α-Silicon effect assisted Curtin-Hammett allylation using allylcopper reagents derived from 1,3-dienylsilanes

Allylsilane as a versatile handle in photoredox catalysis

Organosilanes have secured a special place in the synthetic world for several decades. However, among them, allylsilanes are a choice reagent for organic chemists to develop novel organic transformations. In recent years researchers have proved that visible-light photoredox catalysis has emerged as one of the most mild, sustainable, straightforward, and efficient strategies to construct simple to complex molecules with or without enantioselectivity. This review provides an in-depth analysis of recent advances and strategies employed in visible-light photoredox catalysis for allylsilane and its analogues for the development of various organic transformations. The review is divided into sections, each focused on a specific reactivity of allylsilane under light irradiation with C(sp2) center arene or alkene, C(sp2) center carbonyl, and C(sp3) center carbon functionality. In this review, we present optimization data, reaction scope, and mechanistic aspects to bring forward specific reactivity and selectivity trends of allylsilane in photoredox conditions.

Copper(I)-Catalyzed Asymmetric Allylation of Ketones with 2-Aza-1,4-Dienes

Catalytic asymmetric allylation of ketones under proton-transfer conditions is a challenging issue due to the limited pronucleophiles and the electrophilic inertness of ketones. Herein, a copper(I)-catalyzed asymmetric allylation of ketones with 2-aza-1,4-dienes (N-allyl-1,1-diphenylmethanimines) is disclosed, which affords a series of functionalized homoallyl tertiary alcohols in high to excellent enantioselectivity. Interestingly, N-allyl-1,1-diphenylmethanimines work as synthetic equivalents of propanals. Upon the acidic workup, a formal asymmetric β-addition of propanals to ketones is achieved. An investigation on KIE effect indicates that the deprotonation of N-allyl-1,1-diphenylmethanimines is the rate-determining step, which generates nucleophilic allyl copper(I) species. Finally, the synthetic utility of the present method is demonstrated by the asymmetric synthesis of (R)-boivinianin A and (R)-gossonorol.

Copper-Catalyzed Asymmetric Acylboration of 1,3-Butadienylboronate with Acyl Fluorides

We report herein a Cu-catalyzed regio-, diastereo- and enantioselective acylboration of 1,3-butadienylboronate with acyl fluorides. Under the developed conditions, the reactions provide (Z)-β,γ-unsaturated ketones bearing an α-tertiary stereocenter with high Z-selectivity and excellent enantioselectivities. While direct access to highly enantioenriched E-isomers was not successful, we showed that such molecules can be synthesized with excellent E-selectivity and optical purities via Pd-catalyzed alkene isomerization from the corresponding Z-isomers. The orthogonal chemical reactivities of the functional groups embedded in the ketone products allow for diverse chemoselective transformations, which provides a valuable platform for further derivatization.

Asymmetric Syntheses of (Z)- or (E)-β,γ-Unsaturated Ketones via Silane-Controlled Enantiodivergent Catalysis

Cu-catalyzed highly stereoselective and enantiodivergent syntheses of (Z)- or (E)-β,γ-unsaturated ketones from 1,3-butadienyl silanes are developed. The nature of the silyl group of the dienes has a significant impact on the stereo- and enantioselectivity of the reactions. Under the developed catalytic systems, the reactions of acyl fluorides with phenyldiemthylsilyl-substituted 1,3-diene gave (Z)-β,γ-unsaturated ketones bearing an α-tertiary stereogenic center with excellent enantioselectivities and high Z-selectivities, where the reactions with triisopropylsilyl-substituted 1,3-butadiene formed (E)-β,γ-unsaturated ketones with high optical purities and excellent E-selectivities. The products generated from the reactions contain three functional groups with orthogonal chemical reactivities, which can undergo a variety of transformations to afford synthetically valuable intermediates.

Stereoselective syntheses of 2-methyl-1,3-diol acetals via Re-catalyzed [1,3]-allylic alcohol transposition

Rhenium-catalyzed stereoselective transposition of allylic alcohols is reported. In the presence of 1 mol% of Re2O7, (E)- or (Z)-δ-hydroxymethyl-anti-homoallylic alcohols were converted into the acetals of 2-methyl-1,3-syn-diols with excellent diastereoselectivities. 1,3-syn-Diol acetals can also be synthesized from (E)-δ-hydroxymethyl-syn-homoallylic alcohols.

