Recent advances in "formal" ruthenium-catalyzed [2+2+2] cycloaddition reactions of diynes to alkenes

"Formal" and standard RuII-catalyzed [2+2+2] cycloaddition of 1,6-diynes to alkenes gave bicyclic 1,3-cyclohexadienes in relatively good yields. When terminal 1,6-diynes 1 were used, two isomeric bicyclic 1,3-cyclohexadienes 4 or 6 were obtained, depending on the acyclic or cyclic nature of the alkene partner. When unsymmetrical substituted 1,6-diynes 7 were used, the reaction with acyclic alkenes took place regio- and stereoselectively to afford bicyclic 1,3-cyclohexadienes 8. A cascade process that behaves as a "formal" RuII-catalyzed [2+2+2] cycloaddition explained these results. Initially, a Ru-catalyzed linear coupling of 1,6-diynes 1 and 7 with acyclic alkenes occurs to give open 1,3,5-trienes of type 3, which after a thermal disrotatory 6e(-) pi-electrocyclization led to the final 1,3-cyclohexadienes 4 and 8. When disubstituted 1,6-diyne 10 was used with electron-deficient alkenes, new exo-methylene cyclohexadienes 12 arose from a competitive reaction pathway.     

1,4‐Cyclohexadienes—Easy Access to a Versatile Building Block via Transition‐Metal‐Catalysed Diels–Alder Reactions

1,4‐Cyclohexadiene derivatives are easily accessed via transition‐metal cycloadditions of 1,3‐dienes with alkynes. The mild reaction conditions of several transition‐metal‐catalysed reactions allows the incorporation of various functional groups to access functionalised 1,4‐cyclohexadienes. The control of the regiochemistry in the intermolecular cobalt‐catalysed Diels–Alder reaction is realised utilising different ligand designs. The functionalised 1,4‐cyclohexadiene derivatives are valuable building blocks in follow‐up transformations. Finally, the oxidation of the 1,4‐cyclohexadienes can be accomplished under mild conditions to generate the corresponding arene derivatives.     

Mechanism, regioselectivity, and the kinetics of phosphine-catalyzed [3+2] cycloaddition reactions of allenoates and electron-deficient alkenes

With the aid of computations and experiments, the detailed mechanism of the phosphine-catalyzed [3+2] cycloaddition reactions of allenoates and electron-deficient alkenes has been investigated. It was found that this reaction includes four consecutive processes: 1) In situ generation of a 1,3-dipole from allenoate and phosphine, 2) stepwise [3+2] cycloaddition, 3) a water-catalyzed [1,2]-hydrogen shift, and 4) elimination of the phosphine catalyst. In situ generation of the 1,3-dipole is key to all nucleophilic phosphine-catalyzed reactions. Through a kinetic study we have shown that the generation of the 1,3-dipole is the rate-determining step of the phosphine-catalyzed [3+2] cycloaddition reaction of allenoates and electron-deficient alkenes. DFT calculations and FMO analysis revealed that an electron-withdrawing group is required in the allene to ensure the generation of the 1,3-dipole kinetically and thermodynamically. Atoms-in-molecules (AIM) theory was used to analyze the stability of the 1,3-dipole. The regioselectivity of the [3+2] cycloaddition can be rationalized very well by FMO and AIM theories. Isotopic labeling experiments combined with DFT calculations showed that the commonly accepted intramolecular [1,2]-proton shift should be corrected to a water-catalyzed [1,2]-proton shift. Additional isotopic labeling experiments of the hetero-[3+2] cycloaddition of allenoates and electron-deficient imines further support this finding. This investigation has also been extended to the study of the phosphine-catalyzed [3+2] cycloaddition reaction of alkynoates as the three-carbon synthon, which showed that the generation of the 1,3-dipole in this reaction also occurs by a water-catalyzed process.     

The role of phosphine and 1,2-diimine complexes of nickel in the oxidation states 0, +1, and +2 in the catalyzed di-, oligo-, and polymerization of ethylene

The turnover frequency and number have been determined for eighteen catalytic systems based on triphenylphosphine and 1,4-diazo-1,3-butadiene complexes of nickel in the formal oxidation states 0, +1, and +2 in the oligoand polymerization of lower alkenes. The main catalytic characteristics are almost independent of the oxidation state of nickel in the precursor and depend on the nature and concentration of the cocatalyst (Lewis acid). The catalytic systems have been studied by ESR. The ESR spectral parameters are presented for nickel(I) 1,4-diazo-1,3-butadiene complexes and radical anions resulting from the reactions of the cocatalyst with nickel α-diimine complexes. Reactions describing the formation, functioning, decomposition, and regeneration of the catalytically active nickel hydride complexes are proposed.     

