Synthesis of Biindene Derivatives via Pd-Catalyzed Alkene Difunctionalization Reactions

The palladium-catalyzed cross-coupling of 2-allylphenyl triflate and related electrophiles with substituted indenes affords biindene derivatives in moderate to good yields with high selectivity for thermodynamically preferred alkene isomers. The transformations involve alkene nucleopalladation with indenyl anions, and we also demonstrate that 2-allylphenyl triflates can be transformed to indenes under similar conditions. The scope of this transformation, along with the mechanism of formation of both indene and biindene products, is described.

Influence of CUREs on STEM retention depends on demographic identities

Research has shown that undergraduate research experiences can have substantive effects on retaining students in science, technology, engineering and mathematics (STEM). However, it is impossible to provide individual research experiences for every undergraduate student, especially at large universities. Course-based undergraduate research experiences (CUREs) have become a common approach to introduce large numbers of students to research. We investigated whether a one-semester CURE that replaced a traditional introductory biology laboratory course could increase retention in STEM as well as intention to remain in STEM, if the results differed according to demography, and investigated the possible motivational factors that might mediate such an effect. Under the umbrella of the Authentic Research Connection (ARC) program, we used institutional and survey data from nine semesters and compared ARC participants to non-participants, who applied to ARC but either were not randomly selected or were selected but chose not to enroll in an ARC section. We found that ARC had significant effects on demographic groups historically less likely to be retained in STEM: ARC participation resulted in narrowing the gaps in graduation rates in STEM (first vs continuing-generation college students) and in intention to major in STEM [females vs males, Persons Excluded because of Ethnicity or Race (PEERs) vs non-PEERs]. These disproportionate boosts in intending STEM majors among ARC students coincide with their reporting a greater sense of student cohesiveness, retaining more interest in biology, and commenting more frequently that the course provided a useful/valuable learning experience. Our results indicate that CUREs can be a valuable tool for eliminating inequities in STEM participation, and we make several recommendations for further research.

Palladium-Catalyzed Synthesis of 1-Alkylidene-2-dialkylaminomethyl Cyclobutane Derivatives via Pd-Catalyzed Alkene Difunctionalization Reactions: Influence of Nucleophile and Water on the Reaction Mechanism

The Pd-catalyzed coupling of 1,5-diene-2-yl triflates with amine nucleophiles affords exomethylenecyclobutanes bearing dialkylaminomethyl groups at C2. The strained carbocyclic products are obtained in moderate to excellent yields, with regioselectivities of up to >95:5 for four-membered ring formation. The mechanism of these reactions, which provides products resulting from anti-addition to alkenes, differs from related reactions involving malonate nucleophiles that provide syn-addition products.

Synthesis of 2-Cyanomethyl Indane Derivatives via Pd-Catalyzed Alkene Difunctionalization Reactions of Alkyl Nitriles

The synthesis of indanes bearing substituted cyanomethyl groups at C2 is achieved through Pd-catalyzed coupling reactions between 2-allylphenyl triflate derivatives and alkyl nitriles. Related partially saturated analogues were generated from analogous transformations of alkenyl triflates. The use of a preformed BrettPhosPd(allyl)(Cl) complex as a precatalyst was essential for the success of these reactions.

Regiodivergent Palladium-Catalyzed Alkene Difunctionalization Reactions for the Construction of Methylene Cyclobutanes and Methylene Cyclopentanes

Regiodivergent palladium-catalyzed alkene difunctionalization reactions between diethyl malonate and 1,5-dienes bearing a triflate group at C2 are described. Use of tris(2,4-di-tert-butylphenyl)phosphite as a ligand leads to 4-exo-cyclization/functionalization to afford malonate-substituted methylene cyclobutanes. In contrast, the 1,2-bis(diphenylphosphino)benzene ligand provides methylene cyclopentanes via 5-endo-cyclization/functionalization. The five-membered ring-forming reactions occur via anti-carbopalladation of the enolate nucleophile, whereas four-membered ring-forming reactions proceed through syn-4-exo-migratory insertion of the tethered alkene, followed by C(sp3)-C(sp3) bond-forming reductive elimination from an (alkyl)Pd(II)(malonate) complex.

