Migration, insertion

Hence there is no direct reaction between the methyl manganese unit and the newly incoming carbon monoxide. Later, when we discuss the activation of a coordinated substrate molecule toward nucleophilic attack, the latter will turn out to be an alternative for the insertion/migration process. In the reaction involving nucleophilic attack at the coordinated, activated, unsaturated substrate, the anionic fragment is an uncomplexed species. There is, as yet, no proven example of an insertion of an uncomplexed unsaturated substrate into a metal-carbon a bond. 

It has been suggested that intermolecular incorporation, i.e. oxidative addition and complexation of a substrate by a metal should be favored, intramolecular reactions, i.e. insertion, migration and deinsertion reactions should be invariant, and extmsion reactions such as reductive elimination or decomplexation should be disfavored by pressure [13], However, decomplexation reactions are in most cases ligand exchange reactions, which can proceed by associative mechanisms, and indeed, there is ample evidence that ligand exchange reactions can be accelerated by pressure [2]. 

Rates of insertion are controlled by several factors. Firstly, the insertion of an alkene to a Pd complex is faster when a cationic complex is used. The addition of a Ag salt to a chloro complex generates a cationic complex and hence the insertion is accelerated probably owing to facile coordination of the alkene. For insertion (migration), cis coordination is necessary. Thus the tran -acyl-alkene complex 21 must be isomerized to a rather unstable cis complex 22 to give the insertion product 23. Secondly coordination of a bidentate ligand forms the cis complex 24... 

The hydroliquefaction of coal in the Bergius process is not a simple reaction or process in fact, it involves a series of organometallic transformation, such as insertion, migration, and reductive elimination. Therefore, only a small part of the reaction scheme is displayed here to show a possible conversion. 

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

CO is a representative species for Q, -insertion its insertion into C—Pd bonds affords acylpalladium complexes such as 15. Mechanistically, the CO insertion is 1.2-alkyl migration to coordinated CO. This is an important step in carbonyiation. SO , isonitriies, and carbenes are other species which undergo a.a-insertion. 

Phenyldiazonium salts react with malonaldioxime to produce a 2-isoxazoline (7 IGEPl 920245), and the diazo ketone (484) when photolyzed gave a mixture of 2-isoxazoline and an isoxazole by a 1,5 carbon-hydrogen insertion. A phenyl migration was apparently not involved (Scheme 124) (66CC689). 

Replacement of halides with deuterium gas in the presence of a surface catalyst is a less useful reaction, due mainly to the poor isotopic purity of the products. This reaction has been used, however, for the insertion of a deuterium atom at C-7 in various esters of 3j -hydroxy-A -steroids, since it gives less side products resulting from double bond migration. Thus, treatment of the 7a- or 7j5-bromo derivatives (206) with deuterium gas in the presence of 5% palladium-on-calcium carbonate, or Raney nickel catalyst, followed by alkaline hydrolysis, gives the corresponding 3j3-hydroxy-7( -di derivatives (207), the isotope content of which varies from 0.64 to 1.18 atoms of deuterium per mole. The isotope composition and the stereochemistry of the deuterium have not been rigorously established. 

Benzonitrile with [(i -Cp )P W(CO)5 2] gives 82, the result of migration of the phosphorus atom, insertion of the nitrile moiety into the P-C bond and further C-H bond activation (01AGE3413). 

The next step is the insertion of a lattice oxygen into the allylic species. This creates oxide-deficient sites on the catalyst surface accompanied hy a reduction of the metal. The reduced catalyst is then reoxidized hy adsorbing molecular oxygen, which migrates to fill the oxide-deficient sites. Thus, the catalyst serves as a redox system. ... 

An approach for improving the response of conjugated polymcr/fullerene bilayer devices, which is based on an additional excitonic middle layer inserted into the D-A interface, was suggested by Yoshino el al. [94. In the middle layer light absorption produces electron-hole pairs, which migrate towards the interlace and... 

Compact, ready-prepared calomel electrodes are available commercially and find wide application especially in conjunction with pH meters and ion-selective meters. A typical electrode is shown in Fig. 15.1(h). With time, the porous contact disc at the base of the electrode may become clogged, thus giving rise to a very high resistance. In some forms of the electrode the sintered disc may be removed and a new porous plate inserted, and in some modern electrodes an ion exchange membrane is incorporated in the lower part of the electrode which prevents any migration of mercury(I) ions to the sintered disc and thus... 

Whereas exo-norbornene oxide rearranges to nortricyclanol on treatment with strong base through transannular C-H insertion (Scheme 5.11), endo-norbornene oxide 64 gives norcamphor 65 as the major product (Scheme 5.14) [15, 22]. This product arises from 1,2-hydrogen migration very little transannular rearrangement is observed. These two reaction pathways are often found to be in competition with one another, and subtle differences in substrate structure, and even in the base employed, can have a profound influence on product distribution. 

The formation of the tricarbonylchromium-complexed fulvene 81 from the 3-dimethylamino-3-(2 -trimethylsilyloxy-2 -propyl)propenylidene complex 80 and 1-pentyne also constitutes a formal [3+2] cycloaddition, although the mechanism is still obscure (Scheme 17) [76]. The rf-complex 81 must arise after an initial alkyne insertion, followed by cyclization, 1,2-shift of the dimethylamino group, and subsequent elimination of the trimethylsilyloxy moiety. Particularly conspicuous here are the alkyne insertion with opposite regioselectivity as compared to that in the Dotz reaction, and the migration of the dimethylamino functionality, which must occur by an intra- or intermo-lecular process. The mode of formation of the cyclopenta[Z ]pyran by-product 82 will be discussed in the next section. 

Alkynylcarbene complexes react with strained and hindered olefins yielding products that incorporate up to four different components by the formation of five new carbon-carbon bonds [15b]. This remarkable transformation is explained by an initial [2+2] cycloaddition followed by CO insertion. The resulting intermediate suffers a well precedented [1,3]-migration of the metal fragment to generate a non-heteroatom-stabilised carbene complex intermediate which reacts with a new molecule of the olefin through a cyclopropana-tion reaction (Scheme 85). 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

A related situation is found in the case of P-substituted cycloketones here, the electronic difference between the two a-carbons is almost insignificant, resulting in unselective migration upon chemical oxidation. BVMOs have a particularly different behavior, as they can influence the stereo- and/or regioselectivity of the biooxidation. In the latter case, the distribution of proximal and distal lactones is affected by directing the oxygen insertion process either into the bond close or remote to the position of the P-substituent. Consequently, a regioisomeric excess (re) can be defined for this biotransformation, similar to enantiomeric excess or diastereomeric excess values [143]. 

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