Synthesis of Cyclobutenes and Allenes by Cobalt‐Catalyzed Cross‐Dimerization of Simple Alkenes with 1,3‐Enynes

Cobalt‐catalyzed cross‐dimerization of simple alkenes with 1,3‐enynes is reported. A [2+2] cycloaddition reaction occurred, with alkenes bearing no allylic hydrogen, by reductive elimination of a η³‐butadienyl cobaltacycle. On the other hand, aliphatic alkenes underwent 1,4‐hydroallylation by means of exo‐cyclic β‐H elimination. These reactions can provide cyclobutenes and allenes that were previously difficult to access, from simple substrates in a highly chemo‐ and regioselective manner.     

Formal and standard ruthenium-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,6-diynes to alkenes: a mechanistic density functional study

"Formal" and standard Ru(II)-catalyzed [2 + 2 + 2] cycloaddition of 1,6-diynes 1 to alkenes gave bicyclic 1,3-cyclohexadienes in relatively good yields. The neutral Ru(II) catalyst was formed in situ by mixing equimolecular amounts of [Cp*Ru(CH3CN)3]PF6 and Et4NCl. Two isomeric bicyclic 1,3-cyclohexadienes 3 and 8 were obtained depending on the cyclic or acyclic nature of the alkene partner. Mechanistic studies on the Ru catalytic cycle revealed a clue for this difference: (a) when acyclic alkenes were used, linear coupling of 1,6-diynes with alkenes was observed giving 1,3,5-trienes 6 as the only initial reaction products, which after a thermal disrotatory 6e-pi electrocyclization led to the final 1,3-cyclohexadienes 3 as probed by NMR studies. This cascade process behaved as a formal Ru-catalyzed [2 + 2 + 2] cycloaddition. (b) With cyclic alkenes, the standard Ru-catalyzed [2 + 2 + 2] cycloaddition occurred, giving the bicyclic 1,3-cyclohexadienes 8 as reaction products. A complete catalytic cycle for the formal and standard Ru-catalyzed [2 + 2 + 2] cycloaddition of acetylene and cyclic and acyclic alkenes with the Cp*RuCl fragment has been proposed and discussed based on DFT/B3LYP calculations. The most likely mechanism for these processes would involve the formation of ruthenacycloheptadiene intermediates XXIII or XXVII depending on the alkene nature. From these complexes, two alternatives could be envisioned: (a) a reductive elimination in the case of cyclic alkenes 7 and (b) a beta-elimination followed by reductive elimination to give 1,3,5-hexatrienes 6 in the case of acyclic alkenes. Final 6e-pi electrocyclization of 6 gave 1,3-cyclohexadienes 3.     

Cycloaddition of Alkenes and Alkynes to the P-centered Singlet Biradical [P(μ-NTer)]2

The reaction of biradical [P(μ-NTer)]₂ ( 1 , Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl) towards different alkenes (R = 2,3-dimethyl–butadiene, 2,5-dimethyl-2,4-hexadiene, 1,7-octadiene, 1,4-cyclohexadiene) and alkynes (R = 1,4-diphenyl-1,3-butadiyne) was studied experimentally. Although these olefins can react in different ways, only [2+2] cycloaddition products ( 1R ) were observed. The reaction with 2,3-dimethylbutadiene also led to the [2+2] product ( 1dmb ). Thermal treatment of 1dmb above 140 °C resulted in the recovery of biradical 1 upon homolytic bond cleavage of the two P–C bonds and the release of 2,3-dimethylbutadiene. In contrast to this reaction, all other [2+2] additions products ( 1R , R = 1,7-octadiene, 1,4-cyclohexadiene, 1,4-diphenyl-1,3-butadiyne) began to decompose at temperatures between 200 °C and 300 °C. Only unidentified products were obtained but no temperature-controlled equilibrium reactions were observed. Computations were carried out to shed light into the formal [2+2] as well as the possible [4+2] addition reaction.     

Silylated Cyclohexadienes in Radical Chain Hydrosilylations

A new method for the mild radical hydrosilylation of alkenes and alkynes is described. Silylated cyclohexadienes that can be readily prepared on large scale are used as radical hydrosilylating reagents. Non‐activated alkenes and alkynes are hydrosilylated in high yields. The reaction can be combined with C C bond formation, as demonstrated for the preparation of silylated cycloalkanes from the corresponding dienes. Furthermore, radical hydrosilylations in combination with β‐fragmentation reactions for the synthesis of allylsilanes and hydrosilylations of aldehydes and ketones providing protected alcohols can be readily performed by this strategy.     

