Resonance enolate

Carbonyl compounds are more acidic than alkanes for the same reason that carboxylic acids are more acidic than alcohols (Section 20.2). In both cases, the anions are stabilized by resonance. Enolate ions differ from carboxylate ions, however, in that their two resonance forms are not equivalent—the form with the negative charge on oxygen is lower in energy than the form with the charge on carbon. Nevertheless, the principle behind resonance stabilization is the same in both cases. 

A perhaps more exotic substrate for the Heck reaction is 1,2-cyclohexanedione [25], The reactivity of this molecule under Heck coupling conditions can probably be attributed to its resonance enol form. This reaction is attractive, because the literature contains relatively few examples of the preparation of 3-aryl-l,2-cyclohexane-diones. Yields varied from good to modest when classic heating and electron-rich aryl bromides were used, and reaction times typically ranged from 16 to 48 h. Similar yields were obtained under continuous microwave irradiation with a single-mode microwave reactor for 10 min at 40-50 W (Eq. 11.10) [25],... 

Racemization may occur in molecules in which structural changes, such as those due to resonance, enolization, substitution or elimination of groups, temporarily destroy the asymmetry needed to maintain the optical activity. Also, Walclen inversion of half of an optically active isomer can yield a racemate without the destruction of the center of asymmetry this phenomenon is observed in the reaction of n-butanol-2 with HCI(.)4. 

The best nucleophiles for the SrnI mechanism can make a relatively stable radical in the initiation part, either by resonance (enolates) or by placing the radical on a heavy element (second-row main-group or heavier nucleophiles). The best electrophiles are aryl bromides and iodides. If light is required for substitution to occur, the mechanism is almost certainly SrnI. Substitution at alkenyl C(sp2)-X bonds can also occur by an SrnI mechanism. 

The Claisen condensation is initiated by deprotonation of an ester molecule by sodium ethanolate to give a carbanion that is stabilized, mostly by resonance, as an enolate. This carbanion makes a nucleophilic attack at the partially positively charged carbon atom of the e.ster group, leading to the formation of a C-C bond and the elimination ofan ethanolate ion, This Claisen condensation only proceeds in strongly basic conditions with a pH of about 14. 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

It IS important to recognize that an enol is a real substance capable of mdepen dent existence An enol is not a resonance form of a carbonyl compound the two are constitutional isomers of each other... 

The proton transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids Figure 18 3 illustrates the roles of hydroxide ion and water m a base catalyzed enolization As m acid catalyzed enolization protons are transferred sequentially rather than m a single step First (step 1) the base abstracts a proton from the a carbon atom to yield an anion This anion is a resonance stabilized species Its negative charge is shared by the a carbon atom and the carbonyl oxygen... 

An a hydrogen of an aide hyde or a ketone is more acidic than most other protons bound to carbon Aldehydes and ketones are weak acids with pK s in the 16 to 20 range Their enhanced acidity IS due to the electron withdrawing effect of the carbon yl group and the resonance stabi lization of the enolate anion... 

Resonance forms illustrating charge delocalization in enolate of a p keto ester... 

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

The benzene rings A and B derived from the H NMR spectrum can be completed using Table 41.1. The way in which the enol ether is bonded is indicated by the correlation signal of the proton at Sh = 8.48. The structural fragment C results. Incorporating the C atom resonating at 5c = 123.3, which has not been accommodated in ring A or B and which is two bonds Jch) removed from the enol ether proton. 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

Carbanions derived from carbonyl compoimds are often referred to as etiolates. This name is derived from the enol tautomer of carbonyl compounds. The resonance-stabilized enolate anion is the conjugate base of both the keto and enol forms of carbonyl... 

In this paper Speziale and Smith 109) described experiments which led them to modify the mechanism proposed earlier 108) for the reaction of trivalent phosphorus compounds with haloamides. The first step is considered to be attack of the trivalent phosphorus compound on a chlorine atom of the halo amide (132) to produce a resonance-stabilized enolate ion (133). This is reasonable since under conditions where the trichloroamide... 

Examine the geometry and electrostatic potential map for acetone enolate. Are the CC and CO bonds in the enolate more similar to those in acetone or propen-2-ol precursors Is the negative charge primarily located on oxygen or on carbon Assuming this enolate is a hybrid of the two resonance contributors as shown above, which, if either, appears to be the major contributor ... 

Compare electrostatic potential maps of enolates derived from 2-butanone, 4,4-dimethyl-2-pentanone, 4,4,4-trifluoro-2-butanone and l-phenyl-2-propanone with those of acetone. Which substituents cause significant changes in the electronic structure of these enolates and what are the nature of these changes Justify your answers by making drawings of any important resonance contributors. 

Which of the two enolates enolate A or enolate B) is lower in energy Rationalize your observation by comparing their structures, charge distributions and electrostatic potential maps. Draw all of the resonance contributors needed to describe each enolate. Which enolate is generated by reaction with NaH ... 

Snyder and his co-workers assigned structures 48 and 49 to these j6-hydroxythiophene derivatives on the basis of chemical evidence and infrared and nuclear magnetic resonance spectral data. Infrared and nuclear magnetic resonance spectra further indicate that compounds of type 49 exist as dimers, probably hydrogen bonded, when R = OC2H5 or CH3, but as monomeric enols when R = H. ... 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

This formulation is supported by the proton resonance spectrum of the trifluoromethyl compound 101 which shows that it exists in the CH form shownd However, strong electron-withdrawing groups in the 4-position apparently lead to enolization, and compound 102, for example, gives an intense color with ferric chloride, - Other 4-acylated oxazol-5-ones are often formulated as 103 (see, e.g, reference 113). Tautomerism of the type illustrated by the equilibrium 104 103 has been discussed (see reference 115 for further references). 

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