Decarbonylation kinetic studies

The overall enthalpy change of the insertion process contains contributions from four bonds (M-CO, M-COR, M-R and CO-R). As there is no significant difference between (Mn-R) and Zs(Mn-COR) then, at least in the case of manganese and hydrocarbon groups, R, the dominant factor will be the difference between T (Mn-CO) and E R-COX) [for R = CH3, E = 339 kJ mop1 (X = H), 370 kJ mol"1 (X = Cl) (Ref.23 )] which suggests that the insertion reaction is thermodynamically favoured with respect to decarbonylation. Kinetic studies of the carbonyl insertion reaction in solution have shown87) that the enthalpy of activation is 62 kj mol-1 for inser-... 

Kinetic studies have been carried out on the decarbonylation of RCOMn(CO)5... 

Replacement of CO in MeCOMn(CO)5 with PPh3 seems to have little effect on the rate of the decarbonylation. As shown in Table IV, MeCO-Mn(CO)4PPh3 (an isomeric mixture) reacts only slightly faster than MeCOMn(CO)5 after provision is made for the difference in temperature 169). However, a recent kinetic study on the decarbonylation of CpMo-(CO)2L(COMe) (L = a tertiary phosphine) has shown that both inductive and steric properties of L are important 19a). Sterically demanding and weakly a-bonding phosphines increase the reaction rate. 

Also neglected have been kinetic studies concerned with comparison of the rates of the decarbonylation of different M(COR)CO systems. Heck 109) reports that at 25 °C MeCOCo(CO)4 dissociates CO about 2250 times more rapidly than MeCOMn(CO)5. This may well be an approximate ratio of the respective rates of the decarbonylation. Qualitative, synthetically oriented experiments seem to indicate that RCOMn(CO)5 decarbonylate faster than their rhenium counterparts (Section V,C). 

Recently a kinetic study has been reported of the decarbonylation of various CpMo(CO)2L(COMe) (L = a tertiary phosphine) to CpMo(CO)2-LMe (19a). 

When heated, 5-arylfuran-2,3-diones (346) readily evolve carbon monoxide to give 3-aroyl-6-aryl-4-hydroxypyran-2-ones (75KGS1468). Based on a kinetic study of this thermal decarbonylation, it is proposed that the formation of pyranones involves an initial cheletropic cycloreversion. The benzoylketene so formed dimerizes through a [4 + 2]-cycloaddition to the pyranone (Scheme 107) (78JOU2245). 

Kinetic studies on the decarbonylation of 113 and 114 are of interest in terms of the homoaromatic character of 105/106. It has been reported that the rate of decarbonylation of 113 was 1 x 10s times greater than that of 114247. It was suggested that this reactivity difference was due to partial opening of the cyclopropane in the transition state and overlap between the Walsh orbitals of the cyclopropane and the rehybridizing s-orbitals of the breaking bonds. Homoaromatic electron delocalization in a norcaradiene/cyclohepta-triene-like transition state is only possible with the anti-isomer 113. 

The results discussed above lead to the conclusion that the mechanism of decarbonylation of aldehydes is very similar to that postulated for the decarbonylation of acid chlorides. However, kinetic studies of the reaction show that a different rate -limiting step is operative with aldehydes. With acid chlorides, the rate-limiting step is thought to be migration or reductive elimination, depending on the R-group. " A detailed kinetic study on the stoichiometric decarbonylation of aldehydes with RhCl(PPh3)3 has... 

The kinetic study " " was performed on RhCl(PPh3)3 and the chlorine-bridged dimer, Rh2Cl2(PPh3)4, with a variety of aldehydes. The progress of the reaction was monitored by UV-VIS electronic absorption spectroscopy at 25 C, and decarbonylation products RhCl(CO)(PPh3)2 and alkane were analyzed. 

Lower boiling aldehydes can be decarbonylated in a gas phase. Kinetic studies were carried out on the decarbonylation of butyraldehyde over Pd film to give propene and propane.f Commercial hydrofonnylation of propene catalyzed by either Co or Rh... 

