Enantioselective Cyclopropanation Research Articles

Facile Recovery and Recycling of a Soluble Dirhodium Catalyst in Asymmetric Cyclopropanation via a Catalyst-in-Bag System.

A catalyst-in-bag system facilitates the recovery and recycling of chiral dirhodium carboxylate catalysts used for enantioselective, intermolecular cyclopropanation. The catalyst-in-bag system incorporates a soluble enantioselective dirhodium complex catalyst within a reusable, commercial dialysis membrane. Dirhodium catalysts of different sizes are examined, and two catalysts with molecular weights above 2400 Da are well-retained by the membrane. The catalyst Rh2(S-TPPTTL)4 [TPPTTL = (1,3-dioxo-4,5,6,7-tetraphenylisoindolin-2-yl)-3,3-dimethylbutanoate] is explored in enantioselective cyclopropanation reactions under a variety of conditions. The Rh2(S-TPPTTL)4 catalyst, when contained in the catalyst-in-bag system, provides high yields and enantioselectivities, akin to the homogeneous catalyst in solution, with negligible rhodium permeation out of the bag over five catalytic cycles. The catalyst-in-bag approach facilitates recovery of the expensive rhodium metal and ligand, with only ppm level Rh detected in the reaction products. The flexible and expandable catalyst-in-bag system can be accommodated in vessels of different shapes and dimensions.

Asymmetric Dirhodium-Catalyzed Modification of Immunomodulatory Imide Drugs and Their Biological Assessment.

Cereblon (CRBN) has been successfully co-opted to affect the targeted degradation of "undruggable" proteins with immunomodulatory imide drugs (IMiDs). IMiDs act as molecule glues that facilitate ternary complex formation between CRBN and a target protein, leading to ubiquitination and proteasomal degradation. Subtle structural modifications often cause profound and sometimes unpredictable changes in the degradation selectivity. Herein, we successfully utilize enantioselective cyclopropanation and cyclopropenation on intact glutarimides to enable the preparation of stereochemically and regiochemically matched molecular pairs for structure-activity relationship (SAR) analysis across several classical CRBN neosubstrates. The resulting glutarimide analogs were found to reside in unique chemical space when compared to other IMiDs in the public domain. SAR studies revealed that, in addition to the more precedented impacts of regiochemistry, stereochemical modifications far from the glutarimide can lead to divergent neosubstrate selectivity. These findings emphasize the importance of enabling enantioselective methods for glutarimide-containing compounds to tune the degradation selectivity.

Rh-Catalyzed Enantioselective Single-Carbon Insertion of Alkenes.

The interest in the discovery and development of skeletal editing processes that selectively insert, exchange, or delete an atom in organic molecules has significantly increased over the last few years. However, processes of this class that proceed through the creation of a chiral center with high asymmetric induction have been largely unexplored. Herein, we report an enantioselective single-carbon insertion in aryl- and alkyl-substituted alkenes mediated by a catalytically generated chiral Rh-carbynoid and phosphate nucleophiles that produce enantioenriched allylic phosphates (enantiomeric ratio (e.r.) = 89.5:10.5-99.5:0.5). The key to the process was a diastereo- and enantioselective cyclopropanation of the alkene with a chiral Rh-carbynoid and the formation of a transient cyclopropyl-I(III) intermediate. The addition of the phosphate nucleophile provided a cyclopropyl-I(III)-phosphate intermediate that undergoes disrotatory ring opening following the Woodward-Hoffmann-DePuy rules. This process led to a chiral intimate allyl cation-phosphate pair that evolved with excellent enantioretention. The evidence of an SN1-like SNi mechanism is provided by linear free-energy relationship studies, kinetic isotope effects, X-ray crystallography, and control experiments. We demonstrated the utility of the enantioenriched allylic phosphates in late-stage N-H allylations of natural products and drug molecules and in cross-coupling reactions that occurred with excellent enantiospecificity.

Synthesis of fluorine-containing α-alkyl-α-diazoesters and their utility in enantioselective cyclopropanation

The synthesis and utilization of fluorine-containing α-alkyl-α-diazoesters has been reported. Access to fluoroalkyl cyclopropanes was achieved utilizing Rh2(S-PTTL)4 as a catalyst with good yields, enantioselectivities, and diastereoselectivities across a series of styrenes and α-alkyl-α-diazoesters. This study expands the accessibility of fluorine-containing building blocks for synthesis.

