Rhodium(I)‐Catalyzed Reductive Cyclocarbonylation of Internal Alkynes: Atom‐Economic Process for Synthesis of 2‐Cyclopenten‐1‐ones, 5‐Alkylidenefuran‐2(5 H)‐ones and Indan‐1‐ones

Economical atoms : 2‐Cyclopenten‐1‐ones, 5‐alkylidenefuran‐2(5 H)‐ones and indan‐1‐ones have been synthesized by atom‐economic reductive cyclocarbonylation of internal alkynes with carbon monoxide catalyzed by [{RhCl(CO)₂}₂]/CO(NH₂)₂ in the presence of water (see scheme).

     

Six‐ and Eightfold Palladium‐Catalyzed Cross‐Coupling Reactions of Hexa‐ and Octabromoarenes

Palladium‐catalyzed sixfold coupling of hexabromobenzene ( 20 ) with a variety of alkenylboronates and alkenylstannanes provided hexaalkenylbenzenes 1 in up to 73 % and 16 to 41 % yields, respectively. In some cases pentaalkenylbenzenes 21 were isolated as the main products (up to 75 %). Some functionally substituted hexaalkenylbenzene derivatives containing oxygen or sulfur atoms in each of their six arms have also been prepared (16 to 24 % yield). The sixfold coupling of the less sterically encumbered 2,3,6,7,10,11‐hexabromotriphenylene ( 24 ) gave the desired hexakis(3,3‐dimethyl‐1‐butenyl)triphenylene ( 25 ) in 93 % yield. The first successful cross‐coupling reaction of octabromonaphthalene ( 26 ) gave octakis‐(3,3‐dimethyl‐1‐butenyl)naphthalene ( 27 ) in 21 % yield. Crystal structure analyses disclose that, depending on the nature of the substituents, the six arms are positioned either all on the same side of the central benzene ring as in 1 a and 1 i , making them nicely cup‐shaped molecules, or alternatingly above and below the central plane as in 1 h and 23 . In 27 , the four arms at C‐1,4,6,7 are down, while the others are up, or vice versa. Upon catalytic hydrogenation, 1 a yielded 89 % of hexakis(tert‐butylethyl)benzene ( 23 ). Some efficient accesses to alkynes with sterically demanding substituents are also described. Elimination of phosphoric acid from the enol phosphate derived from the corresponding methyl ketones gave 1‐ethynyladamantane ( 3 b , 62 % yield), 1‐ethynyl‐1‐methylcyclohexane ( 3 c , 85 %) and 3,3‐dimethylpentyne ( 3 e , 65 %). 1‐(Trimethylsilyl)ethynylcyclopropane ( 7 ) was used to prepare 1‐ethynyl‐1‐methylcyclopropane ( 3 d ) (two steps, 64 % overall yield). The functionally substituted alkynes 3 f – h were synthesized in multistep sequences starting from the propargyl chloride 11 , which was prepared in high yields from the dimethylpropargyl alcohol 10 (94 %). The alkenylstannanes 19 were prepared by hydrostannation of the corresponding alkynes in moderate to high yields (42–97 %), and the alkenylboronates 2 and 4 by hydroboration with catecholborane (27–96 % yield) or pinacolborane (26–69 % yield).     

6,13‐Diethynyl‐5,7,12,14‐tetraazapentacene

A new relative of pentacene : The dialkynylated tetraazapentacene (see figure) was prepared by a two‐step synthesis from the corresponding quinone derivative. The heteroacene is an air‐stable, dark‐blue, crystalline material and is of great interest as a potential organic n‐electron‐transport material.

     

Gold‐catalyzed Intermolecular Oxidations of 2‐Ketonyl‐1‐ethynyl Benzenes with N‐Hydoxyanilines to Yield 2‐Aminoindenones via Gold Carbene Intermediates

Gold‐catalyzed oxidations of 2‐ketonyl‐1‐ethynyl benzenes with N‐hydroxyanilines yield 2‐aminoindenone derivatives efficiently. Experimental data suggests that this process involves an α‐oxo gold carbene intermediate, generated from the attack of N‐hydroxyaniline on furylgold carbene intermediate, rather than the typical attack of oxidants on π‐alkynes.     

