Allylboronate reagents

Finally, allylboron reagents have been synthesized via reactions of bromoboranes and allylstan-nanes n. 

I.3.3.3.3.I.I.4. Other Methods of Synthesis of Allylboron Reagents Lacking Stereogenic... 

The reaction of triallylborane31 with diols or acidic amino alcohols provides a convenient method of synthesis of allylboron reagents, especially when the diol or amino alcohol is a valuable chiral auxiliary32-34. Two representative cases arc summarized below. 

Allylboron Reagent Aide- 1 2 A Percentage Reaction at Low Temp. Ref... 

I.3.3.3.3.I.3. Relative Asymmetric Induction Reactions of Chiral Aldehydes with Achiral Allylboron Reagents... 

The enantioselectivities of the reactions of representative achiral aldehydes and chiral allylboron reagents arc compared in Table 4. A comparison of the enantioselectivities of the (Z )-2-butenyl reagents appears in Table 5, while Table 6 provides a similar summary of the reactions of the (Z)-2-butenyl and 3-methoxy-2-propcnyl reagents. A 3-diphenylamino-2-propenyl reagent was recently reported102. 

One additional chiral allylboron reagent has been described that is worthy of mention. This is a chiral y-alkoxyallylboronate75. 

Table 6. 1-Substituted 2-Methyl- and 2-Methoxy-3-butenols from (Z)-Allylboron Reagents and Achiral Aldehydes...

On the other hand, high levels of diastereoselectivity are relatively easy to achieve in matched double asymmetric reactions since the intrinsic diastereofacial preference of the chiral aldehyde reinforces that of the reagent, and in many cases it has been possible to achieve synthetically useful levels of matched diastereoselection by using only moderately enantioselective chiral allylboron reagents. Finally, it is worth reminding the reader that both components of double asymmetric reactions need to be both chiral and nonracemic for maximum diastereoselectivity to be realized. 

Many of the chiral allylboron reagents discussed in Section 1.3.3.3.3.1.4. have been utilized in double asymmetric reactions with chiral aldehydes. Chiral 2-(2-butenyl)-3.5-dioxa-4-boratri-cyclo[5.2.1.02-6]decanes were among the first chiral reagents of any type to be used in double asymmetric reactions52a,b. 

For additional examples in which reagent control is exercised in the reactions of chiral allylboron reagents and chiral aldehydes ... 

Conceptually related methods have been employed in the synthesis of a-substituted allylboron reagents 85 and 106. In the case of 8, a benzimidazoleoxy leaving group is introduced as part of the a-alkoxy-2-butenyllithium reagent. Fragmentation of the ate complex must be conducted at — 100 °C in order to avoid the isomerization of 8, which occurs readily if 8 is allowed to warm to — 78 °C for one hour before addition of an aldehyde5. 

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