Lutidine sites

Fig. 14 Plots of observed pseudo-first-order rate constants for the methanolysis of increasing and equimolar [La3 + ] = [32, HPNPP] at 25 °C and pH 5.0 (iV,jV-dimethylaniline buffer, , right axis) or pH 6.7 (2,6-lutidine buffer, , left axis). Lines through the data computed from fits to a standard one-site binding model. Reproduced from ref. 81 with permission.

Figure 35 Perspective view of the green (orthorhombic) form of [V0(lut)(H20)]-2H20 (H2lut = lutidinic acid). The V atom is six-coordinate in a distorted octahedral site which closely approximates C symmetry the remaining two water molecules (w) link the chelate molecules by hydrogen bonding. Some other intemuclear distances are...

Special care has to be taken, however, that the quinoline titer truly represents the minimum amount of catalyst poison. In most cases this type of base is adsorbed by inactive as well as active sites. Demonstration of indiscriminate adsorption is furnished by the titration results of Roman-ovskii et al. (52). These authors (Fig. 13) showed that introduction of a given dose of quinoline at 430°C in a stream of carrier gas caused the activity of Y-zeolite catalyst (as measured by cumene conversion) to drop with time, reach a minimum value, then slowly rise as quinoline was desorbed. The decrease in catalytic activity with time is direct evidence for the redistribution of initially adsorbed quinoline from inactive to active centers. We have observed similar behavior in carrying out catalytic titrations of amorphous and crystalline aluminosilicates with pyridine, quinoline, and lutidine isomers. In most cases, we found that the poisoning effectiveness of a given amine can be increased either by lengthening the time interval between pulse additions or by raising the sample temperature for a few minutes after each pulse addition. 

The external surfaces of H-FER [143-145], H-MFl [146, 147] and H-MOR [147, 148] have been studied by IR spectroscopy of adsorbed hindered aliphatic nitriles, pyridine [145, 146], lutidine and aromatic hydrocarbons [149]. Terminal silanols and Lewis acid sites exist at the external surface of H-FER, H-MFl and H-MOR. Interestingly, the acidity of the external silanol OH groups of zeolites can be enhanced with respect to those of silica, appearing similar to those of SA. The very... 

Figure 12.21 shows pyridine adsorption at 25°C on an alumina pre-treated at 450 C. Only Lewis sites are visible because the Bronsted sites are too weak to be shown by pyridine. The existence of two Lewis sites L] and L2 should be noted (bands at 1 622 cm and 1617 cm ) corresponding respectively to aluminium atoms in tetrahedral and octahedral coordination sites. Lutidine adsorption on the same alumina support is used to show the Bronsted sites (Fig. 12.22 and Table 12.4). 

According to this picture, the elimination reaction occurs from a distonic radical cation in which the positive charge is localized on the nitrogen atom and is maintained at this site throughout the reaction. Thus, the formal deprotonation step can be described as an hydrogen-atom transfer from the 9-methyl group to an incipient 2,6-lutidine radical cation. 

Such high selectivity induced by bromine on zeolite 5 A may be explainable by the idea that bromine is first activated to form Br with a OH site on zeolite SA, and thus the most active and less hindered para position of the aniline nucleus has dominant access to the Br that is located near a pore window of the zeolite, as an aniline molecule is too large to enter the pores of zeolite SA. It is interesting that the presence of organic bases such as pyridine or 2,6-lutidine not only improved the conversion, owing to neutralization of the generated HBr, but also increased the para-bromination selectivity. 

When both ortho sites on the pyridine substrate are blocked, activation of more remote C—H bonds is possible in the coordination sphere of (C5H5)2Zr(CH3)" (Scheme 6). The reaction of 3 with 2,6-lutidine results in activation of the methyl C—H bonds, leading to the four-membered metallacycle (C5Hs)2Zr( / -C,A -CH2(6-Me-pyridine)] (23), which is isolated free of coordinated THF. Similarly, the reaction of 3 with 2,3-benzo-... 

In order to more precisely differenciate the acid sites, adsorption of pyridine (pKa=5.25), 3,5-dimethylpyridine (pKa=6.15) and 2,6-dimethylpyridine (pKa=6.72) was carried out at 353 K on the samples. These three basic probes display a lower pKa than ammonia (pKa=9.25) and should titrate less weak acid sites. 2,6-lutidine (2,6-DMP) is supposed to adsorb on Bronsted sites preferently to 3,5-lutidine (3,5-DMP) which should adsorb, as pyridine, on both Lewis and Bronsted sites. This behavior can be explained by the steric hindrance due to the methyl groups, the nitrogen atom being less accessible. For example. Figure 4 shows the differential heats of adsorption of the three probe molecules on the sample with Ti=249 pmol/g pretreated at 773 K. All the curves show a sharp decrease till... 

The interaction of different nitriles (acetonitrile, pivalonitrile and benzonitrile), of branched aliphatic compounds (2,2-dimethylbutane, reri-butyl-alcohol, methyl-rert-butylether, methyl and dimethylcyclohexanes) and of aromatics (benzene, toluene, ortho-, meta- and para-xylene, pyridine, picolines and lutidine) has been studied over four different ZSM5 zeolites, over silicalite-1 and titanium silicalite-1 and over boralite BOR-C. Internal, external and extraframework sites have been characterized and the access to the cavities discussed. 

Similar results were obtained upon interaction with 2,6-lutidine, which was claimed to be more selective with respect to Bronsted acid sites. 