Dialkylaluminium‐, ‐Gallium‐, and ‐Indium‐Based Poly‐Lewis Acids with a 1,8‐Diethynylanthracene Backbone

Potential host systems based on a rigid 1,8‐diethynylanthracendiyl backbone were synthesised by treatment of 1,8‐diethynylanthracene with the Group 13 trialkyls AlMe₃, GaMe₃, InMe₃, AlEt₃ and GaEt₃. The resulting products were characterised by IR and multinuclear NMR spectroscopy, elemental analyses and determination of their crystal structures by X‐ray diffraction. The compounds are dimeric in the solid state and comprise two M₂C₂ heterocycles. Depending on the steric demand of the alkyl substituents at the metal atom, different types of binding modes were observed, which can be classified to lie between the ideals of side‐on coordination with almost linear primary M? C?C units and the 3c–2e coordination with symmetrically bridging alkynyl units in M‐C‐M bonds. As a solution in THF the dimers are broken into monomers and some are found to undergo ligand scrambling reactions.     

Dual Gold‐Catalyzed Head‐to‐Tail Coupling of Iodoalkynes

Various haloalkynes are converted in the presence of a dual activation gold catalyst. Via a dual activation process a completely atom economic head‐to‐tail coupling delivers gem‐dihalogenated conjugated enynes as valuable building blocks for organic synthesis.     

Synthesis and Reactivity of 5‐Bromopenta‐2,4‐diynenitrile (BrC5N): an Access to π‐Conjugated Scaffolds

The synthesis of 5‐bromopenta‐2,4‐diynenitrile (BrC₅N) in three steps from commercially available compounds is reported. Reacting 5‐bromopenta‐2,4‐diynenitrile with secondary amines led to the formation of stable butadiynamines or enynenitriles, depending on the nature of the amine reactant. The reaction of 5‐bromopenta‐2,4‐diynenitrile with simple terminal alkynes in the presence of secondary amines, copper, and palladium catalysts, provided a straightforward access to original polyfunctional carbon‐rich scaffolds. In this work, different alkynes and secondary amines were tested, which allowed for the preparation of a family of substituted dienes. Given the high synthetic potential of 5‐bromopenta‐2,4‐diynenitrile, we also prepared iodinated counterparts of this compound, that is, 5‐iodopenta‐2,4‐diynenitrile and its lower homologue 3‐iodopropiolonitrile. The UV‐visible spectrum of some relevant compounds was also recorded.     

Gold‐Catalyzed Cycloisomerization of 1,6,8‐Dienyne Carbonates and Esters to cis‐Cyclohepta‐4,8‐diene‐Fused Pyrrolidines

A synthetic approach that provides access to cis‐cyclohepta‐4,8‐diene‐fused pyrrolidines efficiently through Au^I‐catalyzed cycloisomerization of 1,6,8‐dienyne carbonates and esters at a low catalyst loading of 2 mol % is reported. Starting carbonates and esters with a pendant alkyl group on the terminal alkenyl carbon center were found to favor tandem 1,2‐acyloxy migration/cyclopropanation followed by Cope rearrangement of the resulting cis‐3‐azabicyclo[3.1.0]hexane intermediate. On the other hand, substrates containing a terminal diene or starting materials in which the distal alkene moiety bears a phenyl substituent were observed to undergo competitive but reversible 1,3‐acyloxy migration prior to the nitrogen‐containing bicyclic ring formation. The delineated reaction mechanism also provides experimental evidence for the reversible interconversion between the oft‐proposed organogold intermediates obtained in this step of the tandem process.     