Unusual Enantiodivergence in Chiral Brønsted Acid‐Catalyzed Asymmetric Allylation with β‐Alkenyl Allylic Boronates

We report herein a rare example of enantiodivergent aldehyde addition with β-alkenyl allylic boronates via chiral Brønsted acid catalysis. 2,6-Di-9-anthracenyl-substituted chiral phosphoric acid-catalyzed asymmetric allylation using β-vinyl substituted allylic boronate gave alcohols with R absolute configuration. The sense of asymmetric induction of the catalyst in these reactions is opposite to those in prior reports. Moreover, in the presence of the same acid catalyst, the reactions with β-2-propenyl substituted allylic boronate generated homoallylic alcohol products with S absolute configuration. Unusual substrate-catalyst C-H⋅⋅⋅π interactions in the favoured reaction transition state were identified as the origins of observed enantiodivergence through DFT computational studies.

Palladium-Catalyzed Asymmetric Sequential Hydroamination of 1,3-Enynes: Enantioselective Syntheses of Chiral Imidazolidinones

Pd-catalyzed sequential hydroamination of readily available 1,3-enynes is reported. The redox-neutral process provides an efficient route to synthesize a broad scope of imidazolidinones, thiadiazolidines, and imidazolidines. Asymmetric sequential hydroamination generates a series of synthetically valuable, enantioenriched imidazolidinones. Mechanistic studies revealed that the transformation occurred via an intermolecular enyne hydroamination pathway to give an allene intermediate. Subsequent intramolecular hydroamination of the allene intermediate proceeded under the Curtin-Hammett principle to provide enantioenriched imidazolidinone products.

Development of α-Borylmethyl-(Z)-crotylboronate Reagent and Enantioselective Syntheses of (E)-δ-Hydroxymethyl-syn-homoallylic Alcohols via Highly Stereoselective Allylboration

We report herein the development of α-borylmethyl-(Z)-crotylboronate reagent and the application in highly stereo- and enantioselective syntheses of (E)-δ-hydroxymethyl-syn-homoallylic alcohols. Starting from 1,4-pentadiene, α-borylmethyl-(Z)-crotylboronate was synthesized in two steps with high Z-selectivity and enantioselectivity. Subsequent aldehyde allylboration with the developed boron reagent gave highly enantioenriched (E)-δ-hydroxymethyl-syn-homoallylic alcohols upon oxidative workup.

Development of a Modified System to Provide Improved Diastereocontrol in the Linear-Selective Cu-Catalyzed Reductive Coupling of Ketones and Allenamides

Chiral γ-lactones are prevalent organic architectures found in a large array of natural products. In this work, we disclose the development of a modified catalytic system utilizing a commercially available Cu-phosphite catalyst for the diastereoselective reductive coupling of chiral allenamides and ketones to afford chiral γ-lactone precursors in 80:20 to 99:1 dr.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Stereochemical Control via Chirality Pairing: Stereodivergent Syntheses of Enantioenriched Homoallylic Alcohols

We report herein the development of stereodivergent syntheses of enantioenriched homoallylic alcohols using chiral nonracemic α-CH₂ Bpin-substituted crotylboronate. Chiral phosphoric acid (S)-A-catalyzed asymmetric allyl addition with the reagent gave Z-anti-homoallylic alcohols with excellent enantioselectivities and Z-selectivities. When the enantiomeric acid catalyst (R)-A was utilized, the stereoselectivity was completely reversed and E-anti-homoallylic alcohols were obtained with high E-selectivities and excellent enantioselectivities. By pairing the chirality of the boron reagent with the catalyst, two complementary stereoisomers of chiral homoallylic alcohols can be obtained selectively from the same boron reagent. DFT computational studies were conducted to probe the origins of the observed stereoselectivity. These reactions generate highly enantioenriched homoallylic alcohol products that are valuable for rapid construction of polyketide structural frameworks.

Catalytic asymmetric transformations of racemic α-borylmethyl-(E)-crotylboronate via kinetic resolution or enantioconvergent reaction pathways

We report herein catalytic asymmetric transformations of racemic α-borylmethyl-(E)-crotylboronate. The Brønsted acid-catalyzed kinetic resolution-allylboration reaction sequence of the racemic reagent gave (Z)-δ-hydroxymethyl-anti-homoallylic alcohols with high Z-selectivities and enantioselectivities upon oxidative workup. In parallel, enantioconvergent pathways were utilized to synthesize chiral nonracemic 1,5-diols and α,β-unsaturated aldehydes with excellent optical purity.

