Carbon monoxide insertions 18-electron complexes

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

Although the insertion of carbon monoxide into arylpaladium complexes at ambient pressure is well documented the analogous reaction of azinylpalladium complexes usually requires elevated pressures due to the electron deficient nature of these heterocycles. The reaction is commonly used to convert the haloazine into the appropriate carboxylic acid or hetaryl-ketone derivative. 

The thermal stability of the acyl complexes Co(COR)(CO)4 increases if the group R contains electron-withdrawing substituents for example while Co(COMe)(CO)4 starts to decompose at about - 20°C and decarbonylates at even lower temperatures, the corresponding perfluoro derivative Co(COCF3)(CO)4 is reasonably stable. Also the tertiary phosphine-substituted products are stable, and the following carbon monoxide insertion in a PR3-substituted tricarbonyl derivative has been reported ... 

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

In the next step of the reaction cycle, the carbon monoxide is inserted into the carbon-cobalt bond. At this time, the subsequent aldehyde can be considered as preformed. This step leads to the 16 electron species (VI). Once again, carbon monoxide is associated to end up in the 18 electron species (VII). In the last step of the reaction cycle, hydrogen is added to release the catalyti-cally active hydrido-cobalt-tetracarbonyl complex (I). Likewise, the aldehyde is formed by a final reductive elimination step. 

It is also called dissociative because one of the rate-determining steps is the dissociation of carbon monoxide. The cycle is started by the dissociation of a ligand, which results in the release of the planar 16 electron species (I). In analogy to the cobalt mechanism (see Wiese KD and Obst D, 2006, in this volume), the next step is the addition of an olefin molecule to form the r-complex (II). This complex undergoes a rearrangement reaction to the corresponding reaction steps decide whether a branched or a linear aldehyde is the product of the hydroformylation experiment. The next step is the addition of a carbon monoxide molecule to the 18 electron species (IV). Now, the insertion of carbon monoxide takes place and... 

Complex exo-60 is then protonated to give the 773-vinylcarbene complex exo-64, which subsequently inserts carbon monoxide in the well-established manner (see Sections II,B, V,B, VI,B, VI,C, VI,E, VI,J, and VII), affording the 16-electron species endo-65. Anion trapping of the unsaturated species finally yields the vinylketene complex endo-62. 

The reaction of two alkynes in the presence of pentacarbonyliron affords via a [2 + 2 + 1]-cycloaddition tricarbonyl(ri4-cyclopentadienone)iron complexes (Scheme 1.6) [5, 21-23]. An initial ligand exchange of two carbon monoxide ligands by two alkynes generating a tricarbonyl[bis(ri2-alkyne)]iron complex followed by an oxidative cyclization generates an intermediate ferracyclopentadiene. Insertion of carbon monoxide and subsequent reductive elimination lead to the tricarbonyl(T 4-cyclopentadienone)iron complex. These cyclopentadienone-iron complexes are fairly stable but can be demetallated to their corresponding free ligands (see Section 1.2.2). The [2 + 2 + l]-cycloaddition requires stoichiometric amounts of iron as the final 18-electron cyclopentadienone complex is stable under the reaction conditions. 

Under some reaction conditions, cycloadditions without insertion of carbon monoxide have been observed. Reaction of complex (178) having an electron-deficient alkene under thermal conditions furnished the expected bicyclic enone (179) (Scheme 279). However, reaction promoted by TMANO gave cyclopentadiene (180). This... 

Alkyl- and arylisocyanides are electronically similar to CO and give insertions with transition and non-transition metal complexes. When a metal-alkyl derivative containing coordinated carbon monoxide is treated with an isocyanide, two products are possible [reactions (a) and (b)], depending on whether the inserted fragment is carbon monoxide or the isocyanide, respectively. 

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