Metal atoms insertion reactions

The reactions for C-H, 0-H and S-H bond activation typically involve metal atom insertion reactions. In the presence of solution, the hydrogen that forms can be directly transferred into solution as proton. The site dependence for these reactions, which are at the heart of many electrochemical processes, may not be very strong. The reactivity of terraces, steps and kinks may be quite similar. This is different to the activation of the molecules over a metal in the gas phase, which is structure sensitive. The electrochemical behavior will, of course, be strongly dependent upon the potential. 

The mechanism involves a metal atom insertion into the O—H bond, thus resulting in the formation of an adsorbed metal—OH species (at the same or similar binding site) and a new metal—H bond. This is a classic bond activation process, which involves a significant stretch of the O—H bond in order to lower the antibonding ooh orbital to enable it to accept electron density from the metal. The reaction has been calculated by DFT to be endothermic by +90 kJ/mol over Pt(lll) surfaces with an activation barrier of +130 kJ/mol [Desai et al., 2003b]. 

Transition metal catalyzed insertion reactions offer a variety of alternate approaches for the preparation of heterocyclic rings, of which Heck reactions were utilised extensively to prepare rings with more than 6 atoms. At the end of this Chapter some examples of the use of insertion reactions in the formation of the carbacyclic part of condensed heterocyclic systems will also be presented. 

Oxidative addition reactions lead to products that appear to have had a metal atom inserted into a bond, but the term insertion has generally been reserved for reactions which do not involve changes in metal oxidation state. These reactions are enormously important in catalytic cycles (see page 708). Special emphasis in this section... 

The reactions of Ca, Sr, and Ba with alcohols [43-45] are also different in that only the insertion channel is seen for both ground- and excited-metal atoms. The reaction intermediate is HMOR in all cases and the H atom leaves to give the MOR product. Very recently, the production of a small amount of CaOH was reported in the reaction of Ca with alcohols [50,51]. The bulky R group suppresses the production of the energetically favored MO + HR products. Under both single collision and multiple collision conditions the most important dynamical event is the insertion of an alkaline earth metal atom in an H—OR bond. 

It is presumed that the production of other monovalent derivatives such as CaNH2 or CaC5H5 from NH3 or C5H6 may involve similar excited metal atom insertions into N—H or C—H bonds. Unlike the reactions with H20 and alcohols, however, no studies of the dynamics have been carried out and even the thermochemistry is very uncertain. Clearly, more experimental and theoretical work is necessary before any firm conclusions can be drawn about mechanisms. 

Almost every metal atom can be inserted into the center of the phthalocyanine ring. Although the chemistry of the central metal atom is sometimes influenced in an extended way by the phthalocyanine macrocycle (for example the preferred oxidation state of ruthenium is changed from + III to + II going from metal-free to ruthenium phthalocyanine) it is obvious that the chemistry of the coordinated metal of metal phthalocyanines cannot be generalized. The reactions of the central metal atom depend very much on the properties of the metal. 

One other point to note in regard to this study (141) is that any evidence of oxidative addition, particularly with the chloro-olefins, was absent. The similarity of the spectra, coupled with the nonobservation of any bands in the visible region, as well as the observation of vc-c in the region commonly associated with 7r-complexation of an olefin (141, 142), all argue in favor of normal ir-coordination, rather than oxidative insertion of the metal atom into, for example, a C-Cl bond. Oxidative, addition reactions of metal atoms will be discussed subsequently. 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

The products are, in turn, starting materials for a rich chemistry, only superficially explored to date. The phosphorus atoms serve as Lewis bases toward metals. Coordination complexes with one or two tungsten atoms have been isolated (Eq. 34). They also readily undergo insertion reactions... 

Transition metal atoms react much more readily with alkenes than with alkanes because the initial interaction between the metal atom and an alkene is much less repulsive than for M+alkanes. To insert into a C-H bond of an alkane, the metal atom has to break a C-H bond and form an M-C and an M-H bond. The first step in a reaction with an alkene, however, is formation of a 7r-complex in which the C=C bond is merely weakened, not broken.119 The availability of the DCD bonding scheme (Sec. 1.1) leads... 

We have also observed competition between products resulting from C-C and C-H bond activation in reactions of Y with propene,138 propyne,143 2-butyric,143 four butene isomers,138 acetaldehyde,128 acetone,128 ketene,144 and two cyclohexadiene isomers,145 as well as for Zr, Nb, Mo, and Mo with 2-butyne.143 In this chapter, we use the term C-C activation to describe any reaction leading to C-C bond fission in which the hydrocarbon reactant is broken into two smaller hydrocarbon products, with one hydrocarbon bound to the metal. It is important to note, however, that C-C activation does not necessarily require true C-C insertion. As will be shown in this chapter, the reaction of Y, the simplest second-row transition metal atom, with propene leads to formation of YCH2 +C2H4. The mechanism involves addition to the C=C bond followed by H atom migration and C-C bond fission, rather than by true C-C insertion. 

In studies of vibrationally excited hydrocarbons with transition metal atoms to be carried out in our laboratory, reaction of the unpumped molecules cannot occur at collision energies below the C-H insertion barrier for v = 0. Thus, no background signal from unpumped molecules will be... 

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