Insertion reactions metal alkoxides

Carbon monoxide is also known to insert into metal alkoxides (M—OR), di-alkylamides (M—NR2), and some hydroxymethyls (M—CH2OH). For the reaction of the alkoxide (PPh3)2Ir(CO)(OMe) with CO the intermediate [Ir(CO)3(PPh3)2]+ OMe has been identified, and the insertion therefore proceeds via external nucleophilic attack of OMe" on Ir—CO rather than intramolecular OMe migration. True intramolecular transfer is, however, evident for (dppe)PtMe(OMe) where the rate of OMe migration is much faster than for Me migration, to give (dppe)PtMe(COOMe). 

Zinc phthalocyanine (PcZn) is prepared from phthalonitrile in solvents with a boiling point higher than 200 C, e.g. quinoline277,278 or 1-bromonaphthalene,137 or without solvent in a melt of phthalonitrile.83,116 The zinc compound normally used is zinc(ll) acetate or zinc powder. The reaction of zinc(II) acetate with phthalic acid anhydride, urea and ammonium mo-lybdate(VI) is also successful.262 The metal insertion into a metal-free phthalocyanine is carried out in an alcohol (e.g.. butan-l-ol).127,141,290 This reaction can be catalyzed by an alkali metal alkoxide.112,129... 

This scheme is remarkably close to the coordination insertion mechanism believed to operate in the metal alkoxide-catalyzed ring-opening polymerization of cyclic esters (see Section 2.3.6). It shares many features with the mechanism proposed above for the metal alkoxide-catalyzed direct polyesterification (Scheme 2.18), including the difficulty of defining reaction orders. 

In transfer hydrogenation with 2-propanol, the chloride ion in a Wilkinson-type catalyst (18) is rapidly replaced by an alkoxide (Scheme 20.9). / -Elimination then yields the reactive 16-electron metal monohydride species (20). The ketone substrate (10) substitutes one of the ligands and coordinates to the catalytic center to give complex 21 upon which an insertion into the metal hydride bond takes place. The formed metal alkoxide (22) can undergo a ligand exchange with the hydride donor present in the reaction mixture, liberating the product (15). 

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

The known C02 insertion reactions involving metal-carbon bonds have all resulted in carbor. -carbon bond formation with possibly one exception. Infrared spectral and chemical evidence has been presented for the formation of the metallocarboxylate ester Co(C03) (COOEt)(PPh3), n = 0.5-1.0 from the reaction of Co(CO)(C2H5XPPh3)2 with carbon dioxide from Vol-pin s laboratory (68). Although these studies are not conclusive for abnormal C02 insertion, metallocarboxylate esters are well-known compounds which result from the nucleophilic addition of alkoxides on the carbon center in metal carbonyls (69). 

The Cu(l) bicarbonate complex previously mentioned (77) was synthesized by the reactions summarized in Scheme 7, which includes C02 insertion into copper hydroxide and alkoxide species. The insertion reaction of C02 with metal hydroxides to form bicarbonates is believed to occur... 

Metal-catalyzed reactions of C02 and epoxides that give polycarbonates and/or carbonates have been extensively investigated as a potentially effective C02 fixation (Beckman, 1999 Inoue, 1987). The possible reaction mechanism is illustrated in Figure 3.8 (Darensbourg et al., 1999). The repetition of the reaction sequence in which C02 inserts into a metal-alkoxide bond, followed by ring-opening of the epoxide with the metal carbonate forms the alternating copolymer. In 1969, this copolymerization was first reported by Inoue and Tsuruta who used a Zn catalyst derived from... 

In order that an alternating copolymer is produced, the metal alkoxide must undergo faster insertion of carbon dioxide than reaction with a second equivalent of epoxide. If the metal alkoxide reacts with a second epoxide or undergoes decarboxylation reactions, ether linkage(s) may be formed. Ether linkages are undesirable as they compromise the properties of the polymer and reduce the carbon dioxide uptake. 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

In this case, the silylation of the metal alkoxide initially formed represents the key step of the overall process which releases the chromium salt from the organic product. The other crucial parameter is the use of the stoichiometric reducing agent for the regeneration of the active Cr" species. Commercial Mn turned out to be particularly well suited, as it is very cheap, its salts are essentially non-toxic and rather weak Lewis acids, and the electrochemical data suggest that it will form an efficient redox couple with Cr . Moreover, the very low propensity of commercial Mn to insert on its own into organic halides guarantees that the system does not deviate from the desired chemo- and diastereoselective chromium path. Thus, a mixture of CrX ( = 2, 3) cat., TMSCl and Mn accounts for the first Nozaki reactions catalytic in chromium [13]. 

Carbon dioxide readily undergoes insertion reactions probably via initial C02 complexing, with metal alkoxides, hydroxides, oxides, dialkylamides, and metal hydrides and alkyls 103... 

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