Alkene Acceptors Research Articles

Photocatalytic Reductive Olefin Hydrodifluoroalkylation Enabled by Tertiary Amine Reductants Compatible with Complex Systems.

Noncanonical amino acids (NCAAs) are imperative to many facets of chemistry and biology. Herein, we report a method for the reductive hydrodifluoroalkylation of olefins that utilizes triethylamine base as the terminal reductant. The alkene acceptors include a range of electronically diverse alkenes, chief among them, dehydroalanine in variously protected forms, which provides access to synthetically relevant NCAA scaffolds under mild and general reaction conditions. We have demonstrated that a chiral auxiliary may be incorporated to provide diastereocontrol for pro-stereogenic substrates. Mechanistically motivated experiments provide some insight into the reaction mechanism, which supports a terminal step involving proton transfer for electron-poor olefins, while H atom transfer assisted by a thiol cocatalyst may complete the catalytic cycle for electron-rich olefins. The protocol is found to be compatible with additions to complex molecules, including the natural product thiostrepton.

Discovery and mechanistic investigation of photoinduced sp3 C–H activation of hydrocarbons by the simple anion hexachlorotitanate

The selective transformation of hydrocarbons into more chemically complex materials is an evergreen challenge in chemistry and catalysis. This report finds that the dianion hexachlorotitanate (TiCl 6 2− ) catalyzes the C–H activation of saturated hydrocarbons under 390 nm light irradiation. Investigations into the mechanism of this reaction are detailed. The photolysis event affords the formation of a chlorine radical and a mixture of Ti III Cl 4 (NCMe) 2 − and Ti III Cl 5 (NCMe) 2− . The Ti III Cl x species were characterized by spectrophotometry, electrochemistry, and X-ray crystallography. Alkyl radicals generated by these means were trapped by a range of alkene acceptors. Notably, the Ti III Cl x species were shown to be more reducing (Δ E pa = 0.72 V) than related Ce III Cl x species, enabling access to more electron-rich acceptors by facilitating the reduction of the alkyl radical trapped intermediate. Density functional theory calculations correctly identified the reactivity of alkene substrates using the CeCl 6 2− and TiCl 6 2− photoredox catalysts. The results herein demonstrate the impact of metal identity on C–H activation. • Identification of a simple TiCl 6 2− dianion that photolytically produces chlorine radicals • Identification of the titanium species produced after Ti–Cl bond homolysis • Reactivity studies with the gaseous ethane and methane • Increased scope of radical acceptors relative to the previously reported CeCl 6 2− dianion Recent reports on photochemical C–H functionalization of methane and other hydrocarbons via chlorine radical intermediates have renewed interest in metal photocatalysts that efficiently deliver such reactive, electrophilic radicals. Moreover, little mechanistic work has been performed to understand the impact of metal identity in the context of photocatalysis. This paper reports on the discovery of hexachlorotitanate (TiCl 6 2− ) as an efficient photocatalyst for C–H functionalization and examines reactivity differences between isostructural TiCl 6 2− and hexachlorocerate (CeCl 6 2− ) anions. The breadth of radical acceptors is notably increased as a result of the superior performance characteristics of TiCl 6 2− . Mechanistic studies implicate titanium species produced by Ti–Cl bond homolysis and indicate that subsequent reduction of radical intermediates is essential to the increased reactivity scope. We expect this work to have broad implications for photolytic C–H functionalization chemistry. The functionalization of simple hydrocarbons into more complex compounds is an important chemical goal. Photolysis of hexachlorotitanate (TiCl 6 2− ) produces highly reactive chlorine radicals that can react with hydrocarbon C–H bonds, including the strong C–H bonds of methane, allowing for the capture of methyl radicals. An exploration of the mechanism of hexachlorotitanate’s reactivity with alkene radical acceptors is explored via a combination of spectrophotometry, electrochemistry, X-ray crystallography, and DFT calculations.

Photocatalyst-free hydroacylations of electron-poor alkenes and enones under visible-light irradiation.

Herein we present a photocatalyst- and additive-free radical hydroacylation of electron-poor double bonds under mild reaction conditions. Using 4-acyl-Hantzsch ester radical reservoirs, various Michael acceptors, enones and para-quinone methide substrates could be used. The protocol enabled further derivatizations and it could also be extended to a few unactivated alkenes. Moreover, the nature of the radical process was also investigated.

