Nickel-catalyzed dehydrogenative [4 + 2] cycloaddition of 1,3-dienes with nitriles

Pyridines, which comprise one of the most important classes of the six-membered heterocyclic compounds, are widely distributed in nature, and the transition-metal-catalyzed [2 + 2 + 2] cycloaddition reaction of two alkynes and a nitrile is one of the most powerful methods for preparing versatile, highly substituted pyridine derivatives. However, the lack of chemo- and regioselectivity is still a crucial issue associated with fully intermolecular [2 + 2 + 2] cycloaddition. The present study developed the Ni(0)-catalyzed intermolecular dehydrogenative [4 + 2] cycloaddition reaction of 1,3-butadienes with nitriles to give a variety of pyridines regioselectively.     

Rh-catalyzed enantioselective [2 + 2] cycloaddition of alkynyl esters and norbornene derivatives

[reaction: see text] The enantioselective [2 + 2] cycloaddition of alkynes possessing an ester functionality and norbornene derivatives proceeded efficiently using a chiral rhodium catalyst. The chiral tri- and tetracyclic cyclobutenes were obtained in moderate to high ee.     

Nickel-catalyzed [4+3] cycloaddition of ethyl cyclopropylideneacetate and 1,3-dienes

The [4+3] cycloaddition of ethyl cyclopropylideneacetate (1) with 1,3-dienes proceeded in the presence of Ni(cod)₂-TOPP (tri-o-biphenylyl phosphite). The reaction provided a new method for the synthesis of cycloheptene derivatives. The mechanism of the reaction was proposed.     

Catalytic enantioselective [2 + 4] and [2 + 2] cycloaddition reactions with propiolamides

We report here the catalytic and highly enantioselective [2 + 4] and [2 + 2] cycloaddition reactions of electron-rich dienes or silyl enol ethers with electron-deficient propiolamide derivatives induced by copper(II).3-(2-naphthyl)-L-alanine amide complex.     

Regioselective preferential CH activation of sterically hindered 1,3-dienes over [4+2] cycloaddition

The study of Pd-catalyzed preferential CH activation of sterically hindered α, β-olefinic indoles onto alkenes beyond [4 + 2] cycloaddition has been described. The carbazole derivatives were readily synthesized via activation of vinylic CH bonds with excellent regioselectivity. Further, the one-pot strategy has been employed for the synthesis of tricyclic carbazoles. The double and triple CH activation followed by concomitant Michael addition provides an economical approach for the synthesis of N-protected carbazole. A wide range of alkenes at the α- and β-position are compatible with this reaction. The mechanistic and X-ray crystallographic studies supported the designed chemistry of CH activation.     

Catalytic enantioselective intermolecular [2 + 2 + 2] cycloaddition of an alkene and two alkynes

A catalytic enantioselective intermolecular [2 + 2 + 2] cycloaddition of one molecule of alkene (enone) and two molecules of alkyne was developed in the presence of a nickel complex modified by chiral monodentate oxazoline ligands, which have not previously been used as chiral ligands for transition metals in asymmetric catalysts, and an aluminium phenoxide.     

[4+2]Cycloadditions of organometallic-substituted siloles with dimethyl acetylenedicarboxylate and tetracyanoethylene

1-Sila-2,4-cyclopentadienes (siloles) bearing five organyl groups and a diethylboryl group in 3-position (3), four organyl groups, a trimethylstannyl and a diethylboryl group in 2,4-positions (4), four organyl groups, a diethylboryl group in 3-position and a hydrido function at the silicon atom (5) react by [4+2]cycloaddition with dimethyl acetylenedicarboxylate, MeOC(O)CCC(O)OMe (1), and tetracycanoethylene, (NC)₂CC(CN)₂ (2), to give 7-silanorbornadienes (6–8) and 7-silanorbornenes (10–12), respectively. The silole 5 is converted into isomers 8 and 8′, and 12 and 12′, in which the SiMe group in each major isomer (8 and 12) occupies the syn-position with respect to the C(2)C(3) bond. The molecular structure of 10a was determined by X-ray analysis. The 7-silanorbornadiene (7) rearranges into a benzene derivative by formation of an SiO bond and 1,3-migration of the trimethylstannyl group. All products were characterised in solution by multinuclear magnetic resonance (¹H-, ¹¹B-, ¹³C-, ²⁹Si-, ¹¹⁹Sn-NMR spectroscopy). The geometries of 1,4,7,7-tetramethyl-7-silanorbornadiene, -7-silanobornene, and -7-silanorbornane were optimised by ab initio MO calculations (RHF/6-311+G(d,p) and chemical shifts δ²⁹Si were calculated (GIAO-RHF/6-311+G(d,p)).     

