"Formal" ruthenium-catalyzed [4+2+2] cycloaddition of 1,6-diynes to 1,3-dienes: formation of cyclooctatrienes vs vinylcyclohexadienes

Recent advances in 8π electrocyclization reactions

Medium-ring systems, which constitute a class of structurally intriguing and biologically important molecules, are present in many natural products and pharmaceuticals. However, the construction of these skeletons tends to be difficult because of the torsional strain of the medium-sized ring, and control of the selectivity is also challenging in these flexible skeletons. Electrocyclization is one of the most straightforward methods to construct medium-sized rings and this process typically proceeds in a stereospecific manner, resulting in the stereo-controlled formation of two neighboring stereocenters. At present, there are few studies on 8π electrocyclization, mainly focusing on the synthesis of small molecules, while the applications in the synthesis of functional materials and biological contexts are rare. This feature article highlights recent advances, from 2000 to 2022, in the 8π electrocyclization reaction. This study is organized into four sections based on the size/composition of the target ring, including the synthesis of aza-seven-membered, cycloheptene, cyclooctene and bicyclo[4,2,0]octane frameworks. We expect that this feature article will provide beneficial guidance for the selective construction of medium-ring skeletons.

Hydroelementation of diynes

This review highlights the hydroelementation reactions of conjugated and separated diynes, which depending on the process conditions, catalytic system, as well as the type of reagents, leads to the formation of various products: enynes, dienes, allenes, polymers, or cyclic compounds. The presence of two triple bonds in the diyne structure makes these compounds important reagents but selective product formation is often difficult owing to problems associated with maintaining appropriate reaction regio- and stereoselectivity. Herein we review this topic to gain knowledge on the reactivity of diynes and to systematise the range of information relating to their use in hydroelementation reactions. The review is divided according to the addition of the E-H (E = Mg, B, Al, Si, Ge, Sn, N, P, O, S, Se, Te) bond to the triple bond(s) in the diyne, as well as to the type of the reagent used, and the product formed. Not only are the hydroelementation reactions comprehensively discussed, but the synthetic potential of the obtained products is also presented. The majority of published research is included within this review, illustrating the potential as well as limitations of these processes, with the intent to showcase the power of these transformations and the obtained products in synthesis and materials chemistry.

Bioorthogonal Azide-Thioalkyne Cycloaddition Catalyzed by Photoactivatable Ruthenium(II) Complexes

Tailored ruthenium sandwich complexes bearing photoresponsive arene ligands can efficiently promote azide-thioalkyne cycloaddition (RuAtAC) when irradiated with UV light. The reactions can be performed in a bioorthogonal manner in aqueous mixtures containing biological components. The strategy can also be applied for the selective modification of biopolymers, such as DNA or peptides. Importantly, this ruthenium-based technology and the standard copper-catalyzed azide-alkyne cycloaddition (CuAAC) proved to be compatible and mutually orthogonal.

Metal and Non-Metal Catalysts in the Synthesis of Five-Membered S-Heterocycles

BACKGROUND

A wide variety of biological activities are exhibited by N, O and S containing heterocycles and recently, many reports appeared for the synthesis of these heterocycles. The synthesis of heterocycles with the help of metal and non-metal catalyst has become a highly rewarding and important method in organic synthesis. This review article concentrated on the synthesis of S-heterocylces in the presence of metal and non-metal catalyst. The synthesis of five-membered S-heterocycles is described here.

OBJECTIVE

There is a need for the development of rapid, efficient and versatile strategy for the synthesis of heterocyclic rings. Metal, non-metal and organocatalysis involving methods have gained prominence because traditional conditions have disadvantages such as long reaction times, harsh conditions and limited substrate scope.

CONCLUSION

The metal-, non-metal-, and organocatalyst assisted organic synthesis is a highly dynamic research field. For ßthe chemoselective and efficient synthesis of heterocyclic molecules, this protocol has emerged as a powerful route. Various methodologies in the past few years have been pointed out to pursue more sustainable, efficient and environmentally benign procedures and products. Among these processes, the development of new protocols (catalysis), which avoided the use of toxic reagents, are the focus of intense research.

Ruthenium-Catalyzed [2+2+2] Cycloaddition of 1,6-Enynes and Unactivated Alkynes: Access to Ring-Fused Cyclohexadienes

The [2+2+2] intermolecular carbocyclization reactions between 1,6-enynes and alkynes catalyzed by [RuCl(cod)(Cp*)] (cod=1,5-cyclooctadiene, Cp*=pentamethylcyclopentadienyl) are reported to provide bicyclohexa-1,3-dienes. The presented reaction conditions are compatible with internal and terminal alkynes and the chemo- and regioselectivity issues are controlled by the presence of substituents at the propargyl carbon center of the alkyne(s) partner(s).

Nucleophilic addition of amines to ruthenium carbenes: ortho-(alkynyloxy)benzylamine cyclizations towards 1,3-benzoxazines

A new ruthenium-catalyzed cyclization of ortho-(alkynyloxy)benzylamines to dihydro-1,3-benzoxazines is reported. The cyclization is thought to take place via the vinyl ruthenium carbene intermediates which are easily formed from [Cp*RuCl(cod)] and N2 CHSiMe3 . The mild reaction conditions and the efficiency of the procedure allow the easy preparation of a broad range of new 2-vinyl-2-substituted 1,3-benzoxazine derivatives. Rearrangement of an internal C(sp) in the starting material into a tetrasubstituted C(sp(3) ) atom in the final 1,3-benzoxazine is highly remarkable.

