Intermediate Trans Research Articles

Preparation and reactions of palladium(II) complexes with C2-bonded heteroaromatic ligands trans[PdCl(R N)(PPh 3) 2] (R N = 2-pyridyl, 2-pirazyl, 2-pyrimidyl group). A new reaction pathway in the insertion of isocyanides into the Pdc bond of trans-[PdXR(L) 2] compounds

The complexes trans-[PdCl(R N)(PPh 3) 2] (I) [R N = 2-pyridyl (2-Py), 2-pyrazyl (2-pyz), 2-pyrimidyl (2-pym) group] have been prepared in high yield by deprotonation with NEt 3 of the corresponding cationic compounds trans[PdCl(R NH) (PPh 3) 2] + (R NH = N-protonated C 2-heteroaromatic ligand) in the presence of an excess of PPh 3. In chlorinated solvents, complexes I undergo a slow reversible dimerization into the binuclear derivatives [PdCl(μ-R N)(PPh 3)] 2 (II) (μ-R N = C 2, N 1-bridging ligand). From the 31P NMR spectra in 1,2-dichloroethane the following dissociation constants were obtained: 1.9 mol 1 −1 (R N= 2-py), 5.1 × 10 −2 (2-pym), 6.6 × 10 −3 (2-pyz). The dimerization becomes fast and quantitative if the PPh 3, involved in the equilibrium is removed by oxidation or by reaction with [PdCl(η 3-2-MeC 3H 4)] 2. Only the 2-pyridyl complex Ia reacts (slowly) with CO yielding the migratory insertion product trans-[PdCl{C(2-py)O}(PPh 3) 2], together with the dimer IIa. All the complexes I undergo migratory insertion of t-butylisocyanide with formation of trans-[PdCl{C(R N) = NCMe 3}(PPh 3) 2]] at rates which depend on the heterocyclic group (R N = 2-py > 2-pyz ⪢ 2-pym). The reaction of the 2-pyrazyl complex Ib with CNCMe 3 has been studied in detail by conductivity measurements and by IR and 31P NMR spectroscopy. The data suggest a complex mechanism in which insertion occurs through rearrangement of a four-coordinate intermediate [PdCl(2-pyz)(CNCMe 3)(PPh 3)], and through interaction of a cationic intermediate trans-[Pd(2-pyz)(CNCMe 3)(PPh 3) 2] + (Vb) with Cl − and with the free isocyanide of the initial equilibria. The occurrence of the latter reactions is confirmed by independent experiments in which the cationic complex Vb (isolated as perchlorate salt) is treated with an equimolar amount of [AsPh 4]Cl or CNCMe 3. The isocyanide-promoted insertion step represents a new mechanistic pathway for isocyanide insertion into the PdC bond of trans-[PdXR(L) 2] complexes.

Pulsed laser photolysis of spiropyrans in the presence of acids

Intermediate unstable trans isomers of the merocyanine species participate in the protolytic reaction of spiropyrans. A scheme for the photochromic transformations of spiropyrans in the presence of an acid was proposed.

Effect of Different Levels of Dietary <italic>Trans</italic> Fat or Corn Oil on Azoxymethane-Induced Colon Carcinogenesis in F344 Rats<xref ref-type="fn" rid="FN1">2</xref><xref ref-type="fn" rid="FN2">3</xref>

The effect of various levels of dietary corn oil or trans fat on azoxymethane (AOM; CAS: 25843-45-2)-induced carcinogenesis was investigated in female F344 rats fed the AIN-76 semipurified diets. Starting at 5 weeks of age, groups of rats were fed the low-fat diet containing 5% corn oil (designated as low-fat control diet). At 7 weeks of age, all animals except the vehicle-treated controls, were given sc injections of AOM (15 mg/kg body wt, once weekly) for 3 weeks. After 1 week, groups of animals were transferred to semipurified diets containing 13.6% corn oil and 23.5% corn oil or high-fat diets containing 5.9% corn oil plus 5.9% trans fat plus 11.8% Oleinate (low trans fat), 5.9% corn oil plus 11.8% trans fat plus 5.9% Oleinate (intermediate trans fat), and 5.9% corn oil plus 17.6% trans fat (high trans fat). Fecal bile acids were measured in vehicle-treated rats. All animals were necropsied 34 weeks after the last AOM injection. The animals fed the 23.5% corn oil diet had a higher incidence of colon tumors than did those in the groups fed the 5 and 13.6% corn oil diets. There was no difference in colon tumor incidence between the 5 and 13.6% corn oil diet groups. The animals fed the high-fat diets containing low trans fat, intermediate trans fat, and high trans fat developed significantly fewer liver and colon tumors and more small intestinal tumors than did the rats fed 23.5% corn oil diet. The excretion of fecal deoxycholic acid, lithocholic acid, and 12-ketolithocholic acid was higher in animals fed the 23.5% corn oil diet compared to the excretion in animals fed the other diets.

