C-H Functionalization and Allylic Amination for Post-Polymerization Modification of Polynorbornenes.

Post-polymerization modification (PPM) via direct C-H functionalization is a powerful synthetic strategy to convert polymer feed-stocks into value-added products. We found that a metal-free, Se-catalyzed allylic C-H amination provided an efficient method for PPM of polynorbornenes (PNBs) produced via ring-opening metathesis polymerization. Inherent to the mechanism of the allylic amination, PPM on PNBs preserved the alkene functional groups along the polymer backbone, while also avoiding transposition of the double bonds. Amination using a series of aryl sulfonamides led to good control over the degree of functionalization, access to a range of functionalities, and tunable thermal properties from the resulting polymers.

Upcycling of Polybutadiene Facilitated by Selenium-Mediated Allylic Amination.

Accumulation of end-of-life plastics presents ongoing environmental concerns. One strategy to solve this grand challenge is to invent new techniques that modify post-consumer waste and impart new functionality. While promising approaches for the chemical upcycling of commodity polyolefins and polyaromatics exist, analogous approaches to repurpose unsaturated polymers (e.g., polybutadiene) are scarce. In this work, we propose a method to upcycle polybutadiene, one of the most widely used commercial rubbers, via a mild, metal-free allylic amination reaction. The resulting materials have tunable thermal and surface wetting properties as a function of both sulfonamide identity and grafting density. Importantly, this approach maintains the parent alkene microstructure without evidence of olefin reduction, olefin transposition, and/or chain scission. Based on these findings, we anticipate future applications in the remediation of complex elastomers and vulcanized rubbers.

Metal-Free Allylic C-H Amination of Vinylsilanes and Vinylboronates using Silicon or Boron as a Regioselectivity Switch.

Vinylsilanes and vinylboronates are common building blocks for organic synthesis, but direct functionalization of these species without the participation of either the C=C or C-Si/B bonds is rare. Herein, we report a metal-free allylic C-H amination reaction of these vinylmetalloid species that installs a new C-N bond without competing transmetallation or alkene addition. In this transformation, the silicon or boron substituent inverts the usual regioselectivity, directing amination to the site distal to that group. Subsequent cross-coupling or demetallation allows access to complementary regioisomeric products. Density Functional Theory computations revealed that the observed regioselectivity is due to a subtle combination of electronic and counterintuitive steric factors that favor initial attack of selenium at the silicon-bearing carbon atom.

Catalytic Metal-free Allylic C-H Amination of Terpenoids.

The selective replacement of C-H bonds in complex molecules, especially natural products like terpenoids, is a highly efficient way to introduce new functionality and/or couple fragments. Here, we report the development of a new metal-free allylic amination of alkenes that allows the introduction of a wide range of nitrogen functionality at the allylic position of alkenes with unique regioselectivity and no allylic transposition. This reaction employs catalytic amounts of selenium in the form of phosphine selenides or selenoureas. Simple sulfonamides and sulfamates can be used directly in the reaction without the need to prepare isolated nitrenoid precursors. We demonstrate the utility of this transformation by aminating a large set of terpenoids in high yield and regioselectivity.

Chelation-driven rearrangement of primary alkyl aminopalladation products to stable trisubstituted alkyl-palladium complexes.

The formation of highly substituted carbon centers using catalysis has been a widely sought after goal, but complexes of highly substituted carbon atoms with transition metals are rare, and the factors that affect the relative stability of complexes with differentially substituted carbon atoms are poorly understood. In this study, a set of equilibrating alkyl-palladium complexes were subtly tuned to form either a primary or trisubstituted alkyl complex as the more thermodynamically favored state, depending on either the substrate or reaction conditions. An X-ray crystal structure of the trisubstituted alkyl-palladium complex is presented and compared with the corresponding primary alkyl complex. The mechanism for rearrangement and the factors that drive the change in stability are discussed.

Palladium-catalyzed cross-coupling of N-sulfonylaziridines with boronic acids.

