Insertion concerted

There are three general mechanisms for insertions concerted, free radical, and heterolytic addition. In the 1,2-insertion, the concerted mechanism proceeds via interaction of the 7t system of the unsaturated compound directly with the intact E-H bond, with each end of the n system directed at either the E or the H atom (Scheme 1). This interaction may or may not be preceded by precoordination of the unsaturated molecule to the element. The transition state for this reaction is considered to be four-centered, and yields products that are cis-substituted on the reduced unsaturated substrate. 

The stereoregulating capability of Ziegler-Natta catalysts is believed to depend on a coordination mechanism in which both the growing polymer chain and the monomer coordinate with the catalyst. The addition then occurs by insertion of the monomer between the growing chain and the catalyst by a concerted mechanism [XIX] ... 

The cyclohexylpyrazole (376) and the azlrlne (377) are formed by irradiation of 3-dlazo-4-methyl-5-phenylpyrazolenine (378) in cyclohexane (Scheme 35) (77JA633). The former is the result of carbene insertion into cyclohexane followed by a [1,5] hydrogen shift, whereas the latter arises by ring cleavage of nltrene (379) or by a concerted pathway. 

A particularly elegant route to metallacarbor-anes is the direct oxidative insertion of a metal centre into a c/oso-carborane cluster the reaction uses zero-valent derivatives of Ni, Pd and Pt in a concerted process which involves a nett transfer of electrons from the nucleophilic metal centre to the cage ... 

Generation of (excited state) singlet S in the presence of parafHns yields the corresponding mercaptan by a concerted single-step insertion ... 

Although the crystal structure of CODH or CODH/ACS has not yet been solved, a great deal of work has been done on these enzymes and plausible catalytic mechanisms have been proposed. Concerted action between the Ni ion and one of the Fe centers of a 4Fe-4S cluster are thought to elicit the formation of CO2 from CO. But perhaps the most extraordinary reaction is the one catalyzed by Cluster A the insertion of CO to a Ni-CHs complex. Through the two reactions catalyzed by CODH/ACS, the highly toxic, CO is not only removed, but is used as a source of carbon and electrons. 

Rh(OEP)H reacts with CNR (R = Me, n-Bu,) to give the adduct Rh(OEP)-(H)CNR (which has no parallel in CO chemistry) which then slowly transforms to the formimidoyl insertion product, Rh(OEP)C(H)=NR. The dimer Rh(OEP))2 reacts with CNAr (Ar = 2.6-Cf,H3Mc2) in aqueous benzene to give the carbamoyl product. Rh(OEP)C(0)NHAr (characterized by an X-ray crystal structure) together with the hydride, which it.self reacts further with the isocyanide. This is suggc.sted to form via a cationic carbene intermediate, formed by attack of HiO on coordinated CNAr in concert with disproportionation to Rh(III) and Rh(l). 

The mechanism that has been developed for the conversion of methane to methanol by FeO+ is an excellent example of the synergy between experiment and theory. This mechanism includes two key concepts concerted reaction involving the critical [HO—Fe—CH3] insertion intermediate and two-state reactivity. The reaction proceeds as follows electrostatic interaction between FeO+ and methane produces the [OFe- GHJ entrance channel complex. 

Leconte and Basset [161-166] proposed two other possible mechanisms (Scheme 39) the first one implies a 1,2 carbon-carbon activation which invokes the de-insertion of a methylidene fragment from a surface metal-alkyl species, and the second implies a 1,3 carbon-carbon bond activation in which the key steps are the formation of a dimetallacyle by y-H activation from a metal-alkyl followed by carbon-carbon bond cleavage via a concerted electron transfer. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

The dynamic olefin insertion process has been modeled using various quantum mechanical methods. A concerted four-center mechanism involving a frontal copla-nar attack of the C=C unit on the Zr-H bond of 1 is associated with a low activation energy of 0-15 kcal mol and has been proposed for the reaction of ethylene (Scheme 8-2) [37]. 

Intramolecular insertion and addition reactions are very rare for alkyl nitrenes. In fact, it is not clear that the nitrenes are formed as discrete species. The migration may be concerted with elimination, as is often the case in the Wolff rearrangement.251... 

