Rhodium halide complex formation

Enolates can also be prepared by rhodinm-catalyzed isomerization of allylic Uthinm alcoholates, such as 14 (equation 5)". Subsequent treatment of the intermediately formed rhodium hydride complexes (15 and 16) with an electrophile led to the formation of various products. For example, alkyl halides gave a-alkylated ketones (17) in good yields, as shown in Table 4. Interestingly, addition of benzaldehyde under kinetically controlled... 

By careful investigation of the reaction between acyl halides and chlorotris(triphenylphosphine) rhodium, we found that a new acylrho-dium complex (XIII) could be isolated in good yield. It forms by the oxidative addition of acyl halide, with the elimination of one mole of triphenylphosphine (25). This is the first example of acyl complex formation by direct oxidative addition of acyl halides. 

James, Rempel and Rosenberg [201-203] as weU as Stanko, Petrov and Thomas [204] have studied the carbonylation of rhodium halides in aqueous acid solutions under mild conditions. Reactions appear to proceed via a rhodium(III) carbonyl complex which is believed to react with water to form an insertion product prior to CO2 formation, equation (156). 

Rhodium.—The formation of rhodium(i) complexes with allenes has been much studied for rhodium(i)-halide compounds. Allenes also react with the diketone complexes Rh(LL)(C2H4)a where LL = acetylacetonato or dibenzoylmethanato, to form rhodium(i)-allene compounds. In this case an A"-ray crystal structure determination has shown that the product contains the allene tetramer (19) bonded to the rhodium by two w-allyl bonds. ... 

Efforts to tune the reactivity of rhodium catalysts by altering structure, solvent, and other factors have been pursued.49,493 50 Although there is (justifiably) much attention given to catalysts which provide /raor-addition processes, it is probably underappreciated that appropriate rhodium complexes, especially cationic phosphine complexes, can be very good and reliable catalysts for the formation of ( )-/3-silane products from a air-addition process. The possibilities and range of substrate tolerance are demonstrated by the two examples in Scheme 9. A very bulky tertiary propargylic alcohol as well as a simple linear alkyne provide excellent access to the CE)-/3-vinylsilane products.4 a 1 In order to achieve clean air-addition, cationic complexes have provided consistent results, since vinylmetal isomerization becomes less competitive for a cationic intermediate. Thus, halide-free systems with... 

TPP)Rh(L)J+C1 in the presence of an alkyl halide leads to a given (P)Rh(R) or (P)Rh(RX) complex. The yield was nearly quantitative (>80X) in most cases based on the rhodium porphyrin starting species. However, it should be noted that excess alkyl halide was used in Equation 3 in order to suppress the competing dimerization reaction shown in Equation 1. The ultimate (P)Rh(R) products generated by electrosynthesis were also characterized by H l MR, which demonstrated the formation of only one porphyrin product(lA). No reaction is observed between (P)Rh and aryl halides but this is expected from chemical reactivity studles(10,15). Table I also presents electronic absorption spectra and the reduction and oxidation potentials of the electrogenerated (P)Rh(R) complexes. 

An indirect method for the hydroformylation of olefins involves formation of the tri-alkylborane (5-12) and treatment of this with carbon monoxide and a reducing agent (see 8-26). Hydroacylation of alkenes has been accomplished, in variable yields, by treatment with an acyl halide and a rhodium complex catalyst, e.g.,587... 

Rhodium, incorporated in the silver halide grains, decreases sensitivity and increases contrast. This action has been attributed to depression of latent image formation because of deep electron trapping by the trivalent rhodium ion (183-185). Eachus and Graves (184) showed that rhodium, probably as a complex, acts as a deep trap for electrons at room temperature. Weiss and associates (186) concluded that the rhodium salts introduce deep traps for both electrons and holes. Monte Carlo simulation showed that the photographic properties could be accounted for in this way over a wide range of exposure times. 

