Dissociative mechanisms

Octahedral substitution reactions (e.g. those involving cobalt(III) complexes) may proceed by both Sf l or 8 2 reactions. In the S l case a slow dissociative mechanism (bond breaking) may take place. Reaction with the substituting... 

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 proposed scenario is mainly based on the molecular approach, which considers conjugated polymer films as an ensemble of short (molecular) segments. The main point in the model is that the nature of the electronic state is molecular, i.e. described by localized wavefunctions and discrete energy levels. In spite of the success of this model, in which disorder plays a fundamental role, the description of the basic intrachain properties remains unsatisfactory. The nature of the lowest excited state in m-LPPP is still elusive. Extrinsic dissociation mechanisms (such as charge transfer at accepting impurities) are not clearly distinguished from intrinsic ones, and the question of intrachain versus interchain charge separation is not yet answered. 

The best understood compounds are cis- and fra s-RuX2(DMSO)4 (X = Cl, Br). The fra s-isomers are thermodynamically less stable and isomerize in DMSO solution to the m-isomer, with first-order kinetics, probably via a dissociative mechanism. The reverse process, cis to trans, is catalysed by light. Syntheses for these and other DMSO complexes are shown in Figure 1.37 [108],... 

The steric crowding introduced in the latter by the four ethyl substituents inhibits nucleophilic attack at platinum, so that complexes of this type tend to undergo substitution by a dissociative mechanism [89]. The complex of the more rigid ligand, 2,2, 2"-terpyridyl, Pt(terpy)Cl+, is found to be about 103 to 104 times more reactive to substitution than the dien analogue this is ascribed to steric strain [90], which is reflected in the short Pt—N bond to the central nitrogen (Pt-N some 0.03 A shorter than the other two Pt-N bonds) and N—Pt—N bond angles of 80-82°). 

A dissociative mechanism is indicated by the fact that excess phosphine inhibits elimination from molecules like m-Pd(PPh3)2Me2 and Pt(PPh3)2Bu2. On thermolysis of mixtures where one molecule contains deuterium, such as... 

The evidence is that the thermolytic route does not involve radicals but the photochemical one does. A dissociative mechanism for the thermolytic route is indicated by its inhibition by added phosphine it is likely that once a phosphine group has dissociated, a metal-hydrogen bond is formed, with generation of a coordinated alkene (Figure 3.58). 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

Nickel carbonyl has been shown to have a very high retention— 98.7%—both in the pure hquid and as 10% solution in n-heptane. It was argued that this represents the results following essentially complete isotopic exchange. Since the exchange of CO with Ni(CO)4 is known to occur quite rapidly by a dissociation mechanism, reformation of nickel carbonyl following the nuclear reaction would proceed rapidly by the reaction... 

The role of the support on hydrogen chemisorption on supported rhodium catalysts was studied using static and frequency response techniques. In all Instances, several klnetlcally distinct H2 cheml-sorptlve sites were observed. On the basis of the kinetics, at least one site appears to sorb H2 molecularly at temperatures below 150°C, regardless of the support. At higher temperatures, a dissociative mechanism may become dominant. Inducement of the SMSI state In Rh/T102 does not significantly alter Its equilibrium H2 chemisorption. 

L—Pd° (Dissociative mechanism) Scheme 6.2 Formation of the catalytic species... 

More recently, a number of reports dealing with 1,3-sulfonyl shifts which proceed by other mechanisms have been published. For example, Baechler and coworkers suggested that the higher activation enthalpy observed for the isomerization of the deuterium labeled methallyl sulfone 72 in nitrobenzene at 150°C as compared to the corresponding sulfide, together with the positive entropy of activation may be taken as evidence for a homolytic dissociation mechanism (equation 44). A similar mechanism has also been suggested by Little and coworkers for the gas-phase thermal rearrangement of deuterium labelled allyl sec-butyl sulfone, which precedes its pyrolysis to alkene and sulfur dioxide. 

It is clear that the water reaction of p-nitrophenyl sulfate monoanion should occur by a dissociative mechanism, because 17 is too high in energy. (This was... 

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. 

SCHEME 9.13 A dissociative mechanism releases the 15N-labeled dA from its QM adduct. 

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 kinetics and mechanism of chloride and bromide substitution of diaquonickel(III) complexes with several ligands of the cyclam type have been studied. The data reveal a reversible process yielding [NiL(H20)X]2+ and are consistent with a dissociative mechanism, sterically more demanding axial groups substantially increasing the rate. In addition, a rearrangement step is observed subsequent to the formation of the monohalide complex.146... 

It has been known for some time that the spectroscopic signature of Ob-vacs can be healed by exposure to 02 [42-46], In addition, Epling etal. [47] show that temperature-programmed desorption (TPD) spectra of water and ammonia are perturbed when the surface is predosed with 02. This implies that oxygen is left on the surface in some form when Ob-vacs are healed by 02, As such, Epling et al. proposed that one Ob-vac is healed per 02 molecule with the other O atom being adsorbed at a Ti5c site (Oad), a dissociation mechanism supported by theoretical calculations [48, 49]. 

In marked contrast, AS of the exchange reaction of oxygen in a six-coordinated complex, trans-[Re02(py)4] + is negative, ( - 32 JKT1 mol-1), suggesting that the reaction proceeds by a dissociative mechanism. This is also supported by the kinetic analysis of the exchange reaction of pyridine on trans-[M02(py)4]+ (M = Tc or Re), which was monitored by H-NMR [15] ... 

The rearrangement of 24 to 19 is an intramolecular process with 1st order kinetics. The rearrangement does not occur by reductive elimination of TMS from 24 and by readdition of free TMS to the [(dtbpm)Pt(O)] fragment in a subsequent C-Si activation step. This possibility can be excluded rigorously by performing the 24 to 19 transformation in fully deuterated Si(CD3)4 or in hexamethydisi-loxane as solvents 24 only yields undeuterated 19 in d -TMS and does not lead to 21 in hexamethyl-disiloxane (which would have to be seen as a dissociative mechanism, as [(dtbpm)Pt(O)] had been previously shown to activate hexamethydisiloxane under the same reaction conditions). 

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