[3,3]‐Sigmatrope Umlagerungen von Bor‐haltigen Allylalkoholen: Synthese von Allyladditionsreagentien

Pietruszka, Jörg; Schöne, Niklas

doi:10.1002/ange.200352210

Cited by 20 publications

(2 citation statements)

References 38 publications

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“…Attempts to control the geometrical selectivity typically involve varying the nature of the catalysts and tuning the steric and electronic properties of the chiral boronate fragment, which frequently requires a laborious synthetic input 5. 6 However, these efforts brought only a partial solution to the problem—providing facile access to ( E )‐homoallylic alcohols 6 5a, 6a–d—whereas a general catalytic approach to Z isomers 5 , where R 1 is a simple alkyl group, remains underdeveloped.…”

Section: Methodsmentioning

confidence: 99%

Highly Stereoselective Synthesis of Z‐Homoallylic Alcohols by Kinetic Resolution of Racemic Secondary Allyl Boronates

2013

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Section: Methodsmentioning

confidence: 99%

Highly Stereoselective Synthesis of Z‐Homoallylic Alcohols by Kinetic Resolution of Racemic Secondary Allyl Boronates

2013

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“…[1] Moreover, the chiral ligands employed in these reactions are often expensive. Therefore, new enantioselective reactions that are catalyzed by earth-abundant metals, and that proceed efficiently at very low catalyst loadings to minimize the quantity of chiral ligand employed, are in high demand.Given the ability of electron-deficient dienes to serve as effective substrates for various catalytic asymmetric 1,6addition reactions, [2,3] we became interested in the enantioselective 1,6-boration of a,b,g,d-unsaturated carbonyl compounds as a potential method to prepare functionalized chiral allylboronates [4] and allylic secondary alcohols, [5] which are versatile building blocks for synthesis. Although enantioselective 1,4-borations of electron-deficient alkenes are well-established using chiral catalysts based upon copper, [6][7][8] other metals, [9] or by using organocatalysts, [10] the enantioselective 1,6-boration of electron-deficient dienes is not well-developed.…”

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confidence: 99%

Enantioselective Synthesis of Allylboronates and Allylic Alcohols by Copper‐Catalyzed 1,6‐Boration

et al. 2014

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Catalytic Enantioselective Preparation of α‐Substituted Allylboronates: One‐Pot Addition to Functionalized Aldehydes and a Route to Chiral Allylic Trifluoroborate Reagents

Carosi

Hall

2007

Angewandte Chemie

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Additions of allylic boron reagents to aldehydes have evolved into one of the most popular methods for stereoselective CÀC bond formation.[1] Compared to dialkyl allylic boranes, allylic boronic esters are often more advantageous as a class of reagents because of their superior stability. Three strategies have been developed for the control of enantiofacial selectivity in additions of allylic boronates to achiral aldehydes: 1) the use of a chiral diol or a diamine auxiliary as the two nonallylic substituents on the boron center; [2] 2) the use of chiral Lewis and Brønsted acid catalysis with achiral boronates; [3] and 3) the use of optically pure a-substituted reagents (so-called a-chiral allylboronates).[4] The preparation of chiral a-substituted allylboronates 1 and their additions to aldehydes were pioneered by Hoffmann and co-workers. [4] Regrettably, these reagents have remained underused in part because of their stepwise preparation based on a Matteson asymmetric homologation of chiral alkenylboronates. [5,6] The reagent-controlled additions of 1 to aldehydes proceed with near-complete transfer of chirality to give two diastereomeric products 4 and 5 (Scheme 1). These Z and E allylic alcohols are epimeric, and their ratio is highly dependent on the nature of the a substituent R 1 and the nature of the boronic ester.[4] The ratio of 4 and 5 can be explained in terms of steric and dipolar effects on the two competing transition structures 2 and 3. With a nonpolar alkyl substituent R 1 , steric interactions play a dominant role. The chairlike transition structure 2 can be destabilized by a steric interaction between a large boronic ester and the pseudoequatorial a substituent R 1 . On the other hand, the transition structure 3 features unfavorable allylic interactions that result from the pseudoaxial position of the R 1 substituent. The common use of a hindered ester, such as pinacolate, aggravates the interactions between R 1 and the methyl groups of the pinacol moiety in 2. Thus, in this case transition structure 3 is more probable and leads to mixtures of products 4 and 5 in modest selectivities. [7] In our view, two major issues need to be resolved to render chiral a-substituted allylboronates attractive reagents: a simple catalytic enantioselective method for their preparation and full diastereocontrol of the ratio 4/5 by a suitable optimization of reagent (R 1 , (OR) 2 ) and reaction conditions. Here, we report an approach that successfully addresses these two issues and provides a simple and efficient method for enantioselective aldehyde allylation.Prompted by the recent report of Alexakis and coworkers [8a] on the copper-catalyzed S N 2' allylic alkylation of cinnamyl chloride using chiral phosphoramidite ligands (L*), [8,9] we envisioned that 3-halopropenylboronates 6 [10] could be suitable substrates in this reaction [Eq. (1)]. With these substrates, however, the noncatalyzed (that is, background) allylic Matteson homologation may pose a serious threat to the enantioselectivity of this process.[1...

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[3,3]‐Sigmatrope Umlagerungen von Bor‐haltigen Allylalkoholen: Synthese von Allyladditionsreagentien

Cited by 20 publications

References 38 publications

Highly Stereoselective Synthesis of Z‐Homoallylic Alcohols by Kinetic Resolution of Racemic Secondary Allyl Boronates

Highly Stereoselective Synthesis of Z‐Homoallylic Alcohols by Kinetic Resolution of Racemic Secondary Allyl Boronates

Enantioselective Synthesis of Allylboronates and Allylic Alcohols by Copper‐Catalyzed 1,6‐Boration

Catalytic Enantioselective Preparation of α‐Substituted Allylboronates: One‐Pot Addition to Functionalized Aldehydes and a Route to Chiral Allylic Trifluoroborate Reagents

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