Carbonyl Elimination Research Articles

A 1,2,3-triazole-derived pincer-type mesoionic carbene complex of iron(II): carbonyl elimination and hydrosilylation of aromatic aldehydes via the concerted reaction with hydrosilane and a base.

Iron complexes bearing 1,2,3-triazol-5-ylidene were synthesized and applied to the reaction with hydrosilane and homogeneous catalytic hydrosilylation of aromatic ketones and aldehydes. Addition of a free carbene to a solution of Fe(CO)4Br2 yielded an octahedral, diamagnetic and cationic iron(II) complex [Fe(1,2,3-triazolylidene)(CO)2Br]+. Pyrolysis of the dicarbonyl complex eliminated the two CO ligands to form a paramagnetic four-coordinate complex. A theoretical study using DFT calculations indicated that the spin state changed from singlet to quintet during ligand elimination. Investigations of the successful hydrosilylation of acetophenone and benzaldehyde derivatives using MIC-iron(II) bromide suggested the importance of the base for efficient conversion in the catalytic process. The bromide-to-hydride exchange reaction, transmetallation, of MIC-iron(II) bromide in the presence of KOtBu and HSi(OEt)3 which could occur in the initial process of hydrosilylation was proposed, and supported by a theoretical study.

The mechanistic investigations of photochemical carbonyl elimination and oxidative addition reactions of (η5-C5H5)M(CO)3, (M = Mn and Re) complexes.

We used computational methods to explore the mechanisms of the photochemical decarbonylation and the Si–H bond activation reaction of the group 7 organometallic compounds, η5-CpM(CO)3 (M = Mn and Re). The energies of both conical intersections and the intersystem crossings, which play a decisive role in these photo-activation reactions, are determined. Both intermediates and transition states in either the singlet or triplet states are also computed to furnish a mechanistic interpretation of the whole reaction paths. In the case of Mn, four types of reaction pathways (path I–path IV) that lead to the final insertion product are examined. The theoretical findings suggest that at the higher-energy band (295 nm) the singlet-state channel is predominant. As a result, the conical intersection mechanism (i.e., path I) prevails. However, at the lower-energy band (325 nm) the triplet-state channel occurs. In such a situation, the intersystem crossing mechanism (i.e., path IV) can successfully explain its CO-photodissociation mechanism. In the case of Re, on the other hand, the theoretical evidence reveals that only the singlet state-channel is superior. In consequence, the conical intersection mechanism (i.e., path V) can more effectively explain its photochemical decarbonylation mechanism. These theoretical analyses agree well with the available experimental observations.

Alkynyldicobalt Derivatives of Dibenzosuberenol and Dibenzocyclooctatrien‐5‐ol: Ring Conformations, Ease of Carbonyl Elimination and Relevance to Pauson–Khand Cyclization

Under very mild conditions, (5‐alkynyl‐5H‐dibenzo[a,d]cycloheptatrien‐5‐ol)Co2(CO)6 clusters 1 lose a carbon monoxide ligand and form the corresponding η2‐alkene‐µ‐alkyne‐Co2(CO)5 complexes 2 (the first isolated examples of the initial phase of the Pauson–Khand reaction). This behaviour is not paralleled by the homologue in which an additional methylene group has been inserted within the seven‐membered ring, as in the [5‐alkynyl‐6H‐dibenzo[a,e]cyclooctatrienol]Co2(CO)6 system 8, for which several X‐ray crystal structures are reported. The apparent additional flexibility of this molecular framework does not translate into conformations appropriate for the formation of η2‐alkene‐µ‐alkyne‐Co2(CO)5 complexes. The relative inertness of the ethynyl complex 1a, compared with the ease of carbonyl loss in the trimethylsilylethynyl‐ and the tri(isopropyl)silylethynyl‐substituted hexacarbonyldicobalt clusters, 1b and 11, respectively, is discussed in terms of out‐of‐plane angle strain within the dibenzo[a,d]cycloheptatrienyl moiety. Comparison of the changing conformations of the seven‐membered ring in a series of free and complexed molecules reveals a marked flattening of the organic framework, as bulkier substituents are incorporated at C5. The possibility of η2‐alkene‐µ‐alkyne‐Co2(CO)5 clusters functioning as hemilabile systems was explored by monitoring the ease of carbonyl substitution by triphenylphosphine in 2b (alkynyl substituent C≡CSiMe3).

