Tertiary Carbon Centers Research Articles

Palladium-Catalyzed Asymmetric Tandem Carbonylation-Heck Reaction of Cyclopentenes to Access Chiral Bicyclo[3.2.1]octenes.

A palladium-catalyzed asymmetric tandem carbonylation-Heck reaction of cyclopentenes with carbon monoxide (CO) has been disclosed. This desymmetrization procedure afforded a series of bicyclo[3.2.1]octenes with one chiral quaternary and one tertiary carbon center in good yields with good enantioselectivities. This reaction proceeds via an acyl-palladium intermediate, followed by migratory insertion of the alkenes.

Reduction of Tertiary Carbon Radicals via Asymmetric Hydrogen Atom Transfer (AHAT)†

Comprehensive SummaryIn the past two decades, the development of asymmetric radical reactions has achieved tremendous progress, which has emerged as a powerful tool for the synthesis of chiral molecules in synthetic chemistry. Among the diverse array of radical processes, the transfer of hydrogen atoms to tertiary carbon radicals offers the potential for constructing chiral tertiary carbon centers in a stereoselective fashion. Notwithstanding the challenges associated with the reactive and evanescent nature of radical species, the use of chiral reagents or mediators has enabled the stereocontrol of the asymmetric hydrogen atom transfer (AHAT), which provides novel avenues for advancing the field of asymmetric synthesis. Key Scientists

Site-selective α-C(sp3)-H arylation of dialkylamines via hydrogen atom transfer catalysis-enabled radical aryl migration.

Site-selective C(sp3)-H arylation is an appealing strategy to synthesize complex arene structures but remains a challenge facing synthetic chemists. Here we report the use of photoredox-mediated hydrogen atom transfer (HAT) catalysis to accomplish the site-selective α-C(sp3)-H arylation of dialkylamine-derived ureas through 1,4-radical aryl migration, by which a wide array of benzylamine motifs can be incorporated to the medicinally relevant systems in the late-stage installation steps. In contrast to previous efforts, this C-H arylation protocol exhibits specific site-selectivity, proforming predominantly on sterically more-hindered secondary and tertiary α-amino carbon centers, while the C-H functionalization of sterically less-hindered N-methyl group can be effectively circumvented in most cases. Moreover, a diverse range of multi-substituted piperidine derivatives can be obtained with excellent diastereoselectivity. Mechanistic and computational studies demonstrate that the rate-determining step for methylene C-H arylation is the initial H atom abstraction, whereas the radical ipso cyclization step bears the highest energy barrier for N-methyl functionalization. The relatively lower activation free energies for secondary and tertiary α-amino C-H arylation compared with the functionalization of methylic C-H bond lead to the exceptional site-selectivity.

CAMDOL-enabled diastereoselective synthesis of α-substituted phosphonates.

Enantiopure α-substituted phosphonic acids are widely utilized as drugs, pesticides, and ligands. Despite numerous synthetic approaches having been investigated, precise construction of P-adjacent chiral tertiary carbon centres by the employment of recoverable chiral auxiliaries is traditional and still one of the most reliable and practical synthetic methodologies so far. Herein, we present a highly diastereoselective synthesis of α-substituted phosphonates via the unique CAMDOL-derived P-substrates by an efficient sequential deprotonation with LiHMDS and alkylation/arylation with RI. A wide range of 30 structurally diverse α-substituted phosphonate products, including the well-known P-analogues of naproxen and ibuprofen, were thus afforded conveniently in up to 92% yields and 99 : 1 diastereomeric ratios. The related chiral phosphonic acid could be easily obtained by simple acidic hydrolysis with fully recovered auxiliary. This CAMDOL-enabled asymmetric synthetic protocol exhibits comparative advantages over known chiral-induction methods with easy accessibility and compatibility of furnishing a variety of C-stereogenic centres in the proximity of the phosphorus atom, including some rare examples.