Pd-Catalyzed Alkene Diamination Reactions with O-Benzoylhydroxylamine Electrophiles: Evidence Supporting a Pd(II/IV) Catalytic Cycle, the Role of 2,4-Pentanedione Derivatives as Ligands, and Expanded Substrate Scope

This article describes continued studies on Pd-catalyzed alkene diamination reactions between N-allylguanidines or ureas and O-benzoylhydroxylamine derivatives, which serve as N-centered electrophiles. The transformations generate cyclic guanidines and ureas bearing dialkylaminomethyl groups in moderate to good yield. We describe new mechanistic experiments that have led to a revised mechanistic hypothesis that involves a key oxidative addition of the electrophile to a Pd^II complex, followed by reductive elimination from Pd^IV to form the alkyl carbon-nitrogen bond. In addition, we demonstrate that acac, not phosphine, serves as a key ligand for palladium. Moreover, simple acac derivatives bearing substituted aryl groups outperform acac in the catalytic reactions, and phosphines inhibit catalysis in many cases. These discoveries have led to a significant expansion in the scope of this chemistry, which now allows for the coupling of a variety of cyclic amines, acyclic secondary amines, and primary amines. In addition, we also demonstrate that these new conditions allow for the use of amide nucleophiles, in addition to guanidines and ureas.

Pd-Catalyzed C-C, C-N, and C-O Bond-Forming Difunctionalization Reactions of Alkenes Bearing Tethered Aryl/Alkenyl Triflates

Over the past few years our group has described a new type of alkene difunctionalization reaction in which aryl or alkenyl triflates bearing tethered alkenes are coupled with various nucleophiles to afford carbocyclic products. The products are formed in moderate to good chemical yield, with generally high levels of stereoselectivity. Our progress to date in this area, which includes reactions of amine, alcohol, enolate, and indole nucleophiles, is described in this review.

Pd-Catalyzed Alkene Difunctionalization Reactions of Enolates for the Synthesis of Substituted Bicyclic Cyclopentanes

Palladium-catalyzed alkene difunctionalization reactions between alkenes bearing tethered aryl or alkenyl triflates and enolate nucleophiles are described. The transformations form two C-C bonds, a ring, and up to two stereocenters, while producing substituted cyclopentane derivatives that contain appended carbonyl functionality. Products are formed with up to >20:1 diastereoselectivity, and formation of sterically congested bonds between quaternary carbon atoms is feasible.

Pd-Catalyzed Alkene Difunctionalization Reactions of Malonate Nucleophiles: Synthesis of Substituted Cyclopentanes via Alkene Aryl-Alkylation and Akenyl-Alkylation

The Pd-catalyzed coupling of malonate nucleophiles with alkenes bearing tethered aryl or alkenyl triflates is described. These alkene difunctionalization reactions afford malonate-substituted cyclopentanes that contain fused aryl or cycloalkenyl rings. The transformations generate a five-membered ring, two C-C bonds, and provide products bearing up to three stereocenters with good chemical yield and generally high diastereoselectivity.

Stereocontrolled synthesis of bicyclic ureas and sulfamides via Pd-catalyzed alkene carboamination reactions

The synthesis of bicyclic ureas and sulfamides via palladium-catalyzed alkene carboamination reactions between aryl/alkenyl halides/triflates and alkenes bearing pendant cyclic sulfamides and ureas is described. The substrates for these reactions are generated in 3-5 steps from commercially available materials, and products are obtained in good yield with up to >20:1 diastereoselectivity. The stereochemical outcome of the sulfamide alkene addition is consistent with a mechanism involving anti-aminopalladation of the alkene, whereas the stereochemical outcome of the urea alkene addition is consistent with a syn-aminopalladation mechanism.