Chiral 4-MeO-Pyridine (MOPY) Catalyst for Enantioselective Cyclopropanation: Attenuation of Lewis Basicity Leads to Improved Catalytic Efficiency.

The design of pyridine-derived organocatalysts aims at the increase of their Lewis basicity, however such an approach is not always efficient. For example, strongly Lewis basic DMAP is completely inefficient as catalyst in the cyclopropanation reaction. Herein we disclose an alternative approach that relies on attenuation of DMAP Lewis basicity. Specifically, the replacement of 4-dimethylamino substituent in DMAP for 4-MeO group delivered a highly efficient catalyst for cyclopropanation of electron-deficient olefins with α-bromoketones. Kinetic studies provide compelling evidence that the superior catalytic efficiency of 4-MeO pyridine (MOPY) is to be attributed to the favorable balance between Lewis basicity and leaving group ability. The use of chiral, enantiomerically pure MOPY catalyst has helped to achieve high enantioselectivities (up to 91 : 9 er) in the previously unreported pyridine-catalyzed cyclopropanation reaction.

Nucleophilic Carbenes Derived from Dichloromethane

AbstractNickel PyBox catalysts promote nucleophilic cyclopropanation reactions using CH2Cl2 as a methylene source and Mn as a stoichiometric reductant. The substrate scope includes a broad range of alkenes bearing electron‐withdrawing substituents, including esters, amides, ketones, nitriles, sulfones, phosphonate esters, trifluoromethyl groups, and electron‐deficient arenes. Enantioselective cyclopropanations of α,β‐unsaturated esters have been developed using chiral PyBox ligands. Mechanistic studies suggest the intermediacy of a (PyBox)Ni=CH2 species, which adds to the alkene by a stepwise [2+2]‐cycloaddition/C−C reductive elimination mechanism. DFT models provide a rationale for the nucleophilic character of the nickel carbene and the sense of enantioinduction.

Nucleophilic Carbenes Derived from Dichloromethane.

Nickel PyBox catalysts promote nucleophilic cyclopropanation reactions using CH2 Cl2 as a methylene source and Mn as a stoichiometric reductant. The substrate scope includes a broad range of alkenes bearing electron-withdrawing substituents, including esters, amides, ketones, nitriles, sulfones, phosphonate esters, trifluoromethyl groups, and electron-deficient arenes. Enantioselective cyclopropanations of α,β-unsaturated esters have been developed using chiral PyBox ligands. Mechanistic studies suggest the intermediacy of a (PyBox)Ni=CH2 species, which adds to the alkene by a stepwise [2+2]-cycloaddition/C-C reductive elimination mechanism. DFT models provide a rationale for the nucleophilic character of the nickel carbene and the sense of enantioinduction.

Chromium-Catalyzed Enantioselective Cyclopropanation of Terminal Olefins with gem-Dihaloalkanes

Chromium-Catalyzed Enantioselective Cyclopropanation of Terminal Olefins with gem-Dihaloalkanes

Diruthenium Tetracarboxylate-Catalyzed Enantioselective Cyclopropanation with Aryldiazoacetates.

A series of chiral bowl-shaped diruthenium(II,III) tetracarboxylate catalysts were prepared and evaluated in asymmetric cyclopropanations with donor/acceptor carbenes derived from aryldiazoacetates. The diruthenium catalysts self-assembled to generate C4-symmetric bowl-shaped structures in an analogous manner to their dirhodium counterparts. The optimum catalyst was found to be Ru2(S-TPPTTL)4·BArF [S-TPPTTL = (S)-2-(1,3-dioxo-4,5,6,7-tetraphenylisoindolin-2-yl)-3,3-dimethylbutanoate, BArF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate], which resulted in the cyclopropanation of a range of substrates in up to 94% ee. Synthesis and evaluation of first-row transition-metal congeners [Cu(II/II) and Co(II/II)] invariably resulted in catalysts that afforded little to no asymmetric induction. Computational studies indicate that the carbene complexes of these dicopper and dicobalt complexes, unlike the dirhodium and diruthenium systems, are prone to the loss of carboxylate ligands, which would destroy the bowl-shaped structure critical for asymmetric induction.

