Bis(2-chloroethyl) ethenylphosphonate in the diels-alder reaction

The diene synthesis reactions of bis(2-chloroethyl) ethenylphosphonate with 3-methyl-3-thiolene 1,1-dioxide that generates isoprene under the reaction conditions, as well as with 2,3-dimethylbuta-1,3-diene, cyclohexa-1,3-diene, cyclopentadiene, furan, and anthracene are studied. A series of cycloalkenyl-and heterylphosphonates derived from cyclohexene, bicyclooctene, norbornene, oxanorbornene, and 9,10-dihydro-9,10-ethanoanthracene are synthesized.     

The structure and dynamic behaviour in solution of the trihydridodienerhenium complexes (Ph3P)2(η-1,3-diene)ReH3

The structure and fluxionality of the trihydridodiene complexes (Ph₃P)₂(η-1,3-<di-ene)ReH₃ have been studied by NMR spectroscopy (η-1-3-diene = buta-1,3-diene, 2-methylbuta-1,3-diene, 2,3-dimethylbuta-1,3-diene, cyclohexa-1,3-diene, penta-1,3-diene, hexa-1,3-diene and hexa-2,4-diene). Several rearrangement processes have been observed; they are, in order of increasing temperature: (a) ligand interchange; (b) reversible migration of a hydride ligand on to the diene ligand, leading to η-allyl species and, in the case of the cyclohexadiene trihydride, degenerate isomerisation of the cyclohexadiene moiety; and (c), in the case of the pentadiene and hexadiene derivatives, isomerisation of the diene ligand.     

Stereochemistry of the formation of π-allylpalladium complexes from dialkylcyclohexa-1,3-dienes

1,4-Dimethyl-, 1-isopropyl-4-methyl- and 1-t-butyl-4-methylcyclohexa-1,3-diene reacted with a palladium salt to form, in each case, a single isomer of the corresponding π-allylpalladium chloride complexes, while 2-isopropyl-5-methylcyclohexa-1,3-diene gave two stereoisomeric complexes. An excess of diene (diene/Pd = 2.5–3.0) was required to produce a high yield of the complex. The hydrogen atom, which is incorporated onto the terminal carbon of the diene system, is shown (i) to come from the excess diene, which in turn is converted to an aromatic compound, and (ii) to attack the diene, stereo- and regio-selectively, from the same side as the palladium chloride portion.     

Diels-Alder reactions of 3-and 5-methyl-1-vinylpyrazoles with cyclohexa-1,3-diene and hydrogenation of the reaction products

The behavior of isomeric vinylpyrazoles in the Diels-Alder reactions with cyclohexa-1,3-diene was studied to show that the spatial location of the vinyl group on the N² atom does not affect the diene synthesis.     

Tetramethyl(perfluoroalkyl)cyclopentadienyl rhodium(I) complexes with ethylene or diene ligands. Crystal structure of [(η-C5Me4C6F13)Rh(CO)2]

Tetramethyl(perfluoroalkyl)cyclopentadienyl rhodium(I) complexes with ethylene or diene (norbornadiene, cycloocta-1,5-diene, 2,3-dimethylbuta-1,3-diene, cyclohexa-1,3-diene) ligands were obtained by reduction of tetramethyl(perfluoroalkyl)rhodium(III) dichloro dimers by zinc in THF or by propan-2-ol/sodium carbonate in the presence of the ligands. Reduction in the presence of cycloocta-1,3-diene gave a different product, an η³-cyclooctenyl complex, which was not reduced further. During the reduction in the presence of ethylene, a new tetramethyl(perfluoroalkyl)-η⁴-cyclopentadiene complex was observed by NMR. This compound, formed by hydrogen transfer from the metal to the ligand, is probably in an equilibrium with the parent hydridocyclopentadienyl complex. Crystal and molecular structure of dicarbonyltetramethyl(perfluorohexyl)cyclopentadienylrhodium(I) complex was determined by X-ray diffraction. The structure shows a moderate ring slippage of the rhodium atom which was not observed in the only other known structure of a complex with the same ligand, the rhodium(III) dichloro dimer.     

