Gold‐Catalyzed Synthesis of Dibenzopentalenes – Evidence for Gold Vinylidenes

A series of easily accessible arene‐1,2‐diynes, bearing one aryl substituent on one of the alkynyl groups, is readily converted to dibenzopentalenes in good yields by gold(I) catalysts. The participation of gold acetylides could be proven by the direct conversion to the corresponding gem‐diaurated dibenzopentalenes with a gold catalyst. From an experiment with a gold acetylide complex and stoichiometric amounts of the gold “catalyst” the corresponding gem‐diaurated complex of a dibenzopentalene could be obtained and characterized by X‐ray crystal structure analysis. Labelling studies with deuterated alkynes show the expected deuteration of the two remaining positions of the pentalene core. All this provides evidence for a dual activation mode of the reaction and gold(I) vinylidene complexes as intermediates of the catalytic cycle.     

Gold(I)‐Catalyzed Decarboxylation of Propargyl Carbonates: Reactivity Reversal of the Gold Catalyst from π‐Lewis Acidity to σ‐Lewis Acidity

A cationic gold(I)‐catalyzed decarboxylative etherification of propargyl carbonates to selectively produce propargyl ethers is reported. In the reaction the gold(I) catalyst shows a distinct σ‐Lewis acidity rather than the commonly observed π‐Lewis acidity, and thus catalyzes the decarboxylation of a variety of propargyl carbonates to give the corresponding propargyl ethers with high selectivity. This reaction represents a rare example of the tunable reactivity of cationic gold(I) complexes between σ‐Lewis acidity and π‐Lewis acidity.

     

Gold(I)‐Catalyzed Cascade for Synthesis of Pyrrolo[1,2‐a:2′,1′‐c]‐/Pyrido[2,1‐c]pyrrolo[1,2‐a]quinoxalinones

An efficient and convenient method was developed for the one‐pot construction of the complex polycyclic heterocycles pyrrolo[1,2‐a:2′,1′‐c]‐/pyrido[2,1‐c]pyrrolo[1,2‐a]quinoxalinones from two simple starting materials via a gold(I)‐catalyzed domino reaction. This strategy presents an atom economical and environmentally friendly transformation, in which two new C N bonds and one new C C bond are formed in a one‐pot reaction process.     

Carbene Transfer – A New Pathway for Propargylic Esters in Gold Catalysis

Gold carbenes generated via 1,2‐migration of a propargylic ester group can be transferred over a tethered alkyne. The use of aromatic backbones leads after a 1,7‐carbene transfer to a benzyl‐stabilized carbene as intermediate. A 1,2‐shift of a methyl group delivers vinyl‐substituted β‐naphthol derivatives as the final products.

     

Synthesis of ω‐amino‐α‐phenylcarbonate alkanes and their polymerization to [n]‐polyurethanes

Aliphatic [n]‐polyurethanes have recently been synthesized from ω‐isocyanato‐α‐alkanols or, more traditionally, by cationic ring‐opening polymerization of cyclourethanes or by the Bu₂Sn(OMe)₂‐promoted polycondensation of ω‐hydroxy‐α‐O‐phenylurethane alkanes. For the latter procedures, the conditions employed do not seem to be suitable for highly functionalized monomers. In contrast, the polymerization of ω‐amino‐α‐phenylcarbonate alkanes is expected to occur under milder conditions. ω‐Amino‐α‐phenylcarbonate alkanes have been synthesized from 6‐aminohexanol (1) and 3‐aminopropanol (6). The procedure involves the N‐Boc protection of the amino group, followed by activation of the alcohol. Removal of the N‐Boc affords the corresponding ω‐amino‐1‐O‐phenyloxycarbonyloxyalkane hydrochlorides. Other oligomeric comonomers between 1 and 6 have been prepared. The polymerization of these precursors takes place in the absence of metal catalysts to afford the corresponding linear and regioregular [n]‐polyurethanes. The procedure described is useful for the preparation of stable ω‐amino‐α‐phenylcarbonate alkane derivatives, which possess varied chain lengths between the terminal functions. These monomers yield [n]‐polyurethanes having various structures starting from just two aminoalkanols. The polyurethanes were obtained in high yields, with reasonable molecular weight and polydispersity values, and they were characterized spectroscopically and thermally. These studies reveal constitutionally uniform structures that are free of carbonate or urea linkages. Copyright © 2010 Society of Chemical Industry     

Peculiar sandwich-like π–π interaction regulating the nonplanarity of the model heme crystals

X-ray crystallographic analysis of bis(pyridine N-oxide) complexes of iron(III) porphyrinates has revealed that the two pyridine rings of the axial ligands correctly sandwich the porphyrin ring to induce the deformation of commonly observed S₄ saddled porphyrin ring.     

