Rich Alkenes Research Articles

Radical-Chain Hydrosilylation of Alkenes Enabled by Triplet Energy Transfer.

Development of mild, robust and metal-free catalytic approach for the hydrosilylation of alkenes is critical to the advancement of modern organosilicon chemistry given their powerful capacity in the construction of various C-Si bonds. Herein, we wish to disclose a visible light-triggered organophotocatalytic strategy, which proceeds via a triplet energy transfer (EnT)-enabled radical chain pathway. Notably, this redox-neutral protocol is capable of accommodating a broad spectrum of electron-deficient and -rich alkenes with excellent functional group compatibility. Electron-deficient alkenes are more reactive and the reaction could be finished within a couple of minutes even in PBS solution with extremely low concentration, which suggests its click-like potential in organic synthesis. The preparative power of the transformations has been further highlighted in a number of complex settings, including the late-stage functionalization and scale-up experiments. Furthermore, although only highly reactive (TMS)3SiH is suitable hydrosilane substrate, our studies revealed the great reactivity and versatility of (TMS)3Si- group in diverse C-Si and Si-Si bond cleavage-based transformations, enabling the rapid introduction of diverse functional groups and the facile construction of valuable quaternary silicon architectures.

With or without a co-solvent? highly efficient ultrafast phenanthrenequinone-electron rich alkene (PQ-ERA) photoclick reactions.

The light-induced photocycloaddition of 9,10-phenanthrenequinone (PQ) with electron-rich alkenes (ERA), known as the PQ-ERA reaction, is a highly attractive photoclick reaction characterized by its operational simplicity and high biocompatibility. One essential aspect of photoclick reactions is their high rate, however the limited solubility of PQs often requires the use of a co-solvent. Evaluating the effect of different co-solvents on the PQ-ERA reaction and their influence on the reaction rate, we discovered that sulfur-containing compounds, in particular the frequently used solubilizing co-solvent DMSO, quench the triplet state of the PQ. These experimental results, supported by nanosecond-microsecond and ultrafast transient absorption data, show that even minimal amounts of DMSO result in a decreased lifetime of the reactive triplet state, essential for the photoclick reaction. Without DMSO as co-solvent, exceptionally high photoreaction quantum yields ( Φ P up to 93% with only 1 equivalent ERA) and complete conversion in seconds can be achieved. With these outstanding efficiencies, the PQ-ERA reaction can be used without excess ERA and at low light intensities, facilitating photoclick transformations in various future applications.

Non-Symmetrical Tetrafluoroalkadienes Synthesized by ROCM of 3,3,4,4-Tetrafluorocyclobutene.

As the first known example of ring-opening cross metathesis (ROCM) of polyfluorinated strained cyclobutenes, ROCM of 3,3,4,4-tetrafluorocyclobutene with electronically rich alkenes, catalyzed by Grubbs or Hoveyda-Grubbs 2nd generation precatalysts, gave a small library of non-symmetrical isolated dienes bearing a tetrafluoroethylene spacer between the double bonds. 1-Butoxy-3,3,4,4-tetrafluorohexa-1,5-diene thus formed underwent subsequent regioselective cross metathesis (CM) with a series of styrenes, catalyzed by Hoveyda-Grubbs 2nd generation precatalyst, leading to non-symmetrically substituted dienes. 6,6-Dibutoxy-3,3,4,4-tetrafluorohex-1-ene, formed by regioselective butoxylation of 1-butoxy-3,3,4,4-tetrafluorohexa-1,5-diene, was dihydroxylated and cyclized to the corresponding 3,3,4,4-tetrafluorohexopyranose.

Copper(II) complexes based on isopropyl ester derivatives of bis(pyrazol-1-yl)acetate ligands with catalytic potency in organic macro(molecules) synthesis

In this work, bis(pyrazol-1-yl)acetic acid (HC(pz)2COOH) and bis(3,5-dimethyl-pyrazol-1-yl)acetic acid (HC(3,5-Me2pz)2COOH) were converted into the isopropyl ester derivatives 1 (L1iPr) and 2 (L2iPr), respectively, and used for the preparation of the Cu(II) complexes 3–6 implementing an existing method for a new field of application. All synthesized compounds were characterized by spectroscopic and physicochemical methods. The crystal structure of the ligand L2iPr was determined by X-ray diffraction analysis. The novel synthesized Cu(II) complexes were applied in the synthesis of oxidized protected olefins via Kharasch-Sosnovsky reaction, using electron rich and electron poor alkenes in order to synthesize allyl benzoates and poly(methyl methacrylate), respectively. Our findings suggest that the developed copper(II) complexes can be promising catalysts for further organic modifications.