Many papers formulate the starting chlororhodium(III) porphyrin just as RhCl(P), as if a trans ligand L in MC1(P)L were easily lost (path c, X = Cl). However, the conditions of preparations point to a predominance of hexacoor-dinate aqua species, RhCI(P)H20. Only in one case the formation of a pentacoordinated rhodium(III) halide, the iodide RhI(TMP), seems well-documented [61], see Sect. 2.1.3. The formation of interesting heterobimetallic porphyrins, e.g. (TPP)RhMn(CO)s, [path c, X = Mn(CO)s] was formulated as starting from RhCl(TPP) [63], but the work referred to [264] clearly stated that hexacoordinate species, namely RhCl(TPP)H20, RhCl(TPP)(EtOH), or RhCl(TPP)CO were involved. On the other hand, the heterobimetallic species appear to be pentacoordinate about the rhodium, in accord with many metal-metal-bonded porphyrin complexes [222] (see also below). 

The sterical hindrance between the two air-ligated phosphetanes could be responsible for the lability of these complexes. This assumption is supported by the easy formation of stable bis(phosphetane)rhodium complexes, such as 66, from less-hindered phosphetanes <2000H(52)905, 2001S2095>, as well as by the observed stability of the trans-complex 67 obtained from phosphetane 64 and [Rh(CO)2Cl]2 by halide bridge cleavage and CO displacement (Figure 12) <1998S1539>. 

Regardless of the initial source of rhodium, that is, whether it is added just as a halide or as a phosphine complex, under the reaction conditions 4.1 is formed. With phosphine complexes, the phosphine is converted to a phospho-nium (PRj" or 11 PR, ) counter-cation. As chelating phosphines bind more strongly than the monodentate ones, the induction time for the formation of 4.1 with chelating phosphines is longer. 

Several chemical transformations of this acyl complex were carried out in order to prove its structure. The reaction of carbon monoxide with the complex gave acyl halide and chlorocarbonylbis (triphenylphosphine)-rhodium (XII). The thermal decomposition of the acyl complex gave rise to a mixture of isomeric olefins. The formation of olefin from the complex can be carried out more smoothly by adding iodine. When iodine was added to the solution of the acyl complex at room temperature, terminal olefin was obtained in high yield. These reactions are summarized below... 

Recently Blum reported that chlorotris(triphenylphosphine) rhodium (XI) is an active catalyst for the decarbonylation of aroyl halides and showed several examples (2). But in this case too, the real catalyst seems to be chlorocarbonylbis(triphenylphosphine)rhodium (XII), which is formed in situ from XI by the stoichiometric reaction with acyl halides. Formation of alkyl halides by decarbonylation of acyl halides can be carried out by the Hunsdiecker reaction, but the reaction is unsatisfactory when applied to aroyl halides. Therefore, the decarbonylation reaction of aroyl halides by the rhodium complex is a new and useful means of introducing halogen onto the aromatic ring. 

In the foregoing, the formation of organic molecules on transition metal complexes is explained by stepwise processes of oxidative addition, insertion, and reductive elimination. One typical example, which can be clearly explained in this way, are the carbonylation and decarbonylation reactions catalyzed by rhodium complexes 10-137). Tsuji and Ohno found that RhCl(PPh3)3 decarbonylates aldehydes and acyl halides under mild conditions stoichiometrically. Also this complex and RhCl(CO) (PPh3)2 are active for the catalytic decarbonylation at high temperature. 

It is clear from the examples reported that carbon monoxide, when coordinated to a metal in a neutral complex, is not sufficiently activated to react with organic nitro compounds under mild conditions. More precisely, the first act of this reaction is the electron transfer from the metal to the nitro group to give a radical couple and this requires a very basic metal. This explains why basic ligands usually activate transition metal carbonyls in these catalytic reactions. Moreover, basic ligands such as Bipy favor the in-situ formation of the [Rh(CO)4] species from rhodium clusters. The effect of co-catalysts such as halide anions is more subtle, but even the action of these might, at least in part, be directed toward an increase of the electron density of the metal. 

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