A mild palladium-catalyzed cross-coupling of unsubstituted and 2-alkyl-substituted aziridines with arylboronic acid nucleophiles is presented. The reaction is highly regioselective and compatible with diverse functionality. A catalytic amount of base, a sterically demanding triarylphosphine ligand, and a phenol additive are critical to the success of the reaction. Coupling of a deuterium-labeled substrate established that ring opening of the aziridine occurs with inversion of stereochemistry.

Enantioselective palladium-catalyzed diamination of alkenes using N-fluorobenzenesulfonimide.

An enantioselective Pd-catalyzed vicinal diamination of unactivated alkenes using N-fluorobenzenesulfonimide as both an oxidant and a source of nitrogen is reported. The use of Ph-pybox and Ph-quinox ligands afforded differentially protected vicinal diamines in good yields with high enantioselectivities. Mechanistic experiments revealed that the high enantioselectivity arises from selective formation of only one of four possible diastereomeric aminopalladation products of the chiral Pd complex. The aminopalladation complex was characterized by X-ray crystallography.

Diastereoconvergent synthesis of anti-1,2-amino alcohols with N-containing quaternary stereocenters via selenium-catalyzed intermolecular C-H amination.

We report a diastereoconvergent synthesis of anti-1,2-amino alcohols bearing N-containing quaternary stereocenters using an intermolecular direct C-H amination of homoallylic alcohol derivatives catalyzed by a phosphine selenide. Destruction of the allylic stereocenter during the selenium-catalyzed process allows selective formation of a single diastereomer of the product starting from any diastereomeric mixture of the starting homoallylic alcohol derivatives, eliminating the need for the often-challenging diastereoselective preparation of starting materials. Mechanistic studies show that the diastereoselectivity is controlled by a stereoelectronic effect (inside alkoxy effect) on the transition state of the final [2,3]-sigmatropic rearrangement, leading to the observed anti selectivity. The power of this protocol is further demonstrated on an extension to the synthesis of syn-1,4-amino alcohols from allylic alcohol derivatives, constituting a rare example of 1,4-stereoinduction.

Stereoretentive and regioselective selenium-catalyzed intermolecular propargylic C-H amination of alkynes.

Herein we report an intermolecular propargylic C-H amination of alkynes. This reaction is operationally convenient and requires no transition metal catalysts or additives. Terminal, silyl, and internal alkynes bearing a wide range of functional groups can be aminated in high yields. The regioselectivity of amination for unsymmetrical internal alkynes is strongly influenced by substitution pattern (tertiary > secondary > primary) and by relatively remote heteroatomic substituents. We demonstrate that amination of alkynes bearing α-stereocenters occurs with retention of configuration at the newly-formed C-N bond. Competition experiments between alkynes, kinetic isotope effects, and DFT calculations are performed to confirm the mechanistic hypothesis that initial ene reaction of a selenium bis(imide) species is the rate- and product-determining step. This ene reaction has a transition state that results in substantial partial positive charge development at the carbon atom closer to the amination position. Inductive and/or hyperconjugative stabilization or destabilization of this positive charge explains the observed regioselectivities.

Metal-free highly regioselective aminotrifluoroacetoxylation of alkenes.

We report a highly regioselective metal-free oxidative cyclization of sulfonamides onto tethered, unactivated alkenes using hypervalent iodine and Brønsted acids. Under these conditions the acid counterion is incorporated into the cyclized products providing an overall aminotrifluoroacetoxylation of the alkene. An unusual preference for endo ring closure is exhibited in contrast to existing exo selective methods. Multiple ring sizes can be formed to access functionalized pyrrolidines, piperidines, and azepanes with a general preference for endo cyclization. A variety of substrate substitution patterns were tolerated to provide nitrogen-containing heterocycles with high regioselectivities and good to excellent diastereoselectivities.

Platinum-catalyzed intramolecular hydrohydrazination: evidence for alkene insertion into a Pt-N bond.

