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The successful [Ph2PN(iPr)PPh2]Cr-catalyzed trimerization and tetramerization of ethylene to 1-hexene and 1-octene requires the presence of a cocatalyst, of which methylaluminoxane (MAO) is particu...

The reaction mechanism and substrate-induced decomposition behavior of three ruthenium olefin metathesis catalysts, viz. first- and second-generation catalysts and the recently developed Phoban catalyst (“Phobcat”) are compared by constructing Δ G surfaces at 298.15 K and 1 atm for the complete ligand systems. From these calculations fundamental insight is gained into the reactivity and stability observed experimentally for the three catalysts. In particular, the higher conversions obtained for the first-generation derived Phobcat catalyst, compared to conventional first-generation catalysts, is attributed to its similarity to the second-generation catalysts instead of first-generation catalyst. Important differences between the calculated Δ G surfaces and previously reported total electronic energy (Δ E ) surfaces for the metathesis mechanism with complete ligand complexes are discussed.

A theoretical study of a mechanism for ethylene trimerization, catalyzed by Cr-pyrrolyl complexes, is proposed and investigated with density functional theory methods. The selective formation of 1-hexene is generally accepted to follow a metallacycle mechanism. A detailed spin state analysis for active species in the mechanism shows that the triplet spin state represents the ground spin state for all stationary points. Complete Gibbs free energy (298.15 K) surfaces are mapped for both η5- and σ-bonding modes of pyrrole, as well as a stripped-down Cl anion model and a full ClAlMe3 anion model. From the calculated results it is shown that the proposed metallacycle mechanism is energetically favorable, with metallacycle growth identified as the rate-determining step. In addition, it is demonstrated that different bonding modes of pyrrole are preferred at different stages in the proposed mechanism, effectively suggesting that ring slippage of the pyrrole occurs on the minimum energy path on the potential ener...

A range of tertiary phosphine ligands derived from (R)-(+)-limonene was studied by HP-31P NMR, HP-IR, batch autoclave reactions and molecular modelling during the hydroformylation of 1-dodecene using a cobalt catalyst system and a metal ∶ ligand ratio of 1 ∶ 2. The phosphorus atom was incorporated in a limonene bicycle where the third substituent was systematically varied, Lim-R (R = (CH2)17CH3, (CH2)9CH3, (CH2)4CH3, (CH2)3CH3, (CH2)3C6H5, (CH2)3CN, (CH2)3OCH2C6H5, (CH2)2OCH2CH3). The activity and selectivity was, to a large extent, governed by the equilibria between the modified and unmodified cobalt catalytic species in the reaction. Linearity ranged from 54 to 71% with Lim-(CH2)3CN yielding the most branched product and Lim-(CH2)4CH3 the most linear product. The n ∶ iso ratio (ratio of linear to 2-methyl branched alcohol) also followed the same trend with a ratio of 2.6 for Lim-(CH2)3CN and 4.9 for Lim-(CH2)4CH3. The rate decreased as the linearity increased with pseudo k values of 0.5 to 1.1. Hydrogenation of the alkene varied within a narrow band from 5–6%.

Dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF) and silver tetrafluoroborate (AgBF 4 ) activate the C 4 S bond in the 4-thioethers of flavan-3-ols toward carbon nucleophiles to permit formation of the interflavanyl bond in procyanidins under neutral conditions.

The hepta-O-methyl ethers 3 and 4 of procyanidin A-1 1 and A-2 2 are subject to facile cleavage of the acetal functionality with sodium cyanoboranuide in trifluoroacetic acid at 0°C. This straight forward chemical method permits the unambiguous establishment of the absolute configuration of the DEF-flavanyl unit and the D-ring carbon and oxygen atoms that are involved in the double linkage of the A-class proanthocyanidins.

Acetic acid-catalyzed cleavage of proanthocyanidins in the presence of phloroglucinol gives a series of 2R procyanidin- and prodelphinidin-phloroglucinol adducts together with a novel 2S all-cis derivative implicating cleavage of the pyran ring and subsequent inversion of stereochemistry at C-2C. These flavan-4-phloroglucinol adducts also suffer dehydration to products with novel fused dihydrobenzopyran–dihydrobenzofuran rings. In addition, cleavage of the flavan C-10A–C-4C bond followed by dehydration results in unique 3,2′-spirobi[dihydro[1]benzofurans].

The natural occurrence of the first oligomeric profisetinidins with (2R,3R)-2,3-cis-epifisetinidol chain extender units is demonstrated in the bark of Pithecellobium dulce (Guamuchil). Semi-synthesis using the appropriate flavan-3-ol and flavan-3,4-diol precursors permits unequivocal structural and stereochemical assignment of the novel dimeric epifisetinidol-(4β,8)-catechin and epicatechins 16 and 18, the trimeric bis-epifisetinidol-(4β,6:4β ,8)-catechin and epicatechins 33 and 35, the fisetinidol-(4α,8)-catechin-(6,4β)- epifisetinidol 37 and fisetinidol-(4α,8)-epicatechin-(6,4β)- epifisetinidol 39.

