α-Substituted Allylboronates Research Articles

Tandem Reactions Involving 1-Silyl-3-Boryl-2-Alkenes. New Access to (Z)-1-Fluoro-1-Alkenes, Allyl Fluorides, and Diversely α-Substituted Allylboronates

The reactions of 1-silyl-3-boryl-2-alkenes with various electrophilic reagents (Selectfluor, N-halosuccinimides, benzhydryl, and propargylic alcohols in the presence of a Lewis acid, N-alkoxycarbonyliminium ion) have been investigated as new routes to α-substituted allylboronates. Further functional transformations, including allylboration, Suzuki coupling, protodeboronation, and cycloisomerization, have been carried out to illustrate the synthetic potential of these γ-borylallylsilanes.

Chiral Zinc‐Catalyzed Asymmetric α‐Alkylallylation and α‐Chloroallylation of Aldehydes

Two birds with one stone: In the presence of Zn(OH)2 and a chiral bipyridine ligand, racemic α-substituted allylboronates 2 reacted with aldehydes 1 (see scheme) exclusively in an α-addition fashion to afford various homoallylic alcohols 3 bearing two neighboring stereogenic centers in high yields with high diastereo- and enantioselectivities.

Stereoconvergent Synthesis of Chiral Allylboronates from an E/Z Mixture of Allylic Aryl Ethers Using a 6-NHC−Cu(I) Catalyst

We present a 6-NHC-Cu(I) complex that provides α-substituted allylboronates using allylic aryl ether substrates. The method was discovered by comparison of the chemoselectivities exhibited by complexes 1a, 1b, 2, and 3. We observed that 1a preferentially reacts with electron-rich alkenes over electron-deficient alkenes. Development of an asymmetric method revealed that 1b reacts with both the E and Z isomers to provide the same absolute configuration without showing E-Z isomerization. This stereoconvergent reaction occurs with high yields (av 86%), high S(N)2' selectivity (>99:1), and high ee (av 94%) and exhibits wide functional-group tolerance using pure E or Z isomer or E/Z alkene mixtures. The stereoconvergent feature enables the use of many different olefination strategies for substrate production, including cross-metathesis. Chiral allylboronates could be purified by silica gel chromatography and stored in the freezer without decomposition.

E-Selective Allylboration of α-Substituted Allylboronates

An example of a highly E-selective allylboration of α-substituted allylboronates is presented. This work is closely related to an earlier report by D. G. Hall (J. Am. Chem. Soc. 2007, 129, 3070) wherein allylboration of α-methylene TMS substituted substrates was explored. This work is noteworthy since earlier reports of similar reactions give low selectivity for α-methylated boro­nates. Notably, even α-chloroallylboronate gives the E-product under the reported conditions; contrary to earlier observations. As commonly seen in these reactions, the chirality transfer from allylboronates to the alcohol is excellent.

Enantioselective Synthesis of Allylboronates Bearing a Tertiary or Quaternary B-Substituted Stereogenic Carbon by NHC-Cu-Catalyzed Substitution Reactions

Allylic substitutions that afford alpha-substituted allylboronates bearing B-substituted tertiary or quaternary carbon stereogenic centers are presented. C-B bond-forming reactions, catalyzed by chiral bidentate Cu-NHC complexes, are performed in the presence of commercially available bis(pinacolato)diboron. Transformations proceed in high yield (up to >98%) and site selectivity (>98% S(N)2'), and in up to >99:1 enantiomer ratio. Trans- or cis-disubstituted alkenes can be used; alkyl- (linear as well as branched) and aryl-trisubstituted allylic carbonates serve as effective substrates. Allylboronates that bear a quaternary carbon center are air-stable and can be easily purified by silica gel chromatography; in contrast, secondary allylboronates cannot be purified in the same manner and are significantly less stable. Oxidation of the enantiomerically enriched products furnishes secondary or tertiary allylic alcohols, valuable small molecules that cannot be easily obtained in high enantiomeric purity by alternative synthesis or kinetic resolution approaches.

Highly (E)-selective BF(3).Et(2)O-promoted allylboration of chiral nonracemic alpha-substituted allylboronates and analysis of the origin of stereocontrol.

Delta-methyl-substituted homoallylic alcohols 2 were prepared in 71-88% yield, E:Z >30:1 and 93% to >95% ee via BF(3).Et(2)O-promoted allylboration with alpha-Me-allylboronate 1. The origin of high (E)-selectivity is proposed.