Asymmetric Syntheses of (E)-δ-Hydroxymethyl-anti-homoallylic Alcohols via Highly Enantio- and Stereoselective Aldehyde Allylation with α-Borylmethyl-(E)-crotylboronate

Highly stereo- and enantioselective synthesis of (E)-δ-hydroxymethyl-anti-homoallylic alcohols is reported. Under the developed conditions, reactions between aldehydes and chiral nonracemic α-borylmethyl-(E)-crotylboronate upon oxidative workup gave δ-hydroxymethyl-anti-homoallylic alcohols with high E-selectivities and enantioselectivities.

Highly stereoselective syntheses of (E)-δ-boryl-anti-homoallylic alcohols via allylation with α-boryl-(E)-crotylboronate

A highly stereoselective synthesis of (E)-δ-boryl-anti-homoallylic alcohols is developed. In the presence of a Lewis acid, aldehyde allylation with α-boryl-(E)-crotylboronate gave δ-boryl-anti-homoallylic alcohols in good yields with excellent E-selectivity. The E-vinylboronate group in the products provides a useful handle for cross-coupling reactions as illustrated in the fragment synthesis of chaxamycins.

Cu-catalyzed coupling of vinylidene cyclopropanes with allyl and allenyl boronates

The development of Cu-catalyzed coupling of vinylidene cyclopropanes with allyl or allenyl boronates is reported. The reaction forms a C-C bond at the terminal carbon atom of the allene moiety of vinylidene cyclopropanes with concurrent opening of the cyclopropane ring. In addition, the resulting Cu-enolate intermediate can be intercepted by external electrophiles.

Copper-Catalyzed Highly Selective Protoboration of CF3 -Containing 1,3-Dienes

The copper-catalyzed highly selective protoboration of CF₃ -containing conjugated diene with proton source and B₂ Pin₂ has been developed. This chemistry could suppress the well-known defluorination and provide borated reagents with an intact CF₃ -group. Further studies indicated that the functional group tolerance of this chemistry is very well, and the products could be used as versatile precursors for different types of transformations. Importantly, using chiral diphosphine ligand, we have developed the first example for using such starting material to synthesis allylic boron-reagents which bearing a CF₃ -containing chiral center. Notably, the reaction mechanism was intensively studied by DFT calculations, which could reveal the reason that defluorination was inhibited.

Cu-Catalyzed Highly Stereoselective Syntheses of (E)-δ-Vinyl-homoallylic Alcohols

Stereoselective synthesis of (E)-δ-vinyl-homoallylic alcohols was developed. Starting from α-vinyl allylboronate, Cu-catalyzed allylation of aldehydes or ketones forms secondary or tertiary δ-vinyl-homoallylic alcohols with high E-selectivities. It is proposed that the reaction operates under the Curtin-Hammett principle via the intermediacy of α-vinyl allylic copper species to give the alcohol products with high E-selectivities.

Copper-Catalyzed Three-Component Carboboronation of Allenes Using Highly Strained Cyclic Ketimines as Electrophiles

A diastereoselective copper and NHC-ligand-catalyzed three-component difunctionalization of allenes with bis(pinacolato)diboron and 2H-azirines to afford borylated allylaziridines is described. The reaction exhibits complete diastereoselectivity and good yields, and the further chlorination of the corresponding borylated products was also performed. It is believed that the high ring-strain force of 2H-azirines facilitates the reaction. More chemical transformations of borylated allylaziridines are also reported.

Enantioselective C2-Allylation of Benzimidazoles Using 1,3-Diene Pronucleophiles

Although substituted benzimidazoles are common substructures in bioactive small molecules, synthetic methods for their derivatization are still limited. Previously, several enantioselective allylation reactions of benzimidazoles were reported that functionalize the nucleophilic nitrogen atom. Herein we describe a reversal of this inherent selectivity toward N-allylation by using electrophilic N-OPiv benzimidazoles with readily available 1,3-dienes as nucleophile precursors. This CuH-catalyzed approach utilizes mild reaction conditions, exhibits broad functional-group compatibility, and exclusively forms the C2-allylated product with excellent stereoselectivity.

Enantioselective anti- and syn-(Borylmethyl)allylation of Aldehydes via Brønsted Acid Catalysis

The enantioselective anti- and syn-(borylmethyl)allylation of aldehydes via phosphoric acid catalysis is reported. Both (E)- and (Z)-γ-borylmethyl allylboronate reagents were prepared via the Cu-catalyzed highly stereoselective protoboration of 1,3-dienylboronate. Chiral phosphoric acid-catalyzed aldehyde allylation with either the (E)- or (Z)-allylboron reagent provided 1,2-anti- or 1,2-syn-adducts in good yields with high enantioselectivities. The application to the synthesis of morinol D was accomplished.

