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.     

Substrate dependence in aqueous Diels-Alder reactions of cyclohexadiene derivatives with 1,4-benzoquinone

A reactivity difference based on the position of substituents on cyclohexa-1,3- diene was observed for the title reaction. The effect of water as solvent was more distinct for 1-methyl-4-isopropylcyclohexa-1,3-diene than for 2-methyl-5-isopropylcyclohexa- 1,3-diene or non-substituted cyclohexa-1,3-diene. The effect of NaCl (salting-out) and guanidium chloride (salting-in) was also large for 1-methyl-4-isopropylcyclohexa-1,3- diene.     

Specific allylic-allylic coupling procedures effected by ligand-induced elimination from di(allylic)palladium species

Bis(allylic)palladium complexes can be induced to undergo reductive elimination by replacement of phosphine ligands in the system with π-acidic ligands. The product 1,5-diencs, formed in high yield, are predominantly the ‘head-to-head’ coupled isomers. The bis(allylic)palladium intermediatesmay be formed by addition of an allylic Grignard or trialkyl(allylic)tin reagent to an (η³-allyl)palladiuin chloride complex, or by 1,3-diene condensation. The latter process leads to cydodimerization, ‘unusual’ for palladium catalysed reactions.     

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.     

Solid-phase synthesis of linked heterocycles from a selenopolystyrene resin

A linked heterocycle library of isoxazoles, 1,2,3-triazoles, bicyclo[2.2.1]hepta-2,5-diene or 4-methylcyclohexa-1,3-diene and 1,2,4-oxadiazoles was prepared by solid-phase organic synthesis. Key steps on resin-bound selenium were electrophilic additions; 1,3-dipolar cycloaddition; Porco's two-step, one-pot condensation of amidoxime and carboxylate; and Diels-Alder reaction.     

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.     

Recent progress in the chemistry of 1,3-diene complexes of niobium and tantalum

This short review describes the preparation, structure and reactivity of 1,3-diene complexes of niobium and tantalum and presents a comparison with the similar diene complexes of metals of Group 2, 3 and 4.     

Stereospecific synthesis of dienone iron tricarbonyl complexes by friedelcrafts acylation

Acylation of diene Fe(CO)₃ complexes using the Perrier complexes RCOCl/AlCl₃ in methylene chloride at 0°C gives dienone complexes in high yield. Substitution occurs only at unsubstituted terminal carbons of the diene unit. Quenching the reaction mixtures in cold aqueous ammonia gives cis dienone complexes only. Trans dienone complexes are prepared by subsequent isomerization in methanolic sodium methoxide. Formylation of diene Fe(CO)₃ complexes proceeds in modest yield using dichloromethylmethyleter/AlCl₃ in methylene chloride to give trans-dienal complexes. Reduction of the dienone and dienal complexes as well as those of dienols and dienoic esters with 4 : 1 AlCl₃/LiAlH₄ results in complete removal of the oxygen function to give trans-diene complexes in good yield.     

Allenes as carbon nucleophiles in intramolecular attack on (pi-1,3-diene)palladium complexes: evidence for trans carbopalladation of the 1,3-diene

Reaction of allene-substituted cyclohexa- and cyclohepta-1,3-dienes with [PdCl(2)(PhCN)(2)] gave eta(3)-(1,2,3)-cyclohexenyl- and eta(3)-(1,2,3)-cycloheptenylpalladium complexes, respectively, in which C-C bond formation between the allene and the 1,3-diene has occurred. Analysis of the (pi-allyl)palladium complexes by NMR spectroscopy, using reporter ligands, shows that the C-C bond formation has occurred by a trans carbopalladation involving nucleophilic attack by the middle carbon atom of the allene on a (pi-diene)palladium(II) complex. The stereochemistry of the (pi-allyl)palladium complexes was confirmed by benzoquinone-induced stereoselective transformations to allylic acetates.     

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.     

Facile Synthesis and Palladium‐Catalyzed Cross‐Coupling Reactions of 2,3‐Bis(pinacolatoboryl)‐1,3‐butadiene

Treatment of 1,1‐bis(pinacolatoboryl)ethene with an excess of 1‐bromo‐1‐lithioethene gave 2,3‐bis(pinacolatoboryl)‐1,3‐butadiene in high yield. Palladium‐catalyzed cross‐coupling of the resulting diborylbutadiene with aryl iodides took place smoothly in the presence of a catalytic amount of Pd(OAc)₂/PPh₃ and aqueous KOH to give 2,3‐diaryl‐1,3‐butadienes in good yields. The coupling reaction with commercially available 4‐acetoxyphenylmethyl chloride under the same conditions followed by hydrolysis of the acetyl groups gave anolignan B in a one‐pot manner. A variety of [3]‐ to [6]dendralenes were synthesized by palladium‐catalyzed coupling of the diene or 1,1‐bis(pinacolato)borylethene with alkenyl or dienyl halides, respectively, in good yields.     