Oxidative addition of trifluoromethanesulfonamide to cycloalkadienes

Reactions of trifluoromethanesulfonamide with cyclopentadiene, cyclohexa-1,3- and -1,4-dienes, cyclohepta-1,3,5-triene, and cycloocta-1,3-diene in the presence of t-BuOCl-NaI were studied. Trifluoromethanesulfonamide added at one double bond of cyclopentadiene and cyclohexa-1,3-diene in regio- and stereoselective fashion to give N-(5-iodocyclopent-2-en-1-yl)trifluoromethanesulfonamide and trans-N,N′-cyclohex-3-ene-1,2-diylbis(1,1,1-trifluoromethanesulfonamide), respectively. The reaction with cyclohexa-1,4-diene involved both isolated double bonds to produce N,N′-(2-chloro-5-iodocyclohexane-1,4-diyl)bis(1,1,1-trifluoromethanesulfonamide) which underwent halophilic reduction of the CHI group by the action of NaI and elimination of HCl, leading to N,N′-(cyclohex-2-ene-1,4-diyl)bis(1,1,1-trifluoromethanesulfonamide). Under analogous conditions, cyclohepta-1,3,5-triene was oxidized to benzaldehyde, while no reaction with trifluoromethanesulfonamide occurred.     

Hexafluorothioacetone based synthesis of fluorinated heterocycles

Cycloadducts of hexafluorothioacetone (HFTA) were prepared in high yield by a CsF catalyzed reaction between readily available 2,2,4,4-tetrakis-(trifluoromethyl)-1,3-dithietane (as a source of HFTA) with conjugated electron-rich hydrocarbon dienes, such as cyclopentadiene, 2,3-dimethylbuta-1,3-diene, cyclohexa-1,3-diene or (1Z,3Z)-cyclohepta-1,3-diene. Cyclohexa-1,4- and (1Z,5Z)-cycloocta-1,5-dienes, also undergo the reaction with in situ generated HFTA, but form the products of insertion of HFTA into the C-H bond of the diene as a result of ene-reaction. The highly selective reaction of HFTA with (1Z,3Z,5Z)-cyclohepta-1,3,5-triene and (1Z,3Z,5Z,7Z)-cycloocta-1,3,5,7-tetraene leads to the formation of cycloadducts derived from exclusive addition of thioacetone to the corresponding bicyclic isomers—bicyclo[4.1.0]hepta-2,4-diene or bicyclo[4.2.0]octa-2,4,7-triene, respectively. The corresponding cycloadducts of HFTA with 2,3-dimethylbutadiene-1,3-cyclohexa-1,3-cyclohexa-1,4-dienes and (1Z,3Z,5Z)-cyclohepta-1,3,5-triene were also prepared by direct reaction of sulfur/hexafluoropropene/KF and the corresponding hydrocarbon substrate at 35-45 °C in DMF.     

Diels-alder reaction of (E)-4-(2-nitroethenyl)-1H-imidazoles and methyl (E)-3-(1-imidazol-4-yl)propenoates

Diels-Alder reaction of methyl (E)-3-(1H-imidazol-4-yl)propenoates 2, 3a-c and (E)4-(2-nitroethenyl)-1H-imidazoles 3d,e with 2,3-dimethyl-1,3-butadiene, cyclopentadiene, and cyclohexa-1,3-diene gave the corresponding cycloadducts 6–9 .     

A ring-closing metathesis approach to the bicyclo[4.3.1]decane core of caryolanes

A synthesis of highly substituted and sterically congested bicyclo[4.3.1]decenes, a structure embedded in the core 4,7,6-tricyclic system of natural caryolanes, was successfully achieved via a ring-closing metathesis (RCM) reaction of syn-1,3-diene substituted cyclohexanols. The construction of the diene substrates, starting from 4-acetoxy-3-methyl-2-cyclohexen-1-one, employed diastereoselective copper-mediated conjugate addition and Grignard reactions. An X-ray crystal structure determination of a key synthetic intermediate confirmed the relative stereochemistry of the RCM bicyclic product.     

