Stepwise insertion mechanism

Generally speaking, there are mainly two types of mechanism that may be considered for the formation of benzene derivatives from metallacyclopentadienes the concerted mechanism (Diels-Alder type, path A) and the stepwise insertion mechanism (path B), as shown in Scheme 11.6. 

In addition to the metallacycle-based mechanism depicted above, stepwise insertion pathways such as alkyne hydrometallation followed by aldehyde... 

When the polymerization proceeds via the repetition of the dissociation at the C-B bond, the addition of monomers to the propagating radical, and primary radical termination with B and/or of chain transfer of the propagating radical to the C-B bond, such polymerization may proceed via a living radical mechanism. As an extreme case, if the polymerization proceeds via a stepwise insertion of one monomer molecule into the C-B bond, it would result in a successive reaction. 

Recently, a general catalytic method forthe conversionof 2-alkyl-l -ethynylbenzenes to indenes was disclosed by the group of Liu [41]. Their proposed mechanism involves the stepwise insertion of a ruthenium vinylidene into a benzylic C— H bond (Scheme 9.21). 

In the present chapter, a classification of the hydrogenation reaction mechanisms according to the necessity (or not) of the coordination of the substrate to the catalyst is presented. These mechanisms are mainly classified between inner-sphere and outer-sphere mechanisms. In turns, the inner-sphere mechanisms can be divided in insertion and Meerweein-Ponndorf-Verley (MPV) mechanisms. Most of the hydrogenation reactions are classified within the insertion mechanism. The outer-sphere mechanisms are divided in bifunctional and ionic mechanisms. Their common characteristic is that the hydrogenation takes place by the addition of H+ and H- counterparts. The main difference is that for the former the transfer takes place simultaneously, whereas for the latter the hydrogen transfer is stepwise. 

Recently, trans insertion of hexafluorobutyne into one of the M—H bonds in some metallocene hydrides, Cp2MH , was studied in some detail (47). Experiments carried out in the presence of various radical-sensitive reagents such as TV-phenyl-a-naphthylamine suggested that a free radical mechanism was unlikely. A stepwise ionic mechanism, involving a zwitter-ionic intermediate, Cp2(H2)M+—C(CF3)==CCF3, is improbable, since (i) the stereochemistry and the apparent rate are not influenced by the polarity of the solvents, (ii) no deuterium is incorporated in the reaction in EtOD, and (iii) the trend in reactivity (Mo > W) does not reflect the trend in v-basicity or M—C bond stability (W > Mo). An essentially concerted trans-insertion mechanism is inferred, which is supported inter alia by the low kinetic deuterium isotope effect (kH/k0 = 1). 

Sulfur reacts with hexamethylsilacyclopropane in tetrahydrofuran (THF) to give 2,3-dithia-l-silacyclopentane in 38% yield <79JOM(164)305>. Exposure of rranj-1,1-di-r-butyl-2,3-dimethyl-silacyclopropane (47) to elemental sulfur at room temperature for 3-6 h gave trans-1,1 -di-i-butyl-1,2-silathietane (48) and rra s-3,3-di-r-butyl-4,5-dimethyl-l,2-dithia-3-silacyclopentane (49) in 60% and 19% yield respectively (Equation (10)). Identical results were obtained with the m-silacyclo-propane, suggesting a stepwise radical mechanism <900M2205>. The presence of r-butyl substituents on silicon was a key factor in this reaction, since sulfur insertion cannot be stopped at a single atom when there are small substituents on silicon <79JOM(164)305>. 

According to the Anderson-Shulz-Flory (ASF) mechanism, the Fischer-Tropsch reaction can be considered as a polymerization reaction involving the stepwise insertion of a monomer unit into a growing chain. The use of the isotopic transient kinetic technique has the potential of determining the rate constants of initiation, propagation, and termination and the concentration of adsorbed intermediate species on the catalyst during... 

The mechanism of polymerization of alkenes using Ziegler-Natta-type catalysts is described as a coordination [239] or insertion [240] polymerization process. The coordination terminology assumes that the growing polymer chain is bonded to a transition metal atom and that insertion of the monomer into the carbon-metal bond is preceded by, and presumably activated by, the coordination of the monomer with the transition metal center. Since coordination of the monomer may or may not be a specific feature of these polymerizations, the insertion terminology focuses on the proposal that these reactions involve a stepwise insertion of the monomer into the bond between the transition metal atom and the last carbon atom of the growing chain. It is important to note that the bonding of carbon atoms and transition metals is... 