Cobalt-catalyzed intermolecular [2 + 2 + 2] cycloaddition for the synthesis of 1,3-cyclohexadienes

A cobalt(I)-catalyzed [2 + 2 + 2] cycloaddition reaction between an internal acceptor-substituted alkyne and a terminal alkene leads to the formation of regiochemically enriched polysubstituted 1,3-cyclohexadiene derivatives in acceptable yields when methyl butynoate is used, whereas regiochemically pure products are formed in good yields form phenyl propyonate. The concurrent cyclotrimerization reaction of the alkyne to the corresponding benzene derivative is dependent on the sterical bulk of the alkyne and is considerably reduced with the sterically more hindered alkyne.     

[2+4] vs [4+2] Cycloaddition reactions of o-thioquinones with 1,3-dienes

Under kinetic control, the reaction of o-thioquinones with acyclic 1,3-dienes afforded, as main products, the spiro cycloadducts deriving from the participation of the thiones as dienophiles. Under thermodynamic control, or with cyclic dienes, the thioquinones behave as hetero dienes to give the benzoxathiin derivatives with complete regioselectivity.     

An efficient, overall [4+1] cycloaddition of 1,3-dienes and nitrene precursors

Intermolecular cycloadditions of conjugated dienes and nitrene precursors usually produce aziridines. A generally useful method was lacking to directly provide the [4+1] cycloadducts, 3-pyrrolines. We have realized this transformation by using an uniquely active catalyst, copper(II) 1,1,1,5,5,5-hexafluoroacetylacetonate ([Cu(hfacac)(2)]). The method is applicable to a wide array of dienes with good yields. When 1,4-disubstituted dienes are used as substrates, good-to-excellent cis or trans selectivity can be obtained. Interestingly, the cis or trans preference depends on the nature of the substituents, rather than diene geometry. Mechanistic studies reveal that the [4+1] cycloaddition proceeds through diene aziridination and subsequent ring expansion. Among common copper catalysts, only [Cu(hfacac)(2)] can efficiently catalyze both steps, which explains the unique efficiency of the catalyst.     

[2+2]- AND [4+2] cycloaddition reactions of 1,3-diazabutadienes with diphenylketene

1,3-Diazabutadienes react with diphenylketene in [2+2] - and [4+2] cycloaddition reactions, depending on the substitution pattern. Spectroscopic data (IR, ¹H- and ¹³C-NMR) and results of quantummechanical model calculations (ab initio 3-21G) are presented.     

Synthesis of 3,5-diaryl-4-chlorophthalates by [4+2] cycloaddition of 1-ethoxy-2-chloro-1,3-bis(trimethylsilyloxy)-1,3-diene with dimethyl acetylenedicarboxylate and subsequent site-selective Suzuki-Miyaura reactions

The [4+2] cycloaddition of 1-ethoxy-2-chloro-1,3-bis(trimethylsilyloxy)-1,3-diene with dimethyl acetylenedicarboxylate (DMAD) afforded dimethyl 4-chloro-3,5-dihydroxyphthalate. Site-selective Suzuki-Miyaura reactions of its bis(triflate) provide a convenient approach to 3,5-diaryl-4-chlorophthalates containing two different aryl groups.     

