Low-valent

Low valent titanium McMurry carbonyl coupling is believed to go through the vic-diol. vic-diols are smoothly converted to the corresponding olefins under these conditions. JOC 1976, 41, 896... 

Common reducing agents are hydrogen in the presence of metallic or complex catalysts (e.g. Ni, Pd, Pt, Ru, Rh), hydrides (e.g. alanes, boranes, LIAIH, NaBHJ, reducing metals (e.g. Li, Na, Mg, Ca, Zn), and low-valent compounds of nitrogen (e.g. NjHj, NjHJ, phosphorus (e.g. triethyl phosphite, triphenyiphosphine), and sulfur (e.g. HO-CHj-SOjNa = SFS, sodium dithionite = Na S O. ... 

Low-valent nitrogen and phosphorus compounds are used to remove hetero atoms from organic compounds. Important examples are the Wolff-Kishner type reduction of ketones to hydrocarbons (R.L. Augustine, 1968 D. Todd, 1948 R.O. Hutchins, 1973B) and Barton s olefin synthesis (p. 35) both using hydrazine derivatives. 

Formation o( oleltns by coupling or cross coupling of ketones, mediated by low valent titanium Also coupling ol enol ethers of 1,3-dicarbonyl compounds. 

The methodology of heteroatom-directed lithiation has been applied to the synthesis of a variety of organochalcogen compounds, including unstable, low-valent compounds (Section 15.6). ° The Se N interactions in the diselenide 15.4 activate the Se-Se bond toward insertion of a sulfur atom or a methylene group to give the derivatives 15.21a,... 

Boroles are the ri -complex-forming ligands. However, in the presence of the TT-donor substituents at the heteroatom, they tend to give V or species, especially for the low-valent early transition metals. 

Low-valent chromium compounds in synthesis of natural 0-heterocycles 99CSR169. 

Stoichiometric closure of furan and pyrrole cycles on McMurry reaction induced by low-valent transition metals 98PAC1071. 

Another aspect of this problem is the existence of heterocyclic carbenes containing a low-valent gallium center, 49 (95JA5421,98EJIC305,99JA9758, 01 JCS(D)3459). 1,4-Bis(2,6-di-Ao-propylphenyl)-l, 4-diazabuta-l, 3-diene... 

They form a monolayer that is rich in defects, but no second monolayer is observed. The interpretation of these results is not straightforward from a chemical point of view both the electrodeposition of low-valent Ge Iy species and the formation of Au-Ge or even Au Ge h compounds are possible. A similar result is obtained if the electrodeposition is performed from GeGl4. There, 250 20 pm high islands are also observed on the electrode surface. They can be oxidized reversibly and disappear completely from the surface. With Gel4 the oxidation is more complicated, because the electrode potential for the gold step oxidation is too close to that of the island electrodissolution, so that the two processes can hardly be distinguished. The gold step oxidation already occurs at -i-lO mV vs. the former open circuit potential, at h-485 mV the oxidation of iodide to iodine starts. 

The reductive coupling of the dicarbonyl compound 6 induced by a low-valent titanium species gives initially the dihydroporphycenes 7 which then spontaneously aromatize in the presence of oxygen to yield the porphycenes 8. 

Low-valent titanium was prepared from TiCl4 (9.5 g, 50 mmol) and activated zinc (6.55 g, 100 mmol) in the presence of pyridine (2.5 mL) in THF (400 mL) To this mixture was added all at once the corresponding bipyrroledicarbaldehyde (6 1.36 g, 5 mmol) and the mixture was refluxed for 30 min. After hydrolysis with a 10% aq K.2C03 (125 mL), aqueous extraction and chromatography (silica gel, CH2Cl,/hexane 1 1), the porphycene was obtained as the only nonpolymerized product. Recrystallization (hexane) gave red-violet needles of the porphycene yield 120 mg (10%). 

High-valent ruthenium oxides (e. g., Ru04) are powerful oxidants and react readily with olefins, mostly resulting in cleavage of the double bond [132]. If reactions are performed with very short reaction times (0.5 min.) at 0 °C it is possible to control the reactivity better and thereby to obtain ds-diols. On the other hand, the use of less reactive, low-valent ruthenium complexes in combination with various terminal oxidants for the preparation of epoxides from simple olefins has been described [133]. In the more successful earlier cases, ruthenium porphyrins were used as catalysts, especially in combination with N-oxides as terminal oxidants [134, 135, 136]. Two examples are shown in Scheme 6.20, terminal olefins being oxidized in the presence of catalytic amounts of Ru-porphyrins 25 and 26 with the sterically hindered 2,6-dichloropyridine N-oxide (2,6-DCPNO) as oxidant. The use... 

The titanium(IV) chloride catalyzed addition of allylic silanes to (E)-(2-nitroethenyl)benzene affords y,<5-unsaturated nitronates which, on treatment with low valent titanium species [generated in situ from titanium(IV) and zinc], give y,<5-unsaturated nitriles. For example, [(Zs)-2-butenyl]-(dimethyl)phenylsilane underwent reaction with ( )-(2-nitroethenyl)benzene to give 3-methyl-2-phenyl-4-pentenenitrile in 65 % yield as a 3 1 mixture of diastereomers of unassigned configuration22. 

The Cr—C tr-bond seems to result from the alkylation of low valent surface ions of chromium by the monomer this process may be represented by the following overall scheme ... 

The formation of surface compounds of low-valent ions of transition metals on the surface of the support. In particular, fixing organometallic compounds on the support surface, it may be possible to stabilize coordi-natively insufficient complexes of transition metals and to obtain highly active catalysts. In the ideal case a complete use of the transition metal in the formation of the propagation centers can be achieved. 

Alkylation of Low-Valent Ions in the Process of Formation of the Active Bond... 

The formation of the active metal-carbon bond as a result of the interaction of low-valent ions of the transition metal with olefin is the most intriguing step of the polymerization process by one-component catalysts. The possibility of the formation of the transition metal-carbon bond resulting from the reaction of titanium low-valent ions with ethylene is shown in Dzsabiev et al. (182) ... 

However, these reactions remain hypothetical, and the mechanism of alkylation of low-valent coordinatively insufficient ions during their interaction with hydrocarbons calls for a detailed study. When the activation by some additives is performed the formation of the active transition metal-carbon bond by oxidative addition is also possible, e.g. in the case of such additives as alkylhalogenides or diazocompounds according to the schemes ... 

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