Allylboration asymmetric

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). 

The computational results testify that the level of enantioselection in this reaction is determined via the difference in stabilities of TSIR and TS2S, that is, of the transition states with significantly different structures. Thus, the TSIR is stabilized by the weak hydrogen bond of the 

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The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

Z)-l-Methyl-2-butenylboronate 7 undergoes an exceptionally enantioselective reaction with benzaldehyde (99% ee), propanal (79%. 98% ee), 2-methyl-2-propenal (85%, 99% ee), and ( )-2-methyl-2-pentenal (81 %, 99% ee)10 38. Excellent enantioselectivity is also realized in reactions of the analogous chiral a-methyl-) y-disubstituted allylboronate27 40. Whether the l,2-dicyclohexyl-l,2-ethanediol auxiliary plays a beneficial role in this reaction, as suggested above for the asymmetric allylboration reactions of 6, has not yet been determined. 

Hanessian reported the synthesis of enantiomerically pure or highly enriched allylglycine and its chain-substituted analogs from the reaction of the sultam derivatives of O-benzyl glyoxylic acid oxime with ally he bromides in the presence of zinc powder in aqueous ammonium chloride (Eq. 11.41).72 Brown noticed the critical importance of water in the asymmetric allylboration of /V-trimethylsilyIbcnzaldimines with B-allyldiisopinocampheylborane.73 The reaction required one equivalent of water to proceed (Eq. 11.42). 

The tandem use of asymmetric allylboration to give enantiomerically pure ho-moallyHc alcohols followed by cross-metathesis of homoallylic silyl ethers with p-substituted styrenes has been reported [120] (Eq. 19). Exclusively trans cross-coupled products were formed in 50-75% yields. 

During the past 2 years several research groups have published research that either uses or expands upon Crowe s acyclic cross-metathesis chemistry. The first reported application of this chemistry was in the synthesis of frans-disubstitut-ed homoallylic alcohols [30]. Cross-metathesis of styrenes with homoallylic silyl ethers 15, prepared via asymmetric allylboration and subsequent alcohol protection, gave the desired trans cross-metathesis products in moderate to good yields (Eq. 15). 

Astroquartz, fiber reinforcement for ceramic- matrix composite, 5 558t Asymmetric allylboration, 13 669-671 Asymmetric cellulose acetate membranes, 21 633... 

We began these studies with the intention of applying this tandem asymmetric epoxidation/asymmetric allylboration sequence towards the synthesis of D-olivose derivative 63 (refer to Figure 18). As the foregoing discussion indicates, our research has moved somewhat away from this goal and we have not yet had the opportunity to undertake this synthesis. This, as well as the synthesis of the olivomycin CDE trisaccharide, remain as problems for future exploration. Because it is the enantioselectivity of the tartrate ester allylboronates that has limited the success of the mismatched double asymmetric reactions discussed here, as well as in several other cases published from our laboratorythe focus of our work on chiral allyiboronate chemistry has shifted away from synthetic applications and towards the development of a more highly enantioselective chiral auxiliary. One such auxiliary has been developed, as described below. 

A more detailed study has been made by Brown et al. [14]. They found the critical importance of water in the asymmetric allylboration of N-trimethyl-silylaldimines, and concluded that the reaction takes place during the aqueous workup. The allylboration of 19 with 20 proceeded only in the presence of one molar equivalent of water to give 22 in 92% ee and 90% yield (Scheme 8). They suggested that the reactive aldimines could be generated in situ from N-trimethylsilylimines upon addition of one equivalent of water and captured by the allylborating agent. 

Homochiral borolanes 99-102 can asymmetrically allylborate aldehydes164. The better enantioselectivity exhibited by borolane 102 than others is due to the steric origin offered by the Me3Si group. 