Theoretical studies of cobalt(I)-catalyzed hydroacylation of vinylsilanes and alkyl aldehydes

Density functional theory (DFT) was used to investigate computationally cobalt(I)-catalyzed hydroacylation of vinylsilanes and alkyl aldehydes to give ketones. Calculation indicated that cobalt(I)-catalyzed hydroacylation had eight possible reaction pathways. In the cobalt-hydride complexes IM2a and IM2b, the hydrogen migration occurred prior to the carbon–carbon bond-forming reaction. In the complexes IM3a1 and IM3b1, the carbonyl elimination reaction occurred prior to the direct reductive elimination reaction. In the cobalt–carbonyl complexes IM4a and IM4b, the carbonyl insertion reaction was much easier to achieve than the decarbonylation reaction. The dominant reaction pathway was the reaction channel IM1a TS1a IM2a TS2a1 IM3a1 TS4a IM4a TS5a IM5a TS6a IM6a, and the reductive elimination reaction was the rate-determining step for this channel, so the dominant product predicted theoretically was the linear ketone. Furthermore, the solvation effect was remarkable, and it decreased generally the free energies of the species. Copyright © 2015 John Wiley & Sons, Ltd.

Theoretical investigation of Ni(PMe3)4-catalyzed intermolecular hydroacylation of alkynes with benzaldehydes

Density functional theory (DFT) was used to investigate the Ni(PMe3)4-catalyzed intermolecular hydroacylation of alkynes with benzaldehyde. All intermediates and transition states were optimized completely at the B3LYP/6-31G(d,p) level. The results indicate that oxidative addition of benzaldehyde to the nickel center occurs prior to the coordination of the alkyne. In the five-coordinate Ni(PMe3)2(H)(benzoyl)(alkyne) complexes, hydrogen migration occurs prior to C–C bond formation. In the resulting four-coordinate Ni(PMe3)2(benzoyl)(alkenyl) complexes, reductive elimination to give the four-coordinate Ni(PMe3)2(enone) is more dominant than the carbonyl elimination reaction to the five-coordinate Ni(PMe3)2(phenyl)(CO)(alkenyl) complexes. Hence, hydroacylation is more favorable than decarbonylation. The theoretically predicted dominant product is the (E)-α,β-enone, which is consistent with experimental data.

Effect of deep UV (172 nm) irradiation on PET: ToF/SIMS analysis

Irradiation of PET with 172 nm UV light in the absence of oxygen induces carbonyl elimination and carboxylic acid formation. Using ToF/SIMS to examine irradiated and as received materials with and without deuteration provides insight into the mechanisms. A Norrish type II mechanism is indicated for acid group formation and the mechanism of Day and Wiles [J. Appl. Polym. Sci. 16 (1972) 175; Polym. Lett. 9 (1971) 665] for decarbonylation.

η5↔η3Hapticity interconversion in cycloheptadienyl molybdenum complexes

A series of reactions of the general type [Mo(CO)2L2(η5-C7H9)]++ L′→[Mo(CO)2(L′)L2(η3-C7H9)]+, involving η5→η3 hapticity conversion at a molybdenum co-ordinated Cycloheptadienyl ring, have been examined. The specific examples investigated comprise reactions of [Mo(CO)2L2(η5-C7H9)]+[L2= 2,2′-bipyridyl (bipy)1, Ph2PCH2PPh2(dppm)6, Ph2PCH2CH2PPh2(dppe)7; L = CNBut17 or PPh318] with NCMe or CNBut. Complex 1 reacts reversibly with NCMe to give [Mo(CO)2(NCMe)(bipy)(η3-C7H9)]+2 and, in NCMe, 7 exists as an equilibrium mixture with [Mo(CO)2(NCMe)(dppe)(η3-C7H9)]+8. The complexes 6, 17 and 18 do not react with NCMe at ambient temperature. Treatment of 1, 6 and 7 with CNBut affords [Mo(CO)2(CNBut)L2(η3-C7H9)]+(L2= bipy 4, dppm 9 or dppe 11) although 4 is better prepared by reaction of [Mo(CO)2(CNBut)(NCMe)2(η3-C7H9)]+5 with bipy. Complex 4 is resistant to carbonyl elimination but 9 and 11 undergo facile loss of CO to give [Mo(CO)(CNBut)L2(η5-C7H9)]+(L2= dppm 10 or dppe 12) so establishing examples of associative ligand-substitution processes which proceed with η5→η3→η5 hapticity interconversion at the C7H9 dienyl ligand. The reaction between 17 and CNBut affords [Mo(CO)(CNBut)3(η5-C7H9)]+19 with no observable η3-C7H9 intermediate but 19 reacts with further CNBut to yield [Mo(CO)(CNBut)4(η3-C7H9)]+20. The crystal structures of complexes 1 and 4 have been determined. Complex 1 adopts an asymmetric ligand arrangement similar to that previously described for 6. Complex 4 has a pseudo-octahedral molecular geometry in which CNBut is located trans to the η3-bonded C7H9 ligand.