Asymmetric diene synthesis of bicyclo[2.2.2]octenes in the presence of Lewis acids

Asymmetric diene synthesis of substituted bicyclo[2.2.2]octenes from (?)-menthyl acrylate and cyclohexa-1,3-diene in the presence of achiral Lewis acid catalysts (TiCl₄, AlCl₃, BBr₃, AlCl₃ · OEt₂, BBr₃ · OEt₂) was studied. The influence of various factors on the isomeric composition, yield, and enantiomeric purity of the compounds prepared was examined.     

Some reactions of tetrafluoroethylene oligomers

As part of a programme to prepare and evaluate a series of perfluoro- chemicals for use as inert fluids, the fluorinations of some tetrafluoroethylene oligomers over cobalt (III) fluoride have been studied.Fluorination of perfluoro-3,4-dimethylhex-3-ene (tetramer) and perfluoro-4-ethyl-3,4-dimethylhex-2-ene (pentamer) over CoF₃ at 230°C and l45°C respectively afforded the corresponding saturated fluorocarbons however, at 250°C, pentamer gave predominantly the saturated tetramer. The thermal behaviour of these saturated fluorocarbons alone and in the presence of bromine and toluene has been studied.Pyrolysis of pentamer over glass beads at 500°C gave perfluoro-1,2,3- trimethylcyclobutene and isomers of perfluoro-2,3-dimethylpenta-1,3- diene. Under similar conditions perfluoro-2-(1-ethyl-1-methylpropyl). 3-methylpent-1-ene (hexamer) gave perfluoro-1-methyl-2-(1-methyl- propyl)-cyclobut-1-ene and perfluoro-2-methyl-3-(1-methylpropyl)-buta- 1,3-diene.These reactions and the structural elucidation of the products will be discussed.     

Low-lying excited states and primary photoproducts of [Os3(CO)10(s-cis-L)] (L=cyclohexa-1,3-diene, buta-1,3-diene)] clusters studied by picosecond time-resolved UV/Vis and IR spectroscopy and by density functional theory

Combined picosecond transient absorption and time-resolved infrared studies were performed, aimed at characterising low-lying excited states of the cluster [Os(3)(CO)(10)(s-cis-L)] (L=cyclohexa-1,3-diene, 1) and monitoring the formation of its photoproducts. Theoretical (DFT and TD-DFT) calculations on the closely related cluster with L=buta-1,3-diene (2') have revealed that the low-lying electronic transitions of these [Os(3)(CO)(10)(s-cis-1,3-diene)] clusters have a predominant sigma(core)pi*(CO) character. From the lowest sigmapi* excited state, cluster 1 undergoes fast Os-Os(1,3-diene) bond cleavage (tau=3.3 ps) resulting in the formation of a coordinatively unsaturated primary photoproduct (1 a) with a single CO bridge. A new insight into the structure of the transient has been obtained by DFT calculations. The cleaved Os-Os(1,3-diene) bond is bridged by the donor 1,3-diene ligand, compensating for the electron deficiency at the neighbouring Os centre. Because of the unequal distribution of the electron density in transient 1 a, a second CO bridge is formed in 20 ps in the photoproduct [Os(3)(CO)(8)(micro-CO)(2)(cyclohexa-1,3-diene)] (1 b). The latter compound, absorbing strongly around 630 nm, mainly regenerates the parent cluster with a lifetime of about 100 ns in hexane. Its structure, as suggested by the DFT calculations, again contains the 1,3-diene ligand coordinated in a bridging fashion. Photoproduct 1 b can therefore be assigned as a high-energy coordination isomer of the parent cluster with all Os-Os bonds bridged.     

Chemodivergent,Regio- and Enantioselective Cycloaddition Reactions between 1,3-Dienes and Alkynes

Alkynes and 1,3-dienes are among the most readily available precursors for organic synthesis. We report two distinctly different, catalyst-dependent, modes of regio- and enantioselective cycloaddition reactions between these classes of compounds providing rapid access to highly functionalized 1,4-cyclohexadienes or cyclobutenes from the same precursors. Complexes of an earth abundant metal, cobalt, with several commercially available chiral bisphosphine ligands with narrow bite angles catalyze [4+2]-cycloadditions between a 1,3-diene and an alkyne giving a cyclohexa-1,4-diene in excellent chemo-, regio- and enantioselectivities. In sharp contrast, complex of a finely tuned phosphino-oxazoline ligand promotes unique [2+2]-cycloaddition between the alkyne and the terminal double bond of the diene giving a highly functionalized cyclobutene in excellent regio- and enantioselectivities.     

