Stability carbonyls

Instead of stabilizing the carbonyl group by electron donation as alkyl substituents do trifluoromethyl groups destabilize it by withdrawing electrons A less stabilized carbonyl group IS associated with a greater equilibrium constant for addition... 

Most methods for their preparation convert one class of carboxylic acid derivative to another and the order of carbonyl group stabilization given m Figure 20 1 bears directly on the means by which these transformations may be achieved A reaction that converts one carboxylic acid derivative to another that lies below it m the figure is pracfical a reacfion fhaf converts if fo one fhaf lies above if is nol This is anofher way of saying fhaf one carboxylic acid derivative can be converted to another if the reaction leads to a more stabilized carbonyl group Numerous examples of reacfions of fhis fype will be pre senfed m fhe secfions fhaf follow... 

Conversions of acid anhydrides to other carboxylic acid derivatives are illustrated m Table 20 2 Because a more highly stabilized carbonyl group must result m order for nucleophilic acyl substitution to be effective acid anhydrides are readily converted to carboxylic acids esters and amides but not to acyl chlorides... 

Nitrogen is a better electron parr donor than oxygen and amides have a more stabilized carbonyl group than esters and anhydrides Chlorine is the poorest electron pair donor and acyl chlorides have the least stabi lized carbonyl group and are the most reactive... 

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). 

FIGURE 20.1 Structure, reactivity, and carbonyl-group stabilization in carboxylic acid derivatives. Acyl chlorides are the most reactive, amides the least reactive. Acyl chlorides have the least stabilized carbonyl group, amides the most. Conversion of one class of compounds to another is feasible only in the direction that leads to a more stabilized carbonyl group that is, from more reactive to less reactive. 

One of the major drawbacks to many promising copolymers is their unsatisfactory electrochemical stability. Carbonyl groups which feature in many of the back-bone/chain linking groups are likely to cause stability concerns. Likewise, urethane, alcohol, and siloxane functions are sensitive to lithium metal. With this in mind, a recent trend has been to find synthetic routes to amorphous structures with... 

Co(en)2(/3-AlaOi-Pr)](C104)3 in aqueous solution (49). We add some further information on these two studies here. Both esters are bound via terminal amino-iV as well as carbonyl-O. Such chelation stabilizes carbonyl-0 coordination and the O-donor is not displaced from the metal at any stage. The other amine ligands surrounding Co(III) provide the necessary octahedral ligand field (27), and we have found the... 

Deuterium and C labeling studies demonstrate that ylide 1021 rapidly rearranges to the A -methyl-2-dehydro-pyridinium ylide 1028, by an intermolecular mechanism (Scheme 201) <1997JA5091>. Ylide 1028 can be trapped with added acids or with O2 to form l-methyl-2(l//)-pyridinone 1030 via stabilized carbonyl oxide 1029. 

Stabilized carbonyl ylides such as 3.52 have been prepared by the treatment of the silyl substituted a-chloro ethers 3.51 with fluoride ions under neutral conditions. 

Non-stabilized carbonyl ylides such as 3.55 can be prepared from 1-iodoalkyl triethylsilyl ethers (3.53), probably via bis( 1-iodoalkyl) ethers (3.54). 

The carbocation that is formed upon protonation of a carbonyl compound can lose H+ from the a-carbon to give an enol. Enols are good nucleophiles. Thus, under acidic conditions, carbonyl compounds are electrophilic at the carbonyl C and nucleophilic at the a-carbon and on oxygen, just like they are under basic conditions. Resonance-stabilized carbonyl compounds such as amides and esters are much less prone to enolize under acidic conditions than less stable carbonyl compounds such as ketones, aldehydes, and acyl chlorides in fact, esters and amides rarely undergo reactions at the a-carbon under acidic conditions. 

The tetrahedral intennediate has three potential leaving groups on carbon two hydroxyl groups and a chlorine. In the second stage of the reaction, the tetrahedral intermediate dissociates. Loss of chloride from the tetrahedral intermediate is faster than loss of hydroxide chloride is less basic than hydroxide and is a better leaving group. The tetrahedral intermediate dissociates because this dissociation restores the resonance-stabilized carbonyl group. 

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