One-Pot Cooperation of Single-Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization-Hydrosilylation Process

Realizing the full potential of oxide-supported single-atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one-pot combination of Ru₁/CeO₂ and Rh₁/CeO₂ SACs enables a highly selective olefin isomerization-hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double-bond migration and anti-Markovnikov α-olefin hydrosilylation, respectively. First-principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single-pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio-selectivity (>95 %) even from industrially-relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide-supported single-atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes.     

Synthesis and thermolysis of tetrasubstituted cyclopropenes

Photolysis of 5-benzyl-2-methoxy-2,5-dimethyl-³-1, 3,4-oxadiazoline and of the 5,5-dibenzyl analogue with 300-nm light afforded 1-phenyl-2-diazopropane and 1,3-diphenyl-2-diazopropane, respectively. The diazoalkanes were intercepted, in situ, with dimethyl acetylenedicarboxylate to afford 3-benzyl-4,5-bis(methoxycarbonyl)-3-methyl-3H-pyrazole and 3,3-dibenzyl-4,5-bis(methoxycarbonyl)-3H-pyrazole, respectively. Those pyrazoles are short-lived under the reaction conditions and undergo two major reactions. Photolysis prior to rearrangement affords the corresponding 3,3-dialkyI-1,2-bis(methoxycarbonyl)-cyclopropenes. Thermal 1,5-benzyl migration converts the two 3H-pyrazoles in part into the corresponding 4H-pyrazoles, which undergo photolysis to 2,3-dialkyl-1,3-bis(methoxycarbonyl)cyclopropenes.Thermolysis of the 3,3-dialkyl-1,2-bis(methoxycarbonyl) cyclopropenes affords conjugated dienes, presumably through the sequence cyclopropene vinyl carbene diene. The stereochemistry of the dienes was determined and a mechanism consistent with that stereochemistry is proposed.The 2,3-dialkyl-1,3-bis(methoxycarbonyl)cyclopropanes are very stable under conditions that convert isomeric 3,3-dialkyl-1,2-bis(methoxycarbonyl)cyclopropenes to conjugated dienes. It is proposed that the effect of substitution pattern on the thermolysis rate constants is the result of combined ground state and transition state factors.     

Cobalt-catalysed asymmetric hydrovinylation of 1,3-dienes

In the presence of bidentate 1,n-bis-diphenylphosphinoalkane-CoCl₂ complexes {Cl₂Co[P ∼ P]} and Me₃Al or methylaluminoxane, acyclic (E)-1,3-dienes react with ethylene (1 atmosphere) to give excellent yields of hydrovinylation products. The regioselectivity (1,4- or 1,2-addition) and the alkene configuration (E- or Z-) of the resulting product depend on the nature of the ligand and temperature at which the reaction is carried out. Cobalt(ii)-complexes of 1,1-diphenylphosphinomethane and similar ligands with narrow bite angles give mostly 1,2-addition, retaining the E-geometry of the original diene. Complexes of most other ligands at low temperature (–40 °C) give almost exclusively a single branched product, (Z)-3-alkylhexa-1,4-diene, which arises from a 1,4-hydrovinylation reaction. A minor product is the linear adduct, a 6-alkyl-hexa-1,4-diene, also arising from a 1,4-addition of ethylene. As the temperature is increased, a higher proportion of the major branched-1,4-adduct appears as the (E)-isomer. The unexpectedly high selectivity seen in the Co-catalysed reaction as compared to the corresponding Ni-catalysed reaction can be rationalized by invoking the intermediacy of an η⁴-[(diene)[P ∼ P]CoH]⁺-complex and its subsequent reactions. The enhanced reactivity of terminal E-1,3-dienes over the corresponding Z-dienes can also be explained on the basis of the ease of formation of this η⁴-complex in the former case. The lack of reactivity of the X₂Co(dppb) (X = Cl, Br) complexes in the presence of Zn/ZnI₂ makes the Me₃Al-mediated reaction different from the previously reported hydroalkenylation of dienes. Electron-rich phospholanes, bis-oxazolines and N-heterocyclic carbenes appear to be poor ligands for the Co(ii)-catalysed hydrovinylation of 1,3-dienes. An extensive survey of chiral ligands reveals that complexes of DIOP, BDPP and Josiphos ligands are quite effective for these reactions even at –45 °C and enantioselectivities in the range of 90–99% ee can be realized for a variety of 1,3-dienes. Cobalt(ii)-complex of an electron-deficient Josiphos ligand is especially active, requiring only <1 mol% catalyst to effect the reactions.     