The ‘Aldol Condensation’ of Citral and Related Reactions

The reaction of citral with anhydrous base leads initially to a cyclohexa-1,3-dienecarbaldehyde. Stronger base and longer reaction times result in deconjugation to a cyclohexa-1,4-diene-carbaldehyde, together with oxidative loss of six carbon atoms to yield 2-methyl-4-(4-methylpent-3-enyl)benzaldehyde. A mixed aldol reaction between citral and 3-methyl-2-butenal (= senecia aldehyde) is described.     

DFT-calculation of the structural isomers of (1-methyl-4-phenyl-1-azabuta-1,3-diene)tetracarbonyliron(0) and (4-phenyl-1-oxabuta-1,3-diene)tetracarbonyliron(0)

DFT calculations (B3LYP/LANL2DZ/6-31 G*) were used to investigate the ways in which 1-methyl-4-phenyl-1-azabuta-1,3-diene and 4-phenyl-1-oxabuta-1,3-diene bind to a Fe(CO)(4) moiety. As possible coordination modes, eta(2)-coordination across the C=C or C=N/C=O bond, sigma-coordination to the lone pair of the heteroatom, or eta(3)-coordination through the C=C-C or the N=C-C/O=C-C moiety were considered. The latter forms involve coupling of the non-coordinated atom of the heterodiene with one of the carbonyl ligands to an acyl species. The calculated geometric parameters of all structures compare well with X-ray crystallographic data of similar complexes. The species in which the ligand is transoid and sigma-coordinated is lowest in energy, for both compounds studied. However, the eta(2)-alkene bound 1-oxabuta-1,3-diene complex is practically equal in energy to the sigma-transoid form and thus competes. This agrees with experimental observations that the heterodiene is sigma-bonded in Fe(CO)(4)(1-methyl-4-phenyl-1-azabuta-1,3-diene) but eta(2)-coordinated in Fe(CO)(4)(4-phenyl-1-oxabuta-1,3-diene). The solvent dependence was estimated from single point PCM calculations, for CH(2)Cl(2) as solvent. For the 1-azabuta-1,3-diene complexes, the relative energies of eta(2)-olefin and eta(3)-allyl forms are inverted, with the eta(3)-allyl form being more stable in polar solvents. The 1-oxabuta-1,3-diene complexes in their eta(2)-olefin and sigma-O forms change order of relative energy, and conversion to the sigma-O form is expected in a polar medium for these complexes. Calculated IR vibrational stretching frequencies of the carbonyl ligands and the C[double bond, length as m-dash]N/C[double bond, length as m-dash]O bond were compared with experimental data, to produce the best fits for the sigma-transoid form of Fe(CO)(4)(1-methyl-4-phenyl-1-azabuta-1,3-diene) and eta(2)-olefin bonded Fe(CO)(4)(4-phenyl-1-oxabuta-1,3-diene). These results are again consistent with the experiment and show that the DFT method applied in this work can be used as an aid for structural validation.     

A mild and efficient aza-Diels-Alder reaction of N-benzhydryl imines with trans-1-methoxy-2-methyl-3-trimethylsiloxybuta-1,3-diene catalyzed by Yb(OTf)3

The aza-Diels-Alder reaction of trans-1-methoxy-2-methyl-3-trimethylsiloxybuta-1,3-diene with N-benzhydryl imines in the presence of Yb(OTf)₃ in toluene at room temperature gave the corresponding 2,5-disubstituted 2,3-dihydro-4-pyridones in high yields. This reaction can also be carried out with diene, aldehydes, and amine in a three-component one-pot reaction manner in moderate to high yields under solvent-free conditions. The relationship between Lewis acids and activity, solvents and catalyst loading were studied. Some investigation towards the reaction mechanism was discussed.     

Kinetics,mechanism, and stereochemistry of Diels-Alder reactions of carbonyl-substituted ethenes with cyclohexa-1,3-diene in the gas phase

The kinetics of the Diels–Alder additions of CH₂ ? CHCHO, CH₂? C(CH₃)CHO, and CH₂? CHC(CH₃)O to cyclohexa-1,3-diene (CHD) have been studied in the gas phase. The stereochemistry and the mechanism of these reactions are discussed. In contrast with other Diels–Alder additions involving CHD as diene, a biradical mechanism does not fit the experimental results.     

