Topics in Stereochemistry, Volume 21 by Ernest L. Eliel, Samuel H. Wilen

By Ernest L. Eliel, Samuel H. Wilen

A part of a sequence which supplies in-depth assurance of natural stereochemistry, this quantity comprises dialogue of aldol diastereoselection lower than thermodynamic and kinetic keep watch over, enantioselection in aldol condensations regarding chiral reactants, purposes of molecular mechanics calculations to natural chemistry, and masses extra. The members objective to assemble the disciplines of floor chemistry and stereochemistry to illustrate that the houses of stereoisomers will be helpful in extending the certainty of the susceptible but vital molecular forces that function in floor monolayers.

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VEDEJS AND M. J. 301 A, respectively) led these workers to conclude that the stabilized ylide a-carbon is sp2 hybridized. The ylide electron distribution can be approximated by a charge-separated phosphonium enolate representation. IV. CHARACTERIZATION OF OXAPHOSPHETANES The discovery that oxaphosphetanes are the low-temperature intermediates in representative Wittig reactions was based largely on the observation of characteristic 'P chemical shifts for pentacoordinated phosphorus (1 9a). Although the phosphorus chemical shifts rule out isomeric, tetravalent phosphorus (betaine) structures, they are not especially informative with regard to other structural details.

This involves a seven center interaction that is shown in the conversion from 43 to the betaine lithium halide adduct 44 or some equivalent aggregated species. Stereochemical equilibration would then occur via betaine C-C bond cleavage to give the corresponding ylide, followed by the usual recombination process to produce the more stable trans-disubstituted oxaphosphetane. A variety of anionic ylides reacts with high E selectivity with the reversalprone aromatic aldehydes. On the other hand, aliphatic aldehyde adducts are more resistant to Li +-induced betaine equilibration.

Table 6 includes entries corresponding to all of the principal families of Wittig reagents: nonstabilized ylides (entries 1-12, 24, 25, 29, and 30), benzylic ylides (entries 13-17 and 28), allylic ylides (entries 22, 23, 26, and 27), and ester-stabilized ylides (entries 18-21). The corresponding Wittig reactions must take place under dominant kinetic control. There are also some examples where significant reversal and stereochemical equilibration of intermediates has been demonstrated in aldehyde Wittig reactions (Table 7, subset 1).

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