Carbonyl exchange mechanisms

When the reaction is run in the presence of 13CO and stopped after one half-life, not only is the label incorporation into recovered starting material negligible, as predicted by the mechanism, but the label incorporation into H2Os2(CO)s exceeds that required by the mechanism. This shows that it is undergoing carbonyl exchange under the reaction conditions and suggests that such dinuclear species are much more labile than their mononuclear counterparts. 

Regardless of the details of the exchange mechanism, it is clear that careful control experiments are necessary when working with deuterated metal carbonyls, be they mononuclear or polynuclear. Also, D2O exchange catalyzed by a chromatographic support may be a convenient method for synthesizing certain metal deuterio complexes. 

It was suggested that an increase in the electron density on the cluster due to substitution weakens the metal-metal bonds and thereby facilitates scrambling within the Fe(CO)3 fragment (9). The mechanism of carbonyl exchange between the metals remains unclear. 

As seen from Table III, iron pentacarbonyl reacts satisfactorily in spite of its inertness towards carbon monoxide substitution under the normal conditions 189). In benzene at 80° C, however, Fe(CO)5 dissociates rapidly (190). The Fe(CO)4 generated displays a nucleophilic reactivity which should promote an A-type mechanism. In spite of the specificities discussed, Maitlis et al. 177) have proposed the following mechanism for the metal carbonyl exchange reactions. 

The knowledge of the axial-equatorial chemical shift separation and of the averaged csa allowed the evaluation of the carbonyl exchange rate in solution at any temperature via the measurement of the longitudinal and transverse relaxation times. In fact the transverse relaxation rates are essentially determined by the sum of the contributions arising from the csa relaxation mechanism (l/Ff ) and from the intramolecular carbonyl exchange (l/T ) ... 

Two mechanisms are possible. Dissociation of PR3 from MC1(C0)(PR3)2 (M = Rh, Ir) would result in a 14-electron three-coordinate intermediate which could then react rapidly with PR 3, or dissociation could occur from a dimeric carbonyl or halide bridged intermediate (as proposed for the halide and carbonyl exchanges). Hard evidence for neither mechanism was presented. 

The various types of carbonyl site exchange mechanisms can be placed in three classes. 

In contrast, intermolecular carbonyl exchange processes seem to play only a secondary role and just a few examples, which take place through either dissociative or associative mechanisms, have been reported. [70]... 

There are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

There have been numerous studies of the rates of deprotonation of carbonyl compounds. These data are of interest not only because they define the relationship between thermodynamic and kinetic acidity for these compounds, but also because they are necessary for understanding mechanisms of reactions in which enolates are involved as intermediates. Rates of enolate formation can be measured conveniently by following isotopic exchange using either deuterium or tritium ... 

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