Metal carbonyls 3-eliminations

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

C-C bonds can be formed by reaction with alkyl iodides or more usefully by reaction with metal carbonyls to give aldehydes and ketones e.g. Ni(CO)4 reacts with LiR to form an unstable acyl nickel carbonyl complex which can be attacked by electrophiles such as H+ or R Br to give aldehydes or ketones by solvent-induced reductive elimination ... 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

The logical basis for employing metal carbonyls as catalysts would be the CO activation through coordination which facilitates nucleophilic attack by water or OH" (6). The key step then may be the formation of a hydroxy-carbonyl species followed by 6-hydrogen elimination reaction (eq. 2,3). Another important elemental re-... 

C02 and H2, presumably from the respective neutralizations of carbonate and of the metal carbonyl anions to give metal hydrides (e.g., H2Fe(C0)tf) which undergo reductive elimination of H2. 

Alkenyl Fischer carbene complexes can serve as three-carbon components in the [6 + 3]-reactions of vinylchro-mium carbenes and fulvenes (Equations (23)—(25)), providing rapid access to indanone and indene structures.132 This reaction tolerates substitution of the fulvene, but the carbene complex requires extended conjugation to a carbonyl or aromatic ring. This reaction is proposed to be initiated by 1,2-addition of the electron-rich fulvene to the chromium carbene followed by a 1,2-shift of the chromium with simultaneous ring closure. Reductive elimination of the chromium metal and elimination/isomerization gives the products (Scheme 41). 

A somewhat related problem is the nature of the bridged carbonyl group between two metal centers. Obvious correlations with organic ketonic behavior in general provide difficulties. In general nucleophilic attack by OR- (R = H or Me) does appear to occur at the carbon center. For R = Me, stable M- C02R complexes may often be isolated, but for R = H, transfer of hydrogen to the metal with elimination of C02 occurs readily, to yield the hydridocarbonyl. 

In the reaction of group 13 element halides with metal carbonyl dianions, the analysis is more complex than observed for the reactions with metal monoanions. Upon addition of metal dianions to EX3 or REX3, either one or two halide ions may be eliminated. When only one halide ion is eliminated per added metal dianion, the complexes may still be viewed as E3+ derivatives (Equations (33)-(36)).19 This may be controlled to some extent by the stoichiometry of the reaction. Comparison of Equations (33)19 and (34)19 shows that the electron demand at the main group element can be satisfied by coordination either to an electron-rich metal center 26 or formation of a halide bridge 27. Ligand-stabilized forms may also be prepared in this fashion (Equation (36)).19... 

The synthesis of a,p-unsaturated carbonyl compounds and nitriles by Pd-catalyzed reaction [245] of allyl p-oxo esters and allyl a-cyano esters is an oxidative process. With the contra-polarizability of the metal ion, elimination of a hydride from the p-position is electronically favorable. 

Elimination of carbon monoxide from metal carbonyls and coordination of the organosilicon olefin has been reported for divinylsilanes (54, 108, 109) (the compounds obtained are air-sensitive) ... 

A very versatile preparation seems to be the elimination of organotin halides in the reaction of organostannyl-organosilylcyclopentadienes with metal carbonyl halides. This reaction is very selective and only Sn—C bonds are cleaved with formation of 7)5-cyclopentadienyl complexes (7) ... 

Perfluoroalkyl transition metal carbonyl complexes146 are prone to a-fluorinc elimination pathways as it is well-known that C-F bonds a to the metal center are weaker than in aliphatic compounds. Thus, a-elimination of fluorine from perfluoroalkyliron(II) tetracarbonyl complexes, e.g. 10,147 has been observed. 

Both HRe(CO)s and H2Os(CO)4 can be oxidatively added to Os3(CO),, (NCMe) (126 -128). This leads to external attachment of the new metal carbonyl unit as in 64 (127), and a second HRe(CO)s molecule can be incorporated the same way (126). In both cases just one metal-metal bond has been formed in the first step. CO elimination from 64 introduces one more metal - metal bond, one possible result of which is rhomboidal 65 (126), whereas further CO elimination under H2 leads to full aggregation to tetrahedral 66 (127). All three steps of a M3 + M capping sequence have thereby been performed. A similar two-step sequence leads from Os6(CO)17(NCMe) and H2Os(CO)4 via H2Os7(CO)21 to H2Os7(CO)20 (128). 

In both the examples given above, there is concomitant loss of one or more neutral ligands. Elimination of CO is the rule in reactions of mononuclear metal carbonyls (e.g., entry 12) and cyclopentadienyl metal carbonyls (e.g., entry 4), but not those of polynuclear carbonyls (e.g., entry 16) or carbonyl halides (e.g., entry 33). Elimination of tertiary phosphines often occurs, especially when more than two molecules are present in the initial complex however, this is not always the case (see entry 24). Clearly, steric requirements and the dictates of the 18-electron rule determine the composition of the product, and normally act in concert when they conflict, as in the case of R3SiRuH3(PR3) (n = 2 or 3 entry 22), variable stoichiometry may result. Chelating diphosphines, with somewhat reduced steric requirements, are usually retained (e.g., entry 19), while complexed olefins are invariably lost the bulky ligand P(cyclohexyl)3 is associated with unusual products (entries 47 and 48). Particular mention may be made of the 17-electron species Cl3SiVH(Cp)2 and (Cl3Si)2V(Cp)2 shown... 

The reaction expressed in Eq. (88) is the reverse of the hydrogen addition [Eq. (84)] and constitutes one of the few demonstrated cases of reductive elimination of H2 from an intact cluster. The tetranuclear clusters that have been studied appear to fragment consistently in the presence of CO, to produce a metal carbonyl trimer and a monomeric fragment in highly specific fashions. 

Other neutral fragments eliminated in the fragmentation processes of tris-(dimethylamino) phosphine metal carbonyl complexes appear to contain phosphorus-hydrogen bonds, e. g. 

The mass spectra of certain metal carbonyl complexes of triphenyl-phosphine and l,2-bis(diphenylphosphino)ethane (Pf-Pf) have been investigated 48>. Besides the usual stepwise loss of carbonyl groups, cleavage of the phosphorus-carbon bond occurs. Thus triphenylphosphine complexes exhibit cleavage of the phenyl-phosphorus bond after all carbonyl groups are lost. The 1,2- bis(diphenylphosphino)ethane complexes (e.g. (Pf-Pf)[W(CO)5]2 and (Pf-Pf)M(CO)4) exhibit elimination of the ethylene bridge between two phosphorus atoms. 

The hydroacylation of olefins with aldehydes is one of the most promising transformations using a transition metal-catalyzed C-H bond activation process [1-4]. It is, furthermore, a potentially environmentally-friendly reaction because the resulting ketones are made from the whole atoms of reactants (aldehydes and olefins), i.e. it is atom-economic [5]. A key intermediate in hydroacylation is a acyl metal hydride generated from the oxidative addition of a transition metal into the C-H bond of the aldehyde. This intermediate can undergo the hydrometalation ofthe olefin followed by reductive elimination to give a ketone or the undesired decarbonyla-tion, driven by the stability of a metal carbonyl complex as outlined in Scheme 1. 

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