Pd-Catalyzed Alkene Carboheteroarylation Reactions for the Synthesis of 3-Cyclopentylindole Derivatives

The Pd-catalyzed alkene carboheteroarylation of aryl and alkenyl triflate electrophiles bearing pendant alkenes with heteroaromatic nucleophiles affords substituted carbocycles with 3-indolyl or 3-pyrrolyl groups. The products are obtained in moderate to good yields, and the use of alkenyl triflate substrates produces products with high diastereoselectivities. The transformation is believed to proceed via a Friedel-Crafts-like reaction between the heteroaromatic nucleophile and an intermediate electrophilic palladium complex.

Pd-Catalyzed Alkene Diamination Reactions of Nitrogen Electrophiles: Synthesis of Cyclic Guanidines and Ureas Bearing Dialkylaminomethyl Groups

The Pd-catalyzed coupling of N-allylguanidines or N-allylureas with O-benzoylhydroxylamine derivatives affords cyclic guanidines or cyclic ureas bearing dialkylaminomethyl groups. The desired products are obtained in good yield, and substrates bearing substituents at the allylic position are transformed with moderate diastereoselectivity. The mechanism of these reactions appears to involve anti-aminopalladation of the alkene, followed by a rare sp³C-sp³N bond-forming reductive elimination from an alkylpalladium complex that contains β-hydrogen atoms.

Palladium-Catalyzed Alkene Carboalkoxylation Reactions of Phenols and Alcohols for the Synthesis of Carbocycles

Intermolecular alkene difunctionalization reactions between terminal alkenes bearing a pendant aryl or alkenyl triflate electrophile and exogenous alcohol or phenol nucleophiles are described. These transformations afford substituted indanyl or alkylidenecyclopentyl ethers in high yield with excellent diastereoselectivity. The transformations proceed through intermolecular capture of an intermediate [Pd(II)-alkene]⁺[OTf]^- complex by the alcohol or phenol nucleophile.

Synthesis of Cyclic Guanidines Bearing N-Arylsulfonyl and N-Cyano Protecting Groups via Pd-Catalyzed Alkene Carboamination Reactions

Palladium-catalyzed carboamination reactions of N-allylguanidines bearing cleavable N-cyano or N-arylsulfonyl protecting groups are described. The reactions afford cyclic guanidine products in good yield, and transformations of substrates bearing internal alkenes proceed with high diastereoselectivity. Deuterium labeling studies indicate these transformations proceed via anti-aminopalladation pathways.

Stereocontrolled Synthesis of Amino-Substituted Carbocycles by Pd-Catalyzed Alkene Carboamination Reactions

Amino-substituted alkylidenecyclopentanes were synthesized through a stereoselective intermolecular Pd-catalyzed alkene carboamination reaction between alkenyl triflates bearing a pendant alkene and exogenous amine nucleophiles. The reactions are effective with a range of different substrate combinations, and proceed with generally high diastereoselectivity. Use of (S)-tBuPhox as the ligand in reactions of achiral substrates provides enantioenriched products with up to 98.5:1.5 e.r.

Recent Developments in Pd(0)-Catalyzed Alkene Carboheterofunctionalization Reactions

This review summarizes recent developments in palladium-catalyzed alkene carboalkoxylation and carboamination reactions. New synthetic methods that have been reported in the past four years are described, along with mechanistic issues and the influence of mechanism on product stereochemistry. The applications of these transformations to the synthesis of natural products and other biologically relevant compounds are also discussed.

Synthesis of 2,3-Dihydrobenzofurans via the Palladium Catalyzed Carboalkoxylation of 2-Allylphenols

A new Pd-catalyzed alkene carboalkoxylation strategy for the preparation of 2,3-dihydrobenzofurans is described. This method effects the coupling of readily available 2-allylphenol derivatives with aryl triflates to generate a wide range of functionalized 2,3-dihydrobenzofurans in good yields and diastereoselectivities (up to >20:1). Use of newly developed reaction conditions that promote anti-heteropalladation of the alkene is essential in order to generate products in high yield.

