Dissociative mechanism intermediate

There are alternatives to the addition-elimination mechanism for nucleophilic substitution of acyl chlorides. Certain acyl chlorides are known to react with alcohols by a dissociative mechanism in which acylium ions are intermediates. This mechanism is observed with aroyl halides having electron-releasing substituents. Other acyl halides show reactivity indicative of mixed or borderline mechanisms. The existence of the SnI-like dissociative mechanism reflects the relative stability of acylium ions. 

The catalysts used in hydroformylation are typically organometallic complexes. Cobalt-based catalysts dominated hydroformylation until 1970s thereafter rhodium-based catalysts were commerciahzed. Synthesized aldehydes are typical intermediates for chemical industry [5]. A typical hydroformylation catalyst is modified with a ligand, e.g., tiiphenylphoshine. In recent years, a lot of effort has been put on the ligand chemistry in order to find new ligands for tailored processes [7-9]. In the present study, phosphine-based rhodium catalysts were used for hydroformylation of 1-butene. Despite intensive research on hydroformylation in the last 50 years, both the reaction mechanisms and kinetics are not in the most cases clear. Both associative and dissociative mechanisms have been proposed [5-6]. The discrepancies in mechanistic speculations have also led to a variety of rate equations for hydroformylation processes. 

Compound (32) phosphorylates hindered alcohols in the presence of EtOH consistent with a dissociative mechanism involving a metaphosphate-like intermediate,since it proceeds with considerable racemization at phosphorus(Scheme 6). The extent of phosphoryl transfer which proceeds with retention of configuration is ca. 357, (i e. ca. 707. racemization) the excess of (S)p configuration would arise from transfer with configurational inversion, and might indicate a relatively free ... 

The above kinetics studies of the thermal reactions provide powerful indirect evidence for the operation of a limiting dissociative mechanism in this solvent and for the formation of a reactive intermediate such as IV. Such studies also allow one to evaluate the relative reactivities of that intermediate with different substrates. For example, k.g/kg, the ratio of the rate constants for reaction of IV with CO or PPI13 in 25° THF, was determined to have the value 15 ... 

However, many other proposals have been published slightly modifying the original mechanisms described above. For example, Inderwildi et al.4 and Ojeda et al.5 recently proposed a modification of the alkyl theory in which the intermediate CH2 is formed through the so-called H-assisted CO dissociation mechanism as an alternative to the classical unassisted CO dissociation. 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

Table XII contains the enthalpy and free-energy differences of the critical intermediate species for the anthraphos Rh catalyst. Although no experimental results are available yet, our predicted energies show a much smaller dissociation energy for H2 loss, the first step in the dissociative mechanism (8 is more stable by 13 kcal/mol relative to 4). In contrast, the oxidative addition intermediate of the first step in the associative mechanism, the M(V) species 5, is 14 kcal/mol less stable...

The dissociative mechanism can explain both facts in that the hydrogen removed in the first step may recombine with an isomeric form of the ally lie intermediate to yield the isomeric olefin. Apparently syn and anti 7T-allylic complexes [67, 68) retain their configurations unless each may be converted into a common a-bonded complex in which the nonterminal carbon atoms of the allyl group are connected by a single bond and the isomerization of the intermediate can be represented as in Fig. 11. However, the recombination of the hydrogen atom with the allylic intermediate must be faster than the rate at which it enters the surface pool of... 

Apparently, in the reaction of olefins with hydrogen on catalysts such as palladium and platinum, both the dissociative and the associative mechanisms operate for isomerization and exchange. However, the dissociative mechanism accompanies those factors which tend to slow the addition or accelerate the removal of hydrogen from either substrate or intermediate. These factors may be any of the independent variables, such as the pressure of hydrogen, the structure of the substrate, or the catalyst (5). 

Flo. 27. Geometrical relationships for a possible dissociative mechanism of exchange involving an allylio intermediate. 

Perhaps the most controversial suggestion is that spontaneous isomerizations may proceed through a dissociative mechanism, invoking two three-coordinated intermediates (probably T-shaped). The labile intermediates may be cis-like (A) or trans-like (B) in structure. Applied to the monoorganoplatinum(II) complexes, PtL2XR, the scheme is ... 

A plausible intermediate in the rearrangement and in the reaction with PhjP is a four-coordinated Ir compound. The similarity in k values suggests a dissociative mechanism for the reactions. 

These, and similar data for other systems, demonstrate the tremendous potential that the MICR technique has for the qualitative elucidation of potential energy surfaces of relatively complex organic reactions. Once implementation of the quadrupolar excitation technique has been effected to relax ions to the cell center, the technique will become even more powerful, in that the determination of highly accurate unimolecular decomposition lifetimes of chemically activated intermediates will also become possible. No other technique offers such a powerful array of capabilities for the study of unimolecular dissociation mechanisms and rates. 

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