Enolates also

The Pd enolates also undergo intramolecular Michael addition when an enone of suitable size is present in the allyl d-keto ester 744[465]. The main product is the saturated ketone 745, hut the unsaturated ketone 746 and ally-lated product 747 are also obtained as byproducts. The Pd-catalyzed Michael... 

Is the most delocalized enolate also the most easily formed enolate Calculate relative deprotonation energies from the enolate precursors using the deprotonation energy of acetone as a standard. 

Enolates also result from the deprotonation of ketones 4 by means of dieyclohcxylchloro-borane. As expected, the (A)-enolboranes 5 formed in this way lead to rw/Z-aldols. Remarkably simple induction of diastereoselectivity is achieved in aldol additions with isobutyraldehyde53d< ... 

Titanium enolates also can be prepared from /V-acyloxazolidinones. These Z-enolates, which are chelated with the oxazolidinone carbonyl oxygen,128 show syn stereoselectivity, and the oxazolidinone substituent exerts facial selectivity. 

Enolates also serve as suitable reaction partners in a directed aldol condensation (Eq. 52) 88) Dehydration of the aldol 51 to give enone (POCl3, HMPA then C5H5N) followed by reduction [LiAlH(nC4H9) (iC4H9)2] produces a vinylogue of a cyclopropylcarbinol 53. Aqueous fluoroboric acid smoothly rearranges 53 to the... 

There is room for further analysis in many traditional areas, as pointed out above during the discussion of enolization. Also, it is noted that the employment of transition state pKf values is very close to the use of the proton activating factors and deprotonating factors, introduced by Stewart (Stewart and Srinivasan, 1978 Stewart, 1985). It is to be hoped that the two approaches can be consolidated in a common view of acid-base catalysis. 

The palladium catalysed conversion of alkenes to enols, also known as the Wacker reaction, has also been used in the formation of oxygen heterocycles. In the example shown in 3.68. the subsequent formation of two carbon-oxygen bonds leads to the desired dioxabicyclo[3.2.1]octane derivative. The first Wacker reaction gives selectively a six membered ring formation (other possible routes would lead to even larger rings), while in the second Wacker reaction the selective formation of the five membered ring is observed.86... 

A 2,2-disubstituted chromanone results from the condensation of o-hydroxyacetophenone with diethyl oxalate. The initially formed 1,3-diketone cyclizes spontaneously to ethyl 2-hydroxy-4-oxochroman-2-carboxylate (77LA1707). The enolate also reacts with aliphatic ketones to give 2,2-disubstituted chroman-4-ones via the diol (79TL3685). 

Other simple alkenois (enols) also rearrange to carbonyl compounds. However, ether and ester derivatives of enols are known and can be prepared by... 

Since enolates also add via a ligand attack process, the regioselectivity that they exhibit is quite comparable to soft caibon nucleophiles. Alkyl or aiyl substituents at the allyl termini direct attack to the less substituted terminus (equations 228-232) functional groups such as COaMe and halogen at one allyl terminus direct attack to the remote terminus (equations 233 and 234). Remote functionalities such as —OR also direct addition to the allyl terminus more removed from the substituent (equations 23S and 236). 

The Pd enolates also undergo the intramolecular Michael addition when the allyl / -keto carboxylate or diallyl malonate 450, in which an enone is present at a suitable position, is treated with Pd(0) catalyst [190], The main product is the saturated ketone 451, and the allylated product 452 is obtained as a byproduct. 

This deprotonation may reform the ketone enolate that was the intermediate en route to the Michael adduct. However, the regioisomeric ketone enolate also can be formed. Figures 13.71-13.74 show such enolate isomerizations B — D, which proceed via the intermediacy of a neutral Michael adduct C. This neutral adduct is a 1,5-diketone in Figure 13.71, a 5-ketoaldehyde in Figure 13.72, and a 5-ketoester in Figure 13.73. 

Lobry de Bruyn-Alberta van Ekenstein reaction) A base-catalyzed tautomerization that interconverts aldoses and ketoses with an enediol as an intermediate. This enolization also epimer-izes C2 and other carbon atoms, (p. 1115)... 

The stability of the phosphonium-stabilized enolates also means that, although they react well with aldehydes, their reactions with ketones are often poor, and it is better in these cases to use phos-phonate-stabilized enolates. Being anionic, rather than neutral, these are more reactive. If an ester enolate equivalent is being used, the best base is the alkoxide ion belonging to the ester with a ketone enolate equivalent, use sodium hydride or an alkoxide. 

Aza-enolates also react cleanly at carbon with acid chlorides. Good examples come from dimethyl-hydrazones of ketones. When the ketone is unsymmetrical, the aza-enolate forms on the less substituted side, even when the distinction is between primary and secondary carbons. The best of our previous regioselective acylations have distinguished only methyl from more highly substituted carbon atoms. 