3,4,5,6-Tetrafluoro-1,2-dehydrobenzene in reactions with 1,2,4-triazines

Tetrafluoro-substituted aryne, 3,4,5,6-tetrafluoro-1,2-dehydrobenzene, has been generated in situ from 2-amino-3,4,5,6-tetrafluorobenzoic acid, and its reactivity in reactions with 1,2,4-triazines as dienes has been studied. In these reactions, the corresponding azine ring transformation products, i.e., 1,2,3,4-tetrafluoro-10-(1H-1,2,3-triazol-1-yl)pyrido[1,2-a]indoles, have been obtained, in the case of triazines activated by the presence of electron-withdrawing groups, such as 6-aryl-3-(2-pyridyl)-5-cyano-1,2,4-triazines. The crystal structure of the obtained products was confirmed by X-ray diffraction analysis.     

Stereospecific 1,4-cis-hydrogenation of conjugated dienes,dienynes, and dienediynes catalyzed by chromium carbonyl complexes in stereocontrolled syntheses of physiologically active olefins

Synthesis of insect pheromones provides a good illustration of versatility, flexiblility, and practical convenience of the chromium carbonyl complexes-mediated 1,4-cis-hydrogenation of conjugated dienes as the tool for for stereocontrolled construction of di-, tri-, and tetrasubstituted olefins and nonconjugated dienes. A new strategy for synthesizing homoconjugated all-Z-dienes and trienes by hydrogenation of diene-alkyne conjugated systems using the same catalysts is proposed.     

anti-Markovnikov addition of tellurium tetrachloride to trimethyl ethynyl silane

An investigation of the TeCl₄ interaction with trimethyl ethynyl silane 1 in CHCl₃ has shown that anti-Markovnikov adduct [Z-1-(trimethylsilyl)-2-chlorovinyl]tellurium trichloride is formed as the only product. In time, it is hydrolyzed to give [Z-1-(trimethylsilyl)-2-chlorovinyl]tellurium (hydroxy) dichloride which, in turn, is dehydrated to afford bis[(2-chloro-1-trimethyl-silylvinyl)dichlorotellurium]oxide. These data revealed that the reaction studied was the first example of anti-Markovnikov syn-addition of TeCl₄ to terminal acetylenes. A computed simulation of the TeCl₄ interaction with ethynyl silane 1 in a gas state using PES method did not reveal dominating orientation of the addition but showed the conditions at which anti-Markovnikov addition can occur and which were probably met in carrying out the reaction in CHCl₃.     

Iron-Catalyzed Stereoconvergent 1,4-Hydrosilylation of Conjugated Dienes

Stereoconvergent transformation of E/Z mixtures of olefins to products with a single steric configuration is of great practical importance but hard to achieve. Herein, we report an iron-catalyzed stereoconvergent 1,4-hydrosilylation reactions of E/Z mixtures of readily available conjugated dienes for the synthesis of Z-allylsilanes with high regioselectivity and exclusive stereoselectivity. Mechanistic studies suggest that the reactions most likely proceed through a two-electron redox mechanism. The stereoselectivity of the reactions is ultimately determined by the crowded reaction cavity of the α-diimine ligand-modified iron catalyst, which forces the conjugated diene to coordinate with the iron center in a cis conformation, which in turn results in generation of an anti-π-allyl iron intermediate. The mechanism of this stereoconvergent transformation differs from previously reported mechanisms of other related reactions involving radicals or metal-hydride species.     

