Enolates sterically congested

The chemoselective desilylation of one of the two different silyi enoi ethers in 10 to give the monosilyl enol ether II is realized by the Pd-catalyzed reaction of Bu3SnF. The chemoselectivity is controlled by steric congestion and the relative amount of the reagent[7,8]. An interesting transformation of the 6-alkoxy-2,3-dihydro-6//-pyran-3-one 12 into the cyclopentenone derivative 13 proceeds smoothly with catalysis by Pd(OAc)2 (10 mol%)[9]. 

It is possible to use a, -disubstituted cycloalkenone substrates for the construction of vicinal, quarternary carbon centers. Equation (20)96 illustrates such a successful application typically, however, enolate equilibration and steric congestion prevent straightforward application of the method. 

Ring contractions are not frequent among the 5-alkoxytriazolines469,521 however, under pressure arylsulfonyl azides react with sterically congested silyl enol ethers to give clean one-carbon, ring-contracted products (Scheme 183).528 For example, cyclohexanone enol ethers give cyclopentanes in 62-87% yield. 

Silenolates 26, i.e. silicon analogues of enolates, are formed, as shown by Ishikawa and coworkers, when sterically congested tris(trimethylsilyl)acylsilanes 25 are treated with silyllithium compounds (Scheme 10)87a b c The silyllithium reagent does not add to the C=0 bond but exclusively cleaves a Si—Si bond, yielding the corresponding silenolate 26. 

Transformation of a substrate into its ion radical enhances the species reactivity. Sometimes, this can overcome steric encumbrance of the substituent to be removed. Thus, l,4-di-iodo-2,6-dimethybenzene expels only one, sterically congested, iodine from position 1 upon the action of the tributylstannyl radical. Upon the action of the enolate ion of pinacolone (Me3CCOCH2) in the photoinitiated SrnI process, both iodines (from positions 1 and 4) are substituted (Branchi and co-authors 2000). 

Scheme 1.20. Double Michael-type addition of dienolate 220 to Cgo, affording after workup the thermodynamically more stable Irans-disubstiluted ketone ( )-219. Due to steric congestion, the corresponding civ-adduct is not available through Diels-Alder reaction with the silyl enol ether corresponding to 220. Reduction of the racemic ketone ( )-219 with DIBAL-H diastereoselec-tively affords alcohol ( )-223.

Molecules can often surprise us in their ability to retain stereochemical information despite what appear to be stereorandomizing processes. In Chapter Three by Takeo Kawabata and Kaoru Fuji a fascinating manifestation of this phenomenon is illustrated, explained and exemplified in the chemistry of eno-lates derived from sterically congested ketones and a-amino acid derivatives. Although these enolates are certainly planar species, the stereochemical consequences of the ancillary substituents have, until now been unappreciated. This... 

Selective desilylation of silyl enol ethers. The silyl enol ethers of methyl ketones undergo desilylation on treatment with Bu,Snl (1 cquiv.). The reaction is markedly accelerated by a palladium catalyst, particularly PdCl2 P(o-CH,C,H4),],. The rate of desilylation is markedly decreased by steric congestion around the double bond, Thus highly selective desilylation is possible. The relative rate decreases in the following order ... 

The position of the equilibrium has been shown by IR, and H 37 101 and 119Sn NMR spectroscopy102 to depend sensitively on the nature of the substituents, particularly their size, bulky groups favouring the less sterically congested enol form. Some typical results are shown in Table 14-6. 

The ketone yields an enolate in the presence of an amine base, but due to steric congestion the enolate has a low reactivity towards the carbonyl of the unenolized ketone. Addition of TiCl4 to the mixture, quenches the enolate, forms a titanium enolate and polarize the unenolized ketone. The reaction then proceeds as in (1). 

Bisaminomethylation, occurring when the active methylene compound contains more than one acidic hydrogen, is another possible side reaction. Reported cases of bisaminomethylation are rather infrequent, however, suggesting that the protonated P-ammonium group slows down the rate of enolization of the Mannich base relative to the unreacted active methylene compound. Thermodynamic factors reflected in the steric congestion of the bisaminomethylated product may also be important. 

In most mixed alkyl silyl zirconium derivatives, CO insertion occurs preferentially into the Zr-C bond83,85,252,253. However, in sterically congested complexes, insertion occurs into the Zr-Si bond to give silaacyl alkyl compounds such as CpJZr [q2-COSiHMes2]Me (48)85 and CpCp Zr[ 2-COSi(SiMe3)3]Me (49)250,253. Complexes 48 and 49 readily and cleanly isomerize to the hydrido, silyl-substituted enolates... 

On the basis of this structural information, the pathway of the MPS reaction can be outlined as follows (Scheme 27) oxalacetate is incorporated into the active site of MPS in a similar way to that of pyruvate. Lewis acidity of the magnesium promotes decarboxylation to form the enolate anion, which is stabilized by an electron sink provided by the divalent cation. Steric congestion of the peptide backbone allows the 2-pyrone... 

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