Supramolecular π–π assembly of a neutral [Cu(salen)] complex via the templating effect of an ionic inorganic complex Na2[Cu(mnt)2] forming a framework type material having well-defined channels

A classical ionic inorganic complex Na₂[Cu(mnt)₂] (mnt²⁻ = maleonitriledithiolate = 1,2-dicyanoethylenedithiolate), that acts as a template in assembling neutral [Cu(salen)] (salen = bis(salicylidene)ethylenediamine) complexes forming a framework type arrangement, is accommodated in the channel formed in the crystal structure of a new type of host–guest compound [Cu(salen)]₄ · Na₂[Cu(mnt)₂] (1). The non-covalent supramolecular interactions among [Cu(salen)] complexes and between [Cu(salen)] and [Cu(mnt)₂]²⁻ complexes in the crystal lattice of 1 result in weak antiferromagnetic coupling.     

Diastereoselective Synthesis of Six‐Membered Carbocyclic Spirooxindoles via 6π‐Electrocyclization of 3‐Dienylidene‐2‐ oxindoles

The Wittig reaction of isatin derivatives with Morita–Baylis–Hillman bromides of cinnamaldehydes afforded 3‐dienylidene‐2‐oxindoles. These trienes were converted into the corresponding spirooxindoles in a stereoselective manner in refluxing toluene in good yields. The diastereomeric spirooxindoles could be obtained stereoselectively by adding a catalytic amount of palladium(II) acetate via the palladium‐catalyzed isomerization of EEE‐trienes to ZEE‐trienes followed by a more facile 6π‐electrocyclization process. The obtained spirooxindoles could be further functionalized by palladium‐catalyzed oxidative arylation, thionation with Lawesson’s reagent, catalytic hydrogenation and Friedel–Crafts‐type reaction.

     

Gold‐Catalyzed Synthesis of 3‐Acylimidazo[1,2‐a]pyridines via Carbene Oxidation

A convenient gold‐catalyzed strategy for the synthesis of imidazo[1,2‐a]pyridine derivatives has been developed via gold carbene complexes. This transformation opens a new synthetic route to a variety of 3‐carbonyl‐substituted imidazo[1,2‐a]pyridines using air as oxidant affording the products in good yields.

     

Asymmetric Synthesis of Bioxindole‐Substituted Hexahydrofuro[2,3‐b]furans via Hydroquinine Anthraquinone‐1,4‐diyl Diether‐Catalyzed Domino Annulation of Acylidenoxindoles/Isatins,Acylidenoxindoles and Allenoates

Hydroquinine anthraquinone‐1,4‐diyl diether [(DHQ)₂AQN]‐catalyzed unprecedented asymmetric domino annulation reactions of acylidenoxindoles/isatins, acylidenoxindoles and allenoates are disclosed in this communication, providing a facile access to hexahydrofuro[2,3‐b]furans containing four contiguous chiral centers in good to excellent yields along with good to excellent ee values and moderate to good dr values. Based on theoretical investigations, a concerted [3+2] ring‐closure process was proposed, in which steric hindrance and π–π stacking interaction between catalyst and substrate subtly co‐control the diastereoselectivity of the reaction.

     

Regioselective Friedel–Crafts Reaction of Electron‐Rich Benzenoid Arenes and Spiroepoxyoxindole at the Spiro‐Centre: Efficient Synthesis of Benzofuroindolines and 2H‐ Spiro[benzofuran]‐3,3′‐oxindoles

Metal triflate‐catalysed intermolecular Friedel–Crafts reactions involving electron‐rich benzenoid arenes and spiroepoxyoxindoles at the spiro‐centre have been developed for the exclusive regioselective synthesis of 3‐aryl‐(3‐hydroxymethyl)oxindoles with an all‐carbon quaternary centre. Selective ring opening of spiroepoxyoxindoles with phenols provided a direct access to 3‐(hydroxymethyl)‐3‐(2‐hydroxyaryl)oxindoles. We have utilized this methodology successfully as the key step for the synthesis of benzofuroindolines and 2H‐spiro[benzofuran]‐3,3′‐oxindoles.

     

Domino Reaction of Acyclic α,α‐Dialkenoylketene S,S‐Acetals and Diamines: Efficient Synthesis of Tetracyclic Thieno[2,3‐b]thiopyran‐Fused Imidazo[1,2‐a]pyridine/Pyrido[1,2‐a]pyrimidines

A series of unusual fused tetraheterocyclic compounds 3 , consisting of a thiopyran (ring A), a thiophene (ring B), a pyridine (ring C), and an imidazole or a pyrimidine (ring D) core, with a bridgehead nitrogen and an angular methyl group, were successfully synthesized by a catalyst‐free, one‐pot, two‐component domino reaction of 4‐(4‐methyl‐1,3‐dithiol‐2‐ylidene)‐1,7‐bis(aryl/heteroaryl)hepta‐1,6‐diene‐3,5‐dione 2 and diamines. In this reaction, up to five new bonds were formed accompanied by the C S bond cleavage of the 1,3‐dithiole ring of 2 , with water as the only by‐product.     