Highly Diastereo- and Enantioselective Synthesis of 3,6'-Bisboryl-anti-1,2-oxaborinan-3-enes: An Entry to Enantioenriched Homoallylic Alcohols with A Stereodefined Trisubstituted Alkene

A Cu-catalyzed regio-, diastereo-, and enantioselective carboboration of 1,1-bisboryl-1,3-butadiene is developed to generate enantioenriched 3,6'-bisboryl-anti-1,2-oxaborinan-3-enes. DFT calculations indicate that the initial diene 1,2-borocupration forms a ³ η-allylic copper as the most stable intermediate. Subsequent aldehyde addition, however, operates under Curtin-Hammett control via a more reactive α,α-bisboryl tertiary allylcopper species to furnish products with high enantioselectivities. The three boryl groups in the products are properly differentiated and can undergo a variety of chemoselective transformations to produce enantioenriched homoallylic alcohols with a stereodefined trisubstituted alkene.

Development of α,α-Disubstituted Crotylboronate Reagents and Stereoselective Crotylation via Brønsted or Lewis Acid Catalysis

The development of α,α-disubstituted crotylboronate reagents is reported. Chiral Brønsted acid-catalyzed asymmetric aldehyde addition with the developed E-crotylboron reagent gave (E)-anti-1,2-oxaborinan-3-enes with excellent enantioselectivities and E-selectivities. With BF₃·OEt₂ catalysis, the stereoselectivity is reversed, and (Z)-δ-boryl-anti-homoallylic alcohols are obtained with excellent Z-selectivities from the same E-crotylboron reagent. The Z-crotylboron reagent also participates in BF₃·OEt₂-catalyzed crotylation to furnish (Z)-δ-boryl-syn-homoallylic alcohols with good Z-selectivities. DFT computations establish the origins of observed enantio- and stereoselectivities of chiral Brønsted acid-catalyzed asymmetric allylation. Stereochemical models for BF₃·OEt₂-catalyzed reactions are proposed to rationalize the Z-selective allyl additions. These reactions generate highly valuable homoallylic alcohol products with a stereodefined trisubstituted alkene unit. The synthetic utility is further demonstrated by the total syntheses of salinipyrones A and B.

Enantioselective Syntheses of (Z)-6'-Boryl-anti-1,2-oxaborinan-3-enes via a Dienylboronate Protoboration and Asymmetric Allylation Reaction Sequence

The enantioselective synthesis of 6'-boryl-anti-1,2-oxaborinan-3-enes is reported. A Cu-catalyzed highly stereoselective 1,4-protoboration of 1,1-bisboryl-1,3-butadiene is developed to generate (E)-α,δ-bisboryl-crotylboronate. The chiral phosphoric-acid-catalyzed asymmetric allylboration of aldehydes with the boron reagent produces 6'-boryl-anti-1,2-oxaborinan-3-enes with excellent Z-selectivities and enantioselectivities. The product contains a vinyl and alkyl boronate unit that can directly participate in a variety of subsequent transformations.

Catalytic Enantioselective Synthesis of anti-Vicinal Silylboronates by Conjunctive Cross-Coupling

Chiral 1,2-bimetallic reagents are useful motifs in synthetic chemistry. Although syn-1,2-bimetallic compounds can be prepared by alkene dimetallation, anti-1,2-bimetallics are still rare. The stereospecific 1,2-metallate shift that occurs during conjunctive cross-coupling is shown to enable a practical and modular approach to the catalytic synthesis of enantioenriched anti-1,2-borosilanes. In addition to reaction development, the synthetic utility of anti-1,2-borosilanes was investigated, including applications to the synthesis of anti-1,2-diols and anti-1,2-amino alcohols.

Ligand-controlled diastereodivergent, enantio- and regioselective copper-catalyzed hydroxyalkylboration of 1,3-dienes with ketones

A copper-catalyzed three-component coupling of 1,3-dienes, bis(pinacolato)diboron, and ketones allows for the chemo-, regio-, diastereo- and enantioselective assembly of densely functionalized tertiary homoallylic alcohols. The relative configuration of the vicinal stereocenters is controlled by the chiral ligand employed. Subsequent transformations illustrate the versatility of these valuable chiral building blocks.