Sulfoxonium Ylides as Carbene Precursors in Rhodium‐Catalyzed Sommelet‐Hauser Rearrangement for the Synthesis of S/Se‐Containing Compounds

AbstractRhodium‐catalyzed Sommelet‐Hauser rearrangement derived from α‐thioesters/α‐selenoesters and sulfoxonium ylides has been developed for the synthesis of various sulfur/selenium‐containing ortho‐substituted aromatic compounds in medium to excellent yields (39 examples; up to 97 %). Sulfoxonium ylides as the safe carbene precursors are firstly used in Sommelet‐Hauser rearrangement reactions, which is superior to traditional diazo compounds. The present reaction has the advantages of mild condition, safety and broad substrate scope, which afford a unique method for the synthesis of sulfur/selenium‐containing compounds with a tertiary carbon center.

Catalyst-Controlled C-H Allylation and Annulation of 2-Aryl Quinazolinones with 2-Methylidene Cyclic Carbonate.

The site-selective modification of quinazolinone as a privileged bicyclic N-heterocycle is an attractive topic in medicinal chemistry and material science. We herein report the ruthenium(II)-catalyzed C-H allylation of 2-aryl quinazolinones with 2-methylidene cyclic carbonate. In addition, tandem C-H allylation and annulation are achieved under rhodium(III) catalysis, resulting in the formation of tetracyclic quinazolinones including a tertiary carbon center. Post-transformations of the synthesized products demonstrate the potential of the developed methodology. A series of mechanistic investigations were also performed.

Enantioselective synthesis of Benzofurans bearing adjacent quaternary and tertiary carbon stereocenters by Organocatalytic asymmetric Michael addition of 3-methylbenzofuran −2(3H)-ones to azadienes

An enantioselective Michael addition of 3-methylbenzofuran-2(3H)-ones to azadienes catalyzed by an L-tert- leucine derived squaramide catalyst was developed, by which a variety of benzofuranones bearing adjacent quaternary and tertiary carbon centers were obtained in high yields (up to 98%) with moderate to excellent diastereoselectivities (up to 97:3) and enantioselectivities (up to 97%).

Synthesis of Aliphatic Nitriles via Desulfonylative Smiles Rearrangement

AbstractHerein, we demonstrate the synthesis of aliphatic nitriles from N-acyl (2-nitrophenyl)sulfonamides via a desulfonylative Smiles rearrangement. The developed reaction routes provide a variety of aliphatic nitriles containing primary, secondary, and tertiary carbon centers in good-to-excellent yields. Our method requires the use of the easy-to-handle reagent potassium acetate and solvent 1,3-dimethyl-2-imidazolidinone, and it does not rely on toxic metal cyanides or transition metals. The process is suitable for large-scale reaction and for one-pot syntheses starting from an acyl chloride or a carboxylic acid.

Asymmetric Mannich reaction of N-Boc aminosulfones with 3-methylbenzofuran-2 (3H) - Ones catalyzed by a chiral phase-transfer catalyst

An enantioselective Mannich reaction of N-Boc aminosulfones with 3-methylbenzofuran-2 (3H)-ones catalyzed by a quinine-derived bifunctional phase transfer catalyst was developed, by which a series of chiral benzofuranones bearing adjacent quaternary and tertiary carbon centers were obtained in high yields (up to 96%) with good diastereoselectivities (up to >99:1) and enantioselectivities (up to 98%).

Enantioselective and stereodivergent hydromonofluoroalkylation of conjugated and remote dienes

Because of the widespread applications of optically active alkyl fluorides in medicinal and agro chemicals, enantioselective and even stereodivergent construction of alkyl fluorides remains highly desirable but underdeveloped. Transition-metal-catalyzed asymmetric hydrofluoroalkylation of readily available dienes represents a novel route to achieve this goal, yet receives scarce study. Here we report an intriguing palladium-catalyzed enantioselective hydromonofluoroalkylation reaction of conjugated dienes. Both monosubstituted and internal dienes proceed well with the transformation and furnish alkyl fluorides in generally >80% yield and >90% ee. A stereodivergent hydromonofluoroalkylation protocol via Pd/Cu co-catalysis is also established for the access to all four stereoisomers of corresponding moieties bearing a fully-substituted F-stereogenic center and vicinal tertiary carbon center. In addition, asymmetric migratory hydromonofluoroalkylation of skipped dienes is developed to realize the direct allylic CH fluoroalkylation. A compound library of enantioenriched cyclic fluorides is thus built to highlight the transformation potential of present methodology.