Synthesis and characterization of a new family of unsaturated triangular rhenium cluster anions of formula [Re3(µ-H)4(CO)9L]–(L = neutral two-electron donor ligand). X-Ray crystal and molecular structures of the tetraethylammonium salts of the anions with L = triphenylphosphine and pyridine

The unsaturated triangular cluster anion [Re3(µ-H)4(CO)10]– can undergo selective substitution of one of the axial carbonyl ligands of the Re(CO)4 vertex by neutral two-electron donor ligands L, giving unsaturated [Re3(µ-H)4(CO)9L]– derivatives. The anions with L = NCMe, PPh3, and pyridine (py) have been synthesized either by thermal activation or, more effectively, by carbonyl elimination using Me3NO·2H2O. X-Ray single-crystal analyses of the PPh3 and py derivatives, as [NEt4]+ salts, have been performed. Both species crystallize in the monoclinic system, space group P21/n, with a= 11.448(2), b= 25.311(6), c= 13.596(3)A, β= 94.00(2)°, and Z= 4 for the PPh3 derivative, and a= 11.087(1), b= 15.951(2), c= 17.598(4)A, β= 104.84(2)°, and Z= 4 for the py derivative. The geometries of the two anions are similar to that of the parent anion, with the PPh3 or the py ligand co-ordinated in an axial position with respect to the Re3 triangle, which presents a short Re–Re bond (close to 2.80 A) doubly bridged by hydrides. The McCN derivative is the most reactive member of this new family, containing two active centres: the labile nitrile and the unsaturated moiety. Its reaction with methanol, leading to the anion [Re3(µ-H)3(CO)9(µ3-OMe)]–, exemplifies the possibility of new reaction paths involving both the active sites of this molecule.

Organic chemistry of dinuclear metal centres. Part 1. Combination of alkynes with carbon monoxide at di-iron and diruthenium centres: crystal structure of [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2Ph2}(η-C5H5)2