2 + 4] and [4 + 2] cycloadditions of o-thioquinones with 1,3-dienes: a computational study

Cycloadditions of o-thioquinones (o-TQs) with 1,3-dienes could proceed via either a [2 + 4] or a [4 + 2] mechanism. Under kinetic control and with acyclic dienes the reaction affords the spiro cycloadducts 5deriving from the [2 + 4] path as the main products. Under thermodynamic control, or with cyclic dienes, the o-TQs behave as heterodienes to give the benzoxathiin derivatives 4, in most cases with complete regioselectivity. In the present computational study, DFT calculations were performed in order to achieve a deep understanding of both [2 + 4] and [4 + 2] paths. The reactions of three o-TQs with six 1,3-dienes were thoroughly investigated at the B3LYP/TZVP//B3LYP6-31G level, and the two reaction mechanisms were then compared, evidencing that [2 + 4] cycloadditions are kinetically favored, strongly asynchronous, or even unconcerted, while [4 + 2] reactions are thermodynamically favored, quite asynchronous, but undoubtedly concerted. Moreover, the observed regioselectivity was rationalized by mean of the FMO theory and by comparison of the activation energies for different pathways.     

Enantioselective intramolecular [2 + 2 + 2] cycloaddition of enediynes for the synthesis of chiral cyclohexa-1,3-dienes

The enantioselective intramolecular [2 + 2 + 2] cycloaddition of various enediynes, where two acetylenic moieties are connected by a trans-olefinic moiety, gave chiral tricyclic cyclohexa-1,3-dienes using Rh-H8-BINAP catalyst. In the case of carbon-atom-tethered enediynes, enantioselectivity was generally good-to-high regardless of the substituents on their alkyne termini. In contrast, with heteroatom-tethered enediynes, appropriate substituents were required to induce the oxidative coupling of alkyne and alkene moieties before that of two alkyne moieties, which would be important for highly enantioselective intramolecular cycloaddition.     

Recent advances in enantioselective [2 + 2 + 2] cycloaddition

Enantioselective cycloaddition using chiral transition metal catalysts is an atom-economical and efficient synthetic tool for the construction of chiral carbo- and heterocyclic skeletons. This short account discloses our recent results of inter- and intramolecular enantioselective [2 + 2 + 2] cycloadditions of alkyne and/or alkene moiety(ies). Chiral iridium complexes catalyzed the alkyne trimerization for the generation of axial chirality(ies), and chiral rhodium ones catalyzed alkyne-alkyne-alkene cyclization for the generation of a quaternary carbon including spirocyclic system.     

Rhodium-catalyzed enantioselective [2+2+2] cycloaddition of diynes with unfunctionalized alkenes

A chiral rhodium complex catalyzed an enantioselective [2+2+2] cycloaddition of unsymmetrical diynes with norbornene, and tetracyclic products were obtained in good to excellent ee. The cycloaddition of a symmetrical diyne with styrene derivatives as coupling partners gave bicyclic products in good ee.     

Synthesis of 4-hydroxy- and 2,4-dihydroxy-homophthalates by [4+2] cycloaddition of 1,3-bis(silyloxy)-1,3-butadienes with dimethyl allene-1,3-dicarboxylate

The reaction of 1,3-bis(trimethylsiloxy)-1,3-butadienes with dimethyl allene-1,3-dicarboxylate provides a convenient and regioselective approach to a variety of functionalized 4-hydroxy- and 2,4-dihydroxy-homophthalates.     

Diastereo- and enantioselective palladium-catalyzed dearomative [4+2] cycloaddition of 3-nitroindoles

A Pd-catalyzed asymmetric decarboxylative [4+2] cycloaddition of 3-nitroindoles and vinyl benzoxazinanones is developed through a dearomatization approach. The reaction provides an efficient protocol for constructing a series of chiral tetrahydro-5H-indolo[2,3-b]quinolines in high yields and with excellent diastereo- and enantioselectivities.     

Gold-catalyzed intermolecular [4C+3C] cycloaddition reactions

In the presence of the N-heterocyclic carbene gold catalyst (NHC-AuIPr, 7), propargyl esters 1a-f and 13 undergo a [4C+3C] cycloaddition reaction with cyclopentadiene and furan under mild conditions. The evidence suggests that the formation of the seven-membered ring occurs by a direct cycloaddition process, rather than a stepwise cyclopropanation/Cope rearrangement sequence.