Facial Synthesis of o‐Carborane‐Substituted Alkenes and Allenes by a Regioselective Ene Reaction of 1,3‐Dehydro‐o‐carborane

1,3‐Dehydro‐o‐carborane is a useful synthon for selective cage boron functionalization of o‐carboranes. It reacts readily with alkenes or alkynes to give a variety of cage B(3)‐alkenyl/allenyl o‐carboranes by ene reactions in very high yields and excellent regioselectivity. This can be ascribed to the highly polarized cage C?B multiple bond, which lowers the activation barriers of the ene reaction.     

Alkylative Carbocyclization of ω‐Iodoalkynyl Tosylates with Alkynyllithium Compounds Through a Carbenoid‐Chain Process Leading to (1‐Iodoprop‐2‐ynylidene)tetrahydrofurans and ‐cyclopropanes

Alkylative carbocyclization reactions of ω‐iodoalkynyl tosylates with alkynyllithium compounds to give products with incorporated iodine atoms are described. Slow addition of 2‐(3‐iodoprop‐2‐ynyloxy)ethyl tosylates to 1‐alkynyllithium compounds in tetrahydrofuran at 40 °C followed by additional stirring at this temperature gives (Z)‐3‐(1‐iodoprop‐2‐ynylidene)tetrahydrofurans stereoselectively in good to moderate yields. Under similar conditions at 0 °C, 4‐iodobut‐1‐ynyl tosylates react with 1‐alkynyllithium compounds to give (1‐iodoprop‐2‐ynylidene)cyclopropanes. The carbocyclization reactions are proposed to proceed through a new carbenoid‐chain process involving the exo cyclization of a lithium acetylide intermediate and the vinylic substitution of the resulting TsO,Li‐cycloalkylidenecarbenoids (Ts=tosyl) by 1‐alkynyllithium compounds.     

Stereoselective Base‐Catalyzed 1,1‐Silaboration of Terminal Alkynes

A base‐catalyzed reaction that enables stereoselective 1,1‐silaboration of terminal alkynes is described. This method not only offers a new strategy to functionalize simple and readily accessible alkynes beyond 1,2‐difunctionalization, but also provides an unconventional atom‐ and step‐economical approach to rapidly and reliably access versatile geminal silylboranes in the absence of transition metals and with exquisite stereoselectivity.     

Reaction of a Dialumene‐Benzene Adduct with Diphenylacetylene: Formation of 3,4‐Dialuminacyclobutene and 5,6‐Dialuminabicyclo[2.1.1]hex‐2‐ene Derivatives

Reaction of a dialumene‐benzene adduct bearing bulky aryl substituents with diphenylacetylene was found to give a novel 5,6‐dialuminabicyclo[2.1.1]hex‐2‐ene derivative in addition to the 3,4‐dialuminacyclobutene derivative, the formal [2+2]cycloadduct of an intermediary dialumene with diphenylacetylene. The molecular structure of the newly obtained 5,6‐dialuminabicyclo[2.1.1]hex‐2‐ene has been elucidated by X‐ray crystallographic analysis.     

Facile Synthesis and Properties of 2‐λ5‐Phosphaquinolines and 2‐λ5‐Phosphaquinolin‐2‐ones

Treatment of 2‐ethynylanilines with P(OPh)₃ gives either 2,2‐diphenoxy‐2‐λ⁵‐phosphaquinolines or 2‐phenoxy‐2‐λ⁵‐phosphaquinolin‐2‐ones under transition‐metal‐free conditions. This reaction offers access to an underexplored heterocycle, which opens up the study of the fundamental nature of the N?P^V double bond and its potential for delocalization within a cyclic π‐electron system. This heterocycle can serve as a carbostyril mimic, with application as a bioisostere for pharmaceuticals based on the 2‐quinolinone scaffold. It also holds promise as a new fluorophore, since initial screening reveals quantum yields upwards of 40 %, Stokes shifts of 50–150 nm, and emission wavelengths of 380–540 nm. The phosphaquinolin‐2‐ones possess one of the strongest solution‐state dimerization constants for a D–A system (130 M ^?1) owing to the close proximity of a strong acceptor (P?O) and a strong donor (phosphonamidate N? H), which suggests that they might hold promise as new hydrogen‐bonding hosts for optoelectronic sensing.