Enantioselective allylboration

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.). 

Increasing interest is expressed in diastereoselective addition of organometallic reagents to the ON bond of chiral imines or their derivatives, as well as chiral catalyst-facilitated enantioselective addition of nucleophiles to pro-chiral imines.98 The imines frequently selected for investigation include N-masked imines such as oxime ethers, sulfenimines, and /V-trimcthylsilylimines (150-153). A variety of chiral modifiers, including chiral boron compounds, chiral diols, chiral hydroxy acids, A-sull onyl amino acids, and /V-sulfonyl amido alcohols 141-149, have been evaluated for their efficiency in enantioselective allylboration reactions.680... 

The real promise of this catalytic reaction is the eventual development of an efficient enantioselective allylboration catalyzed by chiral Lewis acids. A stereoselective reaction using a substoichiometric amount of a chiral director has been reported, but only modest levels of stereo-induction were achieved with an aluminum-BINOL catalyst system (Eq. 19)P Recently, a chiral Brpnsted acid catalyzed system has been devised based on a diol-tin(IV) complex (Eq. 80). In this approach, aliphatic aldehydes provide enantioselectivities (up to 80% ee) higher than those of aromatic aldehydes when using the optimal complex 114. Although the levels of absolute stereoselectivity of this method remain too low for practical uses, promising applications are possible in double diastereoselection (see section on Double Diastereoselection ). 

Chiral addition of allyl metals to imines is one of the useful approaches toward the synthesis of homoallylic amines. These amines can be readily converted to a variety of biologically important molecules such as a-, / -, and y-amino acids. Itsuno and co-workers utilized the allylborane 174 derived from diisopropyl tartrate and cr-pinene for the enantioselective allylboration of imines. The corresponding iV-aluminoimines 173 are readily available from the nitriles via partial reduction using diisobutylaluminium hydride (DIBAL-H) <1999JOM103>. Recently, iV-benzyl-imines 176 have also been utilized for the asymmetric allylboration with allylpinacol boronate 177 in the presence of chiral phosphines as the chiral auxiliaries to obtain homoallylic A -benzylamines 178 in high yield and selectivity (Scheme 29) <2006JA7687>. 

The metal-catalyzed allylboration is efficient to control both diastereoselectivity and enantioselectivity. The Sc(OTf)3-catalyzed reaction of chiral allyl boronates resulted in 90-98% ee for representative aldehydes (Equation (158)).624 628 629 The first catalytic enantioselective allylboration and crotylboration was achieved by a chiral lanthanide catalyst (Equation (159)).630... 

Optically active TV-sulfonylamino alcohols derived from D-camphor or norephedrine were found to be efficient chiral ligands for the enantioselective allylboration of iV-silylimines (Equation (170)) 646-648 B-Allyl(diisopinocampheyl)borane allylated iV-diisobutylaluminum imines with 87% ee (Equation (171)).649,650... 

B-Allyldiisocaranylborane (3). This compound, prepared from (+ )-3-carene, undergoes even more enantioselective allylboration than does 1 to furnish chiral ho-moallylic alochols in 86-99% ee (six examples) ... 

Brown HC, Jadhav PK (1984) S-Allyldiisocaranylborane anew, remarkable enantioselective allylborating agent for prochiral aldehydes. Synthesis of homoallylic alcohols approaching 100% enantiomeric purities. J Org Chem 49 4089 1091... 

Copper(II) salts combined with DuPhos ligands proved to be active catalysts for the enantioselective allylboration of ketones (300). Copper(I) triflate with Me-DuPhos hemioxide has been used as an active catalytic system for the enantioselective addition of dialkylzinc to jS-nitroalkenes (301). 

The stability of allylboronates and the high level of diastereoselectivity in their additions to carbonyl compounds and imines represent very attractive attributes in organic synthesis. The examples in this chapter clearly show that recent advances in the preparation of allylboronates will help in furthering their applications. The development of efficient catalytic enantioselective allylboration methods and the invention of more powerful and more elaborate tandem reaction processes constitute two emerging areas for further development. 

Originally, enantiosdective allylboration was developed using chiral allylbo-ranes and allyl boronates. These reactions require multistep preparahons of chiral reagents that are used in stoichiometric amoimts, and are therefore impractical. Recently, catalytic asymmetric allylborations were developed. These reactions can apply either chiral Lewis bases or BBonsted acids as the catalysts, hi particular, chiral BlNOL-phosphoric acids were demonstrated to provide high optical yields in the enantioselective allylboration reaction between allylboronate 1 and aldehydes. For example, the catalytic asymmetric allylboration of benzaldehyde 2 proceeded quantitatively yielding the corresponding homoallyl alcohol 3 with 98% ee ( heme 3.1). 

Hall and coworkers developed the novel enantioselective allylboration of aldehydes catalyzed by chiral Bronsted acid in 2006 (Scheme 1.10) [12]. Moderate to good enantioselectivities (up to 80% ee) were observed in the reactions of allylboronate and both aromatic and aliphatic aldehydes with 10mol% of chiral Bronsted acid (7) [13], a Lewis acid-assisted chiral Bronsted acid (chiral LBA) developed by Yamamoto group. On the basis of their previous studies on allylboration reactions, the authors proposed that the use of the strong chiral Bronsted acid provides the chiral recognition event through B LA-type cyclic transition state with coordination to the oxygen atom of the allylboronate. 

Scheme 1.10 Catalytic enantioselective allylboration of aldehydes by Hall and coworkers.

Rauniyar V, Hall DG. Rationally improved chiral Brpnsted acid for catalytic enantioselective allylboration of aldehydes with an expanded reagent scope. J. Org. Chem. 2009 74 4236 241. 

---