Nickel-catalyzed [2 + 2 + 2] cycloaddition of arynes and an unactivated alkene: synthesis of 9,10-dihydrophenanthrene derivatives

Aryne Chemistry: Generation Methods and Reactions Incorporating Multiple Arynes

Arynes hold significance for the efficient fusion of (hetero) arenes with diverse substrates, advancing the construction of complex molecular frameworks. Employing multiple equivalents of arynes is particularly effective in the rapid formation of polycyclic cores found in optoelectronic materials and bioactive compounds. However, the inherent reactivity of arynes often leads to side reactions, yielding unanticipated products and underlining the importance of a detailed investigation into the use of multiple arynes to fine-tune their reactivity. This review centers on methodologies and syntheses in organic reactions involving multiple arynes, categorizing based on mechanisms like cycloadditions, σ-bond insertions, nucleophilic additions, and ene reactions, and discusses aryne polymerization. The categorization based on these mechanisms includes two primary approaches: the first entails multiple aryne engagement within a single step while the second approach involves using a single equivalent of aryne sequentially across multiple steps, with both requiring strict reactivity control to ensure precise aryne participation in each respective step. Additionally, the review provides an in-depth analysis of the selection of aryne precursors, organized chronologically and by activation strategy, offering a comprehensive background that supports the main theme of multiple aryne utilization. The expectation remains that this comprehensive review will be invaluable in designing advanced syntheses engaging multiple arynes.

One-Pot Preparation of 9,10-Dihydrophenanthrenes Initiated by Rhodium(III)-Catalyzed C-H Activation and Relay Diels-Alder Reaction

An efficient one-pot synthesis of multisubstituted 9,10-dihydrophenanthrenes from easily available 2-arylazaarenes and cyclohexadienone-tethered terminal alkynes (1,6-enynes) has been successfully achieved. This domino reaction proceeded smoothly through Cp*Rh(III)-catalyzed C-H activation, direct protonation of alkenyl-Rh intermediates, intramolecular Diels-Alder reaction, alkene isomerization, subsequent ring-opening aromatization, and acetylation. This strategy was pot-economical and tolerated a wide range of functional groups. Moreover, the potent anticancer activities against HepG2 cells were observed for these artificial 9,10-dihydrophenanthrene derivatives.

C(sp3)–H activation-enabled cross-coupling of two aryl halides: an approach to 9,10-dihydrophenanthrenes

A palladium-catalyzed cross-coupling reaction of aryl halides with 2-chlorobenzoic acids has been developed through C(sp³)–H activation, which provides an innovative method for the synthesis of 9,10-dihydrophenanthren.

Palladium-catalyzed cocyclotrimerization of arynes with a pyramidalized alkene

A pyramidalized alkene and diverse arynes are generated in the presence of a Pd-catalyst to afford [2+2+2] cocycloaddition products.

Synthesis of biaryls using aryne intermediates

The synthesis of biaryls from benzyne intermediates offers an alternative strategy to conventional metal-catalyzed cross-coupling approaches.

Mechanism, reactivity, and selectivity of nickel-catalyzed [4 + 4 + 2] cycloadditions of dienes and alkynes

Density functional theory (DFT) calculations with B3LYP and M06 functionals elucidated the reactivities of alkynes and Z/E selectivity of cyclodecatriene products in the Ni-catalyzed [4 + 4 + 2] cycloadditions of dienes and alkynes. The Ni-mediated oxidative cyclization of butadienes determines the Z/E selectivity. Only the oxidative cyclization of one s-cis to one s-trans butadiene is facile and exergonic, leading to the observed 1Z,4Z,8E-cyclodecatriene product. The same step with two s-cis or s-trans butadienes is either kinetically or thermodynamically unfavorable, and the 1Z,4E,8E- and 1Z,4Z,8Z-cyclodecatriene isomers are not observed in experiments. In addition, the competition between the desired cooligomerization and [2 + 2 + 2] cycloadditions of alkynes depends on the coordination of alkynes. With either electron-deficient alkynes or alkynes with free hydroxyl groups, the coordination of alkynes is stronger than that of dienes, and alkyne trimerization prevails. With alkyl-substituted alkynes, the generation of alkyne-coordinated nickel complex is much less favorable, and the [4 + 4 + 2] cycloaddition occurs.