Tuning Photoenolization-Driven Cycloadditions Using Theory and Spectroscopy.

The first broad spectrum investigation into the photoenolization/Diels-Alder (PEDA) sequence was carried out using M06-2X/6-31+G(d,p) in conjunction with SMD solvation and supported by experimental UV-vis spectroscopy. A test set of 20 prodienes was chosen to examine the role of the H atom acceptor group (substituted and unsubstituted carbonyl, thiocarbonyl, and imine), the H atom donor group, and bystander ring substituents. As reaction partners for the photogenerated dienes, a diverse test set of 20 dienophiles was examined, comprising electron rich, electron poor, neutral, strain activated, hydrocarbon, and heteroatom-containing molecules including CO2 and CO. A key finding of this work is the demonstration that the PEDA sequence of carbonyl based prodienes is tolerant of most substitution patterns. Another is that thiocarbonyl derivatives should behave analogously to the carbonyls but are likely to do so much more slowly, due to an inefficient intersystem crossing, an endothermic 1,5-hydrogen atom transfer (HAT) step, and a [1,5] sigmatropic H shift to regenerate the starting material that outcompetes the [4 + 2]cycloaddition. In contrast, the T1 state of the ortho-alkyl imines displays the incorrect orbital symmetry for 1,5-HAT and is correspondingly accompanied by higher barriers, even in the excited state. However, provided these barriers can be overcome, the remaining steps in the PEDA sequence are predicted to be facile. The Diels-Alder reaction is predicted to be of much broader scope than reported synthetic literature: while electron poor dienophiles are expected to be the most reactive partners, ethylene and electron rich alkenes should react at a synthetically useful rate. CO is predicted to undergo a facile (4 + 1)cheletropic addition instead of the normal [4 + 2]cycloaddition pathway. This unique photoenolization/cheletropic addition (PECA) sequence could provide metal-free access to benzannelated cyclopentanones.

Dimeric Cyclobutane Formation Under Continuous Flow Conditions Using Organophotoredox‐Catalysed [2+2] Cycloaddition**

AbstractRadical‐cation‐initiated dimerization of electron rich alkenes is an expedient method for the synthesis of cyclobutanes. By merging organophotoredox catalysis and continuous flow technology, a batch versus continuous flow study has been performed providing a convenient synthetic route to an important carbazole cyclobutane material dimer 1,2‐trans‐dicarbazylcyclobutane (t‐DCzCB) using only 0.1 mol % of an organophotoredox catalyst. The scope of this methodology was explored giving a new class of functional materials, as well as an improved synthetic route to styrene‐based lignan dimeric natural products. The cyclobutane dimers could be isolated in higher chemical yields under continuous flow conditions and reaction times were reduced significantly compared to traditional batch reaction conditions.

A Giese reaction for electron-rich alkenes

A general method for the hydroalkylation of electron-rich terminal and non-terminal alkenes such as enol esters, alkenyl sulfides, enol ethers, silyl enol ethers, enamides and enecarbamates has been developed. The reactions are carried out at room temperature under air initiation in the presence of triethylborane acting as a chain transfer reagent and 4-tert-butylcatechol (TBC) as a source of hydrogen atom. The efficacy of the reaction is best explained by very favorable polar effects supporting the chain process and minimizing undesired polar reactions. The stereoselective hydroalkylation of chiral N-(alk-1-en-1-yl)oxazolidin-2-ones takes place with good to excellent diastereocontrol.

Metal Free Amino‐Oxidation of Electron Rich Alkenes Mediated by an Oxoammonium Salt

Abstract4‐Acetamido‐2,2,6,6‐tetramethyl‐1‐oxopiperidinium tetrafluoroborate (Bobbitt's salt) effectively activates electron rich alkenes and promotes the addition of anilines. This transformation provides a direct, transition metal free method for amino‐oxidation of alkenes under mild conditions. The relative stereochemistry of the amino‐oxidation is influenced by solvent effects, with both the syn and anti amino‐oxidation products being accessible from identical starting materials.

