Stereoselective Synthesis of Either Exo- or Endo-3-Azabicyclo[3.1.0]hexane-6-carboxylates by Dirhodium(II)-Catalyzed Cyclopropanation with Ethyl Diazoacetate under Low Catalyst Loadings

Although cyclopropanation with donor/acceptor carbenes can be conducted under low catalyst loadings (<0.001 mol %), such low loading has not been generally effective for other classes of carbenes such as acceptor carbenes. In this current study, we demonstrate that ethyl diazoacetate can be effectively used in the cyclopropanation of N-Boc-2,5-dihydropyrrole with dirhodium(II) catalyst loadings of 0.005 mol %. By appropriate choice of catalyst and hydrolysis conditions, either the exo- or endo-3-azabicyclo[3.1.0]hexanes can be formed cleanly with high levels of diastereoselectivity with no chromatographic purification.

In-situ Kinetic Studies of Rh(II)-Catalyzed C-H Functionalization to Achieve High Catalyst Turnover Numbers

Detailed kinetic studies on the functionalization of unactivated hydrocarbon sp3 C-H bonds by dirhodium-catalyzed reaction of aryldiazoacetates revealed that the C-H functionalization step is rate-determining. The efficiency of this step was increased by using the hydrocarbon as solvent and using donor/acceptor carbenes with an electron-withdrawing substituent on the aryl donor group. The optimum catalyst for these reactions is the tetraphenylphthalimido derivative Rh2(R-TPPTTL)4 and a further beneficial refinement was obtained by using N,N'-dicyclohexylcarbodiimide as an additive. Under the optimum conditions with a catalyst loading of 0.001 mol %, effective enantioselective C-H functionalization (66-97% yield, 83-97% ee) was achieved of cycloalkanes with a range of aryldiazoacetates as long as the aryldiazoacetate was not to sterically demanding. The reaction with cyclohexane using a catalyst loading of 0.0005 mol % could be recharged twice with additional aryldiazoacetate, resulting in an overall dirhodium catalyst turnover number of 580,000.

Tuning Rh(II)-catalysed cyclopropanation with tethered thioether ligands

Dirhodium(ii) paddlewheel complexes have high utility in diazo-mediated cyclopropanation reactions and ethyl diazoacetate is one of the most commonly used diazo compounds in this reaction. In this study, we report our efforts to use tethered thioether ligands to tune the reactivity of RhII-carbene mediated cyclopropanation of olefins with ethyl diazoacetate. Microwave methods enabled the synthesis of a family of RhII complexes in which tethered thioether moieties were coordinated to axial sites of the complex. Different tether lengths and thioether substituents were screened to optimise cyclopropane yields and minimise side product formation. Furthermore, good yields were obtained when equimolar diazo and olefin were used. Structural and spectroscopic investigation revealed that tethered thioethers changed the electronic structure of the rhodium core, which was instrumental in the performance of the catalysts. Computational modelling of the catalysts provided further support that the tethered thioethers were responsible for increased yields.

Donor-Acceptor-Acceptor 1,3-Bisdiazo Compounds: An Exploration of Synthesis and Stepwise Reactivity

Metal carbenes, derived from the decomposition of diazo compounds, are valued for their capacity to perform a variety of transformations. A unique class of acyclic, bis-diazo compounds, the donor-acceptor-acceptor 1,3-bisdiazo compounds, are described herein. These compounds are available from acyclic β-keto esters and especially reactive at the donor-acceptor diazo unit. These bisdiazo compounds react smoothly with rhodium acetate and alcohols to give monodiazo, cyclic orthoesters, presumably through the capture of a transient oxonium ylide.

Finding Opportunities from Surprises and Failures. Development of Rhodium-Stabilized Donor/Acceptor Carbenes and Their Application to Catalyst-Controlled C-H Functionalization

Catalyst-controlled C-H functionalization by means of the C-H insertion chemistry of rhodium carbenes has become a powerful synthetic method. The key requirements for the development of this chemistry are donor/acceptor carbenes and the chiral dirhodium tetracarboxylate catalysts. This perspective will describe the stages involved in developing this chemistry and illustrate the scope of the donor/acceptor carbene C-H functionalization.

Blue light-promoted photolysis of aryldiazoacetates

Aryldiazoacetates can undergo photolysis under blue light irradiation (460-490 nm) at room temperature and under air in the presence of numerous trapping agents, such as styrene, carboxylic acids, amines, alkanes and arenes, thus providing a straighforward and general platform for their mild functionalization.

Collective Approach to Advancing C-H Functionalization

C-H functionalization is a very active research field that has attracted the interest of scientists from many disciplines. This Outlook describes the collaborative efforts within the NSF CCI Center for Selective C-H Functionalization (CCHF) to develop catalyst-controlled selective methods to enhance the synthetic potential of C-H functionalization.

Rh(II)-Catalyzed Cyclopropanation of Furans and Its Application to the Total Synthesis of Natural Product Derivatives

Rh(II)-catalyzed enantioselective cyclopropanations of furans, providing access to synthetically useful building blocks, are reported. After screening of 10 Rh(II) catalysts, Rh₂(S-TCPTTL)₄ was identified as a highly efficient and selective catalyst (up to 98% ee, TON 88000, and TOF 24/s) for the cyclopropanation of furans. These cyclopropanes were successfully applied to the enantioselective synthesis of novel paraconic acid derivatives.

