Insertion elimination

In the reaction of Q,/3-unsaturated ketones and esters, sometimes simple Michael-type addition (insertion and hydrogenolysis, or hydroarylation, and hydroalkenylation) of alkenes is observed[53,54]. For example, a simple addition product 56 to methyl vinyl ketone was obtained by the reaction of the heteroaromatic iodide 55[S5]. The corresponding bromide affords the usual insertion-elimination product. Saturated ketones are obtained cleanly by hydroarylation of o,/3l-unsaturated ketones with aryl halides in the presence of sodium formate, which hydrogenolyses the R—Pd—I intermediate to R— Pd—H[56]. Intramolecular hydroarylation is a useful reaction. The diiodide 57 reacts smoothly with sodium formate to give a model compound for the afla-toxin 58. (see Section 1.1.6)[57]. Use of triethylammonium formate and BU4NCI gives better results. 

The next step involves the generation of the new aUcene by P-hydride elimination, throngh an agostic interaction, and evolution to a hydride-paUadium complex. The calculated potential surfaces for the overall insertion-elimination process are quite flat and globally exothermic [11,15], Finally, the reductive elimination of the hydride-Pd(ll) complex, which is favoured by steric factors related to the buUdness of the iV-substituents on the carbene [13], provides the active species that can enter into a new catalytic cycle. 

Oxidative addition of molecular hydrogen was considered to be involved in the alkyne hydrogenations catalyzed by [Pd(Ar-bian)(dmf)] complexes (4 in Scheme 4.4) [41, 42]. Although the mechanism was not completely addressed, 4 was considered to be the pre-catalyst, the real catalyst most likely being the [Pd(Ar-bian)(alkyne)] complex 18 in Scheme 4.11. Alkyne complex 18 was then invoked to undergo oxidative addition of H2 followed by insertion/elimination or pairwise transfer of hydrogen atoms, giving rise to the alkene-complex 19. 

Insertion of phenyl, trimethylsilyl, and nitrile-stabilized metalated epoxides into zircona-cyclcs gives the product 160, generally in good yield (Scheme 3.37). With trimethylsilyl-substituted epoxides, the insertion/elimination has been shown to be stereospecific, whereas with nitrile-substituted epoxides it is not, presumably due to isomerization of the lithiated epoxide prior to insertion [86]. With lithiated trimethylsilyl-substituted epoxides, up to 25 % of a double insertion product, e. g. 161, is formed in the reaction with zirconacyclopentanes. Surprisingly, the ratio of mono- to bis-inserted products is little affected by the quantity of the carbenoid used. In the case of insertion of trimethylsilyl-substituted epoxides into zirconacydopentenes, no double insertion product is formed, but product 162, derived from elimination of Me3SiO , is formed to an extent of up to 26%. 

Once activated, the substrates are transformed via a number of different possible steps including ligand migration, insertion, elimination or extrusion, and external attack on bound substrate. Of these, the last is most easily envisioned—a reagent not coordinated to the metal center of the catalyst attacks the bound substrate whose coordination has rendered it chemically reactive. 

The addition of arylboronic acids to oxabicyclic alkenes yielded ring-opening products via insertion-elimination process.979,980 Chiral DPPF-P(/-Bu)2 was identified as the ligand giving the best reactivity and enantioselectivity (Equation (221)). 

One of the earliest examples of Pd-promoted vinyl substitution involved transfer of an aryl group from Hg to Pd, followed by insertion/elimination (equation 4). The general process requires reoxidation of the Pd for catalysis, and this is generally possible by using excess of Cu. While the method is applicable to alkyl Hg X reactants, it is limited to those alkyl groups that cannot undergo jS-hydride elimination after transfer to Pd, such as Me and alkoxycarbonyl. 

Hydrogen. The reaction of O ( D2) with H2 takes place on the ground state potential surface of water, HiOf Ai). On the basis of trajectory calculations, (Whitlock et al., 1982) it has been suggested that, as is true for the hydrocarbons, parallel mechanisms involving insertion/elimination and direct abstraction govern the course of this reaction. The observation using laser induced fluorescence spectroscopy (Luntz et al., 1979 Smith and Butler, 1980) of a highly excited, non-Boltzmann rotational distribution and a nearly statistical vibrational distribution for v" = 1 and v" = 0 is consistent with the insertion/elimination... 

This review illustrates the above delineated characteristics of electron-transfer activated reactions by analyzing some representative thermal and photoinduced organometallic reactions. Kinetic studies of thermal reactions, time-resolved spectroscopic studies of photoinduced reactions, and free-energy correlations are presented to underscore the unifying role of ion-radical intermediates [29] in—at first glance—unrelated reactions such as additions, insertions, eliminations, redox reactions, etc. (Photoinduced electron-transfer reactions of metal porphyrin and polypyridine complexes are not included here since they are reviewed separately in Chapters 2.2.16 and 2.2.17, respectively.)... 