Chiral α-substituted allylboronates in a one-pot three-component asymmetric allylic alkylation/carbonyl allylation reaction sequence — Applications to the syntheses of (+)-(3R,5R)-3-hydroxy-5-decanolide and (–)-massoialactone

The use of different organomagnesium reagents in the copper-catalyzed allylic alkylation of 3-chloropropenyl boronates with chiral phosphoramidite ligands produces the desired α-substituted allylic boronate reagents in high regioselectivity and with modest to high enantioselectivities (up to 96% ee). The size of the incoming alkyl substituent from the organomagnesium reagent was found to impact the yield and selectivity of the allylic alkylation. A one-pot procedure for the preparation of these chiral allylic boronates followed by a Lewis acid (BF3) catalyzed addition to aldehydes delivers the desired allylboration products, homoallylic secondary alcohols, in good yields and very high diastereoselectivity. This three-component reaction methodology was applied to the syntheses of two lactone-containing natural products, (–)-massoialactone and (+)-(3R,5R)-3-hydroxy-5-decanolide. The key step of these syntheses involved the one-pot enantioselective copper-catalyzed allylic alkylation/allylboration reaction with a benzylic aldehyde, and afforded the desired product in 87% yield, 92% ee, and high E/Z selectivity in a ratio of 22:1. Remarkably, the allylic alkylation step of this sequential reaction was performed with a low catalyst loading of 2 mol% on a scale of >15 mmol that can provide multiple grams of the three-component product.

Zinc-Catalyzed Asymmetric Addition of Allylboronates

The authors describe a zinc-catalyzed asymmetric allylation of imine derivatives. When α-substituted allylboronates are used, a rare α-addition occurs instead of γ-addition. The products were obtained in high stereoselectivities, and post-modifications led to useful allylglycine derivatives.

Zn-Catalyzed Asymmetric Allylation for the Synthesis of Optically Active Allylglycine Derivatives. Regio- and Stereoselective Formal α-Addition of Allylboronates to Hydrazono Esters

We have developed Zn-catalyzed asymmetric allylation of hydrazono esters with allylboronates. The reactions proceeded smoothly in high yields and high stereoselectivities. Remarkably, formal α-addition occurred for α-substituted allylboronates exclusively, and excellent stereoselectivities were observed. This is the first example of catalytic regio- and stereoselective allylations with formal α-addition. In addition, the reaction proceeded in aqueous media, and the use of water is essential. Zn(OH)2 might be a catalyst in this asymmetric allylation, and the catalytic activity of Zn(OH)2 was confirmed. This is also the first case of chiral metal hydroxide-catalyzed asymmetric reactions.

α-Substituted Allylboronates: One-Pot Addition to Aldehydes

α-Substituted Allylboronates: One-Pot Addition to Aldehydes

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

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

Preparation of α-substituted allylboronates by chemoselective iridium-catalyzed asymmetric allylic alkylation of 1-propenylboronates

Chiral α-substituted allylic boronates are attractive reagents that add to aldehydes with very high stereoselectivity. This study examined the feasibility of an improved method of preparation based on the catalytic asymmetric allylic alkylation of simple 3-hydroxy-1-propenylboronate derivatives with malonate anions. Whereas palladium catalysis failed in promoting the desired process, iridium catalysis led to a regioselective formation of the desired, branched allylboronates with up to 84% ee using a chiral monophosphoramidite ligand. This allylation reagent adds to aldehydes with high chirality transfer. A diastereoselective alkoxycyclization on the resulting homoallylic alcohols allows a separation of the epimeric E/ Z isomers.

Additions of Functionalized α‐Substituted Allylboronates to Aldehydes under the Novel Lewis and Broensted Acid Catalyzed Manifolds.

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Additions of functionalized α-substituted allylboronates to aldehydes under the novel Lewis and Brønsted acid catalyzed manifolds

The stereo- and chemoselectivity in the additions of four model α-substituted allylboronates to benzaldehyde was examined under the standard thermal (uncatalyzed) conditions and the novel Lewis and Brønsted acid-catalyzed conditions. With either of Sc(OTf) 3 or triflic acid as catalysts, an α-ethyl allylboronate, 1a , led to a surprising inversion of stereoselectivity that can be tentatively rationalized through subtle differences in the geometry of the allylboration transition state between uncatalyzed and catalyzed pathways. It was also found that the chemoselectivity of α-silyl reagents ( 1c and 1d ) can be reversed upon use of a catalyst, providing allylsilation products instead of the allylboration product obtained from the thermal uncatalyzed reaction.

Z-selective formation of substituted homoallyl alcohols

The α-substituted allylboronates 4 add to aldehydes giving the high Z-preference the homoallyl alcohols 1. The Z-vinyl bromides obtained were further converted into δ-lactones.