Palladium-catalyzed vicinal difunctionalization of internal alkenes: diastereoselective synthesis of diamines

Internal affairs: the first general palladium-catalyzed intermolecular diamination of internal alkenes employs different nitrogen sources, which add to the alkene in a regio- and diastereoselective fashion. The resulting diamination products can be converted directly into a known ligand motif.     

Radical-mediated difunctionalization of unactivated alkenes through distal migration of functional groups

Radical-mediated controllable difunctionalization of alkenes provides a powerful tool for the manipulation of olefins and has become a hot topic recently. In general, however, the scope of alkene is largely restricted to the activated alkenes. The development of a general strategy for the functionalization of unactivated alkenes is desired, yet remains challenging. In this review, we have summarized the recent advances in the strategy of intramolecularly distal migration of functional groups which has been efficiently applied in the radical-mediated difunctionalization of unactivated alkenes. A portfolio of functionalities including aryl, cyano, heteroaryl, imino, carbonyl, alkynyl, and alkenyl groups showcase the migratory aptitude.     

Enantioselective difunctionalization of alkenes by a palladium-catalyzed Heck/borylation sequence

A palladium catalyzed enantioselective Heck/borylation reaction of alkene-tethered aryl iodides was realized, delivering a variety of 2,3-dihydrobenzofuranyl boronic esters in high yield with excellent enantioselectivity. Asymmetric synthesis of chromane boronic ester, indane boronic ester and indoline boronic ester was also accomplished. The protocol offers an efficient access to the corresponding chiral benzocyclic boronic esters, which are notably important chemical motifs in synthetic transformations.

A palladium catalyzed enantioselective Heck/borylation reaction of alkene-tethered aryl iodides was realized, delivering a variety of 2,3-dihydrobenzofuranyl boronic esters in high yield with excellent enantioselectivity.     

Recent advances on catalytic asymmetric difunctionalization of 1,3-dienes

1,3-Dienes and derivatives are either feedstock chemicals or easily available materials. Catalytic difunctionalization of 1,3-diene is one of the most powerful methods for carboncarbon bond formation with rapid increase of the molecular complexity and synthetic value in an atom economic way. By choosing proper metals and chiral ligands, a variety of catalytic asymmetric difunctionalization of conjugated dienes in a highly regioselective fashion have been reported. In this digest review, we will summarize recent advances on this topic based on different metals. We will also introduce unique phenomena that include reversal of regio- and diastereoselectivity.     

Hypoiodite-mediated metal-free catalytic aziridination of alkenes

Look, no metal: A metal-free catalytic procedure for aziridination of alkenes using tetrabutylammonium iodide as the catalyst, m-chloroperoxybenzoic acid (mCPBA) as the terminal oxidant, and N-aminophthalimide as the nitrenium precursor has been developed (see scheme; right: X-ray structure of one of the products). Control experiments suggests that the active oxidant is in?situ generated hypoiodous acid (HIO).     

Thiiranes: One-pot synthesis from alkenes,and catalytic desulphurization.

The reaction of bis(trimethylsilyl)sulphide with bromine at −78°C forms trimethylsilylsulphenyl bromide which reacts with alkenes to give thiiranes. Trimethylsilyl bromide and iodide catalytically desulphurize thiiranes to alkenes.     

A preliminary screening technique for selected metals at waste sites

Fast neutron activation analysis (FNAA) was investigated as a possible on-site preliminary screening technique for metal contamination of soil. Two metals, Cu and Zn, were used in a laboratory setting to evaluate the possibility of detecting metal contamination of soil at or below the maximum permissible metal concentration in soil. Varying quantities of compounds of the selected metals were mixed into a prepared soil column for analysis of signal intensity as a function of concentration in the soil. Experiments were conducted with a sealed tube neutron generator and a germanium gamma-ray detector. Both metals produced signal levels distinguishable from background soil concentrations at the maximum permissible level.     

Enantioselective catalytic diamination of alkenes with a bisimidoosmium oxidant

A first example of an enantioselective catalytic diamination of olefins has been developed which employs enantiopure titanium complexes as catalysts and bis(tbutylimido)dioxoosmium(VIII) as nitrogen source.     

Alkyl complexes of cobalt in catalytic cycles for hydrogenation of alkenes

The potential of several alkylcobalt complexes as catalysts for hydrogenation and isomerization of alkenes has been investigated. The complexes CH₃Co(CO)₂(Pom-Pom) (Pom-Pom = 1,2 bis(dimethoxyphosphino)ethane), CH₃Co(CO)₃P(OMe)₃ and C₆H₅CH₂Co(CO)₃PPh ₃ are compared to CH₃Co(CO)₂(P(OMe)₃)₂, for their ability to function in catalytic cycles. Each is active for hydrogenation and isomerization of alkenes under conditions where the carbonylation-decarbonylation equilibrium is readily established. The lifetime for the complexes is much shorter than for CH₃Co(CO)₂ (P(OMe)₃)₂ suggesting that two phosphorus donors in trans positions in an intermediate is a requirement for catalyst stability in these alkylcobalt complexes.     

Multicatalytic processes using diverse transition metals for the synthesis of alkenes

A series of cascade processes for the synthesis of alkenes from alcohols is described. Each individual step is catalyzed with a specific transition metal complex. The oxidation-methylenation one-pot procedure took place in the presence of a palladium and a rhodium catalyst to produce the desired terminal alkenes in high yields. A methylenation-ring-closing metathesis allowed the synthesis of cyclic alkenes from carbonyl derivatives, using the second-generation metathesis catalyst. Finally, an oxidation-methylenation-RCM process that involves up to three different transition metal catalysts in the same vessel is presented.     

