Associative and dissociative mechanisms

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. 

Explain the difference with respect to the size of the neighboring groups on substitution in an octahedral complex by associative and dissociative mechanisms. 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

The replacement of X by pyridine in the complex PtX(dien)+ to give Pt(dien)py2+ (equation 549) has been studied under controlled conditions with a range of leaving groups X. These data are shown in Table is.1 87-1988 From these data the leaving group order is NOj > H20 > Cl- > Br > I- > N3 > SCN- > NO2 > CN-. Reactions such as these must be carried out under thermal conditions for accurate comparison since photoaquation can occur, albeit with a rather low quantum yield.1989 The volumes of activation of these reactions (equation 550) are all negative. An associative mechanism is proposed for the nucleophilic dependent path, but for the nucleophile independent pathway both associative and dissociative mechanisms need to be considered.1990... 

In order to establish this supposition of a restricted rotation around the coordination bond, it should be assured that the contribution of the dissociation mechanism is negligibly small in the hydrogen-exchange reaction. For this purpose, a 1 1 mixture of (ZJ-propene-l-rf, (composition d0, 3.6% Z-l-dlt 86.1 % E-l-, 7.5% Z-l, 2-d2,1.4% E-l,2-d2,0% and 1, l-d2,1.4%) and propene-d6 (purity, propene-powder catalyst at room temperature, and HD gas (98 %) was added as a cocatalyst. In this experiment, the amounts of Mo—H and Mo—D sites are approximately equal during reaction. The primarily exchanged products formed by associative and dissociative mechanisms are described respectively in Scheme 11. 

The three-phase test has been used to distinguish between associative and dissociative mechanisms. Thus, in this case the negative results in the trapping of the intermediate with dienic polymers seem to support a stepwise mechanism in the formation of cyclocondensation adducts with siloxydienes and l-azetin-4-one. 

Bottom DFT (B3LYP) Analogue of (2a) 3 Competing pathways support associative and dissociative mechanisms and finds carbenoids significant contributors as intermediates... 

As was the case with other phosphotransfer reactions, the hydrolysis reaction catalyzed by PTPs lies along a mechanistic continuum between the limiting cases of associative and dissociative mechanisms. In the case of Yersina PTP, a crystal structure was solved with nitrate, a mimic of the transition state similar to the use of AIF3 with protein kinase A. In this case, analysis of the structure is proposed to be more consistent with a -1 charged metaphosphate, dissociative mechanism (11). 

Widauer, C., Gruetzmacher, H., Ziegler, T. Comparative Density Functional Study of Associative and Dissociative Mechanisms in the Rhodium(l)-Catalyzed Olefin Hydroboration Reactions. Organometattics 2000,19, 2097-2107. 

A reaction in which the transition state has a greater volume than the initial state shows a positive AV, whereas a negative AV corresponds to the transition state being compressed relative to the reactants. After allowance for any change in volume of the solvent (which is important if solvated ions are involved), the sign of A V should, in principle, distinguish between an associative and dissociative mechanism. 

In organometallic lanthanide complexes, the bonds are partially covalent but remain largely ionic in nature and both associative and dissociative mechanisms may play a relevant part in the ligand scrambling. Considering the electrophilic nature of the lanthanides, dissociative pathways are only possible in co-ordinating solvent (Scheme 6). 

Polanyi and coworkers also developed kinetic equations for both the associative and dissociative mechanisms for comparison with their experimental results. 

Values of AS are particularly useful in distinguishing between associative and dissociative mechanisms. A large negative value of AS is indicative of an associative mechanism, i.e. there is a decrease in entropy as the entering group associates with the starting complex. However,... 

The AH- term is generally not very informative because the enthalpies of activation (which deal with the energy of bond breaking) for associative and dissociative mechanisms are similar. However, the AS term is very useful. When AS Is large and negative, the implication is that there are less particles present in the TS than in the reactants. This would indicate an associative TS, where the transition metal increases its coordination number in the activated complex. On the other hand, if the AS term is large and positive, the TS will have more particles than the reactants, implying a dissociative mechanism. 

Controversy over the interpretation of the kinetics for cis-trans isomerizations of square-planar complexes continues. The topic is important for the understanding of substitution mechanisms in general. Associative and dissociative mechanisms have been proposed, as well as intramolecular rearrangements via tetrahedral intermediates. ... 

Many examples of associative and dissociative mechanisms are presented in later sections. One classic example of ligand substitution that exhibits the hallmarks of an associative substitution process is the replacement of chloride in a 16-electron complex, such as Pt(PEt3)2Cl2. One classic example of a ligand substitution that exhibits the hallmarks of a dissociative mechanism is the substitution of phosphine for carbonyl in Ni(CO). ... 

PIatinum(ii).—General. Activation volumes are often useful guides to the diagnosis of mechanism, frequently permitting distinction between associative and dissociative mechanisms and sometimes permitting more subtle distinctions, as between /a and/) or 7a and A alternatives. Reactions of a series of complexes [Pt(dien)X]+ (X = Cl, Br, I, or N3) with a range of incoming nucleophiles Nu (Nu = OH, I", Ng-, NO2 , SCN, or py) follow the usual rate law (1) characteristic of substitution at square-planar species. Activation volumes have been determined, in aqueous solution, both... 

Associative and dissociative mechanisms have different kinetic outcomes... 

The theoretical results indicated that both mechanisms were possible and were controlled by electrode potential. At high electrode potential, the barrier for O2 dissociation would increase, thus, the dissociative mechanism was less favorable. In another study, Jacob [36] also demonstrated that both associative and dissociative mechanisms might occur simultaneously, with a higher ratio for the associative pathway. 

Zhdanov and Kasemo [41] analyzed ORR kinetics corresponding to both associative and dissociative mechanisms. Their study evaluated the relationship between the reaction order and O2 and O coverage, based on the two different reaction mechanisms and under different adsorption assumptions (Langmuir and lateral interaction). The study concluded that if the reaction steps were described by using Langmuir equations, the kinetic models based... 

---