Carbonylation and decarbonylation reactions of alkyl complexes in catalytic cycles have been reviewed . A full account of the carbonylation and homologation of formic and other carboxylic acid esters catalysed by Ru/CO/I systems at 200 C and 150-200 atm CO/H2 has appeared. In a novel reaction, cyclobutanones are converted to disiloxycyclopentenes with hydrosilane and CO in the presence of cobalt carbonyl (reaction 4) . The oxidative addition of Mel to [Rh(CO)2l2] in aprotic solvents (MeOH, CHCI3, THF, MeOAc), the rate determining step in carbonylation of methyl acetate and methyl halides, is promoted by iodides, such as Bu jN+I", and bases (eg 1-methylimidazole) . A further kinetic study of rhodium catalysed methanol carbonylation has appeared . The carbonylation of methanol by catalysts prepared by deposition of Rh complexes on silica alumina or zeolites is comparable with the homogeneous analogue . 

This is the most extensively, as well as intensively, studied type of the decarbonylation. Nevertheless, kinetic data are scarce. Conversion of metal acyls to their respective alkyls has been investigated for two systems RCOMn(CO), 37, 50, 51) and XCH2COIr(PPh3)2Cl2 (X = Ph or a substituted Ph group) 160). 

The success of the phase space theory in fitting kinetic energy release distributions for exothermic reactions which involve no barrier for the reverse reaction have led to the use of this analysis as a tool for deriving invaluable thermochemical data from endothermic reactions. This is an important addition to the studies of endothermic reactions described above. As an example of these studies, consider the decarbonylation reaction 11 of Co+ with acetone which leads to the formation of the... 

The Dotz reaction mechanism has received further support from kinetic and theoretical studies. An early kinetic investigation [37] and the observation that the reaction of the metal carbene with the alkyne is supressed in the presence of external carbon monoxide [38] indicated that the rate-determining step is a reversible decarbonylation of the original carbene complex. Additional evidence for the Dotz mechanistic hyphotesis has been provided by extended Hiickel molecular orbital [23, 24] and quantum chemical calculations [25],... 

A study of the influence of the solvent, stoichiometry, temperature and kinetics has been carried out31 to optimize the decarbonylation of 2-[r8F]fluoro-4-methoxybenzaldehyde. The highest decarbonylation yield achieved at 150 °C was 85 + 5%. No detailed study of the mechanism of this reaction has been presented and the lack of 100% yield has not been explained as was done in the case of decarbonylation of lactic acid33. 

Provided care is taken to avoid exchange with water, a kinetic isotope effect is noted when CDO groups are decarbonylated. This indicates that the rate-determining step of the reaction is the scission of the C-D bond. Deuteration studies have also shown that decarbonylation of RCDO yields RD. Further, decarbonylation of Ar CDO and Ar CHO mixtures yields only Ar D and Ar H. ... 

In the TR-CIDNP spectrum, the methyl protons of products IV, V, and VI, formed from the secondary biradical, show a combination of net and multiplet polarizations. Morozova et al [28] measured separately the time dependence of the net and multiplet polarizations for this group of protons, and the results are shown in figure Bl.16.12 and figure B 1.16.13 respectively. Clearly, the net and multiplet polarizations develop on different time scales while the net polarization is constant after approximately 1 ps, the multiplet polarization takes much longer to evolve. It was determined that this difference arises because the net polarization in these products of the secondary biradical is actually inherited from the primary biradical, while the multiplet polarization is generated in the secondary biradical. If chemical transformation (decarbonylation in this case) is fast compared with the rate of intersystem crossing, then a secondary biradical or radical pair may inherit polarization from its precursor. This is known as the memory effect in CIDNP, and this work was the first report of the memory effect in biradicals. The polarization inherited from the primary biradical is net because Ag > 0 in the primary biradical because the secondary biradical is symmetric, A g = 0, and only multiplet polarization can be generated. It was also determined in this study that the kinetics of the net effect reflect the decay of the level while the multiplet effect corresponds to the decay of the 7Y and T levels the reasons for these observations are beyond the scope of this presentation, but the interested reader is directed to the references for additional details. 

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