Chiral Phosphine Catalyzed Allylic Alkylation of Benzylidene Succinimides with Morita–Baylis–Hillman Carbonates

Owing to their unique chemical properties, α-alkylidene succinimides generally act as versatile synthons in organic synthesis. Compared with well-established annulations, nucleophilic alkylations of α-alkylidene succinimides are very limited. Accordingly, an organocatalytic allylic alkylation of α-benzylidene succinimides with Morita-Baylis-Hillman (MBH) carbonates was established. In the presence of a chiral phosphine catalyst, α-benzylidene succinimides reacted smoothly with MBH carbonates under mild conditions to furnish a series of optical active succinimides in high yields and enantioselectivities. Different from the reported results, the organocatalytic enantioselective construction of pyrrolidine-2,5-dione frameworks bearing contiguous chiral tertiary carbon centers was achieved via this synthetic strategy. Scaling up the reaction indicated that it is a practical strategy for the organocatalytic enantioselective alkylation of α-alkylidene succinimides. A possible reaction mechanism was also proposed.

Enantioselective Synthesis of (R)-Tiagabine via Asymmetric Hydrogen Atom Transfer Protocol

AbstractAn enantioselective synthesis of tiagabine has been achieved utilizing an asymmetric hydrogen atom transfer protocol to construct its essential chiral tertiary carbon center. A cyclization reaction via double N-substitution is tactically orchestrated as the other key step to install the crucial alkaloid ring. Compared with the previous synthetic strategy, which used commercially available nicotinate as the starting material to ensure a short synthetic route, this strategy uses a readily modifiable and accessible alkyl-substituted acrylate as the starting material and thus provides a scenario for the facile synthesis of analogues and derivatives of tiagabine for further biological research.

β‐ and γ‐C(sp3)−H Heteroarylation of Free Carboxylic Acids: A Modular Synthetic Platform for Diverse Quaternary Carbon Centers

AbstractPdII‐catalyzed C(sp3)−H activation of free carboxylic acids represents a significant advance from conventional cyclopalladation initiated reactions. However, developing a modular synthetic platform for diverse quaternary and tertiary carbon centers based on this reactivity, two challenges remain to be addressed: mono‐selectivity in each consecutive C−H functionalization step; compatibility with heteroatoms. While the exclusive mono‐selectivity was achieved by β‐lactonization/nucleophilic attack, the latter limitation remains to be overcome. Herein, we report the PdII‐catalyzed β‐ and γ‐C(sp3)−H heteroarylation of free carboxylic acids using pyridine‐pyridone ligands capable of overcoming these limitations. A sequence of three consecutive C(sp3)−H activation reactions of pivalic acid provides an unique platform for constructing diverse quaternary carbon centers containing heteroaryls which could serve as an enabling tool for escaping the flat land in medicinal chemistry.

β- and γ-C(sp3 )-H Heteroarylation of Free Carboxylic Acids: A Modular Synthetic Platform for Diverse Quaternary Carbon Centers.

PdII -catalyzed C(sp3 )-H activation of free carboxylic acids represents a significant advance from conventional cyclopalladation initiated reactions. However, developing a modular synthetic platform for diverse quaternary and tertiary carbon centers based on this reactivity, two challenges remain to be addressed: mono-selectivity in each consecutive C-H functionalization step; compatibility with heteroatoms. While the exclusive mono-selectivity was achieved by β-lactonization/nucleophilic attack, the latter limitation remains to be overcome. Herein, we report the PdII -catalyzed β- and γ-C(sp3 )-H heteroarylation of free carboxylic acids using pyridine-pyridone ligands capable of overcoming these limitations. A sequence of three consecutive C(sp3 )-H activation reactions of pivalic acid provides an unique platform for constructing diverse quaternary carbon centers containing heteroaryls which could serve as an enabling tool for escaping the flat land in medicinal chemistry.

Site-selective ring opening of bicyclo[n.1.0]alkanols: an Fe(II)-catalyzed 1,6-conjugate addition to p-quinone methides.

