Combined Experimental and Computational Study of Ruthenium N-Hydroxyphthalimidoyl Carbenes in Alkene Cyclopropanation Reactions

Experimental and Computational Studies for the Synthesis of Functionalized Cyclopropanes from 2-Substituted Allylic Derivatives with Ethyl Diazoacetate

The catalytic asymmetric synthesis of highly functionalized cyclopropanes from 2-substituted allylic derivatives is reported. Using ethyl diazo acetate, the reaction, catalyzed by a chiral ruthenium complex (Ru(II)-Pheox), furnished the corresponding easily separable cis and trans cyclopropanes in moderate to high yields (32-97 %) and excellent ee (86-99 %). This approach significantly extends the portfolio of accessible enantioenriched cyclopropanes from an underexplored class of olefins. DFT calculations suggest that an outer-sphere mechanism is operative in this system.

Decarboxylation of β-boryl NHPI esters enables radical 1,2-boron shift for the assembly of versatile organoborons

In recent years, numerous 1,2-R shift (R = aliphatic or aryl) based on tetracoordinate boron species have been well investigated. In the contrary, the corresponding radical migrations, especially 1,2-boryl radical shift for the construction of organoborons is still in its infancy. Given the paucity and significance of such strategies in boron chemistry, it is urgent to develop other efficient and alternative synthetic protocols to enrich these underdeveloped radical 1,2-boron migrations, before their fundamental potential applications could be fully explored at will. Herein, we have demonstrated a visible-light-induced photoredox neutral decarboxylative radical cross-coupling reaction, which undergoes a radical 1,2-boron shift to give a translocated C-radical for further capture of versatile radical acceptors. The mild reaction conditions, good functional-group tolerance, and broad β-boryl NHPI esters scope as well as versatile radical acceptors make this protocol applicable in modification of bioactive molecules. It can be expected that this methodology will be a very useful tool and an alternative strategy for the construction of primary organoborons via a novel radical 1,2-boron shift mode.

Modular Synthesis of (Borylmethyl)silanes through Orthogonal Functionalization of a Carbon Atom

(Borylmethyl)trimethylsilanes are important building blocks in organic synthesis displaying a unique reactivity. Yet, the synthesis of more advanced derivatives is limited by the advanced silicon intermediates required for their preparation. Herein, a one-pot synthesis of (borylmethyl)silanes is developed, sourced on available alkyl-, aryl-, alkoxy-, aryloxy-, and silyl-hydrosilane materials. The privileged reactivity of N-hydroxyphthalimidyl diazoacetate (NHPI-DA) in Si-H insertion and α-silyl redox-active esters in different decarboxylative borylation reactions are scrutinized.

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.

Control of Redox-Active Ester Reactivity Enables a General Cross-Electrophile Approach to Access Arylated Strained Rings

Strained rings are increasingly important for the design of pharmaceutical candidates, but cross-coupling of strained rings remains challenging. An attractive, but underdeveloped, approach to diverse functionalized carbocyclic and heterocyclic frameworks containing all-carbon quaternary centers is the coupling of abundant strained-ring carboxylic acids with abundant aryl halides. Herein we disclose the development of a nickel-catalyzed cross-electrophile approach that couples a variety of strained ring N-hydroxyphthalimide (NHP) esters, derived from the carboxylic acid in one step, with various aryl and heteroaryl halides under reductive conditions. The chemistry is enabled by the discovery of methods to control NHP ester reactivity, by tuning the solvent or using modified NHP esters, and the discovery that t-Bu BpyCamCN , an L2X ligand, avoids problematic side reactions. This method can be run in flow and in 96-well plates.