Chiral fluoro ketones for catalytic asymmetric epoxidation of alkenes with oxone

Two structurally dissimilar, chiral fluoro ketones have been prepared and their potential as enantioselective catalysts for asymmetric epoxidation with Oxone has been evaluated. The tropinone-based ketone (-)-5 was easily prepared and showed excellent reactivity but only modest enantioselectivity. The biphenyl-based ketone (-)-6 was prepared in a somewhat lengthy synthesis (along with its monofluoro and geminal fluoro analogues). This ketone exhibited only modest reactivity; 30 mol % of (-)-6 was needed to bring about complete conversion in a reasonable time. The enantioselectivity of this catalyst was generally much higher, but again very substrate dependent.     

Chiral pyranoside phosphite-oxazolines: a new class of ligand for asymmetric catalytic hydrogenation of alkenes

We have described the first successful application of a phosphite-oxazoline ligand library in the asymmetric Ir-catalyzed hydrogenation of several unfunctionalized olefins. The introduction of a bulky biaryl phosphite moiety in the ligand design is highly adventitious in the product outcome. By carefully selecting the ligand components, we obtained high activities (TOFs up to >1500 mol x (mol x h)(-1) at 1 bar of H2) and enantioselectivities (ee values up to >99%) and, at the same time, show a broad scope for different substrate types. So, this is an exceptional ligand class that competes favorably with a few other ligand series that also provide high ee values for tri- and disubstituted substrate types.     

Comparing interfacial cation hydration at catalytic active sites and spectator sites on gold electrodes: understanding structure sensitive CO2 reduction kinetics

Hydrated cations present in the electrochemical double layer (EDL) are known to play a crucial role in electrocatalytic CO₂ reduction (CO₂R), and numerous studies have attempted to explain how the cation effect contributes to the complex CO₂R mechanism. CO₂R is a structure sensitive reaction, indicating that a small fraction of total surface sites may account for the majority of catalytic turnover. Despite intense interest in specific cation effects, probing site-specific, cation-dependent solvation structures remains a significant challenge. In this work, CO adsorbed on Au is used as a vibrational Stark reporter to indirectly probe solvation structure using vibrational sum frequency generation (VSFG) spectroscopy. Two modes corresponding to atop adsorption of CO are observed with unique frequency shifts and potential-dependent intensity profiles, corresponding to direct adsorption of CO to inactive surface sites, and in situ generated CO produced at catalytic active sites. Analysis of the cation-dependent Stark tuning slopes for each of these species provides estimates of the hydrated cation radius upon adsorption to active and inactive sites on the Au electrode. While cations are found to retain their bulk hydration shell upon adsorption at inactive sites, catalytic active sites are characterized by a single layer of water between the Au surface and the electrolyte cation. We propose that the drastic increase in catalytic performance at active sites stems from this unique solvation structure at the Au/electrolyte interface. Building on this evidence of a site-specific EDL structure will be critical to understand the connection between cation-dependent interfacial solvation and CO₂R performance.

Site-specific vibrational probes were used to elucidate the interfacial solvation structure between catalytic active sites and inactive sites on a Au electrode to reveal a unique, opposing cation-dependent double layer structure at active sites.     

Oxidation of small alkenes at high temperature

If the mechanism of formation of alkenes, the main primary products of the combustion of alkanes above 1000 K, is now well understood, their ways of degradation have been much less studied. Following a previous modeling of the oxidation of propene in a static and a jet‐stirred reactors by using an automatically generated mechanism, the present paper shows new validations of the same mechanism for ignition delays in a shock tube. It also describes the extension of the rules used for the automatic generation to the case of 1‐butene. The predictions of the mechanism produced for the oxidation of 1‐butene are compared successfully with two sets of experimental results: the first obtained in a jet‐stirred reactor between 900 and 1200 K; the second being new measurements of ignitions delays behind reflected shock waves for temperatures from 1200 up to 1670 K, pressures from 6.6 to 8.9 atm, equivalence ratios from 0.5 to 2, and with argon as bath gas. Flux and sensitivity analyses show that the role of termination reactions involving the very abundant allylic radicals is less important for 1‐butene than for propene. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 666–677, 2002     

Cross-coupling of sp(3) C-H bonds and alkenes: catalytic cyclization of alkene-amide substrates

We herein present a new oxidative cyclization of alkene-amide substrates under neutral and catalytic conditions. This overall transformation requires tandem sp3 C-H activation (at the position adjacent to the amide nitrogen) and C-C bond formation. Specifically, pyrrolidine 1 was converted to pyrrolizidinone 3 and indolizidinone 4 in 66% and 17% yield, respectively, in the presence of [Ir(coe)2Cl]2, the carbene ligand IPr (1:1 metal/ligand ratio, 5-10 mol % of Ir), and the hydrogen acceptor (NBE or TBE, 3-10 equiv). The results presented in this study suggest that complex 10 [IPr-Ir(Cl)(substrate)] is the key intermediate in the catalytic cycle. On the mechanistic front, the key advance was the ability to facilitate C-H activation and alkene insertion in tandem, and in preference to beta-hydride elimination, in the context of amide substrates. With respect to complex synthesis, catalytic and neutral conditions of this method unlock new exciting opportunities as illustrated by regioselective cyclization of the proline-derived substrate 16.