Bond-activation sequences

Table n. Summary of Bond Activation Sequences for C2 and C3 Oxygenates on Rh(l 11)... 

Reactant Volatile Hydrocarbon Product Observed Adsorbed Hydrocarbon Product Observed Proposed Ligand Eliminated Proposed Bond Activation Sequence (Greek letters designate C-H bonds at that position)... 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

Enzymes are proteins, i.e. sequences of amino acids linked by peptide bonds. The sequence of amino acids within the polypeptide chain is characteristic of each enzyme. This leads to a specific three-dimensional conformation for each enzyme in which the molecular chains are folded in such a way that certain key amino acids are situated in specific strategic locations. This folded arrangement, together with the positioning of key amino acids, gives rise to the remarkable catalytic activity associated with enzymes. 

In spite of its formal similarity to the above mentioned annulation processes, the reaction shown in 4.37. includes a unique migration step. Oxidative insertion of the palladium into the phenyl-iodine bond is followed by the migration of the palladium onto the more electron rich indole ring. The 2-indolylpalladium complex than carbopalladates the pendant alkync moiety and the process ends by the formal activation of a C-H bond of the phenyl substituent and subsequent reductive elimination, furnishing the pentacyclic product.48 The same strategy has been utilised in the preparation of the indoloindolone framework from /V-bcnzoyl-3-(o-iodophcnyl)-indolc in an oxidative addition - palladium migration - C-H activation sequence.49... 

A two-step mechanism for catalysis is widely accepted (1) adsorption of the monomer, which may be activated, with the configuration established in this step, and (2) insertion of the activated monomer into a metal-carbon bond. This sequence places Ziegler-type polymerization in the context of what Nature accomplishes with enzymes. 

Each PIR entry consists of Entry (entry ID), Title, Alternate names, Organism, Date, Accession (accession number), Reference, Function (description of protein function), Comment (e.g., enzyme specificity and reaction, etc.), Classification (superfamily), Keywords (e.g., dimer, alcohol metabolism, metalloprotein, etc.), Feature (lists of sequence positions for disulfide bonds, active site and binding site amino acid residues, etc.), Summary (number of amino acids and the molecular weight), and Sequence (in PIR format, Chapter 4). In addition, links to PDB, KEGG, BRENDA, WIT, alignments, and iProClass are provided. 

Of the ten different C2 and C3 oxygenates examined on the Rh(l 11) surface to date, at least five and as many as seven different hydrocarbon ligands must be involved as initial elimination products to explain the diversity of their behavior in TPD and HREELS experiments. A summary of these observations and proposed sequences of bond activation is contained in Table II the logic behind this proposal is described below. 

The activation of sp2- or sp-hybridized CH bonds (for reviews on metal-mediated CH bond activation in catalytic processes see [132-137]) generating an organopalladium species suitable for subsequent transformation is the key step for sequences initiated by CH activation. 

Via a Tandem Allylic C-H Bond Activation-Elimination Sequence. 152... 

Replacement of a trialkyltin group with the acetoxy grotq) using LTA in CH2G2 proceeds with acceptable yields for 0-activated, allylic and vinylic C—Sn bonds. These sequences may involve intermediate organolead triacetates (RPbOAcs), which demetallate to yield carixxiium ions (Scheme 7). ... 

LiCC04 and NaCC04 melt before anion breakdown. The induction periods to the decompositions of K, Rb and CsCC04 are attributed [4] to slow anion breakdown within the solid, followed by a more rapid reaction in the melt formed as a eutectic between reactant and products. Because the activation energies for these reactions are about 270 kJ mol , it was concluded from consideration of bond strengths that the rate-limiting step is the rupture of a CC-0 bond. A sequence of fiirther comparable dismption or dissociation reactions yields oxygen and the metal chloride as the final residual product. 

Many of the C-F activation reactions discussed in this account are complicated by multi-step reaction sequences and secondary reactions with solvent or fluoride ion generated in the course of the transformation. In catalytic systems, the additional reagents needed to achieve catalytic turnover may cause undesired side reactions to take place. Gas-phase ion molecule reactions provide one way to study model systems in the absence of these complicating factors to obtain fundamental information on the C-F activation process [84]. In addition, theoretical treatments of C-F bond activation are beginning to provide insight into these transformations. 

Failure to observe the PdCH intermediate in the first study is due to a delicate balance of the associated rate constants leading to an extremely low steady-state concentration of the PdCH intermediate. The sequence via Reactions (7.3) and (7.4) begins with C-I bond activation and is consistent with all experimental and computational findings (Schwarz et al. 1996b). The lesson from this study is that only the consequent improvement of the theoretical methods leads to a correct interpretation of the experiments. 

According to the mechanism, the active center is formed by the interaction of aluminum alkyl with an octahedral vacancy around Ti. For or-TiCls catalyst the formation of active center can be represented as shown in Fig. 9.3. To elaborate, the five-coordinated Ti on the surface has a vacant J-orbital, represented by -Q, which facilitates chemisorption of the aluminum alkyl followed by alkylation of the Ti " ion by an exchange mechanism to form the active center TiRCU-Q. The vacant site at the active center can accommodate the incoming monomer unit, which forms a r-complex with the titanium at the vacant inserted into the Ti-alkyl bond. The sequence of steps is shown in Fig. 9.4 using propylene as the monomer. 

Since platinum, rhodium, and ruthenium catalysts operate with similar activation energies, their differences in catalytic activity can be directly traced to differences in the A factor, which may be related to the % d-char-acter of the metal bond in the three metals above. Since the % d-character is 50, 50, and 44 for ruthenium, rhodium, and platinum, respectively (S), it is seen that this sequence is similar to that of the catalytic activity. During catalysis, the palladium surface becomes a chemical compound represented by various stages of interstitial hydride formation, whose d-charac-ter is essentially different from that of the metal. Therefore, the position of palladium in the % d-character sequence is not directly comparable to that of palladium in the catalytic activity sequence. 

The snRNPs are small ribonucleoprotein particles that occur in the nucleus. Each is composed of a small RNA molecule and several characteristic proteins, some of which are common to different snRNPs. Distinct snRNPs recognize and bind to splice junctions and the branch site and are involved in assembling the spliceosome in an ATP-depend-ent manner. They are requisite components of the splicing apparatus, and the RNA components of some of them are probably catalytically active. The RNAs of some snRNPs form hydrogen bonds with sequences within introns and exons to help to juxtapose properly the reacting splice junctions. 

Recent wide-spreading of the concept of atom economy in chemical synthesis, and economical and ecological requirements have fueled significant interest to selective oxidative functionalization of substrates with C-H and C=C bonds. Catalytic functionalization of CH bonds or olefin C=C bonds with a platinum(ll) or a palladium(ll) complex leading to products with new C-O bonds may represent a mechanistically more complex case as compared to the reaction sequence shown in Fig. 1. A C-H or C=C bond activation step by a transition metal complex leading... 

---