MUA with one 0 incorporated, and the correlative variation of the yield vs. time curves for MUA and 15, support the stepwise oxygenation mechanism as opposed to one-step insertion via a dioxetane intermediate. 

Reactions of the (RC2H)Co2(CO)6 complexes with carbon monoxide under pressure give lactone complexes (reducible to y-butyrolactones ) by insertion of two CO groups into the Co-C bonds (eq 37) the further stepwise insertion of CO and alkyne provides the mechanism for the efficient Co2(CO)g-catalyzed synthesis of 2,2 -bifurylidene-5,5 -diones (eq 38). Under relatively mild conditions and with a 1 1 ratio of C2H2 CO, the extended bifurandiones of eq 39 also become significant products. ... 

In order to explain the competitive formation of the 1 1 and 1 2 adducts and the formation of the 2,6-octadienyl rather than the 1,6-oc-tadienyl chain, a mechanism was proposed (62, 69) in which the insertion of one mole of butadiene to the Pd—H bond gives the 77-methallyl complex (68) at first, from which 1-silylated 2-butene is formed. At moderate temperature and in the presence of a stabilizing ligand, further insertion of another molecule of butadiene takes place to give C5-substituted n-allyl complex 69. The reductive elimination of this complex gives the 1 2 adduct having 2,6-octadienyl chain. In the usual telomerization of the nucleophiles, the reaction of butadiene is not stepwise and the bis-n--allylic complex 20 is formed, from which the l, 6-octadienyl chain is liberated. 

The four hitherto known routes of the C-H insertion are shown in Scheme 1. In general, the insertion by singlet carbenes proceeds via route a in one step, whereas the reaction by triplet carbenes proceeds sequentially via route b, i.e., hydrogen abstraction followed by recombination of the radical pairs.4 Other stepwise mechanisms are hydride abstraction (route c) and proton abstraction (route d), both being followed by the recombination of ion pairs. However, extended study on routes c and d for synthetic purposes had not been done before we started, except for a few earlier studies on carbanion-promoted P C-H insertion reactions.5,6 Recent advances in transition metal-catalyzed... 

With regard to the C-H insertion process, two mechanisms are possible the first one is a concerted one-step process (a) and the second one is a stepwise process (b). Due to the finding that the reaction of a mixture of a,a-d2 -benzyloxide 6... 

E.Z-Selectivity in the insertion by unsymmetrical carbenoid 24, is specifically indicative of the transition state of the stepwise mechanism. Based on the evidence that carbenoid 24, which is generated from 42 or 43 (E Z = 84 16), exists nearly exclusively in the -configuration under the equilibrium even at —95°C,29 the observed stereoselectivity for E-isomers in the insertion products verifies that hydride abstraction takes place via an Sn2-like transition state 52 with inversion of configuration at the carbenoid carbon, followed by the recombination of menthone 40 and carbanion 53 (Scheme 19). 

Ruthenium complexes do not have an extensive history as alkyne hydrosilylation catalysts. Oro noted that a ruthenium(n) hydride (Scheme 11, A) will perform stepwise alkyne insertion, and that the resulting vinylruthenium will undergo transmetallation upon treatment with triethylsilane to regenerate the ruthenium(n) hydride and produce the (E)-f3-vinylsilane in a stoichiometric reaction. However, when the same complex is used to catalyze the hydrosilylation reaction, exclusive formation of the (Z)-/3-vinylsilane is observed.55 In the catalytic case, the active ruthenium species is likely not the hydride A but the Ru-Si species B. This leads to a monohydride silylmetallation mechanism (see Scheme 1). More recently, small changes in catalyst structure have been shown to provide remarkable changes in stereoselectivity (Scheme ll).56... 

The current mechanistic understanding of these reductive cyclization processes is largely conjecture. Stepwise oxidative addition, migratory insertion, and reductive elimination (see Scheme 26) is a widely proposed mechanism. However, other mechanisms - such as initial cyclometallation - are to afford a rhodacyclopentadiene followed by either oxidative addition to a rhodium(v) intermediate or (perhaps more likely) bond metathesis with an additional molecule of silane (Scheme 28). 

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