Dicationic (bpy)Pt(II) complexes were found to catalyze the intramolecular hydrohydrazination of alkenes. Reaction optimization revealed Pt(bpy)Cl(2) (10 mol %) and AgOTf (20 mol %) in DMF-d(7) to be an effective catalyst system for the conversion of substituted hydrazides to five- and six-membered N-amino lactams (N-amino = N-acetamido at 120 degrees C, N-phthalimido at 80 degrees C, (-)OTf = trifluoromethanesulfonate). Of the four possible regioisomeric products, only the product of 5-exo cyclization at the proximal nitrogen is formed, without reaction at the distal nitrogen or 6-endo cyclization. The resting states were found to be a 2:1 Pt-amidate complex (25, for N-acetamido) of the deprotonated hydrazide and a Pt-alkyl complex of the cyclized pyrrolidinone (20 for N-phthalimido). Both complexes are catalytically competent. Catalysis using 25 as the precatalyst shows no rate dependence on added acid (HOTf) or base (2,6-lutidine). The available mechanistic data are all consistent with a mechanism involving N-H activation of the hydrazide, followed by insertion of the alkene into the Pt-N bond, and finally protonation of the resulting cyclized alkyl complex by hydrazide to release the hydrohydrazination product and regenerate the active Pt-amidate catalyst.

Palladium-catalyzed alkoxyamination of alkenes with use of N-fluorobenzenesulfonimide as oxidant.

A Pd-catalyzed alkoxyamination of protected aminoalkenes promoted by N-fluorobenzenesulfonimide is described. This mild transformation allows the direct formation of ethers from carbon-carbon double bonds. An unusual switch from exo to endo selectivity in polar solvents was discovered, allowing the selective formation of either regioisomer by careful choice of reaction conditions.

Selenophosphoramide-catalyzed diamination and oxyamination of alkenes.

A new selenophosphoramide-catalyzed diamination of terminal- and trans-1,2-disubstituted olefins is presented. Key to the success of this transformation was the introduction of a fluoride scavenger, trimethylsilyl trifluoromethanesulfonate (TMSOTf), to prevent a competitive syn-elimination pathway, as was the use of a phosphoramide ligand on selenium to promote the desired substitution reaction. A screen of catalysts revealed that more electron-rich phosphine ligands resulted in higher yields of the desired product, with selenophosphoramides giving the optimal results. A broad range of substrates and functional groups were tolerated and yields were generally good to excellent. For (E)-1,2-disubstituted olefins, diastereoselectivities were always high, giving exclusively anti products. The conditions were also applied to substrates bearing internal nucleophiles such as esters and carbonates, giving rise to 1,2-aminoesters and cyclic carbonates, respectively.

Palladium-catalyzed carboamination of alkenes promoted by N-fluorobenzenesulfonimide via C-H activation of arenes.

This report describes a unique Pd-catalyzed oxidative carboamination of protected aminoalkenes in which inexpensive unactivated nucleophilic arenes are incorporated to give carboamination products in good yields. A variety of protected amide and carbamate groups are tolerated, and various five-, six-, and seven-membered rings are formed in good yields. Under these conditions, halobenzenes are activated at the C-H bond rather than the C-X bond, and very high regioselectivity for the para substitution product is observed in all cases. We propose that this carboamination takes place via electrophilic aromatic substitution of a Pd(IV) alkyl intermediate.

Mechanism of N-fluorobenzenesulfonimide promoted diamination and carboamination reactions: divergent reactivity of a Pd(IV) species.

The mechanism of the Pd-catalyzed diamination and carboamination of alkenes promoted by N-fluorobenzenesulfonimide (NFBS) was investigated. Stereochemical labeling experiments established that the diamination reaction proceeds via overall syn addition of the two nitrogen groups, whereas carboamination is the result of an anti addition of arene and nitrogen to the alkene. The intermediate Pd-alkyl complex arising from aminopalladation was observed, and an X-ray crystal structure of its 2,2'-bipyridine (bipy) complex was obtained, revealing strong chelation of the amide protecting group to palladium. Aminopalladation was shown to be an anti-selective process in both the presence and the absence of added ligands, proceeding via external attack of the nitrogen on a Pd-coordinated alkene. The intermediate Pd-alkyl complex was converted to diamination product upon exposure to NFBS with inversion of configuration via oxidative addition followed by dissociation of the benzenesulfonimide anion and S(N)2 displacement of the Pd-C bond. Conversely, arylation of the Pd-alkyl complex proceeds via retention of stereochemistry, consistent with C-H activation of the arene at the Pd(IV) center. A small intermolecular isotope effect (k(H)/k(D) = 1.1) and a large intramolecular isotope effect (k(H)/k(D) = 4) were measured for this process, indicating that C-H activation occurs via a poorly selective product-determining coordination of the arene followed by a highly selective C-H activation. Competition between arenes reveals an unusual reactivity order of toluene > benzene > bromobenzene > anisole.