Substituent Effects on Fe+-Mediated [4+2] Cycloadditions in the Gas Phase

The Fe⁺-mediated [4+2] cycloaddition of dienes with alkynes has been examined by four-sector ion-beam and ion cyclotron resonance mass spectrometry. Prospects and limitations of this reaction were evaluated by investigating several Me-substituted ligands. Me Substitution at C(2) and C(3) of the diene, i.e., 2-methylbuta-1,3-diene, 2,3-dimethylbuta-1,3-diene, hardly disturbs the cycloaddition. Similarly, variation of the alkyne by use of propyne and but-2-yne does not affect the [4+2] cycloaddition step, but allows for H/D exchange processes prior to cyclization. In contrast, Me substituents in the terminal positions of the diene moiety (e.g., penta-1,3-diene, liexa-2,4-diene) induce side reactions, namely double-bond migration followed by [3+2] and [5+2] cycloadditions, up to almost complete suppression of the [4+2] cycloaddition for 2,4-dimethylhexa-2,4-diene. Similarly, alkynes with larger alkyl substituents (pent-1-yne, 3,3-dimethylbut-1-yne) suppress the [4 + 2] cycloaddition route. Stereochemical effects have been observed for the (E)- and (Z)-penta-1,3-diene ligands as well as for (E,E)- and (E,Z)-hexa-2,4-diene. A mechanistic explanation for the different behavior of the stereoisomers in the cyclization reaction is developed. Further, the regiochemical aspects operative in the systems ethoxyacetylene/pentadiene/Fe⁺ and ethoxyacetylcne/isoprene/Fe⁺ indicate that substituents avoid proximity.     

Cationic hexamethylbenzene-diene cobalt complexes

Cationic complexes [(diene)Co(??-C₆Me₆)]⁺ (diene is buta-1,3-diene (2a), 5-isopropyl-2-methylcyclohexa-1,3-diene (2b), cycloocta-1,3-diene (2c), and cyclohexa-1,5-diene (2d)) were synthesized by the reaction of [Co(??-C₆Me₆)₂]⁺ (1) with dienes in a CH₂Cl₂-Me₂CO mixture. In the absence of dienes, cation 1 undergoes hydrogenation to form [(1,2,3,4,5,6-HMCD-1,3)-Co(??-C₆Me₆)]⁺ (HMCD is hexamethylcyclohexadiene, 2e). Structures [2c?Ce]PF₆ were determined by X-ray diffraction analysis. According to the DFT calculations, the Co-C₆H₆ bond in the complexes with conjugated dienes is stronger than that in the complexes with nonconjugated dienes.     

Hydrogenation of arenes over silica-supported catalysts that combine a grafted rhodium complex and palladium nanoparticles: evidence for substrate activation on Rh(single-site)-Pd(metal) moieties

The complex Rh(cod)(sulfos) (Rh(I); sulfos = (-)O(3)S(C(6)H(4))CH(2)C(CH(2)PPh(2))(3); cod = cycloocta-1,5-diene), either free or supported on silica, does not catalyze the hydrogenation of benzene in either homogeneous or heterogeneous phase. However, when silica contains supported Pd metal nanoparticles (Pd(0)/SiO(2)), a hybrid catalyst (Rh(I)-Pd(0)/SiO(2)) is formed that hydrogenates benzene 4 times faster than does Pd(0)/SiO(2) alone. EXAFS and DRIFT measurements of in situ and ex situ prepared samples, batch catalytic reactions under different conditions, deuterium labeling experiments, and model organometallic studies, taken together, have shown that the rhodium single sites and the palladium nanoparticles cooperate with each other in promoting the hydrogenation of benzene through the formation of a unique entity throughout the catalytic cycle. Besides decreasing the extent of cyclohexa-1,3-diene disproportionation at palladium, the combined action of the two metals activates the arene so as to allow the rhodium sites to enter the catalytic cycle and speed up the overall hydrogenation process by rapidly reducing benzene to cyclohexa-1,3-diene.