Dissociative initiation mechanism

Figure 7.3 The energy profile for the associative and dissociative initiation mechanisms for models of the first- (dotted lines) and second- (solid lines) generation methylidene catalysts. Calculated at the BP86 level of theory. Adapted from Ref. [4].

The first step in the initiation pathway was found to be similar to the dissociative initiation mechanism of non-chelated catalysts the isopropoxy chelating group dissociated from Ru via rotation of the o-isopropoxyphenyl. Metathesis with a substrate olefin molecule then replaces the o-isopropoxybenzylidene with alkylidene to form the active metathesis catalyst. The metathesis step during initiation of the chelated catalysts was also found to occur via a side-bound mechanism. 

For ROMP of cyclooctadiene, the activity decreased in the order 55b > 55d > 55a, while complex 55c only reached low conversions, even after several days. These differences in reactivity may be due to an improved steric shielding of the metal center by the mesityl group in 55b. Moreover, phenyl groups may undergo cyclometalation, which would deactivate the catalyst. Kinetic studies suggested that an associative or dissociative initiation mechanism might be involved. These data were further supported by the calculated thermodynamic data for the activation parameters, which agreed very well with the experimental results. 

More recently, Grubbs et al. obtained a refined mechanistic picture of the initiating step by conducting a 31P NMR spectroscopic study of the phosphine exchange in precatalysts 12-A. These investigations revealed that substitution of the phosphine proceeds via a dissociative-associative mechanism, i.e., a 14-electron species 12-B is involved that coordinates the alkene to give a 16-electron species 12-C (Scheme 12) [26a]. Increased initiation rates are observed if the substituents R and the phosphine ligands PR3 in precatalysts... 

Let us consider the mechanism of initiation in greater detail, taking benzoyl peroxide as an example. It may be assumed to dissociate initially into benzoate radicals, as previously indicated by reaction (2). 

Lewis et al.106 calculated four possible decomposition pathways of the ot-HMX polymorph N-N02 bond dissociation, HONO elimination, C-N bond scission, and concerted ring fission. Based on energetics, it was determined that N-N02 dissociation was the initial mechanism of decomposition in the gas phase, whereas they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The more recent study of Chakraborty et al.42 using density functional theory (DFT), reported detailed decomposition pathways of p-HMX, which is the stable polymorph at room temperature. It was concluded that consecutive HONO elimination (4 HONO) and subsequent decomposition into HCN, OH, and NO are the most energetically favorable pathways in the gas phase. The results also showed that the formation of CH20 and N20 could occur preferably from secondary decomposition of methylenenitramine. 

Ij mechanism. This is the dissociative interchange mechanism and is similar to the previons one in the sense that dissociation is stiU the major ratecontrolling factor. Therefore, we are stiU dealing with an SnI process. Nevertheless, differently from the D mechanism, no experimental proof exists that an intermediate of lower coordination is formed. The mechanism involves a fast onter sphere association between the initial complex and the... 

There is, however, a much better reagent than bromine to brominate at an allylic position selectively. This reagent is A -bromosuccinimide (NBS), and it also reacts via a radical mechanism. The weak N-Br bond in NBS is susceptible to homolytic dissociation initiated either by light or a chemical initiator, such as a peroxide. This produces a small amount... 

The authors conducted an experiment (now regarded as classical) in Fischer-Tropsch catalysis that supports this initiation mechanism (3,4). Using isotopes, they demonstrated that the carbon chain-growth reaction can occur from Ci species generated by the dissociation of CO. As shown below, this hypothesis implies that the rate of CO dissociation should be fast and should not control the overall Fischer-Tropsch reaction. 

Experiments in cyclohexane and hexane show that benzene cannot be regarded as an inert solvent and that the initiation mechanism differs between aliphatic and aromatic solvents. Studies on intermolecular exchange [38, 48—50], which presumably reflects the primary dissociation process, also indicate differences between the two types of solvent as well as between different lithium edkyls. Recent experiments with tert.-butyl-lithium [46], which has the lowest intermolecular exchange rate of the alkyls, indicate a more complex behaviour in the initiation reaction in benzene. This may be connected with a lower rate of dissociation of this alkyl. 

The initiating mechanism of the cyclopropane-hydrogen reaction presumably results in an adsorbed n-propyl radical, while by reason of its weaker secondary C—H bond, propane probably dissociates into an isopropyl radical and an H atom. The close similarity between the distributions in the two cases su ested that exchange proceeds through the equilibria... 

The initiation mechanisms for the Hoveyda-Grubbs catalysts have been computationally investigated by the groups of Hillier, Percy, and Solans-Monfort [8, 9]. Three possible mechanisms have been investigated, including associative, dissociative, and interchange (concerted) processes. As with the phosphine-containing catalyst systems, an associative mechanism was disfavored due... 

Initiation. In both thermal- and photopol5unerizations, the rate of initiation depends on two processes the dissociation of the initiator and the initiation of the propagating chain. The decomposition rate Ua) of thermal initiators strongly depends on temperature, with the half-life of many thermal initiators at the reaction temperature on the order of minutes or hours. In contrast, for photopolymerizations, the rate at which photons are absorbed at a specific wavelength will determine the decomposition rate of photoinitiators. This process is not temperature-dependent. Thus, in the classic initiation mechanism, the interaction between light of a specific wavelength and a photoinitiator molecule is considered. For a unimolecular photoinitiator, this reaction step can be written as follows ... 

Much effort has been expended recendy to understand how Hoveyda-type precatalysts initiate. Originally, it was simply assumed that the initiation mechanism was dissociative and data to support this was pubHshed by Love et al., in which no dependence of the initiation rate of GH2 on substrate concentration was observed. No entropy change was required to reach the transition state (AH = 19.9(5) kcal moH and AS = 1(2) cal mol for the initiation of GH2 with EVE in toluene). However, activation parameters collected during a later study, also by the Grubbs group, suggested a nondissociative mechanism, due to the negative entropy of activation (AH = 15.2(8) kcal mol and AS = —19(3) cal moH for the... 

---