Au- and pt-catalyzed cycloisomerizations of 1,5-enynes to cyclohexadienes with a broad alkyne scope

We describe the development of gold- and platinum-catalyzed cycloisomerizations of 1,5-enynes. This catalytic process displays a wide alkyne scope and furnishes a range of highly functionalized 1,4- and 1,3-cyclohexadienes. In the case of 1-siloxy-1-yne-5-enes, the reactions are efficiently catalyzed by AuCl (1 mol %) at ambient temperature to afford siloxy cyclohexadienes or the corresponding 1,2- and 1,3-cyclohexenones upon subsequent protodesilylation. We propose that the reaction proceeds via a novel mechanism involving a series of 1,2-alkyl shifts. Elucidation of this unusual reaction mechanism enabled us, in turn, to significantly expand the scope of the cycloisomerization by incorporation of a quaternary center at the C(3) position of the enyne. Indeed, we established that PtCl(2) (5 mol %) efficiently catalyzed the cycloisomerizations of 1,5-enynes containing terminal, internal, and arene-conjugated alkynes. Since a variety of 1,5-enynes are readily accessible, the cycloisomerization provides a rapid approach to a wide range of highy substituted cyclohexadienes for many subsequent synthetic applications.     

Arenophile‐Mediated Dearomative Reduction

A dearomative reduction of simple arenes has been developed which employs a visible‐light‐mediated cycloaddition of arenes with an N‐N‐arenophile and in situ diimide reduction. Subsequent cycloreversion or fragmentation of the arenophile moiety affords 1,3‐cyclohexadienes or 1,4‐diaminocyclohex‐2‐enes, compounds that are not synthetically accessible using existing dearomatization reactions. Importantly, this strategy also provides numerous opportunities for further derivatization as well as site‐selective functionalization of polynuclear arenes.     

Rhodium-catalyzed three-component cross-addition of silylacetylenes, alkynyl esters, and electron-deficient alkenes or isocyanates

'Rh'oad crossing: A cationic Rh(I) /(cod)(2) complex catalyzes the chemo-, regio-, and stereoselective title reaction with electron-deficient alkenes leading to substituted 1,3-dienes. The analogous three-component cross-addition involving isocyanates instead of alkenes has also been developed.     

Phosphine-catalyzed [4 + 2] annulation and vinylogous addition reactions between 1,4-dien-3-ones and 1,1-dicyanoalkenes

Phosphine-catalyzed [4 + 2] annulation and vinylogous Michael addition reactions between 1,4-dien-3-ones and 1,1-dicyanoalkenes are presented. Under the catalysis of PBu(3) (20 mol %), 1,4-dien-3-ones like styryl ketones with 2-aryl 1,1-dicyanoalkenes as doubly activated alkenes readily undergo a formal [4 + 2] cycloaddition reaction, affording polysubstituted cyclohexanones in satisfactory yield and good diastereoselectivity; with the doubly activated alkenes bearing an acidic methyl or methylene at the 2-position, a vinylogous Michael addition of 1,4-dien-3-ones occurs under the same reaction conditions, giving a non-cyclized multifunctional adduct in good yield. These two phosphine-catalyzed transformations represent atom economical carbon-carbon bond forming reactions capable of rapid construction of molecular complexity. Based on experimental results, formation of the products has been mechanistically rationalized, and a phosphonium activation is proposed.     

Theoretical study of multiphoton ionization of cyclohexadienes and unimolecular decomposition of their mono- and dications

Quantum chemical calculations of the geometric structure, vertical excitation energies, and ionization potentials for the isomeric pair of 1,3- and 1,4-cyclohexadienes and their mono- and dications have been performed employing a variety of theoretical methods and basis sets. The computed ionization potentials and electronic excitation energies are used to evaluate the range of internal energies available for fragmentation of the cations following multiphoton resonance ionization of the cyclohexadienes in intense laser field. The conditions governing the competition between multiple ionization and decomposition of the ions are also discussed. Calculations of stationary points on the potential energy surfaces for various fragmentation channels and relative product yields at different available internal energies are then utilized to analyze the trends in branching ratios of major dissociation products of the 1,4-cyclohexadiene(2+) dication, which include C(3)H(3)(+) + C(3)H(5)(+), C(2)H(3)(+) + C(4)H(5)(+), and C(4)H(3)(+) + C(2)H(5)(+).     

Tandem 1,4-addition reactions with benzene and alkylated benzenes promoted by pentaammineosmium(II)

Electrophiles such as dimethoxymethane and 3-penten-2-one react with the complex [Os(NH(3))(5)(eta(2)-benzene)](2+) in the presence of triflic acid to form metastable benzenium intermediates. These benzenium intermediates further react with carbon nucleophiles including silyl ketene acetals, (silyloxy)alkenes, and phenyllithium in an overall tandem 1,4-addition sequence. The metal fragment controls the relative stereo- and regiochemistry for both electrophilic and nucleophilic addition steps. Upon oxidative demetalation with silver triflate, cis-1,4 cyclohexadienes are formed in yields ranging from 16 to 82%. This methodology can also be used to dearomatize toluene and ortho- and meta-xylene with unexpectedly high regio- and stereocontrol.     