Herein, we report an efficient synthetic strategy for an Fe(ii)-catalyzed site-selective ring opening of bicyclo[n.1.0]alkanols and their concomitant 1,6-conjugate addition to p-quinone methides. Access to tertiary carbon centers with appendaged carbocycles of distinct sizes and functional groups are achieved, under a substrate-controlled bond scission of the fused cyclopropanols. Synthetic derivatizations further enhance the utility of the protocol.

Transition-metal free C–N bond formation from alkyl iodides and diazonium salts via halogen-atom transfer

Construction of C-N bond continues to be one part of the most significant goals in organic chemistry because of the universal applications of amines in pharmaceuticals, materials and agrochemicals. However, E2 elimination through classic SN2 substitution of alkyl halides lead to generation of alkenes as major side-products. Thus, formation of a challenging C(sp3)-N bond especially on tertiary carbon center remains highly desirable. Herein, we present a practical alternative to prepare primary, secondary and tertiary alkyl amines with high efficiency between alkyl iodides and easily accessible diazonium salts. This robust transformation only employs Cs2CO3 promoting halogen-atom transfer (XAT) process under transition-metal-free reaction conditions, thus providing a rapid method to assemble diverse C(sp3)-N bonds. Moreover, diazonium salts served as alkyl radical initiator and amination reagent in the reaction. Mechanism studies suggest this reaction undergo through halogen-atom transfer process to generate active alkyl radical which couples with diazonium cations to furnish final products.

(Salen)Titanium-Catalyzed Asymmetric Hydrogen Atom Transfer for Epoxides Reduction.

Asymmetric hydrogen atom transfer (HAT) represents arguably one of the most straightforward access to the significant chiral tertiary carbon centers, however, remains a long-standing challenge. Herein, the first example of catalytic coordination-induced asymmetric HAT reaction has been developed relying on a sequential (salen)TiIII -initiated radical generation and (salen)TiIV -controlled enantioselective HAT process. The reaction transforms easily accessible glycidic esters, thioesters and amides to synthetically valuable formal formaldehyde aldol adducts in an enantioconvergent manner, along with broad scope and highly regio- and enantioselectivities.

(Salen)Titanium‐Catalyzed Asymmetric Hydrogen Atom Transfer for Epoxides Reduction

AbstractAsymmetric hydrogen atom transfer (HAT) represents arguably one of the most straightforward access to the significant chiral tertiary carbon centers, however, remains a long‐standing challenge. Herein, the first example of catalytic coordination‐induced asymmetric HAT reaction has been developed relying on a sequential (salen)TiIII‐initiated radical generation and (salen)TiIV‐controlled enantioselective HAT process. The reaction transforms easily accessible glycidic esters, thioesters and amides to synthetically valuable formal formaldehyde aldol adducts in an enantioconvergent manner, along with broad scope and highly regio‐ and enantioselectivities.

Synthetic and Computational Study of the Enantioselective [3+2]-Cycloaddition of Chromones with MBH Carbonates.

Highly efficient and straightforward access to enantioenriched five-membered ring-fused chromanones is developed via [3+2]-cycloaddition of 3-cyanochromones with Morita-Baylis-Hillman carbonates. Densely functionalized chiral cyclopenta[b]chromanones with three continuous quaternary and tertiary stereogenic carbon centers were obtained in high yields with high ee and dr (≤97% yield, 97% ee, and >20:1 dr). Moreover, density functional theory calculations have been carried out to investigate the mechanism and regio- and diastereoselectivity of the reaction.

Photo‐Catalyzed Redox‐Neutral 1,2‐Dialkylation of Alkenes

AbstractDifunctionalization of alkenes enables construction of complex compounds in one pot. In this report, we presented an intermolecular, redox‐neutral three‐component dicarbofunctionalization of activated alkenes under mild reaction conditions. The mechanistic studies indicated tertiary alkyl radicals from oxalate salts were first added to alkenes in Giese‐type fashion, followed by nucleophilic addition with aldehydes to produce the desired 1,2‐dialkylation products. The catalyst system does not require any additives, with good functional group compatibility, regioselectivity, and constructs two tertiary carbon centers simultaneously.magnified image