Visible Light-Driven Radical trans-Hydrosilylation of Electron-Neutral and -Rich Alkenes with Tertiary and Secondary Hydrosilanes.

A visible light-driven radical hydrosilylation of electron-neutral and -rich alkenes has been investigated on the basis of a newly developed catalytic reaction system composed of eosin Y, thiol, and base additives. A variety of linear and cyclic alkenes with different substitution patterns were found to undergo such metal-free hydrosilylation with tertiary and secondary hydrosilanes in a chemo-, regio-, and stereoselective manner. Comparison of the reactivity of diene compounds and late-stage hydrosilylation of steroid drugs were also explored. Deuterium labeling experiments reveal that a stepwise formation of C-Si and C-H bonds with a trans stereochemistry is preferred, in which the thiol may behave as a hydrogen atom transfer agent.

An example of a stepwise mechanism for the catalyst-free 1,3-dipolar cycloaddition between a nitrile oxide and an electron rich alkene

The present Letter studies possible pathways for the non-catalyzed 1,3-dipolar cycloaddition (1,3-DC) reaction between a nitrile oxide derivative and a highly electron-rich alkene. A diagram of the free energy surface during the reaction coordinate was prepared by considering all routes of the 1,3-DC reaction between methyl nitrile oxide and tetraamino ethylene and by detecting any possible species which could form in the course of the predicted pathways. The results indicated that the stepwise pathway was preferable in comparison with the concerted route. Therefore, it was concluded that a switching of the mechanism from concerted to stepwise was the outcome when using strong electron donor functional groups on the dipolarophile.

Synthesis of heterodinuclear hydride complexes by oxidative addition of a transition-metal hydride to Pt(0) and Pd(0) complexes

Heterodinuclear hydridoplatinum and -palladium complexes, (dppe)HPt–MLn (MLn = MoCp(CO)3 (3a), WCp(CO)3 (3b)), (dppe)Pt(μ-H)(μ-CO)Mn(CO)4 (3c), (dppe)Pt(μ-H)(μ-CO)FeCp(CO) (3d), cis-L′2HPt–MLn (5aa: Lʹ = PPh3; MLn = MoCp(CO)3, 5ba: Lʹ = PPh3; MLn = WCp(CO)3, 5ab: Lʹ = PMePh2; MLn = MoCp(CO)3, 5ac: Lʹ = PMe2Ph; MLn = MoCp(CO)3), (dppe)Pd(μ-H)(μ-CO)MLn (MLn = MoCp(CO)2 (7a), WCp(CO)2 (7b)), (dppe)Pd(μ-H)(μ-CO)Mn(CO)4 (7c) are prepared by the oxidative addition of mononuclear transition-metal hydride complexes to zero-valent platinum or palladium complexes. The reactions of the heterodinuclear hydride complexes 3a, 3d and 7a with electron deficient alkenes and alkynes such as dimethyl fumarate or DMAD cause reductive elimination at the Pt or Pd center to give the Pt(alkene or alkyne)(dppe) and MHLn,, suggesting reversibility of this process. The DFT calculations suggest that these reactions are controlled by the thermodynamic stability and the electron rich alkene complex of Pt(0) (or Pd(0)) are preferable to prepare these heterodinuclear hydride complexes by the oxidative addition.

ChemInform Abstract: Copper(II)—Acid Catalyzed Cyclopropanation of 1,3‐Dienamides by Electrophilic Activation of α‐Aryl Diazoesters.

AbstractA facile protocol for the synthesis of substituted cyclopropanes via reaction of electron rich alkenes with α‐aryl diazoesters is presented.

Unraveling the interplay of backbone rigidity and electron rich side-chains on electron transfer in peptides: the realization of tunable molecular wires.