Ruthenium-catalyzed cycloadditions of 1-haloalkynes with nitrile oxides and organic azides: synthesis of 4-haloisoxazoles and 5-halotriazoles

(Cyclopentadienyl)(cyclooctadiene) ruthenium(II) chloride [CpRuCl(cod)] catalyzes the reaction between nitrile oxides and electronically deficient 1-choro-, 1-bromo-, and 1-iodoalkynes leading to 4-haloisoxazoles. Organic azides are also suitable 1,3-dipoles, resulting in 5-halo-1,2,3-triazoles. These air-tolerant reactions can be performed at room temperature with 1.25 equivalents of the respective 1,3-dipole relative to the alkyne component. Reactive 1-haloalkynes include propiolic amides, esters, ketones, and phosphonates. Post-functionalization of the halogenated azole products can be accomplished by using palladium-catalyzed cross-coupling reactions and by manipulation of reactive amide groups. The lack of catalysis observed with [Cp*RuCl(cod)] (Cp* = pentamethylcyclopentadienyl) is attributed to steric demands of the Cp* (η(5)-C5Me5) ligand in comparison to the parent Cp (η(5)-C5H5). This hypothesis is supported by the poor reactivity of [(η(5)-C5Me4CF3)RuCl(cod)], which serves as a an isosteric mimic of Cp* and as an isoelectronic analogue of Cp.

[4+2+2] Cycloaddition catalyzed by a new cationic rhodium-bisphosphine monooxide complex

A rhodium-BozPHOS based complex is reported. This complex is competent in catalyzing the [4+2+2] cycloisomerization of cyclooctatrienes in moderate to good yields. The X-ray crystal structure of this complex is reported, along with formation of both bicyclic and tricyclic cyclooctatrienes.

An expeditious route to eight-membered heterocycles by nickel-catalyzed cycloaddition: low-temperature C(sp)2-C(sp)3 bond cleavage

A cool break: 3-Azetidinone and a variety of diynes undergo a cycloaddition reaction catalyzed by Ni/IPr to give dihydroazocine compounds (see scheme; IPr=1,3-bis(2,6-diisopropylphenyl)imidazolidene). The reaction involves a challenging C(sp)2-C(sp)3 bond cleavage step, yet, surprisingly, proceeds at low temperature.

Transition-metal-catalyzed cycloadditions for the synthesis of eight-membered carbocycles

Eight-membered carbocycles are found in a wide variety of natural products that exhibit a broad range of biological and medicinal activities (cf. the most potent anticancer drug, taxol). However, the synthesis of eight-membered carbocycles is quite challenging as traditional approaches are met with entropic and enthalpic penalties in the ring-forming transition states. These negative effects can be totally or partially avoided with the implementation of transition-metal-catalyzed/mediated cycloadditions. In this Focus Review, examples of elegant and efficient metal-catalyzed and some metal-mediated cycloadditions (including Ni- catalyzed [4+4] and Rh-catalyzed [4+2+2] and [5+2+1] reactions) are presented to illustrate this. Application of these cycloaddition reactions in total synthesis is also presented to show the significance of these reactions in addressing challenges in natural product synthesis.

Enantioselective rhodium-catalyzed [4+2+2] cycloaddition of dienyl isocyanates for the synthesis of bicyclic azocine rings

A highly enantioselective rhodium-catalyzed [4+2+2] cycloaddition of terminal alkynes and dienyl isocyanates has been developed. The cycloaddition provides a rapid entry to highly functionalized and enantioenriched bicyclic azocines. This reaction represents the first [4+2+2] cycloaddition strategy to construct nitrogen-containing eight-membered rings.

A new Ru-catalyzed cascade reaction forming polycyclic cyclohexadienes from 1,6-diynes and alkenes

Functionalized bicyclic 1,3-cyclohexadienes can be easily prepared by a new cascade reaction which involves the Ru-catalyzed addition of acyclic alkenes to 1,6-diynes to give (Z)-hexatrienes, followed by a pure thermal 6e-pi electrocyclization.

Ruthenium-catalyzed reactions--a treasure trove of atom-economic transformations

The demand for new chemicals spanning the fields of health care to materials science combined with the pressure to produce these substances in an environmentally benign fashion pose great challenges to the synthetic chemical community. The maximization of synthetic efficiency by the conversion of simple building blocks into complex targets remains a fundamental goal. In this context, ruthenium complexes catalyze a number of non-metathesis conversions and allow the rapid assembly of complex molecules with high selectivity and atom economy. These complexes often exhibit unusual reactivity. Careful consideration of the mechanistic underpinnings of the transformations can lead to the design of new reactions and the discovery of new reactivity.

Practical asymmetric approach to medium-sized carbocycles based on the combination of two ru-catalyzed transformations and a lewis Acid-induced cyclization

[Reaction: see text] Ruthenium-catalyzed coupling of allyl ethyl ether to optically active 1-trimethylsilyl-1-alkyn-3-ols, followed by in situ ketalization and Lewis-acid-induced cyclization, affords enantiomerically pure 1,5-oxygen-bridged eight- and nine-membered carbocycles. Opening of the oxygen bridge under basic or electron transfer conditions provides optically pure medium-sized carbocycles, products that are difficult to construct using other currently available methodologies.

Eight-membered ring construction by [4 + 2 + 2] annulation involving beta-carbon elimination

Cyclobutanones underwent a formal [4 + 2 + 2] annulation reaction with 1,6- and 1,7-diynes in the presence of nickel(0) catalysts to provide bicyclic eight-membered ring ketones. The annulation reaction proceeds through a ring-expansion of oxanickelacycloheptadiene via beta-carbon elimination to form a nine-membered nickelacycle. This reaction employing cyclobutanones as a C4 unit constructs cyclooctadienone cores in one synthetic step.