Figure 8.19 shows the flux-time profiles obtained in filtration of 5% yeast cell suspension using a mbular membrane of 6 mm i.d. (inside diameter) and 0.14 pm pore size with a helical baffle (HB), a rod baffle (RB), and the mbular membrane without baffle (NB) [35]. The comparison has been made at the same hydraulic-dissipated power, which is defined as the product of the flow rate and the pressure drop along the mbular membrane, or the energy consumed to generate the crossflow through the mbular membrane. Using the hydraulic-dissipated power rather than the crossflow rate as a control parameter for the comparison of the mbular membrane with and without inserts eliminates the effect of the reduced crossflow section by... 

It has been suggested that the hydroplatinum(II) intermediate is formed [Eq. (101)], with insertion-elimination leading to isomerization 175, 126). The halogen acid is the source of reversibility and can be... 

The formation of heterobutadiene 9 can be explained by an insertion-elimination reaction Insertion of sulfur into the silicon-hydrogen bond yields in the first step (2-Me2NCH2C6H4)(CH=CH2)Si(SH)2 as intermediate. This species eliminates H2S, producing the 1-thia-2-sila-1,3-diene 9, which is one of the rare examples possessing a silicon-sulfiir double-bond unit... 

Facile insertion/elimination is demonstrated by the behavior of Tl[Mo(CO)3Cp]3, which results from the reaction of TlCp with Mo(CO)6... 

We saw in Chapter 6 that a reaction that is the reverse of 1,2-insertion— called 1,2- or (3-elimination—may also occur with metal alkyls, the product of insertion.13 Equation 8.8 details the insertion-elimination reaction that yields a vacant site for coordination in the forward direction (1,2-insertion) and requires a vacant site for the reverse ((3-elimination) pathway. 

On the other hand, the other process without involving the formation of r)3-allylic complexes may operate as an alternative route in the course of the C-O bond cleavage. One is the SN2 type attack of a ligand bound to the metal such as hydride, alkyl or alkoxide on the terminal carbon of the allylic entity. The process is followed by elimination of the OX group (acetate or alkoxide) in a concerted manner as shown in Eq. 4. The other mode of cleavage is insertion-elimination type as shown in Eq. 5. The process proceeds by insertion of the olefinic moiety of the allylic entity into the M-Y bond, such as hydride, alkyl, or alkoxide ligand followed by P-elimination of the acetate or alkoxide moiety. 

The other type of process of C-0 bond activation that is different from the direct oxidative addition of the C-0 bond to M(0) complexes to form r 3-allyltransition metal complexes is insertion-elimination type or SN2 type as shown in Eqs. 4 and 5. Although the two processes are conceptually different, it is sometimes difficult to distinguish the two mechanisms. When the insertion-elimination process... 

Operation of the insertion-elimination mechanism has been demonstrated in the reaction of rhodium hydride complex, RhHL4 (L=PPh3), with two isomeric allyl phenyl carbonates [56]. Unbranched 2-butenyl phenyl carbonate was found to give branched allylic phenyl ether exclusively, whereas the decarboxylation of the branched l-methyl-2-propenyl phenyl carbonate afforded unbranched 2-butenyl phenyl ether. These results can be accounted for by assuming a precata-lytic and catalytic insertion-elimination process as shown in Scheme 7. 

Cleavage of C-0 bonds has also been observed in the reactions of Pd(C6F5)Br(NCMe)2 with diallyl ether via insertion-elimination processes (Eq. 19) [57],... 

Mainly two alternative mechanisms are discussed for the linear dimerization and oligomerization of monoolefins catalyzed by transition metal systems an insertion-elimination mechanism via a metal hydride (alkyl) species " and metallacycle pathways " . 

Since most of the important insertion// -elimination processes affect molecules where A=B is a carbon-carbon double or biple bond, the discussion is organized considering brst the different types of M-X bond involved, and the insertion of alkenes or alkynes. Less abundant cases are discussed later. The reader should have in mind that sometimes the word alkene is used in a more general way, meaning an unsaturated carbon-carbon bond not involved in aromaticity. Also the word alkyl can be used in a more general context meaning hydrocarbyl. 

Thiolates. The thioaldehyde hydrido complexes [(C5Me5)2Ta(H)(j7 -S=CHR)] seem to be in rapid equilibrium with the corresponding 16-electron thiolate species [(C5Me5)2Ta(SCH2R)] through a yd-H migratory insertion/elimination process [100]. The low concentration of the latter precludes its direct observation. 

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