Photoredox Propargylation of Aldehydes Catalytic in Titanium

Catalyst-Free Propargylboration of Ketones with Allenyl-Bpins: Highly Stereoselective Synthesis of tert-Homopropargyl Alcohols Bearing Vicinal Stereocenters

A practical and efficient propargylboration of ketones is presented using general allenylboronic acid pinacol esters (allenyl-Bpins) without a catalyst. This reaction is triggered by in-situ activation of stable allenyl-Bpins through the sequential addition of 1.25 equiv. of n BuLi and the prerequisite 2.0 equiv. of TFAA. Under the optimized reaction conditions, the versatile trisubstituted allenyl-Bpins react with various ketones smoothly to afford a wide range of tert-homopropargyl alcohols bearing vicinal stereocenters in high yields with good to excellent diastereoselectivities. Furthermore, propargylboration of ketones with chiral trisubstituted allenyl-Bpins allows for the asymmetric synthesis of chiral tert-homopropargyl alcohols with a full chirality transfer.

Recent Advances in the Synthesis of Propargyl Derivatives, and Their Application as Synthetic Intermediates and Building Blocks

The propargyl group is a highly versatile moiety whose introduction into small-molecule building blocks opens up new synthetic pathways for further elaboration. The last decade has witnessed remarkable progress in both the synthesis of propargylation agents and their application in the synthesis and functionalization of more elaborate/complex building blocks and intermediates. The goal of this review is to highlight these exciting advances and to underscore their impact.

Visible-Light-Promoted Nickel-Catalyzed Cross-Coupling of Alkyltitanium Alkoxides with Aryl and Alkenyl Halides

Here, we report that alkyltitanium alkoxides generated in situ from Grignard reagents and Ti(OiPr)₄ undergo a photocatalyst-free nickel-catalyzed cross-coupling with organic halides upon irradiation with blue light. Mechanistic studies suggested that the reaction proceeds through radical intermediates formed by photochemical decomposition of the alkyltitanium reagents. Various aryl, heteroaryl, and vinyl halides were efficiently alkylated under the reported conditions, including those containing ester and amide groups.

A Catalytic Alkylation of Ketones via sp3 C-H Bond Activation

We identified a ternary hybrid catalyst system composed of an acridinium photoredox catalyst, a thiophosphoric imide (TPI) catalyst, and a titanium complex catalyst that promoted an intermolecular addition reaction of organic molecules with various ketones through sp³ C-H bond activation. The thiyl radical generated via single-electron oxidation of TPI by the excited photoredox catalyst abstracted a hydrogen atom from organic molecules such as toluene, benzyl alcohol, alkenes, aldehydes, and THF. The thus-generated carbon-centered radical species underwent addition to ketones and aldehydes. This intrinsically unfavorable step was promoted by single-electron reduction of the intermediate alkoxy radical by catalytically generated titanium(III) species. This reaction provided an efficient and straightforward route to a broad range of tertiary alcohols and was successfully applied to late-stage functionalization of drugs or their derivatives. The proposed mechanism was supported by both experimental and theoretical studies.

Diastereoselective and enantioselective photoredox pinacol coupling promoted by titanium complexes with a red-absorbing organic dye

The pinacol coupling reaction, a reductive coupling of carbonyl compounds that proceeds through the formation of ketyl radicals in the presence of an electron donor, affords the corresponding 1,2-diols in one single step. The photoredox version of this transformation has been accomplished using different organic dyes or photoactive metal complexes in the presence of sacrificial donors such as tertiary amines or Hantzsch's ester. Normally, the homo-coupling of such reactive ketyl radicals is neither diastereo- nor enantio-selective. Herein, we report a highly diastereoselective pinacol coupling reaction of aromatic aldehydes promoted by 5 mol% of the non-toxic, inexpensive and available Cp₂TiCl₂ complex. The key feature that allows the complete control of diastereoselectivity is the employment of a red-absorbing organic dye in the presence of a redox-active titanium complex. Taking advantage of the well-tailored photoredox potential of this organic dye, the selective reduction of Ti(iv) to Ti(iii) is achieved. These conditions enable the formation of the d,l (syn) diastereoisomer as the favored product of the pinacol coupling (d.r. > 20 : 1 in most of the cases). Moreover, employing a simply prepared chiral SalenTi complex, the new photoredox reaction gave a complete diastereoselection for the d,l diastereoisomer, and high enantiocontrol (up to 92% of enantiomeric excess).

A metallaphotoredox, diastereoselective and enantioselective pinacol coupling reaction promoted by titanium complexes with the use of a red-absorbing organic dye was developed.

Nickel Mediated Enantioselective Photoredox Allylation of Aldehydes with Visible Light

Here we report a practical, highly enantioselective photoredox allylation of aldehydes mediated by chiral nickel complexes with commercially available allyl acetate as the allylating agent. The methodology allows the clean stereoselective allylation of aldehydes in good to excellent yields and up to 93% e.e. using a catalytic amount of NiCl 2 (glyme) in the presence of the chiral aminoindanol-derived bis(oxazoline) as the chiral ligand. The photoredox system is constituted by the organic dye 3DPAFIPN and a Hantzsch's ester as the sacrificial reductant. The reaction proceeds under visible light irradiation (blue LEDs, 456 nm) at 8-12 °C with excellent stereoselectivities. Compared to other published procedures, no metal reductants (such as Zn or Mn), additives (e.g. CuI) or air-sensitive Ni(COD) 2 are necessary for this reaction. Accurate DFT calculations and photophysical experiments have clarified the mechanistic picture of this stereoselective allylation reaction showing a key role played by Hantzsch's ester for the turnover of the catalyst.

Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.