Catalytic intercept

There are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

The relationship between 9 and its predecessor 10 is close. Oxidation of the allylic C-3 methylene group in 10 and elimination of the methoxy group could furnish enone 9. Retrosynthetic disassembly of ring E in 10 furnishes tertiary alcohol 11 as a viable precursor. That treatment of 11 with a catalytic amount of acid will induce the formation of a transient oxonium ion at C-12 which is then intercepted by the appropriately placed C-4 tertiary hydroxyl group is a very reasonable proposition. As we will see, the introduction of the requisite C-4 hydroxyl group is straightforward from intermediate 12. 

Quantitative information can be drawn from such plots. For the a-th order kinetics the slope is the reaction order a and the intercept is In k. For the catalytic reaction considered above with the surface reaction as the rate-limiting process, linearization of the rate equation (5.4-112) leads to ... 

Lamaty and coworkers described a straightforward combination of three Pd-cata-lyzed transformations first, an intermolecular nucleophilic substitution of an al-lylic bromide to form an aryl ether second, an intramolecular Heck-type transformation in which as the third reaction the intermediate palladium species is intercepted by a phenylboronic acid [124]. Thus, the reaction of a mixture of 2-iodophenol (6/1-253), methyl 2-bromomethylacrylate 6/1-254 and phenylboronic acid in the presence of catalytic amounts of Pd(OAc)2 led to 3,3-disubstituted 2,3-di-hydrobenzofuran 6/1-255 (Scheme 6/1.66). In addition to phenylboronic acid, several substituted boronic acids have also been used in this process. 

The catalytic cycle with Ni catalysts is generally similar. The essential difference is the deactivation process, which in this case occurs not via the formation of a precipitate of Ni°, but rather due to interception of the highly reactive Ni° species by any fortuitous oxidant, such as oxygen. As Ni11 is not so easily reduced to Ni° as Pdn is to Pd°, Ni-catalyzed systems often require the addition of a stoichiometric reducing agent (Zn, DIBAL-H, other hydride transfer agents, BuLi, etc.). 

If hydrogen gas is added to the reaction mixture of J, and 11 the hydrogenolysis reaction of thorium-to-carbon sigma bonds (J-1 22) allows interception of species 13 and thus, catalytic hydrogenation of the inserted carbon monoxide functionality. At 35 C under 0.75 atm initial H2 pressure with [JJ =9.0 x 10" M and [ 1JJ = 6.5 x 10" M, hydrogenation and isomerization are competitive and both the enolate and the alkoxide reduction product 14 are produced (eq.(13)). Under these conditions, turnover fre-... 

The proposed catalytic cycle of the ruthenium-catalyzed intermolecular Alder-ene reaction is shown in Scheme 21 (cycle A) and proceeds via ruthenacyclopentane 100. Support for this mechanism is derived from the observation that the intermediate can be trapped intramolecularly by an alcohol or amine nucleophile to form the corresponding five-or six-membered heterocycle (Scheme 21, cycle B and Equation (66)).74,75 Four- and seven-membered rings cannot be formed via this methodology, presumably because the competing /3-hydride elimination is faster than interception of the transition state for these substrates, 101 and 102, only the formal Alder-ene product is observed (Equations (67) and (68)). 

Several factors and observations support the route proposed in Scheme 10 (1) Due to steric factors, the styrenyl alkene is expected to react preferentially (versus the neighboring disubstituted cyclic olefin see below for further discussion). (2) Involvement of tetracyclic intermediates such as 43 provides a plausible rationale for the reluctance of six-membered ring ethers [46 in Eq. 4] to participate in the catalytic rearrangement and for the lack of reactivity of cyclopen-tenyl substrates [48 in Eq. 5] because of the attendant angle strain, the generation of the tetracyclic intermediate is not favored. (3) Reactions under ethylene atmosphere inhibit dimer formation, since 44 is intercepted with H2CCH2, rather than 41 [19]. 

When the kinetic parameter, kr°S/DA becomes very large (i.e., when the chemical process overcomes substrate diffusion), the intercept tends toward zero. The determination of kr° becomes subsequently less and less precise and ultimately impossible. However, the potential location of the wave remains sensitive to the rate of the catalytic process as illustrated in Figure 4.11. When the plateau current has reached its... 

