Direct insertion mechanisms

Since the two-spin state forms can lead to different products, the products obtained will be a mixture that reflects the initial fractionation of the reaction between the two-spin states. The fractionation in turn is a reflection of the interplay and the probability of cross-over between the two-spin states (8). Thus, the two-state reactivity paradigm resolves the dilemma of whether a radical recombination or a direct insertion mechanism governs cytochrome P450-catalyzed hydroxylation actually they are both involved and the degree to which either is expressed depends upon the specific substrate hydroxylated and the specific enzyme. 

Aromatic hydroxylation may also take place by a mechanism other than epoxidation. Thus, the ra-hydroxylation of chlorobenzene is thought to proceed via a direct insertion mechanism (Fig. 4.7). 

O2COR)2 compounds (29). These reactions were shown to proceed by a direct insertion mechanism (i.e. not by a mechanism involving catalysis by traces of alcohols) with energies of activation of not greater than 20 Kcal mol ... 

The mechanisms can now be evaluated on the basis of these data. First, Mechanism 1 is definitely ruled out by the first experiment. Direct insertion of CO must result in C in the acyl ligand because none is found, the mechanism cannot be a direct insertion. Mechanisms 2 and 3, on the other hand, are both compatible with the results of this experiment. 

Many theoretical studies of insertion concern the very important case of insertion of ethylene (or a-alkenes) into a growing polymer, following the Cossee-Arlman direct insertion mechanism (Scheme 6.2) [6], In this particular case, the resting states of the catalyst before and after each propagation step are similar, and contain an M-alkyl bond stabilized by an agostic interaction. The interactions with the solvent or external ligands must be also rather similar for both resting states. Thus, the values calculated must correspond only to the coordination and insertion of the alkene. 

Scheme 13 Direct insertion mechanism for Ru(II)-catalyzed hydrogenation...

The chain propagation steps of alkene insertion consist of repetitive insertions of the olefin into the growing polymer chain. The mechanism of this olefin insertion process was discussed in detail in Chapter 9, and some of the first data in favor of a direct insertion mechanism for polymerization with early metals obtained by Watson is reiterated in Scheme 22.8. These studies include spectroscopic observation of the insertions of propylene into lutetium alkyl linkages. As noted in this previous chapter, the mechanism of this insertion process is best described as a migratory insertion in which C-C bond formation occurs within an alkyl olefin complex. 

Jolly, C. A. Marynick D. S. The direct insertion mechanism in Ziegler-Natta polymerization A theoretical study of Cp2TiCH+ + C2H4- > Cp2TiC3H+. J. Am. Chem. Soc. 1989, 111, 7968-7974. 

This scheme is remarkably close to the coordination insertion mechanism believed to operate in the metal alkoxide-catalyzed ring-opening polymerization of cyclic esters (see Section 2.3.6). It shares many features with the mechanism proposed above for the metal alkoxide-catalyzed direct polyesterification (Scheme 2.18), including the difficulty of defining reaction orders. 

A/E) (Roth, 1972b). In contrast, no polarization is observed in the absence of a sensitizer, despite the fact that ethylbenzene is produced. This is consistent with a direct insertion of singlet methylene into the C—H bond, but it could also arise from an abstraction-recombination mechanism if the lifetime of the intermediate radical-pair were too short to permit a significant amount of Tq-S mixing. 

That this mechanism can take place under suitable conditions has been demonstrated by isotopic labeling and by other means. However, the formation of disproportionation and dimerization products does not always mean that the free-radical abstraction process takes place. In some cases these products arise in a different manner.We have seen that the product of the reaction between a carbene and a molecule may have excess energy (p. 247). Therefore it is possible for the substrate and the carbene to react by mechanism 1 (the direct-insertion process) and for the excess energy to cause the compound thus formed to cleave to free radicals. When this pathway is in operation, the free radicals are formed after the actual insertion reaction. 

Figure 14 shows a representative 2-TS trajectory, which demonstrates that the 2-TS paths follow a direct S-bend insertion mechanism. The trajectory passes through the middle of the molecule, and avoids the Cl this forces the products to scatter into negative deflection angles. The 2-TS QCT total reaction... 

Waymouth and coworkers used chiral zirconocene complexes such as 56 with Et3Al as the stoichiometric reductant to enantioselectively desymmeter-ize oxabicyclic compounds (Scheme 9) [29]. A reductive coupling mechanism to give 57 followed by (i-alkoxidc ring opening and transmetallation is consistent with the experimental results. Neither direct insertion of the alkene into the M - C bond nor nucleophilic attack mechanisms can be ruled out, however [12]. 

An extensive review of the literature reveals that the only studies of vibrational effects in insertion chemistry have focused on reactions of 0(1D)175-177 and C(1D)177,178 with H2. Since there is no potential energy barrier to insertion in these systems, reaction proceeds readily even for unexcited reactants.179 Since the efficiency of vibrational excitation was 20% in both studies, due to the large cross-sections for ground state reactions, only small changes were observed in the experimental signal. From an analysis of the product distributions, it was concluded that while H2(v = 0) primarily reacted via an insertion mechanism, direct abstraction seemed to become important for = 1). For 0(1D), this is similar to behavior at elevated collision energies.180... 

The third class of mechanisms is that originated by Pichler and Schulz in which chain growth is accomplished by direct insertion of an absorbed CO molecule into a carbon-metal bond produced by hydrogenation of a surface carbonyl (89). A complete representation of this mechanism (see... 

The time-resolved, chemical behavior of FL depends on the solvent. Irradiation of DAF in cyclohexane gives FLH The lifetime of FL in cyclohexane is 1.4 ns, and the ratio of products obtained (26) indicates that both direct insertion and abstraction-recombination mechanisms are operating (Griller et al., 1984b). Replacement of the cyclohexane by its deuteriated counterpart reveals a kinetic isotope effect of ca 2 (Table 5). 

Figure I. Insertion and direct transfer mechanisms of ethene epoxidation by TM peroxo compounds, exemplified for a Mo complex.

With regard to epoxidation activity, the peroxo complex MoO(C>2)2-hinpt exhibits close similarity to peracids. Based on this observation, Sharpless et al. suggested [11] a concerted (direct) mechanism as an alternative to the insertion mechanism. That mechanism, which is assumed to proceed via a... 

An important finding is that all peroxo compounds with d° configuration of the TM center exhibit essentially the same epoxidation mechanism [51, 61, 67-72] which is also valid for organic peroxo compounds such as dioxiranes and peracids [73-79], The calculations revealed that direct nucleophilic attack of the olefin at an electrophilic peroxo oxygen center (via a TS of spiro structure) is preferred because of significantly lower activation barriers compared to the multi-step insertion mechanism [51, 61-67]. A recent computational study of epoxidation by Mo peroxo complexes showed that the metallacycle intermediate of the insertion mechanism leads to an aldehyde instead of an epoxide product [62],... 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

Density functional calculations reveal that epoxidation of olefins by peroxo complexes with TM d° electronic configuration preferentially proceeds as direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a TS of spiro structure (Sharpless mechanism). For the insertion mechanism much higher activation barriers have been calculated. Moreover, decomposition of the five-membered metallacycle intermediate occurring in the insertion mechanism leads rather to an aldehyde than to an epoxide [63]. 

Note that this classification does not cover all the theoretical possibilities. For example, a cleavable signal peptide cannot direct the creation of a topology opposite to a type I (according to the loop model). Why other types are not observed must be explained from their insertion mechanisms. This classification can be naturally expanded into multi-spanning proteins (polytopic proteins) based on the location of their N terminus (therefore, type IV cannot be defined). 

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