Regioselective Monoborylation of Spirocyclobutenes

Rapid Access to Chiral Spiro[2.3] Lactams: Stereoselective Hydroborylation and Hydrosilylation and Remote Control of Axial Chirality by Copper-Catalyzed Desymmetrization of Spirocyclopropenes

Chiral spirocyclopropyl β-lactams are common motifs in bioactive compounds and pharmaceuticals. Here we disclose a diastereoselective and enantioselective hydroborylation and hydrosilylation of spirocyclopropenes, via a Cu-catalyzed desymmetrization strategy, for the rapid preparation of enantio-enriched spirocyclopropyl β-lactams. The efficient desymmetrization strategy allows the remote control of axial chirality, offering the borylated and silylated products bearing central, spiro, and axial chirality. The combination of multichiral elements would provide a novel motif for biological evaluation in potential drug discovery.

Borylated cyclobutanes via thermal [2 + 2]-cycloaddition

A one-step approach to borylated cyclobutanes from amides of carboxylic acids and vinyl boronates is elaborated. The reaction proceeds via the thermal [2 + 2]-cycloaddition of in situ-generated keteniminium salts.

Photochemical [2+2] Cycloaddition of Alkynyl Boronates

A photochemical [2+2] cycloaddition of alkynyl boronates and maleimides is reported. The developed protocol provided 35-70 % yield of maleimide-derived cyclobutenyl boronates and demonstrated wide compatibility with various functional groups. The synthetic utility of the prepared building blocks was demonstrated for a range of transformations, including Suzuki cross-coupling, catalytic or metal-hydride reduction, oxidation, and cycloaddition reactions. With aryl-substituted alkynyl boronates, the products of double [2+2] cycloaddition were obtained predominantly. Using the developed protocol, a cyclobutene-derived analogue of Thalidomide was prepared in one step. Mechanistic studies supported the participation of the triplet-excited state maleimides and ground state alkynyl boronates in the key step of the process.

Palladium-Catalyzed Trans-Selective Synthesis of Spirocyclic Cyclobutanes Using α,α-Dialkylcrotyl- and Allylhydrazones

Herein, we demonstrate the use of E/Z mixtures of α,α-disubstituted crotylhydrazones to obtain spirocyclic vinylcyclobutanes in a diastereoselective fashion. We show 24 examples of a 1,1-insertion/4-exo-trig tandem process to produce these motifs. Additionally, spirocyclic alkylidene cyclobutanes can be obtained by using α,α-disubstituted allylated hydrazones (11 examples). In this study, we show that the aryl migrating group has a dramatic impact on the course of the reaction. Specifically, allylic C-H insertion products can be obtained in good yields using bromoenones as reaction partners. When Pd(0) is used with no aryl or alkenyl bromide, an intramolecular cyclopropanation reaction takes place to afford [2.1.0]-bicycles.

Rhodium-Catalyzed Asymmetric Arylation of Cyclobutenone Ketals

Complex cyclobutanes are important motifs in both bioactive molecules and natural products, yet their enantioselective preparation has not been widely explored. In this work, we describe rhodium-catalyzed enantioselective additions of aryl and vinyl boronic acids to cyclobutenone ketals. This transformation involves enantioselective carbometalation to give cyclobutyl-rhodium intermediates, followed by β-oxygen elimination to afford enantioenriched enol ethers. Overall, this addition serves as a surrogate for Rh-catalyzed 1,4-additions to cyclobutenone.

Updated Progress of the Copper-Catalyzed Borylative Functionalization of Unsaturated Molecules

Borylation has become a powerful method to synthesize organoboranes as versatile building blocks in organic synthesis, medicinal chemistry, and materials science. Copper-promoted borylation reactions are extremely attractive due to the low cost and non-toxicity of the copper catalyst, mild reaction conditions, good functional group tolerance, and convenience in chiral induction. In this review, we mainly updated recent advances (from 2020 to 2022) in the synthetic transformations in C=C/C≡C multiple bonds, and C=E multiple bonds mediated by copper boryl systems.

Cobalt-Catalyzed Diastereo- and Enantioselective Carbon-Carbon Bond Forming Reactions of Cyclobutenes

Catalytic enantioselective functionalization of cyclobutenes constitutes a general and modular strategy for construction of enantioenriched complex cyclobutanes bearing multiple stereogenic centers, as chiral four-membered rings are common motifs in biologically active molecules and versatile intermediates in organic synthesis. However, enantioselective synthesis of cyclobutanes through such a strategy remained significantly limited. Herein, we report a series of unprecedented cobalt-catalyzed carbon-carbon bond forming reactions of cyclobutenes that are initiated through enantioselective carbometalation. The protocols feature diastereo- and enantioselective introduction of allyl, alkynyl, and functionalized alkyl groups. Mechanistic studies indicated an unusual 1,3-cobalt migration and subsequent β-carbon elimination cascade process occurred in the allyl addition. These new discoveries established a new elementary process for cobalt catalysis and an extension of diversity of nucleophiles for enantioselective transformations of cyclobutenes.

