Manfred J. Mirbach

Photochemical hydroformylation of olefins with rhodium catalysts at elevated pressure

Abstract The catalytic activity of RhCl 3 and RhHCO(PPh 3 ) 3 towards hydroformylation, hydrogenation and isomerization of olefins such as 1-octene, styrene, 1,5- and 2,4-hexadiene is enhanced by UV-irradiation. The most active photocatalytic system consists of RhCl 3 and norbornadiene, which catalyses the hydroformylation of 1-octene even at room temperature. The photochemical reaction of norbornadiene itself at 80 bar CO/H 2 and 20 °C lead to a copolymer between norbornadiene and carbon monoxide, in contrast to the thermal reaction at 80 bar CO/H 2 and 80 °C which results in the formation of mono- and dialdehydes. A UV-autoclave allowing photochemical reactions at pressures up to 300 bar is described.

Photocatalytic hydrogenation of dienes with chromium carbonyls

Abstract The photochemical hydrogenation of norbornadiene (NBD) in the presence of Cr(CO) 6 or Cr(CO) 4 NBD at normal pressure yields nortricyclene (NTC) and norbornene (NBN) in a ratio of 3/1. With increasing hydrogen pressure the NTC/NBN ratio changes to 0.8/1 at 100 bar H 2 , due to a faster formation of norbornene. The formation of NBN is more strongly inhibited than that of NTC by 50 bar of CO. Other conjugated dienes such as cyclohexadiene, isoprene, or 1,3-pentadiene give exclusively 1.4-hydrogenation products. The results are rationalized by a mechanism in which the 1.4-addition and the NTC formation are initiated by a CO dissociation, whereas the norbornene is formed via initial cleavage of a Crdiene bond in the Cr(CO) 4 diene catalyst.

Activation parameters for the α cleavage of alkanones and azoalkanes measured by temperature dependence of fluorescence

Abstract The temperature dependence of fluorescence of several alkanones and several azoalkanes has been measured and Arrhenius parameters have been derived from the data. It is shown that the activation energies derived from an Arrhenius treatment refer directly to the rate of reaction from S1 only when reaction is the major pathway for deactivation. When fluorescence and/or intersystem crossing determine the rate of deactivation of S1, fluorescence is generally found to be temperature independent. The previously reported activation energies for α cleavage from S1 are discussed and compared with the values obtained in this work.

Reactions of cobalt carbonyls in methanol under high pressure of carbon monoxide: a reexamination of the hydroesterification mechanism

Abstract High pressure IR and UV spectroscopy was used to study the disproportionation of Co2(CO)8 1 to [Co(MeOH)6] [Co(Co)4] 2 2 in methanol under the conditions of carbonylation reactions. The disproportionation is first order in [1] and inverse first order in [CO]. It is reversible under high pressure of CO. In the equilibrium the formation of 2 is favoured by addition of pyridine and KI, that of 1 by olefins and CO. The rate-determining step for the disproportionation is an associative ligand substitution of 1. The comproportionation of 2 under CO is first order in [2] and in [CO]. This is in agreement with a mechanism in which the resolvation of the cation or the electron transfer from the anion to the cation is rate-determining. The activation energies for the forward and backward reactions are 57 ± 12 kJ mol−1 and 60 kJ mol−1, respectively. The thermodynamic parameters for the equilibrium are ΔH0 = −2.35 ± 0.3 kJ mol−1 and ΔS0 ~ −4 J mol−1 K−1. The spectroscopic investigations concerning the hydroesterification (HE) of 1-octene are in line with a mechanism that starts with the ionic addition of H+ and Co(CO)4−. The H+ concentration is the rate-limiting factor and methanol is probably the source of the protons. Addition of pyridine leads to a higher concentration of Co(CO)4−, but it also solvates the H+ more strongly. Thus a large excess of pyridine increases the activation energy of the HE. In the HE of butadiene, activation of the π-allyl complex η3-C4H7-Co(CO)3 is the slowest step.

The catalytic activity of metal carbonyl cluster compounds. Hydroformylation and hydroesterification catalyzed by Co4CO12 and Co2(CO)8

Abstract The carbonyl complexes Co2(CO)8 and Co4(CO)12 catalyze the hydroformylation and hydroesterification reactions via the same catalytic cycle. The entry into this cycle via HCo(CO)3 is easier with Co4(CO)12 than with Co2(CO)8, and so the former is the more active catalyst.

Solvent isotope effect on the fluorescence of azoalkanes

Abstract A study of fluorescence quantum yields and fluorescence lifetimes of the cyclic azoalkanes I and II reveals a striking dependence of Φ F and τ F on solvent and on isotopic substitution (OH a OD). A mechanism involving specific deactivation of the fluorescent state from a hydrogen bonded complex is proposed to rationalize the data. The observation that the quantum yield for decomposition of I does not correlate with the variation of fluorescence parameters with solvent and isotopic change leads to the conclusion that the state responsible for photoreaction proceeds the fluorescent state.

A radical mechanism for the photochemical iron pentacarbonyl catalyzed hydrogenation of octenes

Abstract A high pressure IR and UV spectroscopic study of the catalysis of hydrogenation of 1-octene by Fe(CO) 5 under UV irradiation reveals that Fe(CO) 5 and (olefin)Fe(CO) 4 are the light absorbing species. The primary photolysis product Fe(CO) 4 reacts with H 2 Fe(CO) 4 to give HFe(CO) 4 radicals, which are suggested to be the active hydrogenation promoters.

The triplet energies of butenedioic acid derivatives

Abstract The oxygen perturbation spectra of butenedioic acid derivatives were measured and the vertical triplet energies were determined to be 62 kcal mol−1 for fumaronitrile, about 66 kcal mol−1 for dimethyl fumarate, 71 kcal mol−1 or more for dimethyl and 72 kcal mol−1 for maleic anhydride.

On the mechanism of the chromium carbonyl photocatalyzed watergas shift reaction

Abstract The chromium hexacarbonyl catalyzed watergas shift reaction is accelerated by UV irradiation and inhibited by increased CO pressure. An activation energy of 30 kJ mol −1 has been determined for the photochemical and one of 145 kJ mol −1 for the thermal reaction. Light accelerates the conversion of Cr(CO) 6 into [Cr(CO) 5 formate] − , which is thermally activated, as evidenced by in situ IR and UV spectroscopy.

Photochemische 2+2-Cycloadditionen von Äthylen und Acetylen an 1.3-Cyclohexadien unter erhöhtem Druck / Photochemical 2+2-Cycloadditions of Ethylene and Acetylene to 1,3-Cyclohexadiene under High Pressure

The photochemical cycloaddition of 1,3-cyclohexadiene (CHD) to ethylene and acetylene at pressures above 10 bar is described. Upon sensitized irradiation (2-acetylnaphthaline) CHD adds to ethylene at room temperature in dichloromethane to give cis-bicyclo[4,2,0]-oct-2-ene (1) along with dimers of cyclohexadiene. The yield of cross adduct increases with ethylene pressure (10-50 bar) whereas dimerisation decreases. Quantum yields of cross addition and dimerisation at 12 M ethylene were determined to be 0.31 and 0.35 respectively. At a pressure of 15 bar acetylene CHD reacts with acetylene to give bicyclo-[4,2,0]octa-2,7-diene (2) and bicyclo[2,2,2]octa-2,5-diene (3) as the major and minor products respectively. In a solvent mixture containing 60 vol-% CH2Cl2 and 40 vol-% acetone (2) is formed with a quantum yield of φ = 0.2. The experimental results are explained by a formal kinetic scheme.