Titanium and Rhodium

The Kelly group showed that TiCl4 could be used to remove a triphenylmethyl (trityl, Tr) protecting group from cysteine residues and effect a subsequent 

Tandem hydroformylation/acetylization reactions have also been examined. Thus, heating a mixture of 696a,b with [Rh(cod)Cl]2, Ph3P, and CH2C12 under a CO/H2 atmosphere afforded 697a,b in 72 and 55% yield, respectively (02OL289). Six-membered rings were also accessible as shown by the conversion of 698 into 699 under similar conditions. 

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Figure 21.4 shows an example of the in situ Diffuse Reflection Infrared Fourier Transform (DRIFT) spectra observed when propene in heUum is fed at 330°C to an Rh-Ti02 catalyst with preadsorbed NO species. After addition of propene, an immediate decrease in the intensity of the band at 1925 cm (Rh-NO " ") and a simultaneous increase in the intensity of two bands at 1842 and 1755 cm (gem-dinitrosyl complex on Rh) was observed. For longer times on stream, the intensity of these bands also decreases, while new bands form at about 1720 and 1654 cm which could be attributed to vC=0 in coordinated acrylic acid and acrolein, respectively. When acrolein and acrylic acid started to be detected, bands near 2200 cm (2235 and 2200 cm attributed to isocyanate coordinated to titanium and rhodium ions, respectively) also were detected. 

Nitric acid reacts with all metals except gold, iridium, platinum, rhodium, tantalum, titanium, and certain alloys. It reacts violentiy with sodium and potassium to produce nitrogen. Most metals are converted iato nitrates arsenic, antimony, and tin form oxides. Chrome, iron, and aluminum readily dissolve ia dilute nitric acid but with concentrated acid form a metal oxide layer that passivates the metal, ie, prevents further reaction. 

A similar type of immobilization was obtained by reacting the phosphonylated 2,2 -bipyridine ligand depicted in Figure 42.10 with excess titanium alkoxide. Rhodium and iridium complexes of this immobilized ligand showed activity for... 

Given the importance of chiral amines to synthetic chemistry as well as other fields asymmetric hydrogenation of imines has attracted wide interest but limited success compared to C=C and C=0 bond reduction. The first asymmetric hydrogenation of imines was carried out in the seventies with mthenium- and rhodium-based catalysts, followed later by titanium and zirconium systems [82]. Buchwald found that... 

Borgarello E, Serpone N, Emo G, Harris R, Pelizzetti E, Minero C (1986) Light-induced reduction of rhodium (III) and palladium (II) on titanium dioxide dispersions and the selective photochemical separation and recovery of gold (III), platinum (IV) and rhodium (III) in chloride media. Inorg Chem 25 4499-4503... 

English chemist and physicist Discoverer of palladium and rhodium Inventor of a process for making platinum malleable. Famous for his researches on force of percussion, gout, diabetes, columbium (niobium), tantalum, and titanium, and his scale of chemical equivalents. 

Among the preformed enol derivatives used in this way have been enolates of magnesium, lithium,526 titanium,527 rhodium,528 zirconium,522 and tin,529 silyl enol ethers,530 enol bori-nates,531 and enol borates R CH=CR"—OB(OR)2.532 In general, metallic Z enolates give... 

The enantioselective addition of dialkylzinc to inline derivatives has been performed using copper,118,119 titanium,120 and rhodium catalysts 121 high yields and excellent enantiocontrol (up to 99% ee) have been achieved. 

Those based on the pH dependent change between oxides of different oxidation states titanium, ruthenium, rhodium, tantalum, platinum, and zirconium. 

Rhodium, " titanium,and tungsten " complexes have also been used for this reaction. The reaction can be promoted photochemically and the rate is enhanced by the presence of primary amines.Coordinating ligands also accelerate the reaction,polymer-supported promoters have been developed " and there are many possible variations in reaction conditions.The Pauson-Khand reaction has been done under heterogeneous reaction conditions, and with... 

Alkyl and aryl substituted imines have received the most attention as substrates for asymmetric hydrogenation, and the development of the field can therefore be outlined by examining their reductions. These are usually catalyzed by chiral complexes of titanium, ruthenium, rhodium, or iridium, though gold catalysts have also recently proven useful for this purpose [31]. New catalysts are generally tested for the reductions of substrates A-D (Scheme 6.1). 

A brief comparison of the advantages and disadvantages of titanium, ruthenium, rhodium, iridium, and gold catalysts for the asymmetric hydrogenation of alkyl and aryl substituted imines is given in Table 6.1. 

The parent oxacalix[3]arenes show little ability to bind alkali metals, however, a range of quaternary ammonium cations are attracted to the symmetric cavity [4]. Deprotonation of the phenol moieties allows them to bind to transition metals (scandium, titanium, vanadium, rhodium, molybdenum, gold etc.) [5-7], lanthanides (lutetium, yttrium and lanthanum) [8,9] and actinides (uranium, as uranyl)... 

The catalytic [2 + 2 + 1]-cycloaddition reaction of two carbon—carbon multiple bonds with carbon monoxide has become a general synthetic method for five-membered cyclic carbonyl compounds. In particular, the Pauson-Khand reaction has been widely investigated and established as a powerful tool to synthesize cyclopentenone derivatives.110 Various kinds of transition metals, such as cobalt, titanium, ruthenium, rhodium, and iridium, are used as a catalyst for the Pauson-Khand reaction. The intramolecular Pauson-Khand reaction of the allyl propargyl ether and amine 91 produces the bicyclic ketones 93, which bear a heterocyclic ring as shown in Scheme 31. The reaction proceeds through formation of the bicyclic metallacyclopentene intermediate 92, which subsequently undergoes insertion of CO to give 93. 

Catalytic reductions have been carried out under an extremely wide range of reaction conditions. Temperatures of 20 C to over 300 C have been described. Pressures from atmospheric to several thousand pounds have been used. Catal3rsts have included nickel, copper, cobalt, chromium, iron, tin, silver, platinum, palladium, rhodium, molybdenum, tungsten, titanium and many others. They have been used as free metals, in finely divided form for enhanced activity, or as compounds (such as oxides or sulfides). Catalysts have been used singly and in combination, also on carriers, such as alumina, magnesia, carbon, silica, pumice, clays, earths, barium sulfate, etc., or in unsupported form. Reactions have been carried out with organic solvents, without solvents, and in water dispersion. Finally, various additives, such as sodium acetate, sodium hydroxide, sulfuric acid, ammonia, carbon monoxide, and others, have been used for special purposes. It is obvious that conditions must be varied from case to case to obtain optimum economics, yield, and quality. 

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