Allylboration reactions

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. 

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.). 

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. 

The reaction of triallylborane with silicon triyne 123 is interesting. A113B attacks both internal and external triple bonds giving rise to silole 124 and two heterocycles with bridgehead boron 125 and 126 in a 1 3 3 ratio as a result of competitive sequential reactions (Scheme 52). When 1,1-allylboration of the internal C C bond followed by intramolecular 1,1-vinyIboration takes place, the silole 124 is formed, while in another case 1,1-allylboration followed by a series of intramolecular 1,2-allylboration reactions leads to boron derivatives 125 and 126 <2002JOM(657)146>. 

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... 

N,N >Dibenzyl-N,N -ethylenetartramide, a Rationally Designed Chiral Auxiliary for the Allylboration Reaction... 

Figure 1. Calculated transition structure for the allylboration reaction between 5-allyl-l, 3,2-dioxaborolane (CH2=CHCH2B[OCH2]2) and formaldehyde at the MP2/6-31G level of theory. Indicated bond distances are in angstrom. Reprinted with permission from J. Org. Chem. 1998, 63, 8331. Copyright 1998 American Chemical Society.

It is well accepted that the high diastereospecificify of aldehyde allylboration reactions is a consequence of the compact cyclic transition structure. Theoretical calculations have shown that the chairlike transition structure shown in Scheme 1 and Fig. 1 is the lowest in energy relative to other possibilities such as the twist-boat conformation. With boronate reagents, it has also been suggested that a weak hydrogen bond between the axial boronate oxygen and the hydrogen of the polarized formyl unit contribntes to the preference for the transition structme with the aldehyde substituent in the psendo-eqnatorial position. ... 

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... 

Table A. Functionalized homoallylic alcohols from olefin cross metathesis and subsequent allylboration reactions with benzaldehyde...

The preparation of allyl boronates via CM is of significant interest due to their widespread use in allylation In 2002, Grubbs and co-workers reported a one-pot GM/allylboration reaction for the preparation of... 

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. 

P.y-Unsaturated (allylic) boron derivatives (not alkyl-, aryl- and vinylboranes) readily react with organic compounds containing multiple bonds to form novel carbon-carbon bonds (Scheme 1). In the last 30 years, allylboration reactions have been widely used in organic synthesis.1... 

Table 4 Temperature effects in the allylboration reaction with 9...

A theoretical study of the effects of structure and substituents on reactivity in allylboration has recently been completed [116]. Electron delocalization from the oxygen of an attacking aldehyde to the boron p-type atomic orbital is crucial in the allylboration reaction. The ab initio molecular orbital study indicates that the complex between the allylic borane and the aldehyde is weak. The relative elec-trophilicity of the boron atom in allylboron reagents is estimated by projecting out the unoccupied reactive orbital having the maximum amplitude onto the boron p-type atomic orbital. Two factors which are considered to be of importance in the reaction include the electron accepting level of the reactive unoccupied orbital and the efficiency of localization of the orbital in the unoccupied MO space. 

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. 

J3,4-trans are 7.0, 3.5, and 2.8 Hz, respectively. These data match well those calculated (7.0, 3.2, and 2.8 Hz) and experimentally determined (6.5, 3.6, 2.8 Hz) for analog 4 53 4. .a 5. Importantly, strong NOE enhancements between HC(4) and HC(3 ) were also observed. A 4.7% enhaneement of HC(3 ) was observed when the HC(4) was irradiated, whereas a 7.4% enhaneement of HC(4) was observed. These results strongly support the conclusion that HC(4) and the alkenyl iodide residue exist in a cis relationship and therefore further confirm the expectation that the R-configuration was obtained in the asymmetric allylboration reaction. 

From (+)-limonene, a hydroboration product, limonylborane (32). was obtained and used in asymmetric allylboration reactions, but with less success than the pinene-derived reagents34 (Section D.2.5.2.). 

Flamme, E.M. and Roush, W.R. (2005) Synthesis oftheC(l)-C(25) fragment of amphidinol 3 application of the double-allylboration reaction for synthesis of 1,5-diols. Organic Letters, 7, 1411-1414. 

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