Polycyclic Ring Formation Using Bis-diazolactams for Cascade Stitching

The chemoselective reaction of donor/acceptor (D/A) and acceptor/acceptor (A/A) diazo moieties in the same molecule was examined using 3-diazo-1-(ethyl 2-diazomalonyl)indolin-2-one under rhodium(II) catalysis. The metallo carbenoid derived from the D/A diazo group is preferentially formed and undergoes selective CH, NH, and OH insertion reactions, cyclopropanation, cyclopropenation, sulfur ylide formation/2,3-sigmatropic rearrangement, as well as nitrogen ylide formation followed by azetidine ring expansion. The initial reaction can be paired with a subsequent tandem cascade sequence involving dipole formation/cycloaddition in either an intra- or intermolecular sense to generate polycyclic N-heterocycles in one pot, with the formation up to three new rings in a single operation. Excellent diastereoselectivity was observed in the intramolecular cycloaddition reaction producing 5 to 7-membered rings.

Highly Selective Ring Expansion of Bicyclo[3.1.0]hexenes

A Ru-carbene-promoted ring expansion of bicyclo[3.1.0]hexenes with terminal alkynes is reported. The reaction delivers seven-membered carbocycles starting from readily available starting materials and was found to be highly regioselective. The resulting seven-membered ring products contain both conjugated diene and cyclopropane substructures that could be selectively reacted in subsequent transformations.

Silver-catalyzed vinylogous fluorination of vinyl diazoacetates

A silver-catalyzed vinylogous fluorination of vinyl diazoacetates to generate γ-fluoro-α,β-unsaturated carbonyls is presented. Application of this method to the fluorination of farnesol and steroid derivatives was achieved.

Rh(II)-catalyzed reactions of differentially substituted bis(diazo) functionalities

The chemoselective reaction of donor/acceptor (D/A) and acceptor/acceptor (A/A) diazo moieties in the same molecule was examined using 3-diazo-1-(ethyl 2-diazomalonyl)indolin-2-one under rhodium(II) catalysis. The D/A diazo group undergoes selective cyclopropanation as well as XH-insertion, leaving behind the second diazo group for a further intramolecular dipolar cycloaddition reaction.

Guide to Enantioselective Dirhodium(II)-Catalyzed Cyclopropanation with Aryldiazoacetates

Catalytic enantioselective methods for the generation of cyclopropanes has been of longstanding pharmaceutical interest. Chiral dirhodium(II) catalysts prove to be an effective means for the generation of diverse cyclopropane libraries. Rh₂(R-DOSP)₄ is generaally the most effective catalyst for asymmetric intermolecular cyclopropanation of methyl aryldiazoacetates with styrene. Rh₂(S-PTAD)₄ provides high levels of enantioinduction with ortho-substituted aryldiazoacetates. The less-established Rh₂(R-BNP)₄ plays a complementary role to Rh₂(R-DOSP)₄ and Rh₂(S-PTAD)₄ in catalyzing highly enantioselective cyclopropanation of 3- methoxy-substituted aryldiazoacetates. Substitution on the styrene has only moderate influence on the asymmetric induction of the cyclopropanation.

Rh(2)(S-PTTL)(3)TPA-A Mixed Ligand Dirhodium(II) Catalyst for Enantioselective Reactions of α-Alkyl-α-Diazoesters

Herein we report the synthesis of the mixed ligand paddlewheel complex dirhodium(II) tris[N-phthaloyl-(S)-tert-leucinate] triphenylacetate, Rh(2)(S-PTTL)(3)TPA, the structure of which bears similarity to the chiral crown complex Rh(2)(S-PTTL)(4). Rh(2)(S-PTTL)(3)TPA engages substrate classes (aliphatic alkynes, silylacetylenes, α-olefins) that are especially challenging in intermolecular reactions of α-alkyl-α-diazoesters, and catalyzes enantioselective cyclopropanation, cyclopropenation, and indole C-H functionalization with yields and enantioselectivities that are comparable or superior to Rh(2)(S-PTTL)(4). Mixing ligands on paddlewheel complexes offers a versatile handle for diversifying catalyst structure and reactivity. The results described herein illustrate how mixed ligand catalysts can create new opportunities for the optimization of catalytic asymmetric processes.

D2-symmetric dirhodium catalyst derived from a 1,2,2-triarylcyclopropanecarboxylate ligand: design, synthesis and application

Dirhodium tetrakis-(R)-(1-(4-bromophenyl)-2,2-diphenylcyclopropanecarboxylate) (Rh(2)(R-BTPCP)(4)) was found to be an effective chiral catalyst for enantioselective reactions of aryl- and styryldiazoacetates. Highly enantioselective cyclopropanations, tandem cyclopropanation/Cope rearrangements and a combined C-H functionalization/Cope rearrangement were achieved using Rh(2)(R-BTPCP)(4) as catalyst. The advantages of Rh(2)(R-BTPCP)(4) include its ease of synthesis, its tolerance to the size of the ester group in the styryldiazoacetates, and its compatibility with dichloromethane as solvent. Computational studies suggest that the catalyst adopts a D(2)-symmetric arrangement, but when the carbenoid binds to the catalyst, two of the p-bromophenyl groups on the ligands rotate outward to make room for the carbenoid and the approach of the substrate to the carbenoid.

Asymmetric Synthesis of Highly Functionalized Cyclopentanes by a Rhodium- and Scandium-Catalyzed Five-Step Domino Sequence

A domino sequence has been developed between vinyldiazoacetates and racemic allyl alcohols, involving five distinct steps. The sequence generates highly functionalized cyclopentanes with four new stereogenic centers as single diastereomers in 64-92% ee. The first step is a rhodium-catalyzed oxygen ylide formation, which is then followed by a [2,3]-sigmatropic rearrangement, an oxy-Cope rearrangement, a keto/enol tautomerization, and then finally a carbonyl ene reaction. With appropriate substrates, a further silyl deprotection and a 6-exo-trig cyclization can be added to the domino process.