Catalytic Diastereo- and Enantioselective Cyclopropanation of gem-Dihaloalkanes and Terminal Olefins.

Chiral cyclopropane derivatives are essential in synthetic chemistry and drug discovery. Their synthesis commonly relies on asymmetric cyclopropanation of diazo compounds, potentially explosive and needing stabilizing substituents. Thus, asymmetric catalytic transformations of non-stabilized carbenes or carbenoids remain a formidable challenge. Herein, we report the unprecedented chromium-catalyzed asymmetric cyclopropanation of readily available gem-dihaloalkanes and terminal olefins. Distinct from previous approaches, gem-dihaloalkanes serve as suitable precursors for non-stabilized carbenes or carbenoids, furnishing various functionalized chiral cyclopropanes. Mechanistic studies, including radical trapping, non-linear effect, and UV/Vis spectroscopy, provide insights into the catalytic process, featuring radical-polar crossover.

Catalytic Diastereo‐ and Enantioselective Cyclopropanation of gem‐Dihaloalkanes and Terminal Olefins

AbstractChiral cyclopropane derivatives are essential in synthetic chemistry and drug discovery. Their synthesis commonly relies on asymmetric cyclopropanation of diazo compounds, potentially explosive and needing stabilizing substituents. Thus, asymmetric catalytic transformations of non‐stabilized carbenes or carbenoids remain a formidable challenge. Herein, we report the unprecedented chromium‐catalyzed asymmetric cyclopropanation of readily available gem‐dihaloalkanes and terminal olefins. Distinct from previous approaches, gem‐dihaloalkanes serve as suitable precursors for non‐stabilized carbenes or carbenoids, furnishing various functionalized chiral cyclopropanes. Mechanistic studies, including radical trapping, non‐linear effect, and UV/Vis spectroscopy, provide insights into the catalytic process, featuring radical‐polar crossover.

Dirhodium(II)-catalyzed enantioselective cyclopropenation of internal alkynes with trifluoromethyl carbene

Catalytic enantioselective reactions of trifluoromethyl carbene represent a powerful method for the construction of chiral trifluoromethyl-functionalized molecules, but the utilization of less reactive internal alkynes remains a persistent challenge in this domain. Herein, we report a chiral dirhodium(II) complex-catalyzed enantioselective cyclopropenation reaction of disubstituted alkynes by using 2,2,2-trifluorodiazoethane (CF3CHN2) as the trifluoromethyl carbene precursor. Achiral axial ligation by solvent molecules proves to be a key to improve the reactivity and control the stereoselectivity of metallocarbene species. This transformation provides efficient access to a wide range of enantioenriched trifluoromethyl group-substituted cyclopropenes, cyclopropanes, fused cyclopropane derivatives, and cyclopropyl boronate. The presented methodology constitutes a practical solution to a long-standing hurdle in catalytic enantioselective cyclopropenation of trifluoromethyl carbene with internal alkynes.

Continuous Process to Safely Manufacture an Aryldiazoacetate and Its Direct Use in a Dirhodium-Catalyzed Enantioselective Cyclopropanation

Continuous Process to Safely Manufacture an Aryldiazoacetate and Its Direct Use in a Dirhodium-Catalyzed Enantioselective Cyclopropanation

Enantioselective Intramolecular Iridium-Catalyzed Cyclopropanation of α-Carbonyl Sulfoxonium Ylides.

Enantioselective cyclopropanation of α-carbonyl sulfoxonium ylides (SY) has so far been limited to addition/ring closure reactions on electron-poor olefins. Herein, we report the iridium-catalyzed intramolecular cyclopropanation of SY in the presence of a chiral diene in up to 96% yield and 98% enantioselectivity. Moreover, density functional theory calculations suggest that the re face of the olefin preferably attacks an iridium carbene intermediate in an asynchronous concerted step that is independent of the geometry of the olefin.

Enantioselective Synthesis of trans-Disubstituted Cyclopropyltrifluoroborate Building Blocks through Ru-Catalyzed Cyclopropanation

Enantioselective synthesis of trans-1,2-disubstituted cyclopropanes containing Lewis basic nitrogen functionalities remains a challenge to metal-catalyzed cyclopropanation reactions. Herein, we reported an alternative strategy to access similar products by a sequence of enantioselective cyclopropanation of potassium vinyltrifluoroborate, followed by cross-coupling of the cyclopropyl boronate product. The cyclopropanation process was readily performed in a 50 g scale from commercially available starting materials. An interesting enrichment of product enantiomeric purity by simple crystallization is also noted.

