Reactions with achiral aldehydes

I.3.3.3.3.I.2. Simple Diastereosclectivity Reactions with Achiral Aldehydes and Ketones... 

Finally, 2-allyl-4,5-tra ,s-diphenyl-l,3-bis(4-methylphenylsulfonyl)-l,3,2-diazaborolidincs have been used74. The 2-propenyl derivative undergoes highly stereoselective reactions with achiral aldehydes (95 - 97% ee) the ( )-2-butenyl derivatives (91-95% ee) and the analogous 2-chloro- and 2-bromo-2-propenyl derivatives (84-99% ee) also give excellent results in reactions with achiral aldehydes. 

Results of the asymmetric 2-propenylborations of several chiral a- and /i-alkoxy aldehydes are presented in Table 11 74a-82 84. These data show that diisopinocampheyl(2-propenyl)borane A and l,3-bis(4-methylphenylsulfonyl)-4,5-diphenyl-2-propenyl-l,3,2-diazaborolidine C exhibit excellent diastereoselectivity in reactions with chiral aldehydes. These results are in complete agreement with the enantioselectivity of these reagents in reactions with achiral aldehydes (Section 1.3.3.3.3.1.4.). In contrast, however, the enantioselectivity of reactions of the tartrate 2-propenylboronate B (and to a lesser extent the tartrate (/i)-2-butenylhoronate)53b is highly... 

Chiral, nonracemic allylboron reagents 1-7 with stereocenters at Cl of the allyl or 2-butenyl unit have been described. Although these optically active a-substituted allylboron reagents are generally less convenient to synthesize than those with conventional auxiliaries (Section 1.3.3.3.3.1.4.), this disadvantage is compensated for by the fact that their reactions with aldehydes often occur with almost 100% asymmetric induction. Thus, the enantiomeric purity as well as the ease of preparation of these chiral a-substituted allylboron reagents are important variables that determine their utility in enantioselective allylboration reactions with achiral aldehydes, and in double asymmetric reactions with chiral aldehydes (Section 1.3.3.3.3.2.4.). 

The greater diastercosclectivity of (Z)-l-methoxy-2-butenylboronate 412-25 compared with the 1-chloro derivative 31 33 demonstrated in reactions with achiral aldehydes (Section 1.3.3.3.3.1.) suggests that double asymmetric reactions of chiral aldehydes with 4 will also be more selective than reactions with 3. The data summarized below provide an indication of the magnitude of this effect. 

Reactions with Achiral Aldehydes. The reaction of tartrate allylboronates with achiral aldehydes proceeds with moderate to excellent enantioselectivity (60-92% ee) and high yield (80-90%). Simple aliphatic aldehydes give good enantioselectiv-ities (decanal 86% ee, CyCHO 87% ee, eq 2), while p-alkoxy and conjugated aldehydes give diminished selectivities (60-80% ee) (eq 3). The enantioselectivity is highly temperature and solvent dependent. Best results for reactions with the vast majority of aldehydes are obtained in toluene at —78 °C. 4°A molecular sieves are included to ensure that the reaction is anhydrous. Other tartrate esters (e.g. diethyl tartrate) may also be used without loss of enantioselectivity. 

The reagent s moderate facial preference makes it an ideal choice for illustrating the concept of double asymmetric induction. The chiral lithium (Z)-enolate, which also exhibits a moderate enantiofacial preference in reaction with achiral aldehydes, reacts with the reagent to afford a 61 28 ratio of products (eq 6). This mismatched case of asymmetric induction indicates that the facial preferences of the two compounds are working at crosspurposes. With the reagent s enantiomer, (45)-2 -dimethyl-l,3-dioxolane-4-carboxaldehyde, a greater than 97 3 ratio of products is obtained, indicating matched facial preferences. 

Asymmetric allylation is a valuable method for constructing chiral functionalized structures, and many chiral allylmetal reagents directed toward a high level of asymmetric induction have, therefore, been designed and synthesized. Although for some of these good to excellent enantio- and diastereoselectivity are obtained in reactions with achiral aldehydes, we developed the first novel method for a catalytic process in 1991 [49a]. 

The use of enantiopure allylic boranes in reactions with achiral aldehydes results not only in high diastereoselection, but also in high enantioselectionP Pure (Z)- and )-crotyldiisopinocampheylboranes can be prepared at low temperature from (Z)- or E) crotylpotassium and S-methoxydiisopinocampheylborane, respectively, after treatment of the resultant ate-complexes with BF3 OEt2. The R-methoxydiisopinocampheylboranes are prepared by reacting (-)-diisopinocampheylborane, derived from (+)-a-pinene, or (+)-diisopinocampheylborane, derived from (-)-a-pinene, with methanol. 

Taddei and coworkers reported that chiral allyltrimethylsilanes containing an optically active ligand derived fiom (-)-myrtenal attached to silicon (96) underwent enantioselective addition reactions with achiral aldehydes (Scheme 46) to give, after acid hydrolysis, optically active homoallyl alcohols (98). A variety of Lewis acids were examined to optimize enantiomeric excess, and TiCU was found to be the most effective catalyst. The results of the TiCU-promoted additions are reported in Table 12. ... 

Roush, W. R., Ando, K., Powers, D. B., Halterman, R. L., Palkowitz, A. D. Enantioselective synthesis using diisopropyl tartrate-modified (E)-and (Z)-crotylboronates reactions with achiral aldehydes. Tetrahedron Lett. 1988, 29, 5579-5582. 

Corey and coworkers have described Ae preparation of allylborane (289) by the reaction of bromobo-rane (2W) and allyltributylstannane and shown that (289) undergoes highly stereoselective reactions with )x>th achiral (95-97% ee) and chiral aldehydes (Scheme 54). The corresponding methallyl, ( )-crotyl and 2-chloro- and 2-bromo-allyl reagents were prepared by similar methods and shown to give excellent results in reactions with achiral aldehydes (84-99% ee in most cases). 

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