The enolate also reacted diastereoselectively with the epoxy-tosylate prepared earlier. The epoxide, being more electrophilic than the tosylate, is opened first, giving an alkoxide, which closes again to give a new epoxide. 

Spiroacylal 2 was designed under the rationale that the constraint of the carbonyl groups into a conformation in which overlap of their 7r-orbitals with the bent bonds of the cyclopropane is assured should dramatically increase the vulnerability of the cyclopropane toward nucleophilic attack.8 Experimental support for this notion is abundant.8 Spiroacylal 2 is considerably more reactive than 1,1-dicarbethoxycyclopropane in such reactions. For instance, reaction of 2 with piperidine occurs at room temperature. The corresponding reaction in the case of the diester is conducted at 110°C.5 Reactions with enolates also occur under mild conditions.8 Compound 2 reacts with the weak nucleophile pyridine at room temperature to give a betaine.8 An illustrative mechanism for the reaction of the acylal 2 with aniline to afford 2-oxo-l-phenyl-3-pyrrolidinecarboxylic acid (3) is... 

In isolated cases diastereoselectivity is reversed 2S>. Either acyclic transition states are transversed, or the usual pericycles are involved, having chair or boat conformations depending upon the enolate geometry, the substituents and the nature of the aldehyde 25>. These possibilities have also been discussed in irregularities reported for other enolates 48 109-m). For example, dicyclopentadienylchlorozirconium-109>, triphenyltin u0> and tris(dialkylamino)-sulfoniumnl> (TAS) enolates also favor... 

Analogous reactions of organomagnesium compounds with ketenes, giving enolates, also appear to be general [A] ... 

Aldol reaction. Quantitative yields of 1,2-adducts of alkyllithiums to ketones can be obtained at -65° in the presence of Cel,. Cerium enolates, formed by reaction of CeCl, with lithium enolates, also show enhanced reactivity in reactions with carbonyl compounds, particularly ketones. Yields of aldols are increased, but the stereoselectivity remains moderate. ... 

P-Hydroxy esters. 4 The lithium enolate of ethyl-N-methoxyacetimidate (2) reacts with (S)-( — )-l to provide (R)-(4-methylphenylsulfinyl)-ethyI-N-methoxyacetimidate (3). The enolate of 3 reacts with an aldehyde to give the adducts 4, which are converted by desulfuration and hydrolysis into P-hydroxy acids (5). The stereochemical outcome depends on the experimental conditions. The reaction of the lithium enolate of 3 with benzaldehyde under thermodynamic control gives (S)-5 in 75% ee. Use of the zinc enolate also gives (S)-5, in 86% ee, but use of zirconium enolate, obtained by addition of Cp2ZrCl2 to the lithium enolate, results in (R)-5 in 88% ee. 

The investi tions summarized earlier do not eliminate the possibility that other equilibrium forms than the diazonium ion as well as the conjugate bases of phenols, naphthols and enols also react. Until now no evidence is available for the reactivity of the cis- and tra s-diazohydroxides and the respective diazotates. 

Next to phosphine and phosphite sUver(I) enolates also methylisocyanide-stabilized species such as M(hfac)(N=CMe) (M = Cu, Ag) can be used as suitable CVD precursors for the deposition of thin metal films on glass or silicon substrates (Table 5). ... 

A perfluoro-enol also reacts with diazoalkanes (Eq. 87). ... 

Another side reaction, caused by changes in the solvent composition, was studied in 1980 (in the former USSR) and, unfortunately, was also published in rather inaccessible journals [37]. The reactions of phenylmagnesium bromide with the aliphatic ketones, 2-butanone and 3,3-dimethyl-2-butanone (methyl-fcrt-butyl ketone), were studied. Besides the carbonyl addition reaction, leading to a tertiary alcohol, enolization also takes place with this type of ketones. The enolate, on hydrolysis, yields the starting ketone however, before hydrolysis, in the reaction mixture of the Grignard reagent and the ketone, the enolate can react further with the ketone to form a condensation product (Scheme 16). 

Seebach, Dunitz and cowoikers fust described the THF-solvated tetrameric aggregates obtained from THF solutions of 3,3-dimethyl-2-butanone (pinacolone) and cyclopentanone lithium enolates. These are represented as (137). The pinacolone enolate also crystallizes as the unsolvated hexamer (138) from hydrocarbon solution, but this hexamer rearranges instantaneously to the tetramer (137) in the presence of THF. Williard and Carpenter completed the characterization of both the Na+ and the K+ pinacolone enolates.Quite unexpectedly the Na pinacolone enolate is obtained from hydrocarbon/THF solutions as the tetramer (139) with solvation of the Na atoms by unenolized ketone instead of by THF. Hie potassium pinacolone enolate is a hexameric THF solvate depicted as (140) and described as a hexagonal prism. A molecular model of (140) reveals slight chair-like distortions of the hexagonal faces in (140) so that the solvating THF molecules nicely fit into the holes between the pinacolone residues. 

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