Catalytic Enantioselective Synthesis of Allylic Boronates Bearing a Trisubstituted Alkenyl Fluoride and Related Derivatives

Electrochemical Synthesis of gem-Difluoro- and γ-Fluoro-Allyl Boronates and Silanes

The electrochemical synthesis of fluorinated allyl silanes and boronates was disclosed. The addition of electrogenerated boryl or silyl radicals onto many α-trifluoromethyl or α-difluoromethylstyrenes in an undivided cell allowed the formation of a large panel of synthetically useful gem-difluoro and γ-fluoroallyl boronates and silanes (64 examples, from 31 % to 95 % yield). In addition, a scale up of the reactions under continuous flow was showcased using an electrochemical reactor with promising volumetric productivity (688 g.L-1 .h-1 and 496 g.L-1 .h-1 ). Moreover, the synthetic utility of these building blocks was highlighted through versatile transformations. Finally, plausible reaction mechanisms were suggested to explain the formation of the products.

Nickel-Catalyzed Stereo- and Enantioselective Cross-Coupling of gem-Difluoroalkenes with Carbon Electrophiles by C-F Bond Activation

Stereo- and enantioselective cross-electrophile coupling involving C-F bond activation is reported. Treatment of gem-difluoroalkenes with racemic benzyl electrophiles in the presence of a chiral nickel complex using B₂ pin₂ as a stoichiometric reductant allows the construction of a C(sp² )-C(sp³ ) bond under mild conditions, affording a broad range of monofluoroalkenes bearing stereogenic allylic centers. Initial mechanistic studies indicate that a radical chain pathway may be operating, wherein the ester group in the gem-difluoroalkene promotes C-F bond activation through oxidative addition to a Ni species.

gem-Difluoroallylation of Aryl Halides and Pseudo Halides with Difluoroallylboron Reagents in High Regioselectivity

We report the palladium-catalyzed gem-difluoroallylation of aryl halides and pseudo halides with 3,3-difluoroallyl boronates in high yield with high regioselectivity, and we report the preparation of the 3,3-difluoroallyl boronate reactants by a copper-catalyzed defluorinative borylation of inexpensive gaseous 3,3,3-trifluoropropene with bis(pinacolato)diboron. The gem-difluoroallylation of aryl and heteroaryl bromides proceeds with low catalyst loading (0.1 mol % [Pd]) and tolerates a wide range of functional groups, including primary alcohols, secondary amines, ethers, ketones, esters, amides, aldehydes, nitriles, halides, and nitro groups. This protocol extends to aryl iodides, chlorides, and triflates, as well as substituted difluoroallyl boronates, providing a versatile synthesis of gem-difluoroallyl arenes that we show to be valuable intermediates to a series of fluorinated building blocks.

Enantioselective Synthesis of Acyclic Stereotriads Featuring Fluorinated Tetrasubstituted Stereocenters

Elaboration of enantioenriched complex acyclic stereotriads represents a challenge for modern synthesis even more when fluorinated tetrasubstituted stereocenters are targeted. We have been able to develop a simple strategy in a sequence of two unprecedented steps combining a diastereoselective aldol-Tishchenko reaction and an enantioselective organocatalyzed kinetic resolution. The aldol-Tishchenko reaction directly generates a large panel of acyclic 1,3-diols possessing a fluorinated tetrasubstituted stereocenter by condensation of fluorinated ketones with aldehydes under very mild basic conditions. The anti 1,3-diols featuring three contiguous stereogenic centers are generated with excellent diastereocontrol (typically >99 : 1 dr). Depending upon the precursors both diastereomers of stereotriads are accessible through this flexible reaction. Furthermore, from the obtained racemic scaffolds, development of an organocatalyzed kinetic resolution enabled to generate the desired enantioenriched stereotriads with excellent selectivity (typically er >95 : 5).

Asymmetric Synthesis of Fluorinated Allenes by Rhodium-Catalyzed Enantioselective Alkylation/Defluorination of Propargyl Difluorides with Alkylzincs

The reaction of propargyl difluorides R¹ CF₂ C≡CR² with alkylzincs R³ ZnCl giving axially chiral fluorinated allenes R¹ FC=C=CR² R³ with high enantioselectivity (up to 99 % ee) was found to be catalyzed by a chiral diene/rhodium complex. A key step in the catalytic cycle is selective elimination of one of the enantiotopic fluorides at the β-position of an alkenyl-Rh intermediate, which is generated by regioselective addition of R³ -Rh onto the triple bond of the starting difluorides.

Stereospecific Reactions Leading to Allylboronic Esters Within Acyclic Systems Bearing Distant Stereocenters

The preparation of acyclic molecules featuring congested stereocenters in a 1,4-relationship in only three catalytic steps from commercially available building blocks is reported. This approach involves a diastereoselective diboration of alkenyl cyclopropyl methanol derivatives followed by a regioselective exergonic ring fragmentation. The starting materials can be prepared enantiomerically enriched and all substituents can be interconverted, therefore, this strategy allows a large variety of diversely functionalized allylboronic esters possessing distant tetrasubstituted stereocenters with high diastereoselectivity.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

Electron transfer in complexes of BII cations with organic π-acceptors: a combined experimental and quantum-chemical study

Due to their combined Lewis acidity and electron-donor capability, B^II cations exhibit an interesting reactivity, which is almost unexplored so far. In this work, we compare the reduction in a dicationic diborane of a series of vicinal diones with different redox potentials, namely 3,5-di-tert-butylbenzoquinone, 3,4,5,6-tetrachlorobenzoquinone, 1,2-naphthalene-dione, 9,10-phenanthrene-dione, 2,2'-dichlorobenzil, benzil and 1,2-acenaphthylene-dione. The experimental work is complemented by quantum-chemical calculations, illuminating the electron-transfer step in the reactions.