Basic hydrolysis of l,4-bis(triphenylphosphonio)buta-1,3-diene dihalides

Basic hydrolysis of 1,4-bis(triphenylphosphonio)buta-1, 3-diene dichloride with 10% NaOH gave isomeric 4-diphenylphosphoryl-4-phenylbut-1(2)-enes and 1-diphenylphosphoryl-1-phe-nylbuta-1, 3-diene, the products of anionotropic migration of a phenyl group from the P atom to the α-position. Hydrolysis with Na₂CO₃ afforded only the diene product. In both cases, triphe-nylphosphine and triphenylphosphine oxide were isolated as secondary products. Dehydro-chlorination of 2-chloro-1,4-bis(triphenylphosphonio)but-2-ene dibromide with triphenylphosphine was proposed as a new convenient route to 1,4-bis(triphenylphosphonio)buta-1,3-diene dibromide.     

Acetylation of dicarbonyl(η-cyclohexadiene)triphenyl- phosphineiron

Friedel-Crafts acetylation of dicarbonyl(η⁴-cyclohexadiene)triphenylphos-phineiron is accomplished in 96% yield under mild conditions to give dicarbonyl-(η⁴-5-endo-acetylcyclohexa-1,3-diene)triphenylphosphineiron. The structure of the product was established by direct comparison with the epimeric complex dicarbonyl(η⁴-5-exo-acetylcyclohexa-1,3-diene)triphenylphosphineiron produced by reaction of methyl magnesium iodide with dicarbonyl(η⁴-5-exo-cyanocyclohexa-1,3-diene)triphenylphosphineiron.     

Structure and redox behavior of Ru(II)-diene complexes

A series of Ru(acac)₂(η⁴-diene) complexes containing cis- and trans-diene coordination have been investigated by cyclic voltammetry to correlate structural bonding and conformation patterns of diene ligands with redox behaviors. The solid-state structure of Ru(acac)₂(2,3-dimethyl-1,3-butadiene) has been determined by single crystal X-ray diffraction methods. Ru(acac)₂(2,3-dimethyl-1,3-butadiene) crystallizes in the monoclinic space group C2/c with a = 12.368(2) Å, b = 17.0600(2) Å, c = 16.0110(2) Å, β = 98.4405(10)° and V = 3341.38(10) ų for Z = 8. A structural comparison between several Ru-trans-η⁴-diene complexes and Ru-η⁴-1,3-cyclohexadiene revealed no difference in the Ru-C(diene) bond distances. However, through cyclic voltammetry experiments these species demonstrated different redox behavior, as function of the coordinated diene ligand.     

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.     

2-alkyl-1,3-butadiene polymerization under the influence of π-allylic complexes of transition metals

Stereoregulation in the polymerization of 2-alkyl-1,3-butadienes with transition metal π-allylic complexes has been studied. The direction of isoprene polymerization is shown to be a function of the nature of the metal and ligands in the allylic compound. The presence of acidic ligands in π-allylic complexes of Zr, Cr, Mo, and Co contributes to 1,4-addition and increases the selectivity of π-allylic nickel complexes, favoring cis-1,4-structure formation. Investigation of the model reaction of 2-alkyl-1,3-butadienes with bis(π-perdeuterocrotyl nickel iodide) revealed that active sites have an π-allylic type structure. The mechanism of formation of π-allylic adducts and the main factors which determine the dependence of direction and rate of polymerization on the nature of a monomer in the diene series: 2-methyl-1,3-butadiene(isoprene), 2-ethyl-1,3-butadiene, 2-isopropyl-1,3-butadiene, and 2-tert-butyl-1,3-butadiene, are discussed.     

Verknüpfung von 1,3-dienen mit acrylsäuremethylester durch photochemische umsetzung mit pentacarbonyleisen

Pentacarbonyliron and methyl acrylate/1,3-diene (2,3-dimethylbutadiene, isoprene, butadiene) mixtures react photochemically via diene—Fe(CO)₃ and methyl acrylate—Fe(CO)₄ to give products in which a methyl acrylate—diene adduct is 1,4,5,6-η-coordinated to the Fe(CO)₃ moiety. (η²-diene)(η²-methyl acrylate)Fe(CO)₃ is proposed to be an intermediate.     