NiH‐Catalyzed Migratory Defluorinative Olefin Cross‐Coupling: Trifluoromethyl‐Substituted Alkenes as Acceptor Olefins to Form gem‐Difluoroalkenes

AbstractWe report a NiH‐catalyzed migratory defluorinative coupling between two electronically differentiated olefins. A broad range of unactivated donor olefins can be joined directly to acceptor olefins containing an electron‐deficient trifluoromethyl substituent in both intra‐ and intermolecular fashion to form gem‐difluoroalkenes. This migratory coupling shows both site‐ and chemoselectivity under mild conditions, with the formation of a tertiary or quaternary carbon center.

NiH-Catalyzed Migratory Defluorinative Olefin Cross-Coupling: Trifluoromethyl-Substituted Alkenes as Acceptor Olefins to Form gem-Difluoroalkenes.

We report a NiH-catalyzed migratory defluorinative coupling between two electronically differentiated olefins. A broad range of unactivated donor olefins can be joined directly to acceptor olefins containing an electron-deficient trifluoromethyl substituent in both intra- and intermolecular fashion to form gem-difluoroalkenes. This migratory coupling shows both site- and chemoselectivity under mild conditions, with the formation of a tertiary or quaternary carbon center.

NiH-Catalyzed Migratory Defluorinative Olefin Cross-Coupling: Trifluoromethyl-Substituted Alkenes as Acceptor Olefins to Form gem-Difluoroalkenes

NiH-Catalyzed Migratory Defluorinative Olefin Cross-Coupling: Trifluoromethyl-Substituted Alkenes as Acceptor Olefins to Form <i>gem</i>-Difluoroalkenes

Nickel-catalyzed intermolecular oxidative Heck arylation driven by transfer hydrogenation

The conventional oxidative Heck reaction between aryl boronic acids and alkenes typically involved the PdII/Pd0/PdII catalytic cycle incorporating an external oxidant and often suffered C=C bond isomerization for internal alkyl-substituted alkenes via chain-walking. Herein, we demonstrate that the regioselectivity (γ-selectivity vs. δ-selectivity) and pathway selectivity (hydroarylation vs. oxidative Heck coupling) of a directed Ni-catalyzed alkene arylation can be controlled by judicious tuning of the coordination environment around the nickel catalyst via optimization of an appropriate phosphine ligand and directing group. In this way, the Ni(0)-catalyzed oxidative Heck arylation that relies on transfer hydrogenation of an acceptor olefin is developed with excellent E/Z selectivity and regioselectivity. Mechanistic investigations suggest that the addition of the acceptor is crucial for lowering the energy for carbometalation and for enabling catalytic turnover.

Kinetic Benchmarking Reveals the Competence of Prenyl Groups in Ring-Closing Metathesis.

A series of prenyl-containing malonates are kinetically benchmarked against the standard allyl-containing congeners using a ruthenium benzylidene precatalyst for ring-closing metatheses. The prenyl grouping is found to be a superior acceptor olefin compared to an allyl group in RCM processes with ruthenium alkylidenes derived from terminal alkenes. The prenyl group is also found to be a highly competent acceptor for a ruthenium alkylidene derived from a 1,1-disubstituted olefin in a RCM process.

Fe-Catalyzed C-C Bond Construction from Olefins via Radicals.

This Article details the development of the iron-catalyzed conversion of olefins to radicals and their subsequent use in the construction of C–C bonds. Optimization of a reductive diene cyclization led to the development of an intermolecular cross-coupling of electronically-differentiated donor and acceptor olefins. Although the substitution on the donor olefins was initially limited to alkyl and aryl groups, additional efforts culminated in the expansion of the scope of the substitution to various heteroatom-based functionalities, providing a unified olefin reactivity. A vinyl sulfone acceptor olefin was developed, which allowed for the efficient synthesis of sulfone adducts that could be used as branch points for further diversification. Moreover, this reactivity was extended into an olefin-based Minisci reaction to functionalize heterocyclic scaffolds. Finally, mechanistic studies resulted in a more thorough understanding of the reaction, giving rise to the development of a more efficient second-generation set of olefin cross-coupling conditions.

Synthesis and reactivity of cis-FeH2(dcpe)2 (dcpe = 1,2-bis(dicyclohexylphosphino)ethane)

A new six-coordinate iron dihydride cis-FeH2(dcpe)2 (1) has been synthesized (dcpe=1,2-bis(dicyclohexylphosphino)ethane). It has been found to react with either 1,4-cyclohexadiene or tert-butylethylene in toluene to give the respected hydrogenated hydrocarbon and the zero valent species Fe(dcpe)(toluene) (2). When this reaction with acceptor olefins was performed in methylcyclohexane, transfer dehydrogenation was observed to give low-yields of iron-bound toluene in 2.

