Insertion addition mechanism

Generally speaking, two mechanisms may be considered for the formation of benzene derivatives from metallacyclopentadienes. These are the concerted mechanism (Path A) and the insertion (addition) mechanism (Path B), as shown in Eq. 2.49. 

There are several examples of the concerted mechanism. However, no report of an insertion of a carbon—carbon triple bond into a metallacyclopentadiene had appeared prior to discovery of this reaction. At low temperatures, during the reaction of zirconacyclopentadienes with DMAD, the formation of trienes (79) is observed upon hydrolysis. This clearly indicates that the benzene formation involves the insertion (addition) reaction of DMAD. As shown in Eq. 2.50, the alkenyl copper moiety adds to the carbon—carbon triple bond of DMAD and elimination of Cu metal leads to the benzene derivatives 72. Indeed, a copper mirror is observed on the wall of the reaction vessel. 

Despite the short lifetimes of most silylenes, improvements in flash photolysis techniques for their generation and time-resolved spectroscopic detection methods in the past decade have made possible direct kinetic measurements on the reactions of silylenes. The purpose of these kinetic studies has been to elucidate the mechanisms of silylene reactions. While considerable work remains to be done, transition state structures and activation barriers are emerging from these experiments, and aspects of silylene insertion and addition mechanisms have been revealed that were not uncovered by product studies and were, indeed, unexpected. 

Scheme 1. Proposed mechanism of migratory insertion/addition of the coordinated alkyne ligand to the /i-CSiMe3 ligand generating the W2(/r-CRCR CSiMe3)moiety. The steric preference for R = H relative to alkyl or aryl is implied in the proposed transition state, B.

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

Some typical examples of cyclooligomerization catalysts other than nickel are listed in Table 5 and 6. Examination of these reactions indicates that the mechanisms are closely related to each other. They all seem to proceed via allylic intermediates in a stepwise oxidative insertion (addition)/reductive coupling (elimination) fashion while the metal center undergoes changes in the formal oxidation state (viz. Fe"-Fe Ti"-Ti Cr -Cr Co -Co" Mn -Mn Mo -Mo" Ni°-Ni [6b]. 

Substitution by the SN2 mechanism and -elimination by the E2 and Elcb mechanisms are not the only reactions that can occur at C(sp3)-X. Substitution can also occur at C(sp3)-X by the SRN1 mechanism, the elimination-addition mechanism, a one-electron transfer mechanism, and metal insertion and halogen-metal exchange reactions. An alkyl halide can also undergo a-elimination to give a carbene. 

Czv-Symmetric Catalysts. Syndiotactic polymers have been formed using metallocene catalysts where the polymer chain end controls the syndiospecificity of olefin insertion. Resconi has shown that Cp 2MCl2 (M = Zr. Hf) derived catalysts produce predominantly syndiotactic poly(l-butene) with an approximate 2 kcal/mol preference for syndiotactic versus isotactic dyad formation." At —20 °C. Cp 2HfCl2/MAO produces poly(l-butene) with 77% rr triads. Pellecchia had reported that the diimine-ligated nickel complex 30 forms moderately syndiotactic polypropylene at —78 °C when activated with MAO ([rr] = 0.80)." " Olefin insertion was shown to proceed by a 1.2-addition mechanism." in contrast to the related iron-based systems which insert propylene with 2.1-regiochemistry. ... 

Gunnoe has also reported examples of catalytic aromatic alkylation using a ruthenium complex and olefins. With propylene and other terminal olefins, a 1.6 1 preference for anti-Markovnikov addition was seen. The proposed mechanism involved olefin insertion into the metal-aryl bond followed by a metathesis reaction with benzene to give the alkylated aromatic and a new metal-phenyl bond (Equation (26)). DFT calculations supported the proposed non-oxidative addition mechanism. The work was extended to include catalytic alkylation of the a-position of thiophene and furan. With pyrrole, insertion of the coordinated acetonitrile into the a-C-H bond was observed. Gunnoe has also summarized recent developments in aromatic C-H transformations in synthesis using metal catalysts. ... 

As shown in Scheme 20, the insertion step from 21 to 22a in route 1 is more favored both kinetically and thermodynamically than the addition step from 21 to 23a in route 2 (9.6 and —37.2 vs. 24.7 and —18.8 kJ/mol, respectively). In addition, route 2 (23b to 23c) has much higher N-H oxidative addition barrier than that of route 1 (22c to 22d) (237.7 vs. 205.9 kJ/mol, respectively), and such high barriers are responsible for the dramatic reaction conditions. Direct comparison indicates that route 1 is more favored and less complicated than route 2. Therefore, the insertion-addition route 1 should be the most likely catalytic mechanism. 

The previously accepted pathway consisted of P-H oxidative addition to Pt(0) to form 19 followed by coordination and insertion of the alkene in the Pt-P bond to form 20 and a final reductive elimination to furnish the product and regenerate the catalyst. Another possibility is the nucleophilic attack of phosphido complex 19 to the alkene ( Michael addition mechanism, as in anionic polymerisation) to generate the zwitterionic intermediate 21. This complex can yield the hydrophosphination product 11 via one of two complementary pathways. Carbanion attack at the cationic platinum hydride i.e. intramolecular hydrogen transfer) would yield the final phosphine complexed to Pt(0) that would be displaced by an equivalent of PHR R to furnish, after oxidative addition, starting complex 19. Alternatively, the anionic carbon atom in 21 could attack the platinum centre directly, forming the cyclic intermediate 22. From here Pt-P bond dissociation would generate 20, which would furnish the product after reductive elimination. 

The Michael addition mechanism offers an easy explanation for by-product formation if zwitterion 21 attacks a second molecule of alkene instead of the proton transfer in Scheme 6.10. In this case (Scheme 6.11), the newly generated zwitterion 21 can yield a by-product with two alkene fragments or attack another alkene to eventually produce by-products derived from the insertion of three or more alkenes (23). 

The function of fhe zone control system is to maintain a specified amount of reactivity in the reactor, this amount being determined by the deviation from the specified reactor power set point. If the zone control system is imable to provide the necessary correction, the program in the reactor regulating system draws on other reactivity control devices. Positive reactivity can be added by withdrawal of absorbers. Negative reactivity can be induced by insertion of mechanical control absorbers or by automatic addition of poison to the moderator. 

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