Mechanism of cycloisomerisation of 1,6-heptadienes catalysed by [(tBuCN)2PdCl2]: remarkable influence of exogenous and endogenous 1,6- and 1,5-diene ligands

The mechanism of the highly regioselective cycloisomerisation of dimethyl hept-1,6-dienyl-4,4-dicarboxylate (1) by a neutral pre-catalyst, [(tBuCN)(2)PdCl(2)] (8), to generate dimethyl 3,4-dimethylcyclopent-2-ene-1,1-dicarboxylate (3) has been investigated by isotopic labelling (reactions involving single and mixed samples of 1,1,2,6,7,7-[(2)H(6)]-1; 3,3,5,5-[(2)H(4)]-1; 1,7-(Z,Z)-[(2)H(2)]-1; [1,3-(13)C(1),5,7-(13)C(1)]-1 and [1,3-(13)C(1),6-(2)H(1)]-1) and by study of the reactions of dimethyl 1-aryl-hept-1,6-dienyl-4,4-dicarboxylates (9 a-e, where aryl is p-C(6)H(4)-X; X=H, OMe, Me, Cl, CF(3)) and dimethyl hept-1,5-dienyl-4,4-dicarboxylate (14), a 1,5-diene isomer of 1. The mechanism proposed involves the generation of a monochloro-bearing palladium hydride which undergoes a simple hydropalladation, carbopalladation, Pd/H dyotropy, beta-H elimination sequence to generate 3. A key point that emerges is that chelation of the 1,6-diene 1 at various stages in the mechanism plays an important role in determining the regioselectivity of the reaction. The selective generation of 3 with pre-catalysts of the form L(2)PdCl(2), as compared to the generation of dimethyl 3-methylene-4-methyl-cyclopentane-1,1-dicarboxylate (2) with pre-catalysts of the form [(MeCN)(2)Pd(allyl)]OTf (5) is ascribed to the absence of chloride ion in the latter, which makes an additional coordination site available throughout turnover. Liberation of the product 3 when [(tBuCN)(2)PdCl(2)] (8) is employed as pre-catalyst, is proposed to proceed via a mono- to bidentate switch in the pi-coordination of diene 1 (eta(2) to bis-eta(2)) displacing pi-coordinated 3 from Pd. When 1-aryl-1,6-dienes 9 are employed as substrates, the electron-donor property of the aryl group is found to influence the regioselectivity of cyclisation. Electron-withdrawing groups favour dimethyl 3-arylmethyl-4-methylcyclopent-2-ene-1,1-dicarboxylates (10), whilst electron-donating aryl groups favour 3-arylidene-4-methyl-cyclopentane-1,1-dicarboxylates (11). The regioselectivity (10/11) correlates with the Hammett sigma(+) values (rho(+)=1.3, r (2)=0.975) indicative of a strong pi-resonance contribution from the aryl ring rather than a simple sigma-inductive effect. Intermolecular modulation of regioselectivity is observed and the net effect proposed to arise through the (pi-->d) donation ability of the vinyl arene in the diene displacing product (10/11) via a mono- to bidentate switch in coordination. The isomerisation process increasingly sequesters Pd as turnover proceeds leading to a powerful inhibition mechanism and ultimately a limitation in turnover number to about 80.     

Umsetzungen des Cyclohex-3-en-carbaldehyd-p-toluolsulfonylhydrazons

The vacuum thermolysis (80–90°) of the sodium salt of cyclohex-3-ene-carbaldehyde p-toluenesulfonylhydrazone ( 1 ) in silicone oil gave diazomethyl-cyclohex-3-ene ( 2 ). Pyrolytic and photolytic decomposition of this diazo compound 2 lead to methylenecyclohex-3-ene ( 5 ) and bicyclo [4.1.0]hept-2-ene ( 6 ) (about 3:1), while the CuCl catalyzed cleavage yielded only 5 . The postulated carbene mechanism should also apply under the direct aprotic decomposition conditions of the sodium salt of 1 in diglyme, where methylenecyclohex-3-ene and bicyclo[4.1.0]hept-2-ene (about 3:1) were formed besides small amounts of 1-methylcyclohexa-1, 3-diene ( 9 ) and bicyclo [4.1.0]hept-3-ene ( 8 ). Under protic conditions (in ethylene-glycol) methylenecyclohex-3-ene, 1-methylcyclohexa-1, 3-diene and 1-methylcyclohexa-1, 4-diene ( 14 ) were produced in a ratio of 1:1:1. The direct mild thermolysis of cyclohex-3-ene-carbaldehyde p-toluenesulfonylhydrazone ( 1 ) in benzene solution afforded N-(p-toluenesulfinyl)-O-(p-toluenesulfinyl)-cyclohex-3-en-yl-α-methanolamine ( 15 ) and di-(cyclohex-3-en-yl-methyl)-ammonium p-toluenesulfonate ( 16 ), the structures of which were supported by their nmr. spectra and by alkaline cleavage.     

Stereoselectivity in nucleophilic addition to unsaturated ligands bound to molybdenum : Allylic alkylation of cyclohexanone

The stereochemistry and regiochemistry of nucleophilic addition to olefinic, allylic, or diene moieties can be controlled in reactions of molybdenum complexes. The synthesis of a wide range of -allylic cyclohexanones is feasible using (η⁵-cyclopentadienyl)Mo(CO)(NO)(allyl) cations. The stereoselective preparation of (RS,SR)-2-(1-methyl-2-butenyl)cyclohexanone from the reaction of 1-pyrrolidino-1-cyclohexene with [CpMo(CO)(NO)(η³-1,3-dimethylallyl)]BF₄ illustrates the methodology.