Under u.v. radiation a variety of alkynes (HC2H, MeC2Me, PhC2Ph, MeO2CC2CO2Me, MeC2H, PhC2H, and PhC2Me) reacts with [Fe2(CO)4(η-C5H5)2] to form complexes [Fe2(CO)(µ-CO){µ-σ : η3-C(O)C2R2}(η-C5H5)2] in 10–90% yields. Only PhC2Ph produces an analogous complex with [Ru2(CO)4(η-C5H5)2], but [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2Ph2}(η-C5H5)2] undergoes alkyne exchange on heating in toluene with HC2H, MeC2Me, MeC2H, PhC2H, or PhC2Me to afford the appropriate [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2R2}(η-C5H5)2] in near quantitative yield. The linking of alkyne and CO to produce a dimetallacyclopentenone ring was established through an X-ray diffraction study of the title compound. Crystals are orthorhombic, space group Pbca, with Z= 8 in a unit cell of dimensions a= 14.797,(3), b= 17.805(8), and c= 16.739(8)A. The structure was solved by heavy-atom methods and refined by least squares to R 0.033 for 3 726 diffractometer-measured reflection intensities. The molecule contains a dimetallacyclopentenone ring in which the ethylenic bond is η2-bound to ruthenium, so that the bridging C(O)C(Ph)C(Ph) ligand is σ-co-ordinated to one ruthenium and η3-co-ordinated to the other. Compounds [M2(CO)(µ-CO){µ-σ : η3-C(O)C(R′)C(R2)}(η-C5H5)2](M = Fe or Ru) in which R1≠ R2 exist as isomers as a result of linking of either end of the alkyne with CO. Steric factors appear to determine the relative stability of the isomers when one of R is H, but electronic factors are influential when neither is H. The dimetallacyclopentenones are fluxional, undergoing synchronous carbonyl ‘insertion’ into, and elimination from, the dimetallacycle. Free energies of activation appear dependent upon the size of the ‘alkyne’ substituents. In boiling toluene, carbonyl elimination from the dimetallacycle in [Fe2(CO)(µ-CO){µ-σ : η3-C(O)C2(CO2Me)2}(η-C5H5)2] becomes irreversible, and the dimetallacyclobutene complex [Fe2(CO)2(µ-CO){µ-C2(CO2Me)2}(η-C5H5)2] is formed quantitatively as cis and trans isomers. Only cis-[Ru2(CO)2(µ-CO){µ-C2(CO2Me)2}(η-C5H5)2] is generated when [Ru2(CO)(µ-CO)(µ-σ : η3-C(O)C2Ph2}(η-C5H5)2] is heated with MeO2CC2CO2Me. The ease with which carbon–carbon bond-making and -breaking occurs at the di-iron and diruthenium centres is recognised.

Molecules organiques coordinees a un metal. : Nouveaux complexes olefiniques d'α-enones par le fer(0)

Heterodienetricarbonyliron complexes react with ligands ( L = PMe 2Ph, P(OMe) 3 or P(OPh) 3) to give the adducts (enone)Fe(CO) 3L with the ethylenic double bond coordinated only to the iron(0). Electron-releasing and low-steric effects of L make the reaction which is specific for enones easier compared to that for dienes. PMe 2Ph allows enone exchange and P(OPh) 3 promotes carbonyl elimination. Ligand influence is shown by infrared spectroscopy and by the shielding of ethylenic protons in NMR spectroscopy.

ChemInform Abstract: A PRIMARY HYDROGEN‐DEUTERIUM ISOTOPE EFFECT STUDY ON THE CARBONYL ELIMINATION REACTION OF 9‐FLUORENYL NITRATE WITH VARIOUS BASES

AbstractDie Kinetik der Titel (ECOD‐Reaktion von 9‐Fluorenylnitrat und [9‐D] Fluorenylnitrat mit 1 1 verschiedenen Aminen wie z.B. Benzylamin und Piperidin in Äthanol bei 0°C zu Fluorenon wird untersucht.

A Primary Hydrogen-Deuterium Isotope Effect Study on the Carbonyl Elimination Reaction of 9-Fluorenyl Nitrate with Various Bases

The carbonyl elimination reaction of 9-fluorenyl nitrate with various nitrogen bases in anhydrous ethanol at 0 °C was examined. In all cases fluorenone was formed in 100% yield indicating that there was not any substitution. A reasonable Brønsted plot was obtained for reaction promoted by structurally similar bases with β = 0.84 which suggests a product-like transition state. As well, deviations from the Brønsted plot are discussed. Primary hydrogen-deuterium isotope effects were measured for reaction promoted by 11 different amine bases. A reasonable correlation was obtained for structurally similar bases when a plot of kH/kDvs. pKa was made. The conclusion is reached that when kH/kD reaches a maximum, ∼9.2 at 0 °C, it remains unchanged and hence is a poor measure of transition state geometry. As well, very poor correlations are found when the abstracting base is tertiary which leads to the conclusion that a comparison of kH/kD values is not warranted for structurally different bases.