Synthesis of Cyclic Guanidines via Silver-Catalyzed Intramolecular Alkene Hydroamination Reactions of N-Allylguanidines

The silver-catalyzed hydroamination of tosyl-protected N-allylguanidines is described. These reactions provide substituted cyclic guanidines in high yields. The reactions are amenable to the construction of quaternary stereocenters as well as both monocyclic and bicyclic guanidine products.

Asymmetric Palladium-Catalyzed Alkene Carboamination Reactions for the Synthesis of Cyclic Sulfamides

The synthesis of cyclic sulfamides by enantioselective Pd-catalyzed alkene carboamination reactions between N-allylsulfamides and aryl or alkenyl bromides is described. High levels of asymmetric induction (up to 95:5 e.r.) are achieved using a catalyst composed of [Pd2 (dba)3 ] and (S)-Siphos-PE. Deuterium-labelling studies indicate the reactions proceed through syn-aminopalladation of the alkene and suggest that the control of syn- versus anti-aminopalladation pathways is important for asymmetric induction.

Development of Enantioselective Palladium-Catalyzed Alkene Carboalkoxylation Reactions for the Synthesis of Tetrahydrofurans

The Pd-catalyzed coupling of γ-hydroxyalkenes with aryl bromides affords enantiomerically enriched 2-(arylmethyl)tetrahydrofuran derivatives in good yield and up to 96:4 e.r. This transformation was achieved through the development of a new TADDOL/2-arylcyclohexanol-derived chiral phosphite ligand. The transformations are effective with an array of different aryl bromides, and can be used for the preparation of products bearing quaternary stereocenters.

Aza-Wittig Rearrangements of N-Benzyl and N-Allyl Glycine Methyl Esters. Discovery of a Surprising Cascade Aza-Wittig Rearrangement/Hydroboration Reaction

Treatment of N-(arylmethyl)-N-aryl or N-allyl-N-aryl glycine methyl ester derivatives with (n)Bu2BOTf and (i)Pr2NEt effects an aza-Wittig rearrangement that provides N-aryl phenylalanine methyl ester derivatives and N-aryl allylglycine methyl ester derivatives, respectively, in good yield with moderate to good diastereoselectivity. Under similar conditions, analogous substrates bearing N-carbonyl groups are converted to 1,4,2-oxazaborole derivatives. Additionally, N-allyl-N-aryl glycine methyl ester derivatives subjected to similar conditions at elevated temperatures undergo an aza-[2,3]-Wittig rearrangement, followed by a subsequent hydroboration oxidation reaction, to afford substituted amino alcohol products.

Asymmetric Pd-Catalyzed Alkene Carboamination Reactions for the Synthesis of 2-Aminoindane Derivatives

A new type of Pd-catalyzed alkene carboamination reaction that provides direct access to enantioenriched 2-aminoindanes from 2-allylphenyltriflate derivatives and aliphatic amines is described. A catalyst generated in situ from Pd(OAc)2 and (S)-tert-butylPHOX provides the functionalized carbocycles in good yield with up to >99:1 er. The transformations occur via a key anti-aminopalladation that involves intermolecular attack of an amine nucleophile on an arylpalladium alkene complex.

Palladium-Catalyzed Alkene Carboamination Reactions of Electron-Poor Nitrogen Nucleophiles

Modified reaction conditions that facilitate Pd-catalyzed alkene carboamination reactions of electron-deficient nitrogen nucleophiles are reported. Pent-4-enylamine derivatives bearing N-tosyl or N-trifluoroacetyl groups are coupled with aryl triflates to afford substituted pyrrolidines in good yield. These reactions proceed via a mechanism involving anti-aminopalladation of the alkene, which differs from previously reported analogous reactions of N-aryl and N-boc pentenylamines. The application of these conditions to a formal synthesis of (±)-aphanorphine is also described.

