C-0 bond activation

In the previous review [1] various reactions involving the cleavage of C-0 bonds in organic compounds activated by transition metal complexes were treated. The present review is mainly concerned with the later development in the C-0 bond activation with inclusion of essential concepts related to the C-O bond cleavage reactions promoted by transition metal complexes. 

The other type of process of C-0 bond activation that is different from the direct oxidative addition of the C-0 bond to M(0) complexes to form r 3-allyltransition metal complexes is insertion-elimination type or SN2 type as shown in Eqs. 4 and 5. Although the two processes are conceptually different, it is sometimes difficult to distinguish the two mechanisms. When the insertion-elimination process... 

The C-0 bond activation in benzoic anhydride has also been applied to benzoyla-tion of styrene and its derivatives catalyzed by a rhodium complex (Eq. 33) [79]. 

C=0 stretch mode is quite strong in Raman or IR spectra and in some cases quantitative correlation between hydrogen bonding energy and C=0 stretch frequency can be determined [8, 9]. Thus, it is ideally suited for studies of the substrate C=0 bond activation in the enzyme complexes. Two dehydrogenases, LDH and LADH, have been studied in detail by vibrational spectroscopy. 

Scheme of C=0 bond activation by electropositive iron atoms on platinum surface... 

Ruthenium complexes attract recent interest as new promising candidates for efficient, specific and environmentally benign allylation catalysts. It is noticeable that some J7 -allylruthenium(II) complexes have an ambiphilic property in catalysis involving the C-0 bond activation [52]. When allyl carboxylates or carbonates are treated with nucleophilic 1,3-dicarboxylates or electrophilic aldehyde in the presence of Ru complexes, catalytic allylations of nucleophiles or electrophiles take place [53]. In both reactions, J7 -allylruthenium complexes are assumed to be intermediates. Independent synthesis and reactions of the model compounds support this observation (Scheme 3.28). This ambiphilicity of the allylruthenium(II) may arise from the different reactivity of and rf forms in the allylic moiety [54]. 

When such C-0 bond activation reaction of vinyl epoxide is carried out in the presence of CO2 [96] or isocyanates [97], nucleophilic attack of the allyloxoanion at the electron deficient carbons takes place to give carbonates and oxazolidine with retention of configuration (Eqs. 3.26 and 3.27). Palladium... 

Scheme 6.3 Rhodium-catalyzed C-C=0 bond activation of 8-acylquinolines.

Finally, some examples of C-0 bond activation have been recently reported. Following their initial work on activation of classically unreactive C-0 bonds. 

Sequences such as the above allow the formulation of rate laws but do not reveal molecular details such as the nature of the transition states involved. Molecular orbital analyses can help, as in Ref. 270 it is expected, for example, that increased strength of the metal—CO bond means decreased C=0 bond strength, which should facilitate process XVIII-55. The complexity of the situation is indicated in Fig. XVIII-24, however, which shows catalytic activity to go through a maximum with increasing heat of chemisorption of CO. Temperature-programmed reaction studies show the presence of more than one kind of site [99,1(K),283], and ESDIAD data show both the location and the orientation of adsorbed CO (on Pt) to vary with coverage [284]. 

Active Figure 18.1 The mechanism of the Claisen rearrangement. The C—0 bond-breaking and C—C bond-making occur simultaneously. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

The dimerization of the parent ketene gives the P-lactone. One molecule of ketene reacts across the C=C bond as a donor and the other molecule reacts across the C=0 bond as an acceptor. This is similar to the concerted [2+2] cycloaddition reaction between bis(trifluoromethyl)ketene and ethyl vinyl ether to afford the oxetane (Scheme 26) [127], A lone pair on the carbonyl oxygen in the ketene molecule as a donor activates the C=C bond as the alkoxy group in vinyl ether. 

Lin Y-S, Yamamoto A (1999) Activation of C-0 Bonds Stoichiometric and Catalytic Reactions. 3 161-192... 

Chelating aldehydes such as 2-pyridine carbaldehyde and 2-dimethylamino benzaldehyde improve the stability of the aldehyde complexes via N,0 chelation. NMR studies show that the complexes are present in solution without an excess of aldehyde and can be formed in the presence of donor ligands. The X-ray structures showed longer and weaker Zn—O bonds when more than one chelating ligand was present. IR demonstrates the variation in C=0 bond strengths and how the environment of the zinc ion will influence potential catalytic activity via reaction rates or pathways. Tetrahedral chelate complexes, and octahedral bis- and tris-chelate complexes, were isolated.843... 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

At the outset it is important to clarify the scope of this discussion by the elimination of areas which will not be considered. When one notes that the term "activation of carbon monoxide" may mean a process as little perturbative of the C-0 bond as its end-on attachment to a metal atom in carbonyls, or as strongly perturbative as its dissociation to atoms on a metal surface, the need for limits becomes obvious. In this discussion we will consider only the activation of carbon monoxide in the sense that isolable products are formed by the addition of hydrogen to the molecule without complete rupture of all carbon-oxygen bonds, oxygenates are formed. 

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