Allyl complexes (pseudo-rotations, dynamic NMR studies, 1, 416 with tungsten carbonyls and isocyanides, 5, 688-689 rc-Allyl complexes with Cr, 5, 305 with Cr(II), 5, 300 with Cr(III), 5, 300 and cyclodextrins, 12, 789 in enyne carbometallation, 10, 328 with rhodium, 7, 220-221 (j-Allyl complexes, with iron, 6, 98 5-Allyldiisopinocampheylboranes, in asymmetric allylboration, 9, 198... 

Asymmetric allylboration, characteristics, 9, 197 Asymmetric allylic alkylation, allylic alcohols with copper, 11, 99 with iridium, 11, 105 with molybdenum, 11, 109 with nickel, 11, 102 with non-palladium catalysts, 11, 98 with platinum, 11, 103 reaction systems, 11, 112 with rhodium, 11, 104 with ruthenium, 11, 108 with tungsten, 11, 111... 

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>. 

Asymmetric allylboration of RCHO. (S)-l reacts with aliphatic or aryl aldehydes or with a,p-enals to form homoallylic alcohols in 92-97% ee and 80-92% chemical yield. The chemical and optical yields are higher than those obtained with B-allyldiisopinocamphenylborane (14,12), with allylboronates modified with tartrates, or with B-allyltrimethylsilylboronates. The high asymmetric induction is believed to result from steric effects rather than electronic effects. 

The haloboration-coupling sequence provided simple access to the stereochemically pure 3,3-disubstituted allylboron compounds 116 via one carbon homologation of the alkenylboron intermediates (Equation (20)).196 Bromoboration of allene afforded 117 which was converted into a tartrate for asymmetric allylboration (Equation (21)).197... 

The ruthenium-catalyzed olefin cross-metathesis to the preparation of functionalized allyl boronates has resulted in a one-pot three-component coupling procedure for the synthesis of functionalized homoallylic alcohols.617,618 The utility of the protocol was demonstrated in asymmetric allylboration using a tartrate ester (Equation (152)).617... 

For the purpose of asymmetric allylborations, chiral diols have been used as excellent directing groups. Chiral i -allyldiisopinocampheylboranes (allylB(Ipc)2) were found to be excellent alternatives. There are useful reviews of... 

Asymmetric allylboration using tartrate- and pinane-derived reagents have been successfully exploited in the synthesis of various natural products.653-673... 

Although Brown and co-workers proposed a six-membered transition state for the asymmetric allylboration reaction in which the aldedyde oxygen initially coordinates to boron followed by an internal transfer of the allyl group from boron to the carbonyl carbon,8 a quantitative analysis to explain the enantioselectivity was not available until 1993, when Gennari et al. conducted a computational study to rationalize the enantiofacial selectivity of Brown allylation9 (Scheme 3.1g). Calculation predicts that transition state A, in which the allyl group attacks the si-lace of the aldehyde, is favored over transition state B by 2.12kcal/mol. 

Asymmetric allylboration has also been applied to y-methoxyallyl derivatives. Isomerically pure (Z)-y-methoxyallyldiisopinocampheylborane (rf31), prepared from Ipc2lSOMe and the lithium anion of allyl methyl ether, reacts with various aldehydes to afford the yyn - j-m e (boxy homoallylic alcohol (32a) in a highly regio- and stereoselective manner17 (Scheme 3.In). This one-pot synthesis of enantiomerically pure 1,2-diol derivatives went as smoothly as the asymmetric Brown crotylation, affording products with uniformly high diastereoselectivity. 

The asymmetric allylboration of representative aldehydes with either 9R or 9S was examined in EE (3h, -78 °C). In all cases, the homoallylic alcohols 10 were obtained in i96% ee. These results are summarized in Table 2. The intermediates 11 were isolated in excellent yields in essentially pure form after solvent removal. For the reactions of 9R, solutions (-0.5 M) of SR and (IS, 25)- (+)- pseudoephedrine (1.0 equiv) in MeCN were heated at reflux temperature to effect the transesterification with crystalline (+)-8 being isolable by simple filtration. An analogous procedure was used for the 95 reactions employing (-)-pseudoephedrine to... 

