Cr , substitution

Plenary 21B. A C Albrecht et al, e-mail address aca7 comell.edu (I CRS) (substituting for plenary 21 A). Discusses four new applications using a third approach to the Class II spectroscopies (see above). Raman... 

As with nucleophilic additions and nucleophilic acyl substitutions, many laboratory schemes, pharmaceutical syntheses, and biochemical pathways make frequent use of carbonyl cr-substitution reactions. Their great value is that they constitute one of the few general methods for forming carbon-carbon bonds, thereby making it possible to build larger molecules from smaller precursors. We ll see how and why these reactions occur in this chapter. 

When an alkene reacts with an electrophile, such as HC1, initial addition of H+ gives an intermediate cation and subsequent reaction with Cl" yields an addition product (Section 6.7). When an enol reacts with an electrophile, however, only the initial addition step is the same. Instead of reading with Cl- to give an addition product, the intermediate cation loses the -OH proton to give an cr-substituted carbonyl compound. The general mechanism is showm in Figure 22.3. 

A particularly common cr-substitution reaction in the laboratory is the halogenation of aldehydes and ketones at their a- positions by reaction with Cl2, Br2, or I2 in acidic solution. Bromine in acetic acid solvent is often used. 

The Hell-Volhard-Zelinskii reaction is a bit more complex than it looks and actually involves substitution of an acid bromide enol rather than a carboxylic acid enol. The process begins with reaction of the carboxylic acid with PBr3 to form an acid bromide plus HBr (Section 21.4). The HBr then catalyzes enolization of the acid bromide, and the resultant enol reacts with Br2 in an cr-substitution reaction to give an cv-bromo acid bromide. Addition of water hydrolyzes the acid bromide in a nucleophilic acyl substitution reaction and yields the a-bromo carboxylic acid product. 

Several syntheses of l,3-dioxoperhydropyrrolo[l,2-c]imidazoles have been developed using different strategies. a-Substituted bicyclic proline hydantoins were prepared by alkylation of aldimines 135 of resin-bound amino acids with a,tu-dihaloalkanes and intramolecular displacement of the halide to generate cr-substituted prolines 136 and homologs (Scheme 18). After formation of resin-bound ureas 137 by reaction of these sterically hindered secondary amines with isocyanates, base-catalyzed cyclization/cleavage yielded the desired hydantoin products <2005TL3131>. 

Example calculation <-> "Cr substitution in the chromate anion. An example calculation on the chromate anion [Cr04] makes use of tabulated G and F matrix elements. 

Most of the catalytic interest in the AlP04-based molecular sieves have centered on the SAPOs which have weak to moderate Bronsted acidity, and two have been commerciahzed SAPO-11 in lube oil dewaxing by Ghevron and SAPO-34 in methanol-to-olefins conversion by UOP/Norsk Hydro. Spurred on by the success of TS-1 in oxidation catalysis, there is renewed interest in Ti, Co, V, Mn and Cr substituted AlP04-based materials, for a review of recent developments in the AlP04-based molecular sieves see [35]. 

The spectra of the doped materials (Cr, Ni, Zn +, Li+, Co +, AP+) are similar to those seen for the nominally stoichiometric materials, and sets of resonances between 500 and 700 ppm are seen on cation doping in addition to that of the normal spinel environment (at ca. 500 ppm). Again, these resonances are assigned to lithium ions near manganese-(IV) cations. The lower intensity of the additional resonances seen on Cr + substitution, in comparison to Zn + or Ni + substitution, is consistent with the oxidation of fewer manganese ions near the depart ions. For the Li- and Zn-doped spinels, resonances at ca. 2300 ppm were also observed, which are assigned to lithium ions in the octahedral sites of the spinel structure. In the case of Zn doping, it is clear that the preference of Zn + for the tetrahedral site of the spinel structure forces the lithium onto the octahedral site. 

Johannsenite, Ca(Mn,Fe )Si206, is a species in whieh p = 0. There is little additional trivalent ion (Al, Fe, or Cr ) substitution. The mineral is most commonly found as spherulitic aggregates of brittle fibers or prisms. Another pyroxene, acmite (NaFe Si206), has been found as large (100 X 35 X 20 cm) single crystals of prismatic shape, as needles, as felted aggregates of fine fibers, and as capillary crystals. 

Chromate adsorbed from solution by green rust was reduced to Cr with the green rust being simultaneously converted to ferrihydrite or possibly a Cr substituted ferrihydrite (Loyaux-Launiezak et al., 2000). 

At the third level, the most detailed partition of luminescence minerals is carried out on the basis of metals in the mineral formulae, hi rare cases we have minerals with host luminescence, such as uranyl minerals, Mn minerals, scheelite, powellite, cassiterite and chlorargyrite. Much more often luminescent elements are present as impurities substituting intrinsic cations if their radii and charges are close enough. Thus, for example, Mn + substitutes for Ca and Mg in many calcium and magnesium minerals, REE + and REE substitutes for Ca, Cr substitutes for AP+ in oxygen octahedra, Ee substitutes for Si in tetrahedra and so on. Luminescence centers presently known in solid-state spectroscopy are summarized in Table 4.2 and their potential substitutions in positions of intrinsic cations in minerals in Table 4.3. 

