CO deinsertion

The reaction with acetyl chloride affords the diagonal methyl derivative after CO deinsertion, as in the case of Mel (Scheme 17) ... 

The vinylketene complexes (259) react thermally with isocyanides to afford -vinylketenimine complexes (262) (Scheme 73). This reaction proceeds via initial substitution of a carbonyl ligand by the isocyanide. The isocyanide complex then undergoes intramolecular CO deinsertion, and... 

A nice example of reversibility of the insertion of both alkenes and CO is provided by the isomerization of the acyl complex [Pd(CO Pr)(PPh3)2(MeCN)]BF4, which takes place also with acyls with different alkyl chains [88]. The complexes isomerize to equilibrium mixtures with a different alkyl chain, where the more stable isomer is that having the least branching in the alkyl group. The isomerization is first order in metal complex and inverse first order in MeCN, and is inhibited by PPha. It involves CO deinsertion followed by reversible H abstraction, and finally CO reinsertion as shown in Scheme 6.25. 

Besides (C) and (D), some Ru(CO)j(DPPE) is also formed in the reaction, even under dry conditions. This is attributed to some dehydrogenation of a coordinated methoxy group, which may be formed from a coordinated methanol molecule or by CO deinsertion from a methoxycarbonyl ligand. Apart, from Ru(CO)3(DPPE), another product of this reaction should be formaldehyde. Although this last product has never been observed directly, its condensation products with aniline, PhN=CH2 and PhNHMe, have been observed after catalytic reactions in some cases [8, 147]. Since aniline and the bis-methoxycarbonyl complexes then react with each other to afford the final carbonylated products (see later), this degradation pathway accounts for the fact that some aniline is also formed that is not later carbonylated and slowly accumulates in solution. This problem is also common to other catalytic systems and will be discussed further later. 

A second feature to be noted is the fact that isocyanate is apparently liberated much more easily from the cationic complex than from the neutral one. This is to be compared with the palladium-phenanthroline system discussed in paragraph 6.3.2., were the presence of an acid is essential to promote the formation of the isocyanate. We recall that, in the absence of an acid, complex (A) (or (B) as previously discussed, see paragraph 6.3.2.) yields products which may derive from CO deinsertion from a coordinated isocyanate complex, a reaction which is known to be easy for mononuclear complexes [225]. The presence of an acid apparently favours isocyanate decoordination before any following reaction can occur. 

Evidence was shown for migration of an alkyl group in carbonyl insertion, and deinsertion steps between the methyl carbonyl rhodium complex [ r/Vi/ -Indenyl-l -(CH2)3PPh2 Rh(CO)-Me](BF4) and the acetyl rhodium complex [ r/5 r/l-(Indenyl-l -(Cl I2)3PPh2) RhI(COMe)] by crystallography as well as by 1H NMR spectroscopy.28... 

However, deinsertion involving two electron ligands is not a general reaction for all the hydride complexes. Indeed, the iron complex (CO)4Fe(H)SiPh3 undergoes CO replacement rather than deinsertion ... 

Another route to neutral bisalkyne complexes is from the trifluoro-methylacyl precursor which deinserts carbon monoxide to yield trifluoro-methyl molybdenum products. Photolysis of CpMo(CO)3[C(0)CF3] in the presence of CF3C=CCF3 forms CpMo(CF3C=CCF3)2(CF3) CpMo-(DMAC)2CF3 is formed without photolysis (98). Addition of hexafluoro-butyne to CpMo(CO)(MeC=CMe)(CF3) forms the mixed bisalkyne via CO substitution. 

Compounds of the class R3SiMnH(CO)2(Cp) have been mentioned already in this connection 117-119, 225, 259) (Ihble X, entries 12, 13, 63, and 87). Reactions with tertiary phosphines, chlorine, methyl-lithium, or an excess of HCl all lead to elimination of RgSiH ("deinsertion ), as in... 

The product of this reaction appears to have formed by insertion of a CO group into an Mn—CHj bond. The reverse of this reaction is called decarbonylation but may also be called deinsertion or, more broadly, elimination. Infrared studies with CO have revealed that the reaction actually proceeds by migration of the methyl ligand rather than by CO insertion. 

Insertion of S02 occurs with a wide variety of transition metal complexes, almost as commonly as CO insertion. Unlike CO, however, deinsertion of S02 (desulfination) is not common. Because S02 reacts with 18-electron complexes, the resulting coordinatively saturated insertion complex must lose a ligand to provide an open site for alkyl migration to occur. Apparently, this is a difficult process, and when it does occur, it is accompanied by substantial decomposition. 

Insertion and deinsertion (extrusion) of unsaturated compounds such as olefin, CO and isocyanide. 

Aldehyde CH bonds are reactive in oxidative addition, so it is not unexpected to find that aldehydes readily undergo catalytic reactions involving this oxidative addition. Several catalysts decarbonylate aldehydes as a result of the acyl hydride formed after the C-H addition undergoing deinsertion of CO, followed by reductive elimination of the alkane product (Eq. 2.49). The hard step in the process is the thermally induced dissociation of the resulting tightly bound CO. One such catalyst is [Rh(triphos)Cl] (triphos = PhP(CH2CH2PPh2)2) [134]. 

For clarifying the factors influencing the ease of CO insertion and its reverse process, it is desirable to know the metal-carbon bond energies in the initial metal alkyl and the product metal acyl species. However, the presently available thermochemical data for the bond dissociation energies in acyl-transition metal complexes are not sufficient to allow us to advance a reasonable argument for the thermodynamic feasibilities of insertion and deinsertion processes [22-24],... 

The nature of the ligand into which the CO is to be inserted strongly influences the ease of CO insertion. The metal to hydride bond is known to be resistant to CO insertion, whereas deinsertion from the formyl to the hydride proceeds readily [26]. However, in certain cases the unfavored insertion into metal hydrido complexes takes place when a -formyl bond is formed, giving an extra stability to the product [27]. 

In general, the iminoacyl complexes are thermodynamically more stable than the acyl complexes, so that deinsertion of isocyanides from iminoacyl complexes has not been observed in contrast to the facile a-elimination of CO from acyl complexes. The removal of a halide ligand from a neutral iminoacylpalladium complex by treatment with AgBp4 affords the corresponding cationic iminoacyl complex from which no deinsertion of the isocyanide proceeded (Eq. 7.12) [61b]. 

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