Mechanistic studies of a palladium-catalyzed intramolecular hydroamination of unactivated alkenes: protonolysis of a stable palladium alkyl complex is the turnover-limiting step.

Mechanistic studies of the intramolecular hydroamination of unactivated aminoalkenes catalyzed by a dicationic [bis(diphenylphosphinomethyl)pyridine]palladium complex highlight the important role that protonolysis plays in this reaction. Coordination of the aminoalkene substrate to this complex activates the alkene toward intramolecular nucleophilic attack to form a dicationic palladium alkyl complex (6). A stable monocationic palladium alkyl complex (7) was isolated by in situ deprotonation of 6 with mild base, and its structure was confirmed by X-ray crystallography. Complex 7 reacted rapidly with a variety of strong acids to undergo protonolysis, resulting in formation of hydroamination product 3 and regenerating the active catalyst. Evidence that formation of the palladium alkyl complex is reversible under the catalytic conditions was obtained from observation of the protonolysis at low temperature. During the course of all catalytic reactions, the resting state of the catalyst was palladium alkyl complex 7, indicating that protonolysis of the Pd-C bond was the turnover-limiting step. Kinetic studies reveal an unusual inverse dependence of the reaction rate on the concentration of the aminoalkene substrate. This effect can be accurately explained by a model in which the carbamate protecting group of the aminoalkene acts as a Brønsted base to remove free protons from the catalytic cycle and thereby inhibits the turnover-limiting protonolysis step. Formation of a 2:1 complex (12) between the carbamate and the proton is most consistent with the kinetic data.

Synthesis of 2,6-disubstituted piperazines by a diastereoselective palladium-catalyzed hydroamination reaction.

A highly diastereoselective intramolecular hydroamination is the key step in a modular synthesis of 2,6-disubstituted piperazines. The requisite hydroamination substrates were prepared in excellent yields by nucleophilic displacement of cyclic sulfamidates derived from amino acids. A variety of alkyl and aryl substituents at the 2-position were tolerated. The stereochemistry of the piperazines was determined to be trans by X-ray crystallography, which also showed the preferred conformation of the 2,6-disubstituted piperazine to be a twist-boat due to A(1,3) strain.

Palladium-catalyzed intramolecular chloroamination of alkenes.

A mild and facile Pd-catalyzed intramolecular chloroamination of unactivated alkenes has been described. This reaction takes place at room temperature and is tolerant of synthetically useful acid-sensitive functional groups. Generally high exo-selectivities are observed in the formation of a variety of 5- and 6-membered rings. This system is unique in its ability to tolerate multidentate ligands on palladium, which opens up the possibility of controlling the absolute sense of induction using a chiral ligand.

Regioselective Metal-Free Aza-Heck Reactions of Terminal Alkenes Catalyzed by Phosphine Selenides.

Phosphine selenides are introduced as an alternate class of selenium-based catalysts for the aza-Heck reaction of alkenes. Using these catalysts, a range of terminal alkenes react with NFBS to give oxidative amination products. Judicious choice of phosphine ligand gives greater regio- and stereoselectivity than with diphenyl diselenide, enabling the selective formation of E terminal enimides in high yields. Isotope-labeling experiments and measurements of kinetic isotope effects reveal that the reaction occurs stereospecifically via irreversible anti addition, followed by rate-determining syn elimination.