Solvent Effect in Reaction of Singlet Oxygen with Conjugated Dienes

The 1,4-cycloaddition of singlet oxygen (¹O₂) to conjugated dienes provides a useful tool for simultaneous functionalization of the diene moiety, and its application to organic synthesis has been reported.¹ For the dienes bearing an allylic hydrogen, especially for the dienes not constrained to s-cis conformation, the 1,4-cycloaddition usually competes with the “ene” reaction as well as the 1,2-cycloaddition.² The efficiency of the oxygenation³ and the selectivity of the reaction modes are essential factors in the synthetic application. We have investigated the sensitized photooxygenation of representative acyclic dienes and vinylarenes i n various solvents.     

(R)-4-phenyloxazolidin-2-thione: an efficient chiral auxiliary for [4+2] cycloaddition of 1-aminodiene and activated phosphonodienophiles

A theoretical model for the facial selectivity of N-dienyl oxazolidin-2-(thi)one and thiazolidin-2-thione 2a-c is presented. Our analysis provides a clear understanding of factors controlling stereoselectivity in reaction of these dienes, and allows predictions of high diastereoselectivity in the case of oxazolidin-2-thionyl diene (2b). The application of this diene to the synthesis of β- and γ-aminophosphonic derivatives is then investigated. Under classical conditions or under microwave activation, the D-A reaction of diene 2b leads to aminophosphonic chirons with high regio- and stereoselectivities.     

1,4-Addition versus 1,2-addition of free germylenes Me2Ge to 1,3-dienes

New cycloadditions of free Me₂Ge to 13 different 1,3-dienes are reported: 1,4-addition is favoured for electron deficient dienes, high s-cis content, and C-1 - C-4 distances around 3.10Å. Factors disfavouring it promote formation of 3,4-disubstituted 1-germacyclopentanes via postulated 1,2-addition, and insertion of a second diene.     

2-Allyl-N-benzyl substituted α-naphthylamines as building blocks in heterocyclic synthesis. New and efficient syntheses of benz[e]naphtho[1,2-b]azepine and naphtho[1,2-b]azepine derivatives

A new series of 13-acetyl-7,12-dihydro-7-ethylbenz[e]naphtho[1,2-b]azepine (4a-d) and 2-aryl-4-hydroxy-2,3,4,5-tetrahydronaphtho[1,2-b]azepine derivatives (6a-d) have been synthesized from N-allyl-N-benzyl substituted α-naphthylamines (1a-d) by utilizing aromatic amino-Claisen rearrangement, intramolecular Friedel-Crafts alkylation and intramolecular dipolar 1,3-cycloaddition nitrone-olefin reactions.     

Aluminum enolates via retroaldol reaction: catalytic tandem aldol-transfer—Tischtschenko reaction of aldehydes with aldol adducts of ketones to ketones

Bidentate aluminum chelates derived from biphenol, binaphthol and catechol were found to be efficient catalysts for aldol-transfer reactions of ketone to ketone aldol adducts with aliphatic or aromatic aldehydes giving rise to the formation of aldol adducts of ketones to the aldehydes. In the presence of an excess of an aliphatic aldehyde, a catalytic tandem aldol-transfer—Tischtschenko reaction is observed. The tandem reaction produces monoesters of 1,3-diols with high anti selectivity and with modest to good chemical yield. 1,2-Unsaturated aldehydes are less reactive in the aldol-transfer reaction and require 2-4 times higher load of the catalyst to be used than aliphatic and aromatic aldehydes. Poor diastereoselectivity was observed in the formation of α-substituted aldols and 2-substituted monoesters of anti-1,3-diols indicating that the aldol-transfer reaction is not diastereoselective with the catalysts studied. The utility of the highly 1,3-anti selective formation of diolmonoesters was found to be limited by acyl migration.     

