Substrate dependence

We should stress that this difference only applies to processes in which palladium species have an undefined coordination shell (as in type 1 and type 2 systems). For true ligand-accelerated type 3 processes, which are controlled by a definite stable Ugand in the coordination shell, the difference between electron-rich and electron-poor ( inactivated versus activated ) substrates is indeed not dramatic, which allows for the formulation of broad-scope robust protocols. 

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The mechanism of the reaction is now well known due to a series of kinetic studies by Katritzky et al. (Table 31). The nature, free base or conjugate acid, of the substrate depends on the substituents in the pyrazole ring and on the acidity of the nitrating mixture. 

The spectra from strong oscillators have special features which are different from those from metallic and dielectric substrates. Different structures in tanf and A are observed on a metallic substrate, dependent on the thickness of the film (Fig. 4.65). For very thin films up to approximately 100 nm the Berreman effect is found near the position of n = k and n < 1 with a shift to higher wavenumbers in relation to the oscillator frequency. This effect decreases with increasing thickness (d > approx. 100 nm) and is replaced by excitation of a surface wave at the boundary of the dielectric film and metal. The oscillator frequency (TO mode) can now also be observed. On metallic substrates for thin films (d < approx. 2 pm) only the 2-component of the electric field is relevant. With thin films on a dielectric substrate the oscillator frequency and the Berreman effect are always observed simultaneously, because in these circumstances all three components of the electric field are possible (Fig. 4.66). 

Alternatively, if detachment is associated with a brittle failure, then one must first determine if the fracture followed an elastic loading where an elastic model such as the JKR theory is appropriate or if it follows a plastic or elastic-plastic loading. In this latter case, the force needed to detach the particle from the substrate depends on the specific properties of the materials and the details of the deformations [63]. 

Numerical solution of Eq. (51) was carried out for a nonlocal effective Hamiltonian as well as for the approximated local Hamiltonian obtained by applying a gradient expansion. It was demonstrated that the nonlocal effective Hamiltonian represents quite well the lateral variation of the film density distribution. The results obtained showed also that the film behavior on the inhomogeneous substrate depends crucially on the temperature regime. Note that the film exhibits different wetting temperatures on both parts of the surface. For chemical potential below the bulk coexistence value the film thickness on both parts of the surface tends to appropriate assymptotic values at x cx) and obeys the power law x. Such a behavior of the film thickness is a consequence of van der Waals tails. The above result is valid when both parts of the surface exhibit either continuous (critical) or first-order wetting. 

Alternatively, the pyruvic acid can first condense with the aldehyde. Addition of the aniline to the P-position of 18 provides the same intermediate (13), as above. The mechanism could be substrate dependent. 

Solladie-Cavallo has recently reported a two-step asymmetric synthesis of dis-ubstituted N-tosylaziridines from (R,R,R,Ss)-(-)-sulfonium salt 2 (derived from Eliel s oxathiane see Section 1.2.1.1) and N-tosyl imines with use of phosphazine base (EtP2) to generate the ylide (Scheme 1.42) [67], Although the diastereoselectiv-ity was highly substrate-dependent, the enantioselectivities obtained were very high (98.7-99.9%). The chiral auxiliary, although used in stoichiometric quantities, could be isolated and reused, but the practicality and scope of this procedure is limited by the use of the strong - as well as expensive and sensitive - phospha-zene base. 

The development of Sharpless asymmetric epoxidation (SAE) of allylic alcohols in 1980 constitutes a breakthrough in asymmetric synthesis, and to date this method remains the most widely applied asymmetric epoxidation technique [34, 44]. A wide range of substrates can be used in the reaction ( ) -allylic alcohols generally give high enantioselectivity, whereas the reaction is more substrate-dependent with (Z)-allylic alcohols [34]. 

An alternative process for the synthesis of vinylepoxides was clearly needed, so reactions with stoichiometric amounts of chiral sulfide were investigated (Scheme 9.16a) [74]. Indeed, when benzyl sulfonium salt 20 was treated with unsaturated aldehydes, the ees and des were high in all cases, whereas the yields [75] were highly substrate-dependent. The same products could be formed by treatment of an unsaturated sulfonium salt with benzaldehyde, but the yields and se-lectivities were generally slightly lower. 

Majetich and Hicks <96SL649> have reported on the epoxidation of isolated olefins (e.g., 61) using a combination of 30% aqueous hydrogen peroxide, a carbodiimide (e.g., DCC), and a mildly acidic or basic catalyst. This method works best in hydroxylic solvents and not at all in polar aprotic media. Type and ratios of reagents are substrate dependent, and steric demand about the alkene generally results in decreased yields. 

The variation of enantioselectivities with temperature and pressure was investigated. The effects of these two factors are very substrate dependent and difficult to generalize even in a single substrate serie. However, it seems that enantioselectivities are shghly better at 25-40 °C than at lower temperatures (0 °C or less). The stereoselectivity can be inverted for specific alkenes (formation of the S or R enantiomer preferentially). For several substrates, the reactions tend to proceed to completion with optimal ee s when performed at lower hydrogen pressure (2 bar) instead of 50 bar (Fig. 13). Pronoimced variation of enantioselectivities with hydrogen concentration in solution may indicate the presence of two (or even more) different mechanisms which happen to give opposite enantiomers for some substrates. 

