Carbon-oxygen bond activation

Carbon-Oxygen Bond Activation by Transition Metal Complexes. 

Keywords Amination Carbon-oxygen bond activation Cross-coupling Nickel Reductive deoxygenation Ruthenium... 

A 1,2,3-triazole-promoted iron(III)-catalysed propargyl alcohol dehydration has been developed for the synthesis of conjugated enynes (Scheme 3) The products were obtained in good to excellent yields (up to 95%) with a large substrate scope and excellent Z stereoselectivity. Addition of readily available 1,2,3-triazole to FeCl3 provided a practical and efficient catalyst system, which gave the critical chemoselectivity for the carbon-oxygen bond activation. 

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. 

In the same research group the cationic hydridopalladium complex [Pd(H)(H20)(PCy3)2] [BF4] has been shown to catalyze the hydroxycarbony-lation of triple bonds. As a representative example the dehydration occurring to give the dienoic acid is displayed in Scheme 3 [35]. The same cationic complex is able to activate a carbon oxygen bond in a-allenic alcohols to provide dienoic acids but with the COOH group in the branched position (Scheme 3) [36]. 

Catalysts suitable specifically for reduction of carbon-oxygen bonds are based on oxides of copper, zinc and chromium Adkins catalysts). The so-called copper chromite (which is not necessarily a stoichiometric compound) is prepared by thermal decomposition of ammonium chromate and copper nitrate [50]. Its activity and stability is improved if barium nitrate is added before the thermal decomposition [57]. Similarly prepared zinc chromite is suitable for reductions of unsaturated acids and esters to unsaturated alcohols [52]. These catalysts are used specifically for reduction of carbonyl- and carboxyl-containing compounds to alcohols. Aldehydes and ketones are reduced at 150-200° and 100-150 atm, whereas esters and acids require temperatures up to 300° and pressures up to 350 atm. Because such conditions require special equipment and because all reductions achievable with copper chromite catalysts can be accomplished by hydrides and complex hydrides the use of Adkins catalyst in the laboratory is very limited. 

Carbon-oxygen bonds adjacent to an aromatic ring or an alkene function can be cleaved by reduction at very negative potentials [1]. The process is often followed by reduction of the activating group as in 1. In these processes, the reduction potential of the activating group controls the electrode potential required. Thus an... 

Apart from the carbon-halogen bond, the carbon-oxygen one is rather active toward the reductive cleavage due to its polarity, so different types of compounds bearing a carbon-oxygen bond are able to undergo this reaction. 

Conjugate addition of methanol to a,/l-unsaturated carbonyl compounds forms a new carbon-oxygen bond to yield valuable ethers (Scheme 26). Kabashima et al. (12) reported the conjugate addition of methanol to 3-buten-2-one on alkaline oxides, hydroxides, and carbonates at a temperature of 273 K. The activities of the catalyst follow the order alkaline earth metal oxides > alkaline earth metal hydroxides > alkaline earth metal carbonates. All alkaline earth metal oxides exhibited high catalytic activities and, as in alcohol condensations and nitroaldol reactions, their catalytic activities were not much affected by exposure to CO2 and air. 

Nitrone-Nitrile Oxide Cycloaddition. Unsaturated sugars have been used for the simultaneous formation of carbon-carbon and carbon-oxygen bonds in a cir-relation. One of the best ways to achieve this transformation is the cycloaddition of nitrone or nitrile oxides. The cycloaddition of nitrones with olefins has been reviewed [133]. The regioselec-tivity is almost complete when using activated double bonds, such as enone, enelactone (see compound 98, Scheme 35), or esters. 

One salient feature of this model of the active site with a bound substrate is that the carbonyl of the peptide bond that is to be broken is coordinated to Zn2+. This feature implies that the metal ion may serve as an electrophile to polarize the carbon-oxygen bond, thereby rendering the carbon succeptible to nucleophilic attack by water. In the x-ray structure a water molecule is... 

If we consider reaction of mesylate ion with methyl bromide, we find that this is an endergic reaction thus the transition state lies along the reaction coordinate farther toward the products titan the reactants (Figure 5.11). The activated complex will therefore have a structure more resembling the products. There will be significant carbon-oxygen bond formation between the mesylate group and carbon and only a weak residual bond between carbon and bromine. The bromine... 

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