Insertion of isocyanides into the palladium—carbon bond of C2-palladated heterocycles. Synthesis of trans-[PdCl{C(R N)NR}(PPh 3) 2] complexes (R N = 2-pyridyl, 2-pyrazyl; R = alkyl or aryl group)

The titles complexes trans-[PdCl{C(R N)NR}(PPh 3) 2] (R N = 2-pyridyl (2-py), R = p-C 6H 4OMe, Me; R N = 2-pyrazyl (2-pyz), R = p-C 6H 4OMe) can be prepared by reaction of the N-protonated compounds, cis-[PdCl 2(R NH)(PPh 3] (R NH = 2-pyridylium (2-pyH) or 2-pyrazylium (2-pyzH) group), with PPh 3, followed by addition of the isocyanide CNR and deprotonation with triethylamine, in a molar ratio Pd/PPh 3/CNR/NEt 3 of 1/1/1/1.1. The reaction sequence involves the successive formation of the cationic intermediates trans-[PdCl(R NH)(PPh 3) 2] +, trans-[Pd(R NH)(CNR)(PPh 3) 2] 2+ and trans-[Pd(R N)(CNR)(PPh 3) 2] +, which were isolated and characterized as perchlorate salts for R N = 2-pyridyl. In the final step the coordinated isocyanide of trans-[Pd(R N)(CNR)(PPh 3) 2] + undergoes migratory insertion into the Pd-R N σ bond, promoted by the chloride ions progressively displaced by the entering neutral ligands from cis-[PdCl 2(R NH)(PPh 3)]. The resulting products were characterized by conventional spectroscopic techniques and, for R N = 2-pyridyl and R = p-C 6H 4OMe, also by ligand substitution reaction at the palladium center and by protonation and coordination of the p-methoxyphenylimino(2-pyridyl)methyl group with strong mineral acids (HClO 4, HCl) and ZnCl 2, respectively.

The facile cyclometallation reaction of 1,3-BIS[(diphenyl-phosphino)methyl]benzene

It is shown that 1,3-bis(diphenylphosphinomethyl)benzene (PCHP) readily undergoes a cyclometallation reaction with nickel(II), palladium(II), and platinum(II) species giving products of the type [MX(PCP)] [M  Ni, Pd, and Pt; X  halide and PCP  2,6-bis(diphenylphosphinomethyl)phenyl]. Cyclometallation requires the formation of intermediates trans-[MX 2(PCHP)].

Thermolysis of complex cyanides—XIII Structural transformations at thermal decomposition of [M(en) 3][M′(CN) 5NO] double complexes

[Cr(en) 3][Cr(CN) 5NO]·3H 2O and [Cr(en) 3][Mn(CN) 5NO]·3H 2O lose water at 130°C evolve three moles of ethylendiamine one mole of NO and half a mole of cyanogen between 200 and 540°C to form Cr(CN) 2, and Cr(CN) 2 and together with Mn(CN) 2, respectively. [Co(en) 3][Cr(CN) 5NO], after one mole of ethylendiamine has been lost in an endothermic process at 190°C is transformed to the intermediate trans-[Co(en) 2][Cr(CN) 3NO] in an exothermal reaction at 220°C. The structural transformation was proved by comparison of the thermal curves of cis- and trans-[Co(en) 2Cl 2]Cl, while the transfer of cyanide ligands from anion to cation has previously been demonstrated by thermal decomposition of other similar double complexes. Because of the less stable intermediate the trans effect is much weaker for [Co(en) 3][Mn(CN) 5NO]·2H 2O than in the case of [Co(en) 3][Cr(CN) 5NO].

Photochemistry of 2,4-Cyclooctadienone I. In Benzene and Toluene

Irradiation of 2,4-cyclooctadienone (1) in benzene or toluene is shown to give in >80% yield two head-to-head dimers. The ring fusions are shown to occur at the α,β-positions of the dienone with trans-cis and trans-anti-trans stereochemistry. Both dimers may be epimerized to the same product with cis-anti-cis ring fusions. Evidence is presented to show that the singlet state is responsible for the cis-trans isomerization of 1 to the highly reactive intermediate trans, cis-2,4-cyclooctadienone. This intermediate can dimerize in a thermal 2 + 2 cycloaddition or can be generated at −78° and then trapped by 1,3-dienes in a Diels–Alder reaction. A mechanism for the dimerization reaction is discussed.

Some insertion reactions of platinum hydrido-complexes with olefins: hydrolysis of platinum–carbon σ-bonds

The insertion of olefins into the Pt–H bond of trans-PtH(NO3)(PEt3)2 occurs via preliminary substitution of nitrate by olefin, and the intermediate trans-[PtH(C2H4)(PEt3)2]BPh4 has been isolated. Butadiene and allene give π-allylic complexes and cyclo-octa-1,5-diene and norbornadiene give enyl-systems. Whereas PhN:CH·CH:CHPh gives [PtH(PhN:CH·CH:CHPh)(PEt3)2]BPh4 with no insertion, CH2:CHCOMe gives [Pt(CH2·CH2·-COMe)(PEt3)2]BPh4, the Pt–C σ-bond of which is readily hydrolysed to give [Pt2(OH)2(PEt3)4]2+ and ethyl methyl ketone.