Fluorinated Sulfinates as Source of Alkyl Radicals in the Photo-Enantiocontrolled ß-Functionalization of Enals.

The generation of sulfonyl radicals has long been known as a flexible strategy in a wide range of different sulfonylative transformations. Meanwhile their use in alkylation processes has been somehow limited due to their inherent difficulty in evolving to less-stable radicals after sulfur dioxide extrusion. Herein we report a convenient strategy that involves gem-difluorinated sulfinates as an "upgrading-mask", allowing these precursors to decompose into their corresponding alkyl radicals. The electron-donor character of sulfinates in the formation of an electron donor-acceptor (EDA) complex with transient iminium ions is displayed, achieving the first example of a stereocontrolled light-driven insertion of gem-difluoro derivatives into unsaturated aldehydes. This methodology is compatible with flow conditions, maintaining identical levels of enantiocontrol.

Recent Visible Light and Metal Free Strategies in [2+2] and [4+2] Photocycloadditions.

When aiming to synthesize molecules with elevated molecular complexity starting from relatively simple starting materials, photochemical transformations represent an open avenue to circumvent analogous multistep procedures. Specifically, light-mediated cycloadditions remain as powerful tools to generate new bonds begotten from non-very intuitive disconnections, that alternative thermal protocols would not offer. In response to the current trend in both industrial and academic research pointing towards green and sustainable processes, several strategies that meet these requirements are currently available in the literature. This Minireview summarizes [2+2] and [4+2] photocycloadditions that do not require the use of metal photocatalysts by means of alternative strategies. It is segmented according to the cycloaddition type in order to give the reader a friendly approach and we primarily focus on the most recent developments in the field carried out using visible light, a general overview of the mechanism in each case is offered as well.

Light-Driven Enantioselective Synthesis of Pyrroline Derivatives by a Radical/Polar Cascade Reaction.

Herein, a light-driven, atom-economical process that provides access to enantiomerically enriched substituted chiral 1-pyrroline derivatives is introduced. The strategy involves the distal functionalization of acyl heterocycles through a hydrogen-atom transfer (HAT) process and the use of tailor-made ketimines as reliable electrophilic partners. This transformation is translated into an enantiomerically controlled radical/polar cascade reaction in which water is produced as the sole by-product and stereoselectivity is dictated by coordination to a chiral-at-rhodium catalyst.

Asymmetric trifluoromethylthiolation of azlactones under chiral phase transfer catalysis.

The first enantioselective method for the installation of the SCF3 group at the C-4 position of azlactones is described in the present communication under quinidinium phase transfer catalysis. The higher performance of substrates containing electron-rich 2-aryl groups at the azlactone was rationalized using DFT calculations.

Stereocontrolled Addition of Scrambling ortho-Sulfinyl Carbanions: Easy Access to Homopropargylamines and α-Allenylamines.

An unprecedented behavior of ortho-sulfinylpropargyl carbanions in the presence of optically active sulfinylimines affords two different families of compounds: this peculiar chemodivergency is importantly affected by the nature of the employed base, and assisted by the configuration of the electrophile, displaying no alteration in the stereocontrol of both reactions. α-Allenylamines are formed exclusively, using R-sulfinyl aldimines as electrophiles, while homopropargylamines result when S-sulfinyl aldimines are employed.

Asymmetric Synthesis of α-Trifluoromethylthio-ß-Amino Acids under Phase Transfer Catalysis.

The first asymmetric α-trifluoromethylthiolation of 2-substituted isoxazolidin-5-ones was developed using Maruoka type N-spiro ammonium catalysts under phase-transfer conditions. The resulting products, containing a trifluoromethylthiolated quaternary chiral carbon, were obtained in moderate to good yields and up to 98:2 enantiomeric ratio. Moreover, the easy N-O bond cleavage provided access to undescribed α-trifluoromethylthio-ß2,2-amino acids, with promising applications in biochemistry and medicinal chemistry.

Asymmetric [2,3]-Wittig Rearrangement: Synthesis of Homoallylic, Allenylic, and Enynyl α-Benzyl Alcohols.

A highly stereoselective [2,3]-Wittig rearrangement of allylic and propargylic ethers controlled by a chiral sulfoxide moiety is presented. The activation provided by the sulfoxide at the remote ortho position allows the rearrangement of less-activated and unexplored benzylic carbanions. Thus, this general methodology gives access to the asymmetric synthesis of homoallylic, enynyl, and allenylic α-benzyl alcohol derivatives.

Asymmetric Synthesis of Secondary and Tertiary Propargylic Alcohols by Umpolung of Acetylenic Sulfones and ortho-Sulfinyl Carbanions.

The generation of diastereomerically enriched secondary benzyl propargyl alcohols by the asymmetric addition of ortho-sulfinylbenzyl carbanions to sulfonylacetylene derivatives via formation of a Csp-Csp3 bond is described. This reaction proceeds through an unusual α-attack (anti-Michael addition) of the ortho-sulfinylbenzyl carbanions, followed by elimination of the arylsulfonyl moiety. The scope of this alkynylation reaction is also discussed. Moreover, the development of a new approach for the synthesis of optically active tertiary benzylpropargyl alcohols is described, discussing the possible stereocourse of the reaction so as the influence of the ether 18-crown-6 and steric importance of acetylenic substituent.