Asymmetric Organocatalytic Synthesis of Fluorinated β-Hydroxy Diazenes

Direct Deprotonative Functionalization of α,α-Difluoromethyl Ketones using a Catalytic Organosuperbase

The deprotonative functionalization of α,α-difluoromethyl ketones is described herein. Using a catalytic organosuperbase and a silane additive, the corresponding difluoroenolate could be generated and trapped with aldehydes to deliver various α,α-difluoro-β-hydroxy ketones in high yields. This new strategy tolerates numerous functional groups and represents the access to the difluoroenolate by direct deprotonation of the difluoromethyl unit. The diastereoselective version of the reaction was also investigated with d.r. up to 93 : 7. Several transformations were performed to demonstrate the synthetic potential of these α,α-difluoro-β-hydroxy ketones. In addition, this method has been extended to the use of other electrophiles such as imines and chalcogen derivatives, and a difluoromethyl sulfoxide as nucleophile, thus leading to a diversity of difluoromethylene compounds.

Brønsted acid catalyzed enantioselective addition of hydrazones to 3-indolylmethanols

A metal-free synthesis of enantiopure β-substituted tryptophan derivatives was developed. A chiral Brønsted acid enabled the addition of donor-substituted hydrazones to 3-indolylmethanols in excellent yields and enantioselectivities.

Catalytic Asymmetric Construction of Tertiary Carbon Centers Featuring an α-Difluoromethyl Group with CF2H-CH2-NH2 as the "Building Block"

We report here for the first time a novel difluoromethylated ketimine building block condensed by thioisatin and difluoroethylamine, offering efficient access to a broad range of enantioenriched products bearing difluoroethylamine units (27 examples, ≤98% yield, >99% ee) in the presence of quinine-derived squaramide. Further transformation of the intermediate would generate a variety of versatile functional blocks like α-difluoromethyl amines, β-amino acid, and β-diamine with retention of the enantiomeric excess at the difluoromethyl-bound carbon.

Room Temperature Ionic Liquids in Asymmetric Hetero-Ene Type Reactions: Improving Organocatalyst Performance at Lower Temperatures

Room temperature ionic liquids (RTILs) have been widely used as (co)solvents in several catalytic processes modifying, in most of the cases, the catalyst activity and/or the selectivity for the studied reactions. However, there are just a few examples of their use in hydrogen bonding organocatalysis. In this paper, we show the positive effect of a set of imidazole-based ionic liquids ([bmim]BF₄ and [hmim]PF₆) in the enantioselective addition of formaldehyde tert-butylhydrazone to prochiral α-keto esters catalyzed by a sugar-based chiral thiourea. Reactions performed in the presence of low percentages of RTILs led to an increase of the catalyst activity, thereby making possible to work at lower temperatures. Thus, the chiral tert-butyl azomethyl tertiary alcohols could be obtained with moderate to good conversions and higher enantioselectivities for most of the studied substrates when using up to 30 vol% of [hmim]PF₆ as a cosolvent in processes performed in toluene.

Stereoselective synthetic strategies of stereogenic carbon centers featuring a difluoromethyl group

The scope of this review is to summarize routine asymmetric synthetic methods which enable the effective and selective introduction of difluoromethyl groups into the desired compounds, providing a general introduction to this important research area.

α-Keto hydrazones in asymmetric aminocatalysis: reactivity through β-amino aza-dienamine intermediates

α-keto hydrazones activated by a bifunctional amino-thiourea catalyst generate β-amino aza-dienamine intermediates which react with nitroalkenes in highly enantioselective fashion.

Formaldehyde tert-butyl hydrazone as a formyl anion equivalent: asymmetric addition to carbonyl compounds

The asymmetric 1,2-addition of formyl anion equivalents to carbonyl compounds is a powerful synthetic tool that ideally provide access to highly functionalizable α-hydroxy aldehydes in an enantioselective fashion. In this context, the nucleophilic character of formaldehyde hydrazones, together with their remarkable stability as monomeric species, has been exploited for the functionalization of diverse carbonyl compounds, using initially auxiliary-based methodologies and, more recently, catalytic enantioselective versions. This feature article highlights our research progress employing formaldehyde tert-butyl hydrazone as a versatile formyl anion equivalent, in combination with bifunctional H-bonding organocatalysis. The design and optimization of different catalytic systems, focusing on a dual activation of both reagents, is reviewed, as well as the racemization free unmasking of the formyl group and representative product transformations for the construction of valuable, densely functionalyzed chiral building blocks.

Photoredox-Catalyzed Synthesis of Alkylaryldiazenes: Formal Deformylative C-N Bond Formation with Alkyl Radicals

Diazenes are valuable compounds that have found broad applicability because of their optical and biological properties. We report the synthesis of alkylaryldiazenes via formal, photoredox-catalyzed, deformylative C-N bond formation. The procedure employs dihydropyridines for the generation of alkyl radicals, which are then trapped by diazonium salts and reduced to the corresponding diazenes. Control experiments were performed to confirm the involvement of radicals in the mechanism. The reaction can be carried out at room temperature and employs readily available reagents; the mild conditions allowed the use of highly functionalized substrates. There was no observed tautomerization of the diazenes to the corresponding arylhydrazones.

Catalytic Enantioselective Addition of an Allyl Group to Ketones Containing a Tri-, a Di-, or a Monohalomethyl Moiety. Stereochemical Control Based on Distinctive Electronic and Steric Attributes of C-Cl, C-Br, and C-F Bonds

We disclose the results of an investigation designed to generate insight regarding the differences in the electronic and steric attributes of C-F, C-Cl, and C-Br bonds. Mechanistic insight has been gleaned by analysis of variations in enantioselectivity, regarding the ability of electrostatic contact between a halomethyl moiety and a catalyst's ammonium group as opposed to factors lowering steric repulsion and/or dipole minimization. In the process, catalytic and enantioselective methods have been developed for transforming a wide range of trihalomethyl (halogen = Cl or Br), dihalomethyl, or monohalomethyl (halogen = F, Cl, or Br) ketones to the corresponding tertiary homoallylic alcohols. By exploiting electrostatic attraction between a halomethyl moiety and the catalyst's ammonium moiety and steric factors, high enantioselectivity was attained in many instances. Reactions can be performed with 0.5-5.0 mol % of an in situ generated boryl-ammonium catalyst, affording products in 42-99% yield and up to >99:1 enantiomeric ratio. Not only are there no existing protocols for accessing the great majority of the resulting products enantioselectively but also in some cases there are hardly any instances of a catalytic enantioselective addition of a carbon-based nucleophile (e.g., one enzyme-catalyzed aldol addition involving trichloromethyl ketones, and none with dichloromethyl, tribromomethyl, or dibromomethyl ketones). The approach is scalable and offers an expeditious route to the enantioselective synthesis of versatile and otherwise difficult to access aldehydes that bear an α-halo-substituted quaternary carbon stereogenic center as well as an assortment of 2,2-disubstituted epoxides that contain an easily modifiable alkene. Tertiary homoallylic alcohols containing a triazole and a halomethyl moiety, structural units relevant to drug development, may also be accessed efficiently with exceptional enantioselectivity.

Catalytic Asymmetric Synthesis of α,α-Difluoromethylated and α-Fluoromethylated Tertiary Alcohols

The catalytic asymmetric synthesis of α,α-difluoromethylated tertiary alcohols is described, using an asymmetric dihydroxylation reaction. This protocol using either the AD-mix-α or AD-mix-β allowed an easy access to these valuable fluorinated chiral building blocks, which have been obtained with excellent yields and er. In addition, the reaction was extended to the α-fluoromethylated analogues.