Three-component coupling reactions of arynes for the synthesis of benzofurans and coumarins

The domino three-component coupling reaction of arynes with DMF and active methylenes or methines was studied as a highly efficient method for preparing heterocycles. Coumarin derivative 5 was formed when diethyl malonate (2) or α-bromomalonate (3) were used as a C2-unit. In contrast, dihydrobenzofurans 7a and 7b were obtained by using α-chloroenolates generated from α-chloromalonates 4a and 4b and Et₂Zn. The benzofuran 15a could be obtained by using ethyl iodoacetate (14) as a C1-unit. The one-pot conversion of dihydrobenzofurans 7a, 7b and 8a into benzofurans 15a and 15b was also studied. The direct synthesis of benzofuran 15b was achieved by using the active methine 18 having ketone and ester groups.

Efficient synthesis of 9-aryldihydrophenanthrenes by a cascade reaction involving arynes and styrenes

A mild, general, and transition-metal-free protocol for the synthesis of 9,10-dihydrophenanthrenes is reported. The aryne generated by the fluoride-induced 1,2-elimination of 2-(trimethylsilyl)aryl triflates undergoes an efficient cascade reaction initiated by the Diels-Alder reaction with the differently substituted styrenes leading to the formation of 9-aryl-9,10-dihydrophenanthrene derivatives in moderate to good yields.

Ruthenium-catalyzed intramolecular [2+2+2] cycloaddition and tandem cross-metathesis of triynes and enediynes

[2+2+2] Cycloadditions can be applied to specifically build up derivatives of benzene and cyclohexadiene and, therefore, have attracted much attention. Herein, we present an intramolecular [2+2+2] cycloaddition of triynes catalyzed by the first-generation Grubbs ruthenium complex (Ru gen-1), which can efficiently afford benzene derivatives in good yields under mild conditions. Moreover, we also report on a novel tandem cross-metathesis transformation of intramolecular enediynes also catalyzed by Ru gen-1, which has not been observed previously in related reports. On the basis of deuterium labeling experiments, a possible reaction mechanism is presented.

Ruthenium-catalyzed intramolecular [2+2+2] cyclization of allene-yne-enes: construction of fused-tricyclic skeletons

Three's not a crowd: An intramolecular [2+2+2] cyclization between three different components, allene, alkyne, and alkene, has been realized using a catalytic amount of [Cp*RuCl(cod)] complex (cod = 1,5-cyclooctadiene), and afforded fused-tricyclic compounds in a highly stereoselective manner.

Total synthesis of (-)-herbindoles A, B, and C via transition-metal-catalyzed intramolecular [2 + 2 + 2] cyclization between ynamide and diynes

The total syntheses of (-)-herbindoles A, B, and C as naturally occurring forms were accomplished for the first time through transition-metal-catalyzed intramolecular [2 + 2 + 2] cyclization between ynamide and diynes. This strategy provided a highly efficient synthetic route to all three herbindoles from an identical indoline derivative as a common intermediate.

Synthetic Methods for the Generation and Preparative Application of Benzyne

Many methods have been developed for generating benzyne. Convenient and reliable precursors extensively studied so far involve benzenediazonium-2-carboxylate and o-dihalobenzenes such as 1,2-bromofluorobenzene and 1,2-dibromobenzene. Recently, in addition to the above precursors, o-(trimethylsilyl)phenyl triflate has been put into frequent use for benzyne reactions, in which benzyne is efficiently generated under mild conditions using fluoride ion. Furthermore, o-(trimethylsilyl)phenyliodonium triflate has been developed as a more efficient benzyne precursor. This mini-review focusses on recent progress in benzyne chemistry from the viewpoint of organic synthesis. The methods for generating benzynes are classified by the conditions into four categories: basic conditions using strong bases, mild conditions using fluoride ion, thermolysis, and oxidation.