Equilibration of carbonyl

Evidence for equilibration of carbonyl compounds with enols... 

EVIDENCE FOR THE EQUILIBRATION OF CARBONYL COMPOUNDS WITH ENOLS... 

Infrared analysis indicates that the photolytically assisted mono- or di-substitution of CO in [M(CO)4(nbd)] (M = Cr, Mo, or W nbd = norbomadiene) occurs stereospecifically in the axial position(s). On standing in the dark conversion into the equatorially substituted complexes takes place. The proposed mechanism initially involves dissociative exchange with labelled carbonyl, and subsequently axial-equatorial equilibration of carbonyls via opening of the norbomadiene chelate ring, rather than by carbonyl scrambling. A brief report of intermolecular redistribution... 

An alternative mode to release the strain of the four membered ring of vinylcyclobutanones arises from the addition of vinylorganometallics to the carbonyl group whereby a divinylcyclobutane is created 78,120,121). Since equilibration of the stereoisomers 78 and 79 is fast relative to rearrangement78), both stereoisomers nicely rearrange even though a cis divinyl orientation is required (Eq. 95). 

The abstraction of a proton a to a carbonyl group is not the only method for generating enolates and these alternative methods also offer possibilities for regio- and stereoselectivity. Thus, cleavage of silyl enol ethers (e.g., 1 and 3)9, 12 17 and enol acetates (e.g., 5)18 has been used for the generation of specific enolates. The conditions for these cleavages have to be chosen so that there is no equilibration of the lithium enolates formed. 

Rapid equilibration of the, sO will take place between the carbonyl and hydroxy] oxygens. 

One aspect of metal carbonyl chemistry that should be mentioned in surveying the more commonly found modes of CO coordination is the stereochemical nonrigidity of carbonyl clusters. This aspect has received considerable attention over the past decade, especially as 13C nmr instrumentation has become more readily available. In many carbonyl clusters, terminal and bridging carbonyls as established by x-ray structural studies are equilibrated on the nmr time scale (37, 39-41). The manner of equilibration takes place in a concerted way in order that each metal center maintains a constant electron count. For example, bridge terminal interconversion, (1), proceeds via complementary unsymmetrical CO bridges. 

Some of the reactions of PO3- parallel enzymatic reactions promoted by adenosine triphosphate (ATP). Pyruvate kinase catalyzes the equilibration of ATP and pyruvate with adenosine diphosphate (ADP) and phosphoenol pyruvate (11,12). In a formal sense, this reaction resembles the preparations of enol phosphate (eqs. 6 and 7). Cytidine triphosphate synthetase catalyzes the reaction of uridine triphosphate with ammonia to yield cytidine triphosphate (13). In a formal sense, this reaction resembles the replacement of the ester carbonyl group of ethyl acetate by the nitrogen of aniline (eq. 8). 

JK 1 mol"1 (Stewart and Teo, 1980). For this series also, a plot of log k for reduction against log K for hydration yields a line of slope 1.12, consistent with a transition state with substantial hydride bonding to carbonyl carbon. A correlation (r = 0.9785) between logk for reduction of a series of aliphatic ketones and log K for equilibration of the ketone and alcohol has a slope of 1.1, which has also been taken to support a productlike transition state (Muller and Blanc, 1980, 1981 Boyer et al., 1979). 

Equilibration of alkenes not conjugated with carbonyl groups requires different reagents... 

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