A study of Cu(I)‐ethylene complexation for olefin–paraffin separation

The current cryogenic distillation technology used for olefin–paraffin separation incurs extensive capital and operating costs. An alternative olefin–paraffin separation process, based on reactive absorption, could yield significant cost reductions. The research efforts described herein explored the structural characteristics of an NMP‐CuCl‐aniline absorption solution with ethylene to aid future development of olefin–paraffin separation systems. Solution IR and ¹H NMR spectroscopy suggested weak and labile Cu(I)‐ethylene and Cu(I)‐aniline coordination, which point to the coexistence of multiple structures in solution. Experiments also revealed solvent‐dependent and temperature‐dependent coordination. The agreement of the collected spectral data with literature implied single ethylene coordination, whereas the Cl^? ion likely remained coordinated with Cu(I). Solvent interference prohibited detailed investigation of IR spectra, but ¹H NMR spectroscopy showed more promise as an analytical technique for the NMP‐CuCl‐aniline‐ethylene system. Finally, a tradeoff appears to exist between ethylene capacity and complex stability, and thus, an optimal ligand must be found that balances these two competing needs. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Zinc‐Catalyzed Selective Reduction of Cyclic Imides with Hydrosilanes: Synthesis of ω‐Hydroxylactams

Cyclic imides were selectively reduced to the corresponding ω‐hydroxylactams in high yields with (EtO)₃SiH (triethoxysilane) or PMHS (polymethylhydrosiloxane) under catalysis of zinc diacetate dehydrate [Zn(OAc)₂⋅2 H₂O] (10%) and tetramethylethylenediamine (TMEDA) (10%). This catalytic protocol showed good functional group tolerance as well as excellent regioselectivity for unsymmetrical imides bearing coordinating groups adjacent to the carbonyl.

     

Oxygen‐Involved Oxidative Deacetylation of α‐Substituted β‐Acetyl Amides – Synthesis of α‐Keto Amides

α‐Substituted β‐acetyl amides could undergo C C bond cleavage to form α‐keto amides when treated with copper(II) chloride (CuCl₂) and boron trifluoride diethyl etherate (BF₃⋅OEt₂) under an oxygen atmosphere. The yield can be increased by the addition of tert‐butyl hydroperoxide which alone can also effect the reaction. The reaction provides a new protocol for the synthesis of α‐keto amides.

     

Thermal Intramolecular [2+2] Cycloaddition: Synthesis of 3‐Azabicyclo[3.1.1]heptanes from Morita–Baylis–Hillman Adduct‐Derived 4,4‐Diaryl‐1,3‐dienes

Various 3‐azabicyclo[3.1.1]heptane derivatives were synthesized from Morita–Baylis–Hillman adduct‐derived 1,3‐dienes bearing a 4,4‐diaryl moiety through a thermal intramolecular [2+2] cycloaddition approach. By using the same approach, bicyclo[3.1.1]heptane, 3‐azabicyclo[3.2.0]heptane, and 3‐oxabicyclo[3.1.1]heptane derivatives could also be synthesized. A structurally similar dimethylallyl derivative underwent an intramolecular ene reaction to afford the pyrrolidine derivative.

     

Regioselective Synthesis of Functionalized Pyrroles via Gold(I)‐Catalyzed [3+2] Cycloaddition of Stabilized Vinyl Diazo Derivatives and Nitriles

The reaction of nitriles with alkenyldiazo compounds in the presence of gold catalysts provides functionalized pyrrole derivatives in moderate to high yields. This formal [3+2] cyclization reaction takes place with complete regioselectivity. The observed regiochemical outcome suggests the attack of the nitrile to the terminal position of the alkenylgold carbenoid (vinylogous reactivity). A broad range of nitriles (including those bearing functional groups) is compatible with this cyclization reaction.     

Palladium‐Catalyzed Highly Chemo‐, Regio‐ and Stereoselective Synthesis of Eight‐ to Ten‐Membered Lactones from Allenyl 3‐Oxoalkanoates and Organic Halides

A highly chemo‐, regio‐, and stereoselective synthesis of eight‐ to ten‐membered lactones via the coupling cyclization of readily available allenyl 3‐oxoalkanoates and organic halides through an anti‐π‐allylic palladium intermediate is reported. The yields ranged from moderate to good.     

Brønsted Acid‐Catalyzed Synthesis of Pyrans via a Formal [3+3] Cycloaddition

Brønsted acids catalyze the addition of enolizable β‐keto esters to α,β‐unsaturated aldehydes leading to substituted 2H‐pyrans in good yields under mild conditions via a formal [3+3] cycloaddition.     

Catalytic Asymmetric Synthesis of 2‐Alkyleneoxetanes through [2+2] Annulation of Allenoates with Trifluoromethyl Ketones

The first example of a β‐isocupreidine (β‐ICD)‐catalyzed highly enantioselective [2+2] annulation of allenoates with trifluoromethyl ketones has been disclosed, allowing the synthesis of optically active 2‐alkyleneoxetanes in moderate to good yields along with good to high enantioselectivities and high diastereoselectivities. Further transformations of the cycloadducts have been also disclosed to afford biologically interesting 6‐trifluoromethyl‐5,6‐dihydropyran‐2‐ones and trifluoromethyl β‐keto acids in good yields.