Fig. 5. Possible mechanisms for the MMO hydroxylation step. Pathway A insertion of the oxygen atom of Q into the C-H bond B concerted addition of the C-H bond to Q followed by reductive elimination C, D homolytic attack of Q on the C-H bond E reaction of the peroxo species with substrate.

Like other metal reactions studied previously in our laboratory, H2 elimination is initiated by insertion into one of the C-H bonds forming HMC3H5. The reaction rate constant for Y + cyclopropane was found to be very small at room temperature, 0.7 x 10 12 cm3 s 1, and it was suggested that the reaction most likely involved termolecular stabilization of C-H or C-C insertion complexes, rather than molecular elimination.22 By analogy with other systems studied, the dynamically most favorable route to H2 loss in this case is likely via H atom migration to the Y-H moiety, with concerted... 

Methylene insertion into C—H bonds is believed to be concerted for the singlet species and stepwise for the triplet.<164,156) The C—H insertion of methylene into the 14C-labeled isobutylene shown below results in 92% unrearranged isopentenes and 8% rearranged isopentene [Eq. (11.22)]. Assuming that an additional 8% of the unrearranged isopentene arises from the stepwise addition, it is clear that 84% of the insertion products result from insertion by singlet methylene and 16% by triplet methylene ... 

The metalloalkyne complex Ru ( )-CH=CH(CH2)4C CH Cl(CO)(P,Pr3)2 exhibits behavior similar to that of cyclohexylacetylene (Scheme 10).40 Thus, it reacts with OsHCl(CO)(P Pr3)2 to give the hydride-vinylidene derivative (P Pr3)2 (CO)ClRu ( )-CH=CH(CH2)4CH=C OsHCl(CO)(P,Pr3)2, which evolves in toluene into the heterodinuclear-pi-bisalkenyl complex (P Pr3)2(CO)ClRu (is)-CH=CH(CH2)4CH=CH-( ) OsCl(CO)(P,Pr3)2. Kinetic measurements between 303 and 343 K yield first-order rate constants, which afford activation parameters ofAH = 22.1 1.5, kcal-mol-1 andAS = -6.1 2.3 cal-K 1-mol 1. The slightly negative value of the activation entropy suggests that the insertion of the vinylidene ligand into the Os—H bond is an intramolecular process, which occurs by a concerted mechanism with a geometrically highly oriented transition state. 

Whether the reaction proceed by the concerted insertion into the C—H bond as in [1 ] or by abstraction and recombination as in [2] depends on whether the carbon atom of the attacking methylene impinges on the C—H bond or on the hydrogen atom to be transferred ... 

A site-inversion mechanism (the key feature of which is that isomerization between diastereomeric and A configurations is rapid on the propylene-insertion time scale) based on theoretical calculations was proposed by Cavallo and coworkers in order to explain the ligand-directed chain-end controlled polymerizations (Fig. 35) [42]. The site-inversion mechanism allows chain-end control to work in concert with the site control effects. Our experimental results and the expected catalytic behavior resulting from the site-inversion mechanism concur with each other very well. 

In the ZSM-5 case, production of the carbene and insertion into the C—H bond is thought to be a concerted bimolecular process ... 

The reaction of carbenes with alcohols can proceed by various pathways, which are most readily distinguished if the divalent carbon is conjugated to a tt system (Scheme 5). Both the ylide mechanism (a) and concerted O-H insertion (b) introduce the alkoxy group at the originally divalent site. On the other hand, carbene protonation (c) gives rise to allylic cations, which will accept nucleophiles at C-l and C-3 to give mixtures of isomeric ethers. In the case of R1 = R2, deuterated alcohols will afford mixtures of isotopomers. 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

Although concerted C=C and C-H insertion into benzene by methylene is possible, several experiments on the effect of various solvents on the product ratio suggest the intermediacy of a complex between methylene and benzene.67-69 Based on the obtained kinetic and thermodynamic data, the transient is believed to be a complex formed between singlet methylene and benzene. 

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... 

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