Kinetic Isotope Effect Studies on the Reaction of Benzyl Nitrate with Base in Varions Solvents

The role of solvent and base on the nature of the transition state for the concerted one-step carbonyl elimination reaction of benzyl nitrate has been examined using rate and kinetic isotope effect techniques. The primary hydrogen–deuterium isotope effect was found to increase with increasing strength of the abstracting base, kH/kD = 5.94 and 6.41 at 30° for reaction with EtO−/EtOH and t-BuO−/t-BuOH, respectively. The nitrogen isotope effect decreased with increasing strength of the abstracting base, 2.07, 1.82, and 1.54% for reaction with OH−/40 vol % EtOH–H2O, EtO−/EtOH, and t-BuO−/t-BuOH, respectively. The conclusion is reached that an increase in the strength of the abstracting base leads to a more reactant-like transition state with decreased rupture of both the C—H and O—N bonds. The role of solvent on the nature of the transition state was examined by determining rates and nitrogen isotope effects for reaction in various ethanol–water and ethanol–dimethyl sulfoxide mixtures. The nitrogen effect was found to remain essentially constant in solvent mixtures ranging from 65 vol % EtOH–H2O on the one hand to 80 vol% EtOH–DMSO on the other hand where the rate constants differed by a factor of 100 fold. It is concluded that solvent plays only a very minor role in determining transition state structure for the concerted elimination reaction of a neutral substrate with base.

Reactivity in nucleophilic substitution. Part IV. A carbonyl elimination reaction of 9-fluorenyl toluene-p-sulphonate

In 90% methanolic acetone, 9-fluorenyl toluene-p-sulphonate undergoes first-order solvolysis leading to 9-fluorenyl methyl ether. For the same system, addition of small concentrations of potassium hydroxide completely changes the reaction mechanism and the products are 9-fluorenone and potassium toluene-p-sulphinate. A detailed kinetic investigation of the latter reaction has been made and an alpha deuterium kinetic isotope effect (kH/kD= 2·3) measured. It is suggested that the carbonyl elimination is of the ECO2 type involving concerted proton removal and carbonyl group formation. Changing the methanol content of the solvent gives an insight into the competing solvolysis and elimination. For the solvent mixtures investigated, unimolecular solvolysis is the predominant reaction in the absence of base. In the presence of potassium hydroxide, carbonyl elimination accounts for 100% of the reaction in 90% methanolic acetone, 50% of the reaction in 80% methanolic acetone, and only 15% of the reaction in 65% methanolic acetone.

ISOTOPE EFFECT STUDIES ON ELIMINATION REACTIONS: V. THE MECHANISM OF THE CARBONYL ELIMINATION REACTION OF 9-FLUORENYL NITRATE WITH ACETATE ION

The carbonyl elimination reaction (ECO2) of 9-fluorenyl nitrate with acetate ion in anhydrous ethanol was found to be second order and to give at 30° primary nitrogen-15 and deuterium isotope effects of 1.0091 ± 0.0007 and 4.3 ± 0.2, respectively. The ester showed no deuterium exchange with solvent under the reaction conditions. A change in solvent to 85 volume % ethanol–water increased the nitrogen isotope effect to 1.0131 ± 0.0007 without changing the deuterium effect. These results exclude any form of two-stage carbanion intermediate mechanism, but are in accord with a concerted process.

THE MECHANISM OF THE CARBONYL ELIMINATION REACTION OF BENZYL NITRATE KINETIC ISOTOPE EFFECTS AND DEUTERIUM EXCHANGE

The carbonyl elimination reaction (ECO2) of benzyl nitrate has been investigated with the object of distinguishing between the concerted and carbanion mechanisms. A deuterium exchange experiment resulted in a very small amount of deuterium pickup. The nitrogen isotope effect, k14/k15, associated with formation of the nitrite ion was found to be 1.0196 ± 0.0007 at 30 °C. The two results taken together exclude the formation of a carbanion intermediate but are consistent with a concerted mechanism.Benzyl-α-d2 nitrate has been prepared and the rate of its carbonyl elimination reaction compared with that of the undeuterated compound. The deuterium isotope effect was 5.04 ± 0.25 at 60 °C. The significance of the magnitude of the nitrogen and deuterium isotope effects and of their interrelationship with the Hammett reaction constant rho is discussed in terms of the nature of the transition state and a comparison is made with other E2 elimination reactions.

Hydrolytic decomposition of esters of nitric acid. Part III. Kinetic evidence for ion exchange between anionic reagents and the solvent in the carbonyl elimination reaction (ECO2)

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