Enantioselective Suzuki cross-coupling of 1,2-diboryl cyclopropanes

Herein, we describe the catalytic enantioselective cross-coupling of 1,2-bisboronic esters. Prior work on group specific cross coupling is limited to the use of geminal bis-boronates. This desymmetrization provides a novel approach to prepare enantioenriched cyclopropyl boronates with three contiguous stereocenters, that could be further derivatized through selective functionalization of the carbon-boron bond. Our results suggest that transmetallation, which is the enantiodetermining step, takes place with retention of stereochemistry at carbon.

Synthesis of 3-borylated cyclobutanols from epihalohydrins or epoxy alcohol derivatives

There is an increasing interest in cyclobutanes within the medicinal chemistry community. Therefore, methods to prepare cyclobutanes that contain synthetic handles for further elaboration are of interest. Herein, we report a new approach for the synthesis of 3-borylated cyclobutanols via a formal [3 + 1]-cycloaddition using readily accessible 1,1-diborylalkanes and epihalohydrins or epoxy alcohol derivatives. 1-Substituted epibromohydrin starting materials provide access to borylated cyclobutanols containing substituents at three of the four positions on the cyclobutane core, and enantioenriched epibromohydrins lead to enantioenriched cyclobutanols with high levels of enantiospecificity (>98%). Finally, derivatization studies demonstrate the synthetic utility of both the OH and Bpin handles.

Arylboration of Enecarbamates for the Synthesis of Borylated Saturated N-Heterocycles

Two catalytic systems have been developed for the arylboration of endocyclic enecarbamates to deliver synthetically versatile borylated saturated N-heterocycles in good regio- and diastereoselectivities. A Cu/Pd dual catalytic reaction enables the synthesis of borylated, α-arylated azetidines, while a Ni-catalysed arylboration reaction efficiently functionalizes 5-, 6-, and 7-membered enecarbamates. In the case of the Cu/Pd-system, a remarkable additive effect was identified that allowed for broader scope. The products are synthetically useful, as demonstrated by manipulations of the boronic ester to access biologically active compounds.

Boosting the Enantioselectivity of Conjugate Borylation of α,β-Disubstituted Cyclobutenones with Monooxides of Chiral C2 -Symmetric Bis(phosphine) Ligands

Chiral bis(phosphine) monooxides (BPMOs) derived from C₂ -symmetric bis(phosphines) have been found to induce superior levels of enantioselection in copper-catalyzed conjugate borylation of α,β-disubstituted cyclobutenones. More precisely, enantiomeric excesses as well as chemical yields are exceedingly high with (R,R)-Bozphos as the chiral ligand while these values are low with parent (R,R)-Me-Duphos. A similar yet less pronounced effect was seen in the corresponding 1,6-addition to para-quinone methides.

Photosensitized [2+2]-Cycloadditions of Alkenylboronates and Alkenes

A new strategy for the synthesis of highly versatile cyclobutylboronates via the photosensitized [2+2]-cycloaddition of alkenylboronates and alkenes is presented. The process is mechanistically different from other processes in that energy transfer occurs with the alkenylboronate as opposed to the other alkene. This strategy allows for the synthesis of an array of diverse cyclobutylboronates. The conversion of these adducts to other compounds as well as their utility in the synthesis of melicodenine C is demonstrated.

Catalytic asymmetric hydrometallation of cyclobutenes with salicylaldehydes

Chiral, substituted cyclobutanes are common motifs in bioactive compounds and intermediates in organic synthesis but few asymmetric routes for their synthesis are known.

A Modular Access to 1,2- and 1,3-Disubstituted Cyclobutylboronic Esters by Consecutive Radical Additions

A modular approach to substituted cyclobutylboronic esters is described. It proceeds by successive intermolecular radical additions of xanthates to pinacolato 1-cyclobutenylboronate and to pinacolato bicyclo[1.1.0]but-1-ylboronate. Success hinges on tuning the stability of the α-boryl radical by exploiting the stabilizing influence of the trivalent boronic ester and the slightly destabilizing cyclobutane, which increases the σ-character of the radical. Reductive removal of the xanthate group finally provides a range of 1,2- and 1,3-disubstituted cyclobutylboronic esters. The contrast with cyclopropylboronic esters is striking, since the strong destabilization by the highly strained cyclopropane ring allows the first radical addition to take place but not the second. Furthermore, the first adducts are geminal xanthyl boronic esters that can be converted into cyclobutanones. This chemistry furnishes cyclobutylboronic esters that would be quite difficult to obtain otherwise and thus complements existing methods.

Catalytic Arylboration of Spirocyclic Cyclobutenes: Rapid Access to Highly Substituted Spiro[3.n]alkanes

A method to achieve the synthesis of highly substituted spirocyclic cyclobutanes is disclosed. The reaction involves the catalytic arylboration of cyclobutenes. Depending on the substitution pattern of the cyclobutene, either a Cu/Pd- or a Ni-catalyzed reaction was utilized. In the case of the Cu/Pd catalyzed reactions, the identification of a Cu-complex for arylboration was crucial to observe high selectivity. The synthetic utility of the products is demonstrated, and the mechanistic details are discussed.