We therefore have qualitative evidence for the dependence of the dewetting speed on the elastic properties of the substrate. Dependence of wetting on the elastic modulus was previously suggested in the case of thin substrates [31], It may be conjectured that cross-linking affects the surface properties of the elastomer and, therefore, wettability. However,... 

Years earlier, Nicholas and Ladoulis had found another example of reactions catalyzed by Fe2(CO)9 127. They had shown that Fe2(CO)9 127 can be used as a catalyst for allylic alkylation of allylic acetates 129 by various malonate nucleophiles [109]. Although the regioselectivites were only moderately temperature-, solvent-, and substrate-dependent, further investigations concerned with the reaction mechanism and the catalytic species were undertaken [110]. Comparing stoichiometric reactions of cationic (ri -allyl)Fe(CO)4 and neutral (rj -crotyl ace-tate)Fe(CO)4 with different types of sodium malonates and the results of the Fe2(CO)9 127-catalyzed allylation they could show that these complexes are likely no reaction intermediates, because regioselectivites between stoichiometric and catalytic reactions differed. Examining the interaction of sodium dimethylmalonate 75 and Fe2(CO)9 127 they found some evidence for the involvement of a coordinated malonate species in the catalytic reactions. With an excess of malonate they... 

The reactivity of these oxidants towards organic substrates depends in a rough manner upon their redox potentials. Ag(II) and Co(III) attack unactivated and only slightly activated C-H bonds in cyclohexane, toluene and benzene and Ce(IV) perchlorate attacks saturated alcohols much faster than do Ce(lV) sulphate, V(V) or Mn(III). The last three are sluggish in action towards all but the active C-H and C-C bonds in polyfunctional compounds such as glycols and hydroxy-acids. They are, however, more reactive towards ketones than the two-equivalent reagents Cr(VI) and Mn(VIII) and in some cases oxidise them at a rate exceeding that of enolisation. 

Kiihner S, L Wohlbrand, I Fritz, W Wruck, C Hultschig, P Hufnagel, M Kube, R Reinhardt, R Rabus (2005) Substrate-dependent regulation of anaerobic degradation pathways for toluene and ethylbenzene in a denitrifying bacterium strain EbNl. J Bacteriol 187 1493-1503. 

FIG. 9 The substrate-dependent changes in cathodic peak current (/p ) of CVs taken similarly to those in Figs. 5 and 7. 

E° and E2° values of +76 and +21 mV, respectively, have been measured for Hox from M. trichosporium OB3b by similar methods (63). These values are more closely spaced and imply that Hmv from this organism is thermodynamically less stable with respect to disproportionation. Addition of protein B lowered the potentials to -52 mV and -115 mV, respectively. The regulation of electron transfer to the hydroxylase with protein B and reductase observed with the M. capsulatus (Bath) MMO was not seen with this system. Instead, it was reported that the potentials of Hox and of Hox with added protein B are shifted slightly to more positive values in the presence of reductase (Table II), and the reduction was not substrate-dependent. 

As the divalent cation effect on CYP3A4 activity appears to be substrate dependent, the benefit of including MgCl2 (1-30 mM) in the oxidative incubation should be examined for each substrate. 

Direct aromatic substitution of unactivated aryl halides is slow and generally requires a catalyst to become a useful synthetic method. Copper reagents have been used in some cases in classical procedures for the formation of products from aromatic substitution. In many cases these copper-mediated reactions occur at high temperatures and are substrate dependent. Since the 1970s, transition metal catalysts have been developed for aromatic substitution. Most of the early effort toward developing metal-catalyzed aromatic substitution focused on the formation of... 

Upon completion of the synthesis of (+)-ll,ll -dideoxyverticillin A (1), it was recognized that there were only a handful of reports of monomeric epitri- or epitetrathiodiketopiperazine syntheses, nearly all of which were accomplished with lack of sulfide chain length control. In the limited cases where selectivity was achieved, the results were highly substrate dependent or the method lacked substrate scope [56],... 

Oxidized Fe-TAML could be the iron(V)oxo complex 6, which as noted above can be produced from la and m-chloroperox-ybenzoic acid at low temperatures (—60°C) in a nonaqueous solvent (51). Presumably such an iron(V)oxo complex can behave in a substrate-dependent way as both a two-electron or one-electron oxidant. In the former case, it is returned in one step to the iron(III) state. In the latter, it must first pass through an iron(IV) intermediate. At pH>12, the likely iron(IV) species would be the same compound as is formed from la and H202, (48) namely the iron(IV)-oxo complex 7, which has similar features with [(H20)sFeIV = 0]2 +, (54) or its water adduct 7". At other pHs, other iron(IV) compounds are known to be formed (48). Both the iron(V)-oxo and iron(IV)-oxo complexes as well as the other iron(IV) species could be involved in catalysis by 1 (see Section V.B). The possible involvement of complexes that are in a higher oxidation state than 6 cannot be ruled out. 

After the precursor stock solution is prepared, various techniques can be used to coat the substrate, depending on the solution viscosity, required film thickness and coverage. The most common methods in the semiconductor industry are spin- and dip-coating. Other processes that are used for deposition include spray coating and stamping. A summary of the uses, limitations, and advantages of the various thin film deposition methods is reported in Table 2.2. 

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