Allyl/crotylboron reagents

FIGURE 10.8. Commonly used allyl/crotylboron reagents. 

The presence of a stereogenic center on the aldehyde can strongly inlinence the diastereoselectivity in allylboration reactions, especially if this center is in the a-position. Predictive rules for nucleophilic addition on snch a-snbstitnted carbonyl substrates such as the Felkin model are not always snitable for closed transition structures.For a-substituted aldehydes devoid of a polar substituent, Roush has established that the minimization of ganche-ganche ( syn-pentane ) interactions can overrule the influence of stereoelectronic effects. This model is valid for any 3-monosubstituted allylic boron reagent. For example, althongh crotylboronate (E)-7 adds to aldehyde 39 to afford as the major prodnct the diastereomer predicted by the Felkin model (Scheme 2), " it is proposed that the dominant factor is rather the minimization of syn-pentane interactions between the Y-snbstitnents of the allyl unit and the a-carbon of the aldehyde. With this... 

Tartrate-derived Chiral Allyl- and Crotylboronate Reagents... 

Chiral crotylboronates (216) and (217) were among the first chiral allyl metal reagents to be used in double asymmetric reactions. The example in Scheme 47, however, shows that (217) induces only modest changes in the stereoselectivity of the reactions of (249), thus underscoring the need for highly enantioselective chiral reagents. 

Results of reactions of chiral a-methyl aldehydes and several chiral crotyl- and allyl-boron reagents are summarized in Tables 8 and 9. It is apparent from these data that the Brown (Ipc)2B(crotyl) and (Ipc)2B(allyl) reagents (51), (52) and (219) consistently give excellent results for the synthesis of each product diastereomer (Table 8, entries 3-6, 11, 16, 20, and 24 Table 9, entries 1,2, 10 and 18). This is true also for their reactions with chiral a- and 3-alkoxy aldehydes (Scheme 49).i. i4S-i50 Thg tartrate crotylboronates (18) and (19) also display excellent selectivity in the synthesis of crotyl diastereomers (136), (137) and (139) (Table 8, entries 7,10,13,17,25 and 28), but are much less selective for the syndesis of crotyl diastereomer (138), especially from -alkoxy-substituted aldehydes such as (253). Tartrate allylboronate (224) is also less effective than (Ipc)2Ballyl (219) for the synthesis of (257) and (258) in Table 9, and of (266) and (267) in Scheme 49.Substantial improvements in selectivity have been realized by using the taitramide-based allylboronate (228), and the results with this reagent (Table 9, entries 4, 7, 9, 12, 14, 17, 20 and 22) compare very favorably with those obtained with (219). The data... 

The synthesis and use of tartrate-modified allylboronate 1 was first reported by Roush and co-workers in 1985. The synthesis and use of the corresponding (E)- and (. -crotylboronate reagents 2 and 3 was published by Roush and co-workers shortly thereafter. The ease of synthesis, stability and efficient reactivity of these reagents offers advantages over many other allyl- and crotylmetal reagents. Roush and co-workers have extensively explored the enantioselective allylations with achiral aldehydes as well as the... 

Stereoselectivity diminished or disappeared altogether as the C(3) substituent was removed (allyl reagent 25) or inverted ((E)-crotylboronate 26 see Figure 10).3 ... 

In general, as the aldehyde a-substituents become more sterically demanding, it becomes more difficult to obtain useful levels of diastereoselection for the product expected from reagent control in mismatched double asymmetric reactions between chiral aldehydes and chiral allyl- and crotylboronates [203]. For this reason, in natural product synthesis, mismatched double asymmetric reactions should be designed to occur early rather than late in a synthetic sequence. 

---