Boryl triflate

In studies not yet published (66), the A/-acyl-oxazolidine-2-one 62 has been found to exhibit exceptionally high levels of (Z)-enolization stereoselection with either amide bases (LDA, THF, -78°C) or boryl triflates [(n-C4H9)2BOTf, CH2CI2, -78°C] in the presence of diiso-propylethylamine (DPEA). Upon aldol condensation, the enolates 63a and 63b afford the aldolates 64 (Scheme 11), which react readily with nucleophiles at the carbonyl function (Table 22). As discussed earlier, the large preference for (Z)-enolate formation in this system can be attributed to allylic strain considerations (37)... 

Although simple alkyl esters (ethyl propionate) fail to enolize with the boryl triflate reagents under normal conditions, the more acidic acyloxyboranes 66 readily form the diboryl enediolates 67 (eq. [52]) (6a,66). Several interesting trends are noted in the data included in Table 23. Since previous studies have demonstrated that enolate geometry strongly correlates with product stereochemistry, enediolate 67 has been employed to directly compare the reactivities... 

Use of the boron azaenolate 3, prepared from achiral 2-ethyl-4,4-dimethyloxazoline and the chiral boryl triflate, undergoes aldol condensation to give mainly threo-fi-hydroxy esters with enantioselectivity of about 80% (equation II). 

Simple alkyl esters do not react with boryl triflate reagents, but acyloxyboranes give diborane enedio-lates under these conditions (Scheme 25). These diborane enediolates usually give more anti aldol than syn aldol product. Because aldol geometry depends on enolate geometry, it can be inferred that ( )-boron enolates are somewhat more reactive than the (Z)-isomers. 

Trimethylsilyl enol ethers react rapidly with boryl triflate reagents (Scheme 26). Subsequent aldol reaction occurs with apparent stereospecificity provided that the by-product trimethylsilyl triflate is... 

Boron ketone enolates 20 are generated from a hindered dialkylboryl triflate (e.g. dicyclohexyl) and diisopropylethylamine at 0 °C whereas the (Z) isomer 21 is prepared by using a less hindered boryl triflate (e.g. dibutyl) at -78 °C (Figure 3.3) [8],... 

Aldol-type reactions of acetonitriles generally proceed only in the presence of strong base at low temperature. It has now been found that the use of such conditions may be circumvented by utilization of 9-BBN triflate or di-n-butyl-boryl triflate in the presence of di-isopropylethylamine to provide aldol products in high yield (Scheme 27). ... 

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