Similarly, Karge et al. [398] employed pyridine and 2,6-di-tert-butylpyridine to differentiate between internal and external acid sites of zeolite crystallites. In this respect, another possibility is the use of lutidine [139] or quinoline. The latter probe was employed by Corma et al. [693] for the determination of external Bronsted and Lewis acid sites of H, Na-Y and Al, Na-Y zeolites (cf. also [694,695]). For a characterization of the external Bronsted and Lewis acidity of ZSM-5 samples, Keskinen et al. [696] utilized as sufficiently bulky bases trimethylsi-lyldiethylamine and, like Karge et al. [398], 2,6-di-ferf-butylpyridine. For the discrimination of external from internal acid sites of shape-selective H-ZSM-5 catalysts,Take et al. [135] utilized pyridine andabulkytrialkylamine (e.g.,Et3N, n-Pr3N and n-Bu3N) as a pair of probes, with the former indicating the total amount of acid sites. For quantitative evaluation they determined the extinction... 

The unambiguous regioselective synthesis of chiral polysiloxane-containing CyDs as chiral stationary phases, with the mono-octamethylene spacer in either the 02, 03, or 06 position was performed and the products applied to enantio-selective GC separations [99]. Subtle differences in the chemistry of the hydroxyl groups at the 2-, 3-, and 6-positions of CyDs can be exploited to direct an electrophilic reagent to the desired site. Selective monoalkylation at the primary side of a-CyDs involves the reaction of a-CyD with 4-methylamino-3-nitrobenzyl chloride in 2,6-lutidine. Monoalkylation at the 2-position of j8-CyD is accomplished by the reaction of yS-CyD with l -bromo-4-methylamino-3-nitroacetophenone [100]. 

Although they are not as widely used as pyridine, substituted pyridines have found their own place in the characterization of hydroxyl groups. It was proposed (470) that, for steric reasons, 2,6-dimethylpyridine (DMP, lutidine) does not interact with Lewis acid sites and is thus a proton-specific probe. Later, it was demonstrated that DMP (247,256,471-473) (as well as 2,4,6-trimethyl pyridine or collidine (474)) stiU forms coordination bonds with coordinatively unsaturated surface cations. This bond is only weakened by the steric interference of the methyl groups with the surface but not prevented. In any case, the steric hindrance leads to preferential interaction ofDMP with hydroxyl groups (471-473). DFT calculations suggest that the proton transfer is promoted by the stabilization of the lutidinium ion on the deprotonated site, rather than by the intrinsic acidity of the acid site itself (475). 

Table 2.20 summarizes the spectral parameters of differently bound DMP. Compared to pyridine, lutidine seems to have some advantages as a probe. The most important aspect is that the spectra of protonated lutidine seem to contain information on the strength of the Bronsted acid sites. It was demonstrated (256), with a series of FAU zeolites, that the positions of the Vga and v(NH) bands (2975-2494 cm ) of DMPH depend on the acid... 

A widely used class of compounds are pyridine and alkylpyridines, including 2,6-lutidine, 2,4,6-coUidine, DTBP, quinoline, 4-methylquinohne, and 2,4-dimethyl-quinoline (565,582). These molecules interact with acidic hydroxyls through their N-atoms with formation of protonated species. Because they are strong bases, the possibility to extract protons from their original positions cannot be ruled out. An advantage (but in some cases disadvantage) of many pyri dines with bulky substituents is that, for steric reasons, they do not interact with Lewis acid sites through the N-atoms, or this interaction at least is hindered. 

In recent years the tentative to go deeper in the analysis of surfaces (in particular, but not exclusively, of zeolites) suggested the use of sets of molecular probes with similar functionality but different steric hindrance. As for example, different nitriles like those reported in Scheme 9.1 allow to explore the position of the sites in porous materials such as zeolites [82]. Similarly, hindered pyridines, such as 2,6-dimethylpyridine (lutidine, [83]) and 2,6-diterbutyl pyridine can be used [84]. The use of these molecules allow to show that some of the adsorption sites detected by CO and previously supposed to be in the interior of zeolite channels are actually out of the channels [85]. 

The IR spectmm of lutidine (DMP) adsorbed on p-AfF2.6(OH)o.4 and displayed in Figure 4.21, shows the presence of coordinated lutidine (z/ga at 1615, j/gb at 1590cm ) and lutidinium species (z/ga at 1652 and z/gb at 1630cm ). The latter bands indicate that surface Br0nsted acid sites are present. In agreement with this... 

To summarize, experiments with CO and lutidine, described above, both indicate that surface OH groups (KOH) at 3680 cm ) are present, which generate homogeneous Br0nsted acid sites with a medium acid strength (Ai/(OH) = 160cm, j/(CO) = 2173 cm ), able to protonate lutidine but not pyridine. 

Prins cyclizations, which proceed by intramolecular addition of alkenes to oxocarbenium ions, provide a simple, efficient method for the stereoselective synthesis of carbocycles and cyclic ethers [77]. Halosilanes and (la) have been used for Prins cyclizations not only as Lewis acids but also as heteroatom nucleophiles. For instance, in the presence of MesSil or MesSiBr, and lutidine, mixed acetals (26) are efficiently cyclized to 4-halotetrahydropyrans (27) with high diastereoselectivity [78]. The halide is introduced into the axial site of the C(4) position. The proposed mechanism for the MesSiBr-promoted reaction involves the initial formation of a-bromoethers (28) from (26). Solvolysis of (28) provides the intimate ion pair (29). Cyclization to the chair transition structure (30) and proximal addition of the bromide produces the observed axial adduct (27). The role of lutidine is to suppress a less selective HBr-promoted cyclization (Scheme 9.23). Acetals bearing an alkyne or allene moiety also undergo the halosilane-promoted cyclization to form haloalkenes [79, 80]. 

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