Alkenes in [2+2+2] Cycloadditions

Participation of alkenes and allenes in [2+2+2] cycloaddition reactions has attracted much attention recently. This version of the well‐established alkyne cyclotrimerization renders interesting products, such as cyclohexadienes and other polycycles, through cascade processes. Many mechanistic variations are observed when using certain metal complexes as catalysts. The frequent generation of stereogenic centers has prompted the development of efficient asymmetric versions. This Minireview summarizes the efforts reported to date on the use of double bonds as partners in [2+2+2] cyclotrimerizations.     

Synthesis of 1,3-cyclohexadienes by tandem diene-alkyne metathesis: improved procedure

A practical synthesis of 2-substituted 1,3-cyclohexadienes by the cross enyne metathesis between alkynes and 1,5-hexadiene is reported. The isolation of the 1,3-cyclohexadienes has been hampered by the formation of an inseparable triene by-product. The use of a second consecutive cross alkene metathesis to give water-soluble products allowed removal of this by-product. Using this one-pot procedure, a synthesis of cyclohexadienes from simple starting materials was developed. The procedure was used in a three-step synthesis of a functionalized tetrahydroquinoline using Pd(II)-catalyzed chloroacetoxylation (Bäckvall reaction) for cyclohexadiene functionalization.     

Reactions of a metallacyclobutene complex with alkenes

The first productive reactions of a characterized metallacyclobutene complex with alkenes are reported. Thus, the metallacyclobutene complex (eta5-C5H5)(PPh3)Co[kappa2-(C,C)-C(SO2Ph) C(Si(CH3)3)CH(CO2CH2CH3)] (2) undergoes reaction with alkenes to give 1,4-diene complexes with a high degree of regio- and stereoselectivity. A mechanism is proposed in which the metallacyclobutene generates a cyclic vinylcarbene intermediate that undergoes [4 + 2]-cycloaddition reactions with activated alkenes. A model of the vinylcarbene intermediate has been examined using quantum mechanical methods.     

Conversions of hydrocarbons treated with neodymium metal under the conditions of mechanochemical activation at room temperature

The conversions of cyclohexene and 1,3- and 1,4-cyclohexadiencs treated with neodymium metal under the conditions of mechanochemical activation at 20 °C have been studied. A complicated cycle of conversions, including isomerization, disproportionation, and polymerization, occurs under the reaction conditions. A mechanism for the conversion of the hydrocarbons involving organornetallic intermediates is proposed.The conversion of hexane under the conditions used for the reactions with cyclohexene and cyclohexadienes varies only slightly, which makes it possible to use hexane as the internal standard.Deceased.Translated fromlzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1386–1390, June, 1996.     

Synthesis of highly substituted 1,3-dienes, 1,3,5-trienes, and 3,6-disubstituted cyclohexenes by the palladium-catalyzed coupling of organic halides, internal alkynes or 1,3-cyclohexadienes, and organoboranes

A number of highly substituted 1,3-dienes and 1,3,5-trienes have been stereoselectively prepared in moderate to good yields by the coupling of vinylic iodides, internal alkynes, and organoboranes in the presence of a palladium catalyst. Optimal reaction conditions for different organoboron substrates have been developed. The analogous three-component coupling of aryl halides, 1,3-cyclohexadiene, and boronic acids provides a synthetically useful route to 3,6-disubstituted cyclohexenes. These methods are very efficient and provide an expeditious way to synthesize the indicated alkenes, dienes, and trienes, whose preparation would normally require multi-step synthesis.     

Facile syntheses of allylic allenethiosulfinates and -sulfonates, and of beta-iodo alpha,beta-unsaturated gamma-sultines

The regiospecificity of the 1,4-addition of the recently reported novel alkoxy chlorodisulfides to 2-methyl-1,3-butadiene has been established. Allyl allenethiosulfinates formed by spontaneous [2,3]-sigmatropic rearrangement of the addition products were oxidized at 4 degrees C to the corresponding thiosulfonates. Periodate oxidation at room temperature, preferably in the presence of I2, resulted in oxidative cleavage and cyclization to beta-iodo alpha,beta-unsaturated gamma-sultines. Such sultines, with varying degrees of gamma-alkyl substitution, were also conveniently prepared by reaction of iodine with alkyl allenesulfinates.