30) via cis addition.By contrast, the analogous platinum complex did not give a phenylation product. The palladium complex 30 underwent reversible ring closure to a nortricyclenyl complex 31 with pyridine. ... 

In a more detailed study, the structure of the catalyst precursor was determined and found to be Pd(Diop),32. Other L2Pd and L2Ni complexes [L = Diop, BPPM, BINAP, etc.] were prepared [e.g., by in situ reduction of Pd(Il)Cl,L with sodium borohydride or as isolated palladium(O) complexes] and used as catalysts for the asymmetric addition of hydrogen cyanide to norbornene. norbornadiene, benzonorbornadiene, and cyclopentadiene dimer. In the presence of excess ( + )-Diop and L,Pd, norbornene gives 91 -95% of exo-2-cyanonorbornane with 24% cc of the ( + )-(15.25,4/ )-isomer. Similarly, use of the ( —)-Diop complex leads to the (-)-(l/ ,2f ,4S)-isomer with 24% ee (95% yield). Lower reaction temperatures, instead of the 120 "C used above, give better ee values (80 =C 32% ee with 94% yield 35 °C 35 % ee with 6% yield)32. 

The reaction can be explained by the transmetalation of the borate with PdCl2 giving phenylpalladium intermediate and the subsequent insertion of a C=C double bond of norbornadiene into the Pd-Ph bond. Cationic 7t-allyl-palladium(II) and -rhodium(I) complexes having a BPhJ counter anion induce phenyl group migration from B to the transition metals at elevated temperature to form phenyl complexes [54,55] (Eq. 5.17). 

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