Selenylated dienes: synthesis, stereochemical studies by Se NMR, and transformation into functionalized allenes

2-Phenylselanyl-1,3-dienes 3-8 were prepared by a Wittig or Wittig-Horner-Emmons procedure starting from α-phenylselanyl α,β-unsaturated aldehydes. Ratio and configuration of each diene isomers were determined by ⁷⁷Se and ¹H NMR. These dienes were then oxidized into selenoxides, which could be isolated in some cases. In THF, [2,3]-sigmatropic rearrangement of allylic selenoxides, selenimides, and dihalo-selenuranes occurred, yielding allenyl alcohols 12-15, allenyl carbamates 16c-19c, and 1-haloalkyl allenes 20c-22c, respectively.     

Sequential transhalogenation and Heck reaction for efficient access to dioxo-tetrasubstituted 2,4 E,E-dienes: synthesis of segment C1-C6 of apoptolidin

Efficient access to dioxo-tetrasubstituted 2,4 E,E-dienes is developed in three steps from commercially available starting materials via sequential transhalogenation and Heck reaction, which provides potentially useful synthons for the synthesis of a tetrasubstituted conjugated diene structure in complex molecules. Thereby, segment C1-C6 of apoptolidin is synthesized.     

One‐Pot Cooperation of Single‐Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization‐Hydrosilylation Process

Realizing the full potential of oxide‐supported single‐atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one‐pot combination of Ru₁/CeO₂ and Rh₁/CeO₂ SACs enables a highly selective olefin isomerization‐hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double‐bond migration and anti‐Markovnikov α‐olefin hydrosilylation, respectively. First‐principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single‐pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio‐selectivity (>95 %) even from industrially‐relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide‐supported single‐atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes.     

Nickel-Catalyzed Cross-Electrophile 1,2-Silyl-Arylation of 1,3-Dienes with Chlorosilanes and Aryl Bromides

Catalytic, three-component, cross-electrophile reactions have recently emerged as a promising tool for molecular diversification, but studies have focused mainly on the alkyl-carbonations of alkenes. Herein, the scope of this method has been extended to conjugated dienes and silicon chemistry through silylative difunctionalization of 1,3-dienes with chlorosilanes and aryl bromides. The reaction proceeds under mild conditions to afford 1,2-linear-silylated products, a selectivity that is different to those obtained from conventional methods via an intermediary of H(C)-η³-π-allylmetal species. Preliminary mechanistic studies reveal that chlorosilane reacts with 1,3-diene first and then couples with aryl bromide.     

Marked difference in singlet-chemiexcitation efficiency between syn-anti isomers of spiro[1,2-dioxetane-3,1′-dihydroisobenzofuran] for intramolecular charge-transfer-induced decomposition

Spiro[1,2-dioxetane-3,1′-dihydroisobenzofuran] syn-3 bearing a hydroxy group at the 6-position (as a model syn-rotamer of parent dioxetane 4 bearing a 3-hydroxyphenyl group) and its isomer anti-3 (as a model anti-rotamer of 4) were synthesized. When these spiro-dioxetanes were treated with tetrabutylammonium fluoride (TBAF) in DMSO, anti-3 emitted light with high efficiency (Φ^CL = 0.41), while the respective value for syn-3 was only 1/10 for anti-3. This significant difference in Φ^CL between syn-3 and anti-3 was attributed to the difference in their singlet-chemiexcitation efficiencies.     

Synthesis of 3-imino and 3-ylideno derivatives of 4-fluoro-5-polyfluoroalkyl-1,2-dithiolenes

Reactions of 4-fluoro-5-polyfluoroalkyl-1,2-dithiol-3-thiones with hydroxylamine and hydrazines occur with replacement of the thiocarbonyl by imino group affording oximes and hydrazones respectively. N-Alkyl-and N-aryl-3-imino-1,2-dithiolenes formed in reactions of 3-chlorothio-1,2-dithiolium salts with primary alkyl-or arylamines. 3-Chlorothio-1,2-dithiolium salts react with compounds possessing an active methylene group yielding 3-ylideno derivatives of 1,2-dithiolenes.     

A new and versatile method for iodofunctionalization of 1,3-dienes

We wish to report the reactions of conjugated dienes with I(py)₂BF₄. This simple and general method allows the regiospecific 1,2-addition of iodine and a nucleophile to terminal dienes and the 1,4-addition to internal dienes.