The best known chiral boron ligation for asymmetric allylborations for which comparative data is available are included in Figure l.2 Comparing the product ee s achieved by these systems with those from 9 reveals that this new asymmetric reagent equals or exceeds the selectivity observed for any of these reagents at -78 °C (c/Table 3). 

Roush, R W, Hunt, J A, Asymmetric allylboration of 2-iV,3-0-isopropylidene-A-Boc-L-serinal diastereoselective synthesis of the calicheamicin 7 aminosugar, J. Org. Chem., 60, 798-806, 1995. 

Brown, H. C., Bhat, K. S., Jadhav, P. K. Chiral synthesis via organoboranes. Part 32. Synthesis of B-(cycloalk-2-enyl)diisopinocampheylboranes of high enantiomeric purity via the asymmetric hydroboration of cycloalka-1,3-dienes. Successful asymmetric allylborations of aldehydes with B-(cycloalk-2-enyl)diisopinocampheylboranes. J. Chem. Soc., Perkin Trans. 1 1991, 2633-2638. 

The asymmetric allylboration of achiral aldehydes with a substituted chiral al-lylborolane 193 and ( )- or (Z)-194 has been reported [128]. The enantioselectiv-ity observed with (5)-193 at -100 C and aldehydes is uniformly high with all of the achiral aldehydes examined (Scheme 10-75). The enantioselection observed with the borolane 193 is proposed to be primarily steric in origin and not from any stereoelectronic component. The reaction likely proceeds via a closed, six-membered transition structure in which the aldehyde is coordinated such that the trimethylsilyl group is oriented anti to the developing B-0 bond. 

Generally the reaction of unsaturated aldehydes (aromatic, olefmic and acetylenic) with chiral boronates has provided homoallylic alcohols in low to moderate enantioselectivity [124]. However, the enantioselectivity of the allyl- and 2-bu-tenylborations of benzaldehyde and unsaturated aldehydes is significantly improved when a metal carbonyl complex is utilized as the substrate [131]. For example, the reaction of iron carbonyl-complexed diene 225, chromium carbonyl-complexed benzaldehyde 226 and dicobalt hexacarbonyl-complexed acetylene 227 all give significantly increa.sed allyl and 2-butenylboration selectivities compared to the parent aldehydes (Fig. 10-6). In the case of chiral substrates 225 and 226, these species can be obtained in enantioenriched form by kinetic resolution by use of the asymmetric allylboration reaction. 

Scheme 3.12 illustrates the polymer-supported aUylboron reagents derived from chiral N-sulfonylamino alcohols and used for the asymmetric synthesis of homoal-lylic alcohols and amines (see Scheme 3.12) ]29]. All of these asymmetric allylbora-tions were performed using the polymeric chiral aUylboron reagent prepared from triallylborane and PS-supported N-sulfonylamino alcohols 38-41. High levels of enantioselectivity were obtained in the asymmetric allylboration of imines with the polymeric reagent derived from norephedrine.

The last class of allylation reactions that are amenable to asymmetric catalysis employs allylboronate derivatives. Schaus reported that several chiral BINOL deri vatives catalyze the enantioselective asymmetric allylboration of acyl imines [97]. This reaction is most effective when 3,3 diphenyl BINOL acts as the catalyst and allyldii sopropoxyborane is the nucleophile. The allylation products are obtained in good yields (75 94%) and excellent enantiomeric excesses (>90% ee) for both aromatic and aliphatic imines (Table 1.13). 

By a similar method, the (Z)-crotylborate is synthesized from cA-2-butene in 70-75% yield with a 98% isomeric purity. The tartrate esters of allylboronic acids are an excellent reagent for asymmetric allylboration of carbonyl compounds. Allyl(diisopinocampheyl)borane [51] and the allylic boron derivatives of ester and amide, such as camphordiol [52], pinanediol [53], 1,2-diphenyl-1,2-ethylenediamine [54], have also been successfully used for asymmetric allylboration of carbonyls. 

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