In zone R, all three phenomena that take place in the film are fast compared to the diffusion of the substrate from the bulk of the solution to the film-solution interface. The concentrations of both Q and A are constant through the film. The RDEV response is similar to that of a monolayer coating (Section 4.3.2), except that more catalytic material is present on the surface of the electrode (it is multiplied by the number of layers in the multilayered coating). A linear Koutecky-Levich plot is obtained from the intercept, from which the kinetics of the catalytic reaction can be characterized. 

The catalytic reaction conditions required some optimization. This was due to competing reaction pathways. The interception of trans-11 results in the formation of the organotitanium intermediate 44, as shown in Scheme 17. Thus, 2 equiv. of Cp2TiCl are consumed and a complete conversion in the presence of 10 mol% Cp2TiCl2 cannot be achieved because catalyst regeneration is prevented. Similar considerations apply for czs-11. 

Depressed catalytic activity occurring when an inhibitor binds more than once to a single enzyme form (or forms). While standard double-reciprocal plots are usually linear, secondary replots of the data (i.e., plots of slopes and/or intercepts vx. [I], the concentration of the inhibitor) will be nonlinear depending on the relative magnitude of the [I], [If,. .., and [If terms in the rate expression. 

The early stages in the chain growth process have been mimicked by Braunstein with the use of a Pd-Me complex supported by an acetamido-derived P-O ligand. Four catalytic intermediates were intercepted by the sequential addition of CO-ethene-CO-ethene, and the occurrence of reversible and irreversible steps was established (Scheme 7.9). Unlike diphosphine ligands [10], the insertion of ethene into the y-chelate acyl complex was found to be a facile process occurring without the intervention of CO [25]. 

Aldol condensation. Anhydrous lithium iodide (ca. 5 equivalents) promotes aldol condensation of ketones with enolizable or nonenolizable aldehydes. The intermediate aldol is usually not isolable, but can be intercepted by addition of ClSi(CH3)3 and N(C2H5)3. In this case Lil can be used in a catalytic amount. The salt cannot be replaced by LiBror LiCl or Nal. 

Interception by various antioxidants of superoxide anions generated by the catalytic action of xanthine oxidase in converting xanthine to uric acid. 

An ESI mass spectrometer coupled online to a microreactor was used to intercept the catalytically active cationic intermediates of the Ziegler-Natta polymerization of ethylene with the homogeneous catalyst system [Cp2Zr(Me)Cl]-MAO (MAO = methylaluminoxane). For the first time these intermediates were studied directly in the solution and their catalytic activity proved.60... 

Ma and co-workers have reported the selective synthesis of pyrrolidine derivatives through a three-component reaction based on a conceptually related strategy (Scheme 8.29) [72], Beginning with the catalytic intermolecular carbopalladation of y-allenic malonate 57 in the presence of a base, they succeeded in intercepting the internal carbonucleophile 58 with an imine such as the N-benzylidene p-toluenesulfonamide 59. The attack of the newly formed heteronucleophile on the 7r-allyl palladium intermediate affords the functionalized pyrrolidine 60 with high... 

It is worthy to note here that the methylidene complex 11 is a poor initiator for olefin metathesis reactions at room temperature. Although this complex can undergo multiple catalytic turnovers, if it is intercepted by free phosphine ligand, it becomes incapable of reentering the metathesis catalytic cycle.32... 

The kinematic viscosity of the solution, v, should not be confused with the stoichiometric coefficient of j, v, [cf. eqn. (LIII)]. A plot of l/ucat against 1 t Jf will therefore be a straight line. Its intercept will yield a value for ucatx, the surface-controlled catalytic rate, which is given by eqn. (89). 

Since both starting organometals and products in carbopalladation are organopalladium derivatives (Schemes 1 and 2), the process of carbopalladation can, in principle, repeat itself as exemplified in the reaction shown in Scheme 4. Thus, unless intercepted by some C - Pd bond cleaving process, this living process will stay alive, and no catalytic process will result. In both Heck and Maitlis reactions, the carbopalladation steps are spontaneously followed by /1-dehydropalladation and dechloropalladation, respectively. In cases where no such process occurs spontaneously, some processes must be deliberately devised usually through addition of appropriate reagents. In addition to dehydropalladation and dechloropalladation shown in Schemes 3 and 4 as well as in Eqs. 1 and 2 in Scheme 5, several other reactions used for this purpose are exemplied with prototypical cases of catalytic cyclic carbopalladation shown in Scheme 5 [9-19]. 

Gases stronger than 13 volume% S02 will always give intercept temperatures in the catalyst degradation range. They must be diluted with air before they are catalytically oxidized. 

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