An acetone extract of the bark of Cassia abbreviata yielded guibourtinidol-(4β → 8)-epicfzelechin, guibourtinidol-(4α → 8)-epiafzelechin, guibourtinidol-(4α → 8)-catechin, guibourtinidol-(4β → 8)-epicatechin and ent-guibourtinidol-(4β → 8)-epicatechin. The structures of the proguibourtinidin dimers were confirmed by synthesis via mild acid-condensation. Guibourtinidol-(4α → 8)-afzelechin, previously isolated from Acacia leuderitzii, was also synthesized. Another two proguibourtinidol oligomers, namely, guibourtinidol-(4β → 8)-afzelechin and guibourtinidol-(4α → 6)-afzelechin were products of the previous synthesis but neither of them have been reported to date.

The range of naturally occurring prorobinetinidins is extended by characterization of the novel robinetinidol-leucorobinetinidins 3, 5, 7, and 9, the robinetinidol-dihydrorobinetins 11, 13, and 15, a robinetinidol-robinetin 17, and a robinetinidol-flavone analogue 19. The diversity regarding the oxidation level of the chain terminating moieties suggests that the biflavanoids in Robinia pseudacacia may be interrelated via oxidation/reduction of these units. The conspicuous absence of the effects of dynamic rotational isomerism about the interflavanyl bond in the 1H NMR spectra of some of the derivatives is explained in terms of a preferred conformation of this bond rather than to ‘free rotation’.

A comprehensive density functional theory (DFT) study analysing the bulk and various low Miller index surfaces of Hägg Fe carbide (Fe(5)C(2)), considered to be the active phase in Fe-catalysed Fischer-Tropsch synthesis (FTS), has been carried out. The DFT determined bulk structure of Hägg Fe carbide (Fe(5)C(2)) is found to be in good agreement with reported monoclinic (C 2/c) XRD data, independently of whether a monoclinic (C 2/c) or triclinic ([Formula: see text]) bulk structure is used as input for calculations. Attention is focused on the construction of a surface energy stability trend with subsequent correlation with particular surface properties. It is found that a (010) Miller index plane results in the most stable surface (2.468 J m(-2)), while a (101) surface is the least stable (3.281 J m(-2)). The systematic comparison of calculated surface energies with surface properties such as the number of dangling bonds and surface atom density (within a broken bond model), as well as unrelaxed surface energies, relative ruggedness of surfaces, degree of surface relaxation upon optimization, total spin density changes of surfaces compared to the bulk, etc, result in only an approximate correlation with the surface stability trend in selected cases. From the results it is concluded that the relative surface energies fall in a narrow range and that a large number of additional surfaces may be defined, e.g. from higher Miller index planes, sharing similar surface energy values. The results serve to demonstrate the rich complexity and diverse nature of the Fe carbide phase responsible for FTS, effectively laying the foundation for further fundamental studies.

The fisetinidol-(4,8) and (4,6)-catechin profisetinidin biflavanoids 1,4 and 5 respectively, are subject to facile cleavage of the interflavanyl linkage with sodium cyanoborohydride in trifluoroacetic acid at 0 °C.

The interflavanyl bond in profisetinidins 1, 4 and 6, and methyl ethers 3, 5, 28 and 29 and procyanidins 24 and 26, and their methyl ethers 25 and 27 is readily subject to cleavage with sodium cyanoboranuide in trifluoroacetic acid at 0 °C. This method will contribute significantly to the structure elucidation of the 5-deoxy (A-ring) proanthocyanidins from important commercial sources. Boltzmann-averaged heterocyclic ring coupling constants as determined by a conformational global search routine (GMMX) and NOE difference spectroscopy were used to assign unequivocally the diastereotopic methylene protons in the 1H NMR spectra of flavan-3-ols, a prerequisite for corroboration of the cleavage mechanism.

The rare series of proguibourtinidins ( 3,4', 7-trihy- droxy functionality) based on the fisetinidol (3,3',4',7-tetrahydroxy functionality) 5-deoxy flavan-3-ol unit is extended by identification of (+)-guibourtinidol-(4β,6)-(-)-fisetinidol and the (+)-guibourtini- dol-(4α,6) and (4β,6)-(+)-epifisetinidols. These metabolites are formed in low yields via acid-catalyzed coupling of (+)-guibourtaca- cidin and (-)-fisetinidol, the high activation energy of generating a 4-C benzylic carbocation being attributable to the poor electron donating properties of the phenol B-ring of the flavan-3,4-diol. Stereo-electronic principles operating in the in vitro process pre- sumably also control their formation in Nature.