Synthesis of Polycyclic Nitrogen Heterocycles via Cascade Pd-Catalyzed Alkene Carboamination/Diels-Alder Reactions

Cascade Pd-catalyzed alkene carboamination/Diels-Alder reactions between bromodienes and amines bearing two pendant alkenes are described. These transformations generate 4 bonds, 3 rings, and 3-5 stereocenters to afford polycyclic nitrogen heterocycles with high diastereoselectivity.

Synthesis of substituted 2-aminoimidazoles via Pd-catalyzed alkyne carboamination reactions. Application to the synthesis of preclathridine natural products

A new method for the synthesis of 2-aminoimidazole products is described. The heterocyclic products are generated in good yields via Pd-catalyzed carboamination reactions of N-propargyl guanidines and aryl triflates. This methodology generates both a C-N and C-C bond during the annulation step and facilitates the rapid construction of 2-aminoimidazole products with different aryl groups. The utility of this methodology was demonstrated in the total synthesis of preclathridine A, preclathridine B, and dorimidazole B from a single intermediate.

Influence of catalyst structure and reaction conditions on anti- versus syn-aminopalladation pathways in Pd-catalyzed alkene carboamination reactions of N-allylsulfamides

The Pd-catalyzed coupling of N-allylsulfamides with aryl and alkenyl triflates to afford cyclic sulfamide products is described. In contrast to other known Pd-catalyzed alkene carboamination reactions, these transformations may be selectively induced to occur by way of either anti- or syn-aminopalladation mechanistic pathways by modifying the catalyst structure and reaction conditions.

Stereocontrolled synthesis of bicyclic sulfamides via Pd-catalyzed alkene carboamination reactions. Control of 1,3-asymmetric induction by manipulating mechanistic pathways

A new annulation strategy for the synthesis of trans-bicyclic sulfamides is described. The Pd-catalyzed alkene carboamination reactions of 2-allyl and cis-2,5-diallyl pyrrolidinyl sulfamides with aryl and alkenyl triflates afford the fused bicyclic compounds in good yields and with good diastereoselectivity (up to 13:1 dr). Importantly, by employing reaction conditions that favor an anti-aminopalladation mechanism, the relative stereochemistry between the C3 and C4a stereocenters of the products is reversed relative to related Pd-catalyzed carboamination reactions that proceed via syn-aminopalladation.

Synthesis of substituted tetrahydroindoloisoquinoline derivatives via intramolecular Pd-catalyzed alkene carboamination reactions

Intramolecular Pd-catalyzed alkene carboamination reactions of substituted 2-allyl-N-(2-bromobenzyl)anilines are described. The substrates for these reactions are generated in two steps from readily available 2-allylanilines and 2-bromobenzaldehyde derivatives. The transformations afford substituted tetrahydroindoloisoquinolines, an uncommon class of fused bicyclic heterocycles, in good yield. The mechanism of these transformations is described, and a model that accounts for the observed product stereochemistry is proposed.

Enantioselective Synthesis of Tetrahydroquinolines, Tetrahydroquinoxalines, and Tetrahydroisoquinolines via Pd-Catalyzed Alkene Carboamination Reactions

A catalyst composed of Pd₂(dba)₃ and (S)-Siphos-PE provides excellent results in Pd-catalyzed alkene carboamination reactions between aniline derivatives bearing pendant alkenes and aryl or alkenyl halides. These transformations generate tetrahydroquinolines and tetrahydroquinoxalines that contain quaternary carbon stereocenters with high levels of asymmetric induction. In addition this catalyst also effects the asymmetric synthesis of tetrahydroisoquinolines via related transformations of 2-allylbenzylamines. In contrast to most other approaches to the asymmetric synthesis of these compounds, which frequently involve functional group interconversion or a single C-C or C-N bond-forming event, the carboamination reactions generate both a C-N bond, a C-C bond, and a stereocenter.

Synthesis of cyclic guanidines via Pd-catalyzed alkene carboamination

A new approach to the synthesis of substituted 5-membered cyclic guanidines is described. Palladium-catalyzed alkene carboamination reactions between acyclic N-allyl guanidines and aryl or alkenyl halides provide these products in good yield. This method allows access to a number of different cyclic guanidine derivatives in only two steps from readily available allylic amines.