Electrochemical studies are reported on a series of peptides constrained into either a 310-helix (1–6) or β-strand (7–9) conformation, with variable numbers of electron rich alkene containing side chains. Peptides (1 and 2) and (7 and 8) are further constrained into these geometries with a suitable side chain tether introduced by ring closing metathesis (RCM). Peptides 1, 4 and 5, each containing a single alkene side chain reveal a direct link between backbone rigidity and electron transfer, in isolation from any effects due to the electronic properties of the electron rich side-chains. Further studies on the linear peptides 3–6 confirm the ability of the alkene to facilitate electron transfer through the peptide. A comparison of the electrochemical data for the unsaturated tethered peptides (1 and 7) and saturated tethered peptides (2 and 8) reveals an interplay between backbone rigidity and effects arising from the electron rich alkene side-chains on electron transfer. Theoretical calculations on β-strand models analogous to 7, 8 and 9 provide further insights into the relative roles of backbone rigidity and electron rich side-chains on intramolecular electron transfer. Furthermore, electron population analysis confirms the role of the alkene as a “stepping stone” for electron transfer. These findings provide a new approach for fine-tuning the electronic properties of peptides by controlling backbone rigidity, and through the inclusion of electron rich side-chains. This allows for manipulation of energy barriers and hence conductance in peptides, a crucial step in the design and fabrication of molecular-based electronic devices.

ChemInform Abstract: Oxetane Synthesis Through the Paterno—Buchi Reaction

AbstractReview: 263 refs.

Oxetane Synthesis through the Paternò-Büchi Reaction

The Paternò-Büchi reaction is a photochemical reaction between a carbonyl compound and an alkene to give the corresponding oxetane. In this review the mechanism of the reaction is discussed. On this basis the described use in the reaction with electron rich alkenes (enolethers, enol esters, enol silyl ethers, enanines, heterocyclic compounds has been reported. The stereochemical behavior of the reaction is particularly stressed. We pointed out the reported applications of this reaction to the synthesis of naturally occuring compounds.

Stereoselective intramolecular cyclopropanation through catalytic olefin activation

A novel stereoselective and convergent catalytic method for the cyclopropanation of electron-neutral and -rich olefins by sulfonium ylides is reported which unusually proceeds through olefin activation rather than by metal carbene formation.

Photoinduced decarboxylation of 3-(N-phthalimido)adamantane-1-carboxylic acid and radical addition to electron deficient alkenes

Direct and sensitized excitation of 3-(N-phthalimido)adamantane-1-carboxylic acid (1) leads to the population of the triplet state that, in the presence of a base, decarboxylates, giving N-(1-adamantyl)phthalimide (2) cleanly and efficiently (Φ = 0.11). The radical initially formed by decarboxylation adds regiospecifically to electron deficient alkenes, whereas radical addition was not observed for electron rich alkenes. The radical addition can also be applied to molecules not bearing adamantanes wherein the electron donor (carboxylate) and the acceptor (phthalimide) are separated by a rigid spacer. The photodecarboxylation induced radical addition of phthalimide derivative 1 to alkenes takes place in good to excellent yields and represents a mild and efficient method for C-C bond formation.

N-Heterocyclic carbene-borane radicals as efficient initiating species of photopolymerization reactions under air

Eight N-heterocyclic carbene-boranes (NHC-boranes) are proposed as new efficient co-initiators for acrylate photopolymerization reactions. They are particularly interesting in aerated conditions, where they help overcome the classical oxygen inhibition. The carbene boryl radicals that are the initiating species have been characterized by their transient absorption spectra obtained in laser flash photolysis (LFP) experiments. Rate constants for the generation of the carbene boryl radicals by hydrogen abstraction with t-butoxyl radical and triplet benzophenone as well as the reactions with oxygen, electron rich and electron poor alkenes, two alkyl halides (CHCl3 and iodopropane) and diphenyliodonium hexafluorophosphate have been measured. The reactivity of N-heterocyclic carbene borane radicals is clearly affected by the NHC substituent.

Organocatalytic diimide reduction of enamides in water

Bridged flavinium organocatalysts have displayed efficacy in the diimide mediated reduction of enamides in aqueous conditions. This represents the first diimide reduction of an electron rich alkene and offers a clean alternative to the use of alkylating agents for N-alkylation.

ChemInform Abstract: (Me3Si)3SiH‐Mediated Intermolecular Radical Perfluoroalkylation Reactions of Olefins in Water

Abstractas a new and convenient route to various perfluorinated compounds