The functionalized 2,3- trans -8,9- trans -9,10- cis -3,4,9,10-tetrahydro-2 H ,8 H -pyrano[2,3- h ]chromenes 4 and 6 represent the first naturally occurring condensed tannins with (2 S ,3 R )-2,3- trans -(−)-catechin ‘terminal’ moieties; the structure of 4 is unequivocally confirmed by synthesis.

The flavan-3-ols (+)-catechin and (−)-epicatechin were selectively transformed to their 3′,4′-di- O -methyl- and 5,7-di- O methyl-ethers respectively via benzyl carbonates. Such a regioselectivity is facilitated by marked differences in the pKa values of the phenolic hydroxyl groups and by the ability of the o -dihydroxy funtionality of the pyrocatechol B-ring to form borate complexes under mild basic conditions.

Ruthenacyclobutane decomposition, involving competitive beta-hydride transfer to Ru and reductive olefin elimination during ruthenium-catalyzed olefin metathesis, is predicted by density functional theory calculations and experimentally confirmed by propene and butene formation during degenerate Ru-methylidene-catalyzed metathesis of ethylene. The results provide new focus on the nature of ruthenium metathesis catalyst decomposition under catalytic conditions.

CD data of all four diastereoisomers of the permethylaryl ether 3- O-acetyl derivatives of a series of flavan-3-ols permit assignment of the absolute configuration at the stereocentres of the heterocyclic ring.

The rare series of proguibourtinidins ( 3,4', 7-trihy- droxy functionality) based on the fisetinidol (3,3',4',7-tetrahydroxy functionality) 5-deoxy flavan-3-ol unit is extended by identification of (+)-guibourtinidol-(4β,6)-(-)-fisetinidol and the (+)-guibourtini- dol-(4α,6) and (4β,6)-(+)-epifisetinidols. These metabolites are formed in low yields via acid-catalyzed coupling of (+)-guibourtaca- cidin and (-)-fisetinidol, the high activation energy of generating a 4-C benzylic carbocation being attributable to the poor electron donating properties of the phenol B-ring of the flavan-3,4-diol. Stereo-electronic principles operating in the in vitro process pre- sumably also control their formation in Nature.

Cobalt complexes involved in the hydroformylation of alkenes using (4R,S)-4,8-dimethyl-2-octadecyl-2-phospha-bicyclo[3.3.1]nonane (LIM-18), which consists of a mixture of two diastereomers, have been studied. Complexation to cobalt has been used to separate or enrich the enantiomers so that spectroscopic parameters can be determined for complexes of the form [Co2(CO)8−n(LIM-18)n] (n = 1 or 2) and [Co(CO)3(LIM-18)2][Co(CO)4] containing the different diastereomers. An X-ray structure of a complex of the form [Co(CO)3(LIM-18)]2 shows it to contain the (4R) isomer, but since this was isolated from a mixed solution of diastereomers, this does not definitively identify whether the (4R) or (4S) isomer is the more strongly coordinating isomer. Experimental studies backed up with molecular modelling suggest that steric and electronic effects determine which isomer reacts preferentially in coordination, protonation and methylation reactions. The (4R) isomer is the more strongly coordinating and is metallated preferentially, although the (4S) isomer is the more basic. Variable temperature and high pressure NMR studies on the acyl complex, [Co(C(O)C5H9)(CO)3(LIM-18)], suggest that product elimination may be accompanied by isomerisation to branched acyl species. Modified cobalt catalysed hydroformylation of reactions suggest that the two diastereomers of LIM-18 do not give greatly different rates or selectivities and that a long chain secondary alkyl substituent on the LIM gives lower linear selectivity and a faster rate than LIM-18. LIM-But gives a selectivity similar to that obtained using LIM-18.

The interflavanyl bond in profisetinidins 1, 4 and 6, and methyl ethers 3, 5, 28 and 29 and procyanidins 24 and 26, and their methyl ethers 25 and 27 is readily subject to cleavage with sodium cyanoboranuide in trifluoroacetic acid at 0 °C. This method will contribute significantly to the structure elucidation of the 5-deoxy (A-ring) proanthocyanidins from important commercial sources. Boltzmann-averaged heterocyclic ring coupling constants as determined by a conformational global search routine (GMMX) and NOE difference spectroscopy were used to assign unequivocally the diastereotopic methylene protons in the 1H NMR spectra of flavan-3-ols, a prerequisite for corroboration of the cleavage mechanism.

The fisetinidol-(4,8) and (4,6)-catechin profisetinidin biflavanoids 1,4 and 5 respectively, are subject to facile cleavage of the interflavanyl linkage with sodium cyanoborohydride in trifluoroacetic acid at 0 °C.