Dialkyl boron triflates

Ketones, in which one alkyl group R is sterically demanding, only give the trans-enolate on deprotonation with LDA at —12°C (W.A. Kleschick, 1977, see p. 60f.). Ketones also enolize regioseiectively towards the less substituted carbon, and stereoselectively to the trans-enolate, if the enolates are formed by a bulky base and trapped with dialkyl boron triflates, R2BOSO2CF3, at low temperatures (D A. Evans, 1979). Both types of trans-enolates can be applied in stereoselective aldol reactions (see p. 60f.). 

A rationale similar to this may be used to explain the selective formation of fO)-enolates of tert-heptylthio- and rer/-butylthiopropionates (Scheme 5.15 and ref. [101]) the Et3CS- replaces the Me3CO- in the Scheme 5.18 rationale) and the selective formation of ketone Z(0)-enolates with dialkyl boron triflates and (0)-enolates with dialkylboron halides (Scheme 5.16 and 5.17 the triflates are more likely to ionize than the halides, thus favoring ionization over direct deprotonation of the zwitterion). 

Masamune has documented the addition of optically active ester enolates that afford lanfi-aldol adducts in superb yields and impressive stereoselectivity (Eq. (8.3)) [4]. The generation of a boryl enolate from 8 follows from groundbreaking studies of ester enolization by Masamune employing dialkyl boryl tri-flates and amines [5]. Careful selection of di-n-alkyl boron triflate (di-n-butyl versus dicyclopentyl or dicyclohexyl) and base (triethyl amine versus Hiinigs base) leads to the formation of enolates that participate in the <2u//-selective propionate aldol additions. Under optimal conditions, 8 is treated with 1-2 equiv of di-c-hex-yl boron triflate and triethyl amine at -78 °C followed by addition of aldehyde the products 9 and 10 are isolated in up to 99 1 antv.syn diastereomeric ratio. The asymmetric aldol process can be successfully carried out with a broad range of substrates including aliphatic, aromatic, unsaturated, and functionalized aldehydes. 

The boron enolates can be prepared by reaction of the ketone with a dialkyl-boron trifluoromethanesulfonate (triflate) and a tertiary amine. The Z-stereoisomer is formed preferentially for ethyl ketones with various R substituents. The resulting aldol products are predominantly the syn stereoisomers. 

Reaction with boronates and aryUrismutii triflates. Dialkyl 2,3-butadienylboronates, which serve as l,3-butadiene-2-yl anion equivalents, are formed from allenylmagnesium bromide with dialkyl (halomethyl)boronates. ... 

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