ChemInform Abstract: Constructing Quaternary Carbons from N‐(Acyloxy)phthalimide Precursors of Tertiary Radicals Using Visible‐Light Photocatalysis.

Tertiary carbon radicals have notable utility for uniting complex carbon fragments with concomitant formation of new quaternary carbons. This article explores the scope, limitations, and certain mechanistic aspects of Okada’s method for forming tertiary carbon radicals from N-(acyloxy)phthalimides by visible-light photocatalysis. Optimized conditions for generating tertiary radicals from N-(acyloxy)phthalimide derivatives of tertiary carboxylic acids by visible-light irradiation in the presence of 1 mol % of commercially available Ru(bpy)3(PF6)2, diethyl 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate (8), and i-Pr2NEt and their coupling in dichloromethane at room temperature with alkene acceptors were developed. Four representative tertiary N-(acyloxy)phthalimides and 15 alkene radical acceptors were examined. Both reductive couplings with electron-deficient alkenes and radical substitution reactions with allylic and vinylic bromides and chlorides were examined with many such reactions occurring in go...

Fragment Coupling and the Construction of Quaternary Carbons Using Tertiary Radicals Generated From tert-Alkyl N-Phthalimidoyl Oxalates By Visible-Light Photocatalysis.

The coupling of tertiary carbon radicals with alkene acceptors is an underdeveloped strategy for uniting complex carbon fragments and forming new quaternary carbons. The scope and limitations of a new approach for generating nucleophilic tertiary radicals from tertiary alcohols and utilizing these intermediates in fragment coupling reactions is described. In this method, the tertiary alcohol is first acylated to give the tert-alkyl N-phthalimidoyl oxalate, which in the presence of visible-light, catalytic Ru(bpy)3(PF6)2, and a reductant fragments to form the corresponding tertiary carbon radical. In addition to reductive coupling with alkenes, substitution reactions of tertiary radicals with allylic and vinylic halides is described. A mechanism for the generation of tertiary carbon radicals from tert-alkyl N-phthalimidoyl oxalates is proposed that is based on earlier pioneering investigations of Okada and Barton. Deuterium labeling and competition experiments reveal that the reductive radical coupling of tert-alkyl N-phthalimidoyl oxalates with electron-deficient alkenes is terminated by hydrogen-atom transfer.

Constructing Quaternary Carbons from N-(Acyloxy)phthalimide Precursors of Tertiary Radicals Using Visible-Light Photocatalysis.

Tertiary carbon radicals have notable utility for uniting complex carbon fragments with concomitant formation of new quaternary carbons. This article explores the scope, limitations, and certain mechanistic aspects of Okada's method for forming tertiary carbon radicals from N-(acyloxy)phthalimides by visible-light photocatalysis. Optimized conditions for generating tertiary radicals from N-(acyloxy)phthalimide derivatives of tertiary carboxylic acids by visible-light irradiation in the presence of 1 mol % of commercially available Ru(bpy)3(PF6)2, diethyl 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate (8), and i-Pr2NEt and their coupling in dichloromethane at room temperature with alkene acceptors were developed. Four representative tertiary N-(acyloxy)phthalimides and 15 alkene radical acceptors were examined. Both reductive couplings with electron-deficient alkenes and radical substitution reactions with allylic and vinylic bromides and chlorides were examined with many such reactions occurring in good yield using only a slight excess (typically 1.5 equiv) of the alkene. In general, the yields of these photocatalytic reactions were higher than the analogous transformations of the corresponding N-phthalimidoyl oxalates. Deuterium labeling and competition experiments reveal that the reductive radical coupling of tertiary N-(acyloxy)phthalimides with electron-deficient alkenes can be terminated by both hydrogen-atom transfer and single-electron reduction followed by protonation, and that this mechanistic duality is controlled by the presence or absence of i-Pr2NEt.

Palladium-Catalyzed Catellani Aminocyclopropanation Reactions with Vinyl Halides

Palladium is shown to catalyze an intramolecular aminocyclopropanation of norbornenes with aliphatic vinyl halides in good yields. The reaction tolerates a variety of amine substituents and gives good results with a variety of carbocyclic and oxabicyclic [2.2.1] alkene acceptors. Notably, stabilized enolate nucleophiles were also employed in cyclopropanation reactions.

ChemInform Abstract: Development of an Asymmetric Trimethylenemethane Cycloaddition Reaction: Application in the Enantioselective Synthesis of Highly Substituted Carbocycles.