Enantioselective Cyclopropanation Catalyzed by Gold(I)-Carbene Complexes.

The formation of polysubstituted cyclopropane derivatives in the gold(I)-catalyzed reaction of olefins and propargylic esters is a potentially useful transformation to generate diversity, therefore any method in which its stereoselectivity could be controlled is of significant interest. We prepared and tested a series of chiral gold(I)-carbene complexes as a catalyst in this transformation. With a systematic optimization of the reaction conditions, we were able to achieve high enantioselectivity in the test reaction while the cis:trans selectivity of the transformation was independent of the catalyst. Using the optimized conditions, we reacted a series of various olefins and acetylene derivatives to find that, although the reactions proceeded smoothly and the products were usually isolated in good yield and with good to exclusive cis selectivity, the observed enantioselectivity varied greatly and was sometimes moderate at best. We were unable to establish any structure-property relationship, which suggests that for any given reagent combination, one has to identify individually the best catalyst.

Asymmetric Synthesis of Chiral Cyclopropanes from Sulfoxonium Ylides Catalyzed by a Chiral-at-Metal Rh(III) Complex.

An enantioselective cyclopropanation reaction of sulfoxonium ylides with β,γ-unsaturated ketoesters catalyzed by a chiral rhodium catalyst has been realized. A variety of optically pure 1,2,3-trisubstituted cyclopropanes was synthesized in 48-89% yields, with up to 99% ee, and with dr >20:1. Furthermore, research shows that a weak coordination between the chiral rhodium catalyst and β,γ-unsaturated ketoesters was responsible for the high diastereoselectivity and enantioselectivity of the corresponding products.

Asymmetric Total Synthesis of a Bioactive Lignanamide Using a 5‐endo‐tet‐Type Cyclization of Activated Cyclopropylcarbinols and Synthetic Support for the Reaction Mechanism

AbstractThe first enantioselective total synthesis of a bioactive lignanamide was achieved with high enantiomeric excess. Key synthetic steps include an organocatalytic enantioselective cyclopropanation and a Lewis‐acid‐mediated chirality‐transferring 5‐endo‐tet‐type cyclization that proceeds with a very high degree of stereoinduction. The proposed mechanism of the key reaction is supported by the experimental results. Based on these experimental results, the 5‐endo‐tet‐type cyclization of a cyclopropylcarbinol proceeds predominantly via an SN1 mechanism with high trans‐selectivity, which arises from the steric hindrance of the neighboring substituent. Minor pathways include the anchimeric participation of (i) the oxygen atom of the benzoyl group in an SN2 mechanism and/or (ii) a benzene‐coordinated transition state in an SN1‐like mechanism.

Catalytic Asymmetric Syntheses of Alkylidenecyclopropanes from Allenoates with Donor-Acceptor and Diacceptor Diazo Reagents.

The first diastereo- and enantioselective cyclopropanation reactions of electron-deficient allenes with donor-acceptor and diacceptor diazo reagents are described. The desired enantioenriched alkylidenecyclopropanes (ACPs) were obtained in high yields with high diastereo- and enantioselectivities in the presence of Rh2 ((S)-TCPTAD)4 or Rh2 ((R)-BTPCP)4 catalysts (up to 95 % yield, >95 : 5 d.r. and 99 : 1 e.r.). This methodology gave a direct access to ACPs bearing multiple electron-deficient substituents and allows to further expand the availability of ACPs chemistry. Interestingly, during the examination of the scope of this reaction, the asymmetric intramolecular C-H insertion reaction into tert-butyl group was observed as a side reaction with up to 94 : 6 e.r.

Catalytic Transformations of 2-Pyridones by Rhodium-Mediated Carbene Transfer.

An enantioselective cyclopropanation reaction of N-substituted 2-pyridones with diazo compounds has been realized by using a chiral rhodium complex as the catalyst, and the corresponding chiral cyclopropanes could be formed in good yields with high enantioselectivities. Moreover, using acceptor-acceptor dimethyl 2-diazomalonate as the carbene precursor, a novel 1,4-rearrangement of a Boc group from N to C has also been discovered under rhodium catalysis.