Katalytische stereoselektive Dihalogenierung von Alkenen: Herausforderungen und Chancen

Catalytic Site-, Diastereo-, and Enantioselective Cascade Iodocyclization of 2-Geranylarenols

A chiral amidophosphate-N-iodosuccinimide cooperative catalysis has been developed for the site-, diastereo-, and enantioselective iodocyclization of 2-geranylarenols with molecular iodine to give the corresponding iodo-containing polycyclic compounds with good levels of selectivity. This is the first example of a catalytic enantioselective iodocarbocyclization. A reactive chiral iodonium species is generated from molecular iodine via the dual halogen-bonding interactions with a chiral Lewis base and Lewis acid. The sterically demanding 3,3'-substituents of the chiral BINOL-derived amidophosphate are critical to induce the site-selective iodination at the less-hindered terminal alkenyl moiety of 2-geranylarenols.

Brønsted Acid versus Phase-Transfer Catalysis in the Enantioselective Transannular Aminohalogenation of Enesultams

We have studied the enantioselective transannular aminohalogenation reaction of unsaturated medium-sized cyclic benzosulfonamides by using both chiral Brønsted acid and phase-transfer catalysis. Under optimized conditions, a variety of bicyclic adducts can be obtained with good yields and high enantioselectivities. The mechanism of the reaction was also studied by using computational tools; we observed that the reaction involves the participation of a conformer of the nine-membered cyclic substrate with planar chirality in which the stereochemical outcome is controlled by the relative reactivity of the two pseudorotational enantiomers when interacting with the chiral catalyst.

Regio- and Enantioselective Intermolecular Aminofluorination of Alkenes via Iodine(I)/Iodine(III) Catalysis

The regio- and enantio-selective, intermolecular vicinal fluoroamination of α-trifluoromethyl styrenes has been achieved by enantioselective II /IIII catalysis. Leveraging C2 -symmetric resorcinol-based aryl iodide catalysts, it has been possible to intercept the transient iodonium intermediate using simple nitriles, which function as both the solvent and nucleophile. In situ Ritter reaction provides direct access to the corresponding amides (up to 89 % yield, e.r. 93 : 7). This main group catalysis paradigm inverts the intrinsic regioselectivity of the uncatalyzed process, thereby providing facile access to tertiary, benzylic stereocenters bearing both CF3 and F groups. Privileged phenethylamine pharmacophores can be generated in which there is complete local partial charge inversion (CF3δ- /Fδ- versus CH3δ+ /Hδ+ ). Crystallographic analyses of representative β-fluoroamide products reveal highly pre-organized conformations that manifest the stereoelectronic gauche effect.

Stereocontrolled Synthesis of Fluorinated Isochromans via Iodine(I)/Iodine(III) Catalysis

The success of saturated, fluorinated heterocycles in contemporary drug discovery provides a stimulus for creative endeavor in main group catalysis. Motivated by the ubiquity of isochromans across the bioactive small molecule spectrum, the prominence of the anomeric effect in regulating conformation, and the metabolic lability of the benzylic position, iodine(I)/iodine(III) catalysis has been leveraged for the stereocontrolled generation of selectively fluorinated analogs. To augment the current arsenal of fluorocyclization reactions involving carboxylic acid derivatives, the reaction of readily accessible 2-vinyl benzaldehydes is disclosed (up to >95 : 05 d.r. and 97 : 03 e.r.). Key stereoelectronic interactions manifest themselves in the X-ray crystal structures of the products, thereby validating the [CH2 -CHF] fragment as a stereoelectronic mimic of the [O-CH(OR)] acetal motif.

The Electrochemical cis-Chlorination of Alkenes

The first example for the electrochemical cis-dichlorination of alkenes is presented. The reaction can be performed with little experimental effort by using phenylselenyl chloride as catalyst and tetrabutylammoniumchloride as supporting electrolyte, which also acts as nucleophilic reagent for the SN 2-type replacement of selenium versus chloride. Cyclic voltammetric measurements and control experiments revealed a dual role of phenylselenyl chloride in the reaction. Based on these results a reaction mechanism was postulated, where the key step of the process is the activation of a phenylselenyl chloride-alkene adduct by electrochemically generated phenylselenyl trichloride. Like this, different aliphatic and aromatic cyclic and acyclic alkenes were converted to the dichlorinated products. Thereby, throughout high diastereoselectivities were achieved for the cis-chlorinated compounds of >95 : 5 or higher.