A protocol for the enantioselective [3+2] cycloaddition of trimethylenemethane (TMM) with electron-deficient olefins has been developed. The synthesis of novel phosphoramidite ligands was critical in this effort, and the preparation and reactivity of these ligands is detailed. The evolution of the ligand design, commencing with acyclic amine-derived phosphoramidites and leading to cyclic pyrrolidine and azetidine structures, is discussed. The conditions developed to effect an asymmetric TMM reaction using 2-trimethylsilylmethyl allyl acetate were shown to be tolerant of a wide variety of alkene acceptors, providing the desired methylenecyclopentanes with high levels of enantioselectivity. The donor scope was also explored, and substituted systems were tolerated, including one bearing a nitrile moiety. These donors were reactive with unsaturated acylpyrroles, giving the product cyclopentane rings bearing three stereocenters in high enantioselectivity and complete diastereoselectivity.

An all-purpose preparation of oxime carbonates and resultant insights into the chemistry of alkoxycarbonyloxyl radicals

Oxime carbonates have been synthesised and their potential studied as a new class of radical precursor. A general synthetic route, avoiding laboratory use of phosgene, was developed. Alkyl or aryl 1H-imidazole-1-carboxylates were prepared from the corresponding alcohols and 1,1-carbonyldiimidazole, then reacted with oximes in the presence of base. UV photolyses of oxime carbonates released particular N- and O-centred radicals suitable for spectroscopic examination. Spiro-cyclisations of alkoxycarbonyloxyl radicals onto aromatic acceptors and 5-exo cyclisations onto alkene acceptors were studied by EPR spectroscopy. EPR data also provided insight to the loss of CO2 from alkoxycarbonyloxyl radicals. The conformational preferences of 2-oxo-1,3-dioxolan-4-yl-methyl and related radicals were established. Modest yields of cyclic carbonates were obtained from preparative scale photolyses.

Development of an Asymmetric Trimethylenemethane Cycloaddition Reaction: Application in the Enantioselective Synthesis of Highly Substituted Carbocycles

A protocol for the enantioselective [3+2] cycloaddition of trimethylenemethane (TMM) with electron-deficient olefins has been developed. The synthesis of novel phosphoramidite ligands was critical in this effort, and the preparation and reactivity of these ligands is detailed. The evolution of the ligand design, commencing with acyclic amine-derived phosphoramidites and leading to cyclic pyrrolidine and azetidine structures, is discussed. The conditions developed to effect an asymmetric TMM reaction using 2-trimethylsilylmethyl allyl acetate were shown to be tolerant of a wide variety of alkene acceptors, providing the desired methylenecyclopentanes with high levels of enantioselectivity. The donor scope was also explored, and substituted systems were tolerated, including one bearing a nitrile moiety. These donors were reactive with unsaturated acylpyrroles, giving the product cyclopentane rings bearing three stereocenters in high enantioselectivity and complete diastereoselectivity.

ChemInform Abstract: Tridentate SCS Palladium(II) Complexes: New, Highly Stable, Recyclable Catalysts for the Heck Reaction.

A new pincer-type SCS ligand containing Pd(II) is a simple, robust catalyst for Heck chemistry using a variety of alkene acceptors and aryl iodides. It is less active with aryl bromides. While certain palladium(II) species insert slowly into the aryl C−H bond of an unsubstituted version of this ligand, the introduction of activating groups into the 5 position of the aromatic ring readily allows quantitative metal insertion. These ligands were synthesized and attached to soluble polymers by simple modification of inexpensive starting materials. For example, both 5-oxy and 5-amido SCS ligands were successfully appended to 5000 Mn poly(ethylene glycol) via ether or amide linkages, respectively. Both the 5-oxo and 5-amido complexes are active as Heck catalysts in DMF solution in air. The PEG-bound 5-amido-SCS−Pd complex was recycled via solvent precipitation three times with no observed catalyst deactivation. While the 5-amido-SCS−Pd complexes are very robust, their 5-oxo counterparts decompose slowly under c...

Triplet energy transfer from d 0 organometallic compounds to unsaturated hydrocarbons and quantum-chemical calculations: Revealing of short-lived intermediates

and M is Zr or Hf)with extremely long lifetimes (τ on the order of milliseconds at 77 K) produced as a result of ligandtometal charge transfer.Based on the results of spectral–luminescent studies [4, 5], in particular, revealing the existence of linearcorrelation between the critical radius of the sphere ofelectronexchange resonance energy transfer and thechain length of the energy acceptor olefin (Fig. 1), wehave assumed that there is innersphere coordinationof the olefin (diene) to the

Rhodium‐Catalyzed Tandem Cyclization: Formation of 1H‐Indenes and 1‐Alkylideneindans from Arylboronate Esters in Aqueous Media.

AbstractFor Abstract see ChemInform Abstract in Full Text.