Desymmetrization of meso-2,5-diallylpyrrolidinyl ureas through asymmetric palladium-catalyzed carboamination: stereocontrolled synthesis of bicyclic ureas

The stereoselective synthesis of fused bicyclic ureas 8 is accomplished via enantioselective Pd-catalyzed desymmetrizing carboamination reactions of meso -2,5-diallylpyrroldinyl urea 7c . The reactions generate a C–N bond, a C–C bond, and afford products bearing three stereocenters with good diastereoselectivity (6–12:1 dr) and enantioselectivity (up to 95:5 er). The N -(p -chlorophenyl) group can be cleaved in good yield using a two step sequence. In addition, 8c was transformed to a tricyclic guanidine product using a four-step (two pot) procedure and was converted to 9-epi -batzelladine k in seven steps.

Synthesis of enantiomerically enriched imidazolidin-2-ones through asymmetric palladium-catalyzed alkene carboamination reactions

Positive water effect: A catalyst composed of [Pd(2)(dba)(3)] (dba=dibenzylideneacetone) and (S)-Siphos-PE is effective for the enantioselective coupling of N-allyl ureas with aryl bromides to afford 4-substituted imidazolidin-2-ones. Added water leads to significantly improved enantioselectivities with electron-poor aryl halide substrates. It is suggested that the C-C bond-forming reductive elimination is the enantiodetermining step in these reactions.

Recent Developments in Pd-Catalyzed Alkene Aminoarylation Reactions for the Synthesis of Nitrogen Heterocycles

This short review describes new developments in Pd-catalyzed aminoarylation reactions between aryl halides and alkenes bearing pendant nitrogen nucleophiles. These transformations provide a novel and powerful method for accessing numerous 3-, 5-, 6-, and 7-membered nitrogen heterocycles.

Synthesis of chromans via Pd-catalyzed alkene carboetherification reactions

A new method for the construction of 2-substituted and 2,2-disubstituted chromans via Pd-catalyzed carboetherification reactions between aryl/alkenyl halides and 2-(but-3-en-1-yl)phenols is described. These reactions effect formation of a C-O bond and a C-C bond to afford the chroman products in good yield, and also provide stereoselective access to tricyclic chroman derivatives.

Stereoselective synthesis of substituted 1,3-oxazolidines via Pd-catalyzed carboamination reactions of O-vinyl-1,2-amino alcohols

The stereoselective synthesis of 2,4- and 2,5-disubstituted 1,3-oxazolidines is accomplished via Pd-catalyzed carboamination of O-vinyl-1,2-amino alcohol derivatives. The transformations generate cis-disubstituted products with good to excellent diastereoselectivity, and enantiomerically enriched substrates are converted without loss of optical purity. In addition to yielding synthetically useful products that are difficult to generate with existing methods, these transformations illustrate that electron-rich enol ethers are viable substrates for alkene carboamination processes.

Cascade intramolecular N-arylation/intermolecular carboamination reactions for the construction of tricyclic heterocycles

A new method for the stereoselective synthesis of tetrahydropyrroloindoles and hexahydropyrroloquinolines of general structure 8 is described. These products are formed through cascade Pd-catalyzed coupling reactions between aryl chlorides and unsaturated amine substrates 5. A single catalyst effects an intramolecular N-arylation reaction followed by an intermolecular alkene carboamination reaction to generate two rings, three bonds, and one stereocenter with good chemoselectivity, diastereoselectivity, and chemical yield.

Intramolecular alkene carboamination reactions for the synthesis of enantiomerically enriched tropane derivatives

The synthesis of tropane derivatives via intramolecular Pd-catalyzed alkene difunctionalization reactions is described. Enantiopure N-aryl-γ-aminoalkenes bearing an aryl or alkenyl halide adjacent to the amino group were converted to benzo- or cycloalkenyl-fused tropane products in good yield and with no loss of enantiopurity.