1 -Trimethylsilyl-2-propanone

Fig. 6.1.3. Trimethylsilyl (TMS) derivatives of l-hydroxy-3-(4-hydroxy-3-methoxyphenyl)-2-propanone (Ja., Fig. 6.1.1)...

Coordinative interaction of the type Si -0=C produces a low field shift of the C resonance of the carbonyl carbon in l-(trimethylsilyl)-2-propanone and in (aryloxymethyl)trifluorosilanes XCgH COOCH SiFj In the latter cases a correlation between C chemical shifts and substituent constants has been established (r = 0.974) 6 C = 175.3 - 2.8 o, - 1.0 a . 

The reaction of lithium aluminum hydride with cyclopropanones, e. g. trans-2, i-6 -tert-h xiy -cyclopropanone (l)A 2-(trimethylsilyl)cyclopropanone (3) ° and 2-(trialkylgermyl)cyclo-propanone (5, Me = Ge)A ° gave rise to the 1,2-addition products which, upon aqueous workup, delivered cyclopropanols 2, 4, and 6. Under forcing conditions the addition reaction may be followed by ring opening. 

Of course, under many conditions, the preformed enolate aldol reaction appears to be significantly exothermic. The additional driving force is presumably provided by the enthalpy of coordination of the ambident aldolate ion with a cation. The importance of cation solvation in providing a driving force for the aldol reaction has been elegantly demonstrated by Noyori and coworkers.In this important experiment, the tris(dimethylamino)sulfonium (TAS" ) enolate of l-phenyl-2-propanone was prepared as shown in equation (6). The naked enolate was obtained as a yellow crystalline material, free of trimethylsilyl fluoride, by concentration of the THF solution. 

The situation in ketones with relatively small alkyl groups (e.g., 3-propanone) is similar to that of esters (Scheme 2.11). The reaction paths via transition states 48a and 48b (ethyl instead of OR ) with a slight preference for 48b, so that the trans-enolate forms in a moderate excess over the cis-diastereomer. It can be taken as a confirmation of Ireland s model that the sterically more demanding base LTMP enhances the 1,3-diaxial repulsion in 48a, so that the formation of trans-enolates is preferred (Table 2.1, entries 14 vs. 15). The fact that bases of similar bulkiness but different electronic properties, LTMP and lithium (trimethylsilyl)anilide, lead to the opposite stereochemical outcome (entries 15 vs. 17) has been explained by the assumption that the weaker sUylamide base prefers a late expanded Ireland transition state. The stronger base lithium A-f-butyl(trimethylsilyl)amide with similar steric demand leads predominantly to the cis-enolates, in accordance with Ireland s closed transition-state model. If ketones with sterically demanding substituents in the a -position, as in 2,2-dimethyl-3-(trimethylsilyloxy)-4-hexanone... 

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