Enantioselective Synthesis of 3-Fluorochromanes via Iodine(I)/Iodine(III) Catalysis

The chromane nucleus is common to a plenum of bioactive small molecules where it is frequently oxidized at position 3. Motivated by the importance of this position in conferring efficacy, and the prominence of bioisosterism in drug discovery, an iodine(I)/iodine(III) catalysis strategy to access enantioenriched 3-fluorochromanes is disclosed (up to 7:93 e.r.). In situ generation of ArIF2 enables the direct fluorocyclization of allyl phenyl ethers to generate novel scaffolds that manifest the stereoelectronic gauche effect. Mechanistic interrogation using deuterated probes confirms a stereospecific process consistent with a type IIinv pathway.

Organocatalytic, Enantioselective Dichlorination of Unfunctionalized Alkenes

The use of a new class of unsymmetrical cinchona-alkaloid-based, phthalazine-bridged organocatalysts enabled the highly enantioselective dichlorination of unfunctionalized alkenes. In combination with the electrophilic chlorinating agent 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) and triethylsilyl chloride (TES-Cl) as the source of nucleophilic chloride, 1-aryl-2-alkyl alkenes were dichlorinated with enantioselectivities of up to 94:6 er. Initial mechanistic investigations suggest that no free chlorine is formed, and by replacement of the chloride by fluoride, enantioselective chlorofluorinations of alkenes are possible.

Chiral Sulfide Catalysis for Desymmetrizing Enantioselective Chlorination

An unprecendented chiral sulfide catalyzed desymmetrizing enantioselective chlorination is disclosed. Various aryl-tethered diolefins and diaryl-tethered olefins afforded teralins and tricyclic hexahydrophenalene derivatives, respectively, bearing multiple stereogenic centers in high yields with excellent enantio- and diastereoselectivities. In contrast, the tertiary amine catalyst (DHQD)₂ PHAL led to a diastereomeric product. The products could be transformed into a variety of compounds, such as spiro-N-heterocycles.

Multiple Halogenation of Aliphatic C-H Bonds within the Hofmann-Löffler Manifold

An innovative approach to position-selective polyhalogenation of aliphatic hydrocarbon bonds is presented. The reaction proceeded within the Hofmann-Löffler manifold with amidyl radicals as the sole mediators to induce selective 1,5- and 1,6-hydrogen-atom transfer followed by halogenation. Multiple halogenation events of up to four innate C-H bond functionalizations were accomplished. The broad applicability of this new entry into polyhalogenation and the resulting synthetic possibilities were demonstrated for a total of 27 different examples including mixed halogenations.

Enantioselective, Catalytic Vicinal Difluorination of Alkenes

The enantioselective, catalytic vicinal difluorination of alkenes is reported by I^I /I^III catalysis using a novel, C₂ -symmetric resorcinol derivative. Catalyst turnover via in situ generation of an ArI^III F₂ species is enabled by Selectfluor oxidation and addition of an inexpensive HF-amine complex. The HF:amine ratio employed in this process provides a handle for regioselective orthogonality as a function of Brønsted acidity. Selectivity reversal from the 1,1-difluorination pathway (geminal) to the desired 1,2-difluorination (vicinal) is disclosed (>20:1 in both directions). Validation with electron deficient styrenes facilitates generation of chiral bioisosteres of the venerable CF₃ unit that is pervasive in drug discovery (20 examples, up to 94:06 e.r.). An achiral variant of the reaction is also presented using p-TolI (up to >95 % yield).

Rhodium-Catalyzed Enantioselective Radical Addition of CX4 Reagents to Olefins

We describe the synthetically useful enantioselective addition of Br-CX₃ (X=Cl or Br) to terminal olefins to introduce a trihalomethyl group and generate optically active secondary bromides. Computational and experimental evidence supports an asymmetric atom-transfer radical addition (ATRA) mechanism in which the stereodetermining step involves outer-sphere bromine abstraction from a [(bisphosphine)Rh^II BrCl] complex by a benzylic radical intermediate. This mechanism appears unprecedented in asymmetric catalysis.