The colors are clear, yellow, orange, red, blue and green, while the main color centers are radiation induced. Violet and violet-red colors in Cr-containing topaz are generated by two absorption bands in the visible part of the spectriun, which are connected with Cr + substituting for AP". Yellow topaz besides Cr +... 

The color of spodumen can vary from pale pink for the kunzite variety to a deep green for the hiddenite variety, but colorless crystals also exist. The green color of hiddenite is connected with Cr substituting for AP+, while also takes part in optical absorption spectrum formation. The green-yellow color of spodumen is connected with Fe " in the Ml position and Fe " in the M2 position. Pink color of kunzite is connected with Mn " impurities in different structural positions Ml and M2 (Platonov et al. 1984). 

Chromium activated ruby was the first laser material and its luminescence properties are carefully studied. It is a classical example of Cr + in octahedral crystal field. Here Cr + substitutes the AP ions, while such a possibihty can be rationalized by an excellent chemical fit of Cr in place of Al. Ruby is a high crystal field material and thus the T2g state Hes above the E2g level. Pumping is accomplished by a spin-allowed transition into the state, while emission occurs from the level without vibrational broadening and almost all excited... 

A new synthesis of cr-substituted and a,a-disubstituted a-amino acid derivatives based on the ammonium ylide formation/[2,3]-sigmatropic rearrangement has been recently reported by Clark s group.Decomposition of a-diazo -keto ester 153 was studied in detail with Rh2(OAc)4, Cu(acac)2, and Cu(hfacac)2 as the catalyst. Cu(acac)2 and Cu(hfacac)2 gave similar results, but Rh2(OAc)4 turned out less effective (Equation (23)). 

Most ozonolysis reaction products are postulated to form by the reaction of the 1,3-zwitterion with the extruded carbonyl compound in a 1,3-dipolar cycloaddition reaction to produce stable 1,2,4-trioxanes (ozonides) (13) as shown with itself (dimerization) to form cyclic diperoxicles (4) or with protic solvents, such as alcohols, carboxylic acids, etc., to form cr-substituted alkyl hydroperoxides. The latter can form odter peroxidic products, depending on reactants, reaction conditions, and solvent. 

Intramolecular allylsilylation of alkynes is a convenient route to cyclic vinylsilanes.211 Propargylsilanes are available for Lewis acid-promoted carbosilylation of alkynes.212 Unlike allylsilanes, both cr-substituted and unsubstituted propargylsilanes react at the a-position, which can be rationalized by in situ isomerization of these reagents to allenylsilanes (Equation (55)). 

In the case of cyclic or cr-substituted tin enolates 358, it was found that E-enolates provide //-products 359, whereas the corresponding Z-tin enolates give syn-aldols (Scheme 105).315 Thus, the reaction seems to follow the classical Zimmerman-Traxler transition states 360/361 whereby the chiral silver catalyst activates the aldehyde. A possible alternative is a tin-silver exchange and formation of an analogous transition state of the silver enolate. 

Nevertheless, Tsushio et al. [34] reported the hydrogenation properties of MgNio.86Mlo.o3 (Ml = Cr, Fe, Co, Mn) alloys. Using the van t Hoff equation based on the desorption plateaus and the standard entropy of hydrogen gas 130.9 J mol they determined an enthalpy of hydride formation of the Cr-substituted alloy of —50kJ (mol H2), which is, for Mg-based alloys, a very high value. 

Few redox studies with cubic mesoporous materials have been reported [52]. The large, complex, three-dimensional pore system offers a unique environment. Ti- and Cr-substituted MCM-48 have been studied for the selective oxidation of methyl methacrylate and styrene to methyl pyruvate and benzaldehyde, respectively, using peroxides as oxidants and were found to outperform TS-1. Ti-MCM-48 has also been found to be better than Ti-MCM-41, TS-1 and Ti02 for the photocatalytic reduction of CO2 and H2O to methane and methanol. Ti-grafted MCM-48 has also been reported as the first functional biomimic of vanadium bromoperoxidase, active at neutral pH and used in the peroxidative halogenation of bulky organic dyes. 

Chemical analysis for Fe and other cations requires total dissolution of the oxides. This is usually achieved by treating the oxides (tenths to hundreds of mg) in cone. HCl (e.g. 0.1 g in 10-20 ml of 6-12 M HCl) at an elevated temperature (60-80 °C). The time needed for complete dissolution is normally less than one hour, but may be longer for crystals >0.5 pm. It may also be considerably longer for Cr-substituted Fe oxides (Schwertmann et al., 1989). 

A carbonyl condensation reaction takes place between two carbonyl partners and involves both nucleophilic addition and cr-substitution steps. One carbonyl partner (the donor) is converted by base into a nucleophilic enolate ion, which adds to the electrophilic carbonyl group of the second partner (the acceptor). The donor molecule undergoes an a substitution, while the acceptor molecule undergoes a nucleophilic addition. 

We shall consider in details how the above described approach can be applied to Cr ion doped in KMgF3 crystal, at perfect octahedral site symmetry [35]. After doping, Cr substitutes for Mg " " ions at the center of an octahedron formed by six fluorine ions. The Cr " — F distance is 1.995 A [36]. We do not discuss here the charge compensating mechanisms required to maintain electrical neutrality of the samples, but, instead, focus on the electronic and optical properties of the [CrFe] " units. The DFT-based treatment of the defects related to the doping and their impact on the JT effects was given in [37]. 

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