Adjacent Basic Sites

Dibromo-9,10-dihydro-9,10-diindaanthracene (52) contains two electrophilic indium centers positioned at the opposite apexes of a flat six-membered ring (Scheme 28). The specific arrangement of the two indium centers in this molecule indicates that the cooperative binding of bifunctional bases with adjacent basic sites might be attainable. 

In view of these observations it would seem sensible that the influence of adjacent superficial antimony and tin ions should also be considered in terms of likely mechanisms. Immediately one would recall the suggestion (72) that the catalytic properties may be related to the blue color of the material, which has been associated with a possible Sb -Sb charge transfer process. Such an association may then be related to the kinetics of butene oxidation, which have been interpreted in terms of the formation of allylic intermediates at active centres containing Sn and Sb ions. Indeed, McAteer (76) has suggested that these active centers have acidic and basic functions and consist of surface oxide ions of different electron density as determined by the coordinated cations. McAteer described the pattern of selectivity for the formation of butadiene and a-ketone according to the depiction in Fig. 7a. The initial step was postulated as the formation at an acid center of a positively charged allyl ion which is ti or a bonded at an adjacent basic site. The formation of butadiene was attributed to proton abstraction from the zr-allyl intermediate, its facile desorption at surfaces... 

The Effect of an Adjacent Basic Site Diarylpiperidones are formed in good to excellent yield by the reaction of piperidones with arene in TfOH (Scheme 2.19) [44], while no product was detected from the reaction of cyclohexanone with benzene in the presence of TfOH. These results suggested that protonation of a strong, adjacent base site can activate an electrophilic functional group such as... 

The kinetics outlined above, first observed empirically by Giintherschulze and Betz, were modelled by Verwey" with the rate-controlling energy barrier being that between to adjacent cation sites within the oxide film. The same basic form can be derived if the rate-controlling energy barrier is that between a metal atom on the metal surface and an adjacent cation site in the film. The rate is then limited by ion injection into the film rather than... 

Extended p-p n bonding between the basic site and an adjacent tt system (resonance) delocalizes the electronic charge. 

This is the case as shown in Table 1, runs 4,5, provided that basic sites are free from adsorbed H20 and C02 (compare runs 3 and 4, Table 1). Notably, once again the ratio hexan-3-one/hexen-3-ols appears unaffected by the catalyst precursor and/or pretreatment (runs 3,4, Table 1). These observations suggest that in the transfer hydrogenation of 4-hexen-3-one, the substrate is coordinated on a weak acid site while propan-2-ol must be coordinated on an adjacent surface basic site [7,24]. This is confirmed by the lack of reduction products observed over Mg(OH)2 and MgCl2 (runs 8,9, Table 1). 

Figure 9.13. The acetaldehyde formation mechanism, where A and B are Lewis acid sites and Bronsted basic sites, respectively. Dehydration requires the combination of an acid and strong base site with an adjacent strong basic site. After Di Cosimo et al. [184].

Calculations for the CO2 adsorption show that, on the naked surfaces, CO2 has to be considered as a basic species that binds to the exposed metallic cation[38, 39]. On Ti02 (110) CO2 is perpendicular to the surface building a OCO...Ti bond while on MgO(lOO) CO2 is flat and parallel to the surface bridging two adjacent Mg sites[24, 40]. The adsorption energies are nearly equal (7 kcal/mol. vs. 6.3 kcal/mol.) on MgO and Ti02- On MgO (100), it is an interaction between soft reactants and two bonds are built. 

Prerequisite coordination of the Lewis basic site to Ru complexes is known to promote bond-deavage reactions via a so-called metallation process (Scheme 14.25) [61]. For example, the pyridinomethyl group effectively guides the C-O bond-cleavage reaction of the adjacent ester group. 

Huang and Kaliaguine interpret spectra such as those in Figure 10 to mean that the strongly basic sites in alkali metal exchanged zeolites are framework oxygen atoms immediately adjacent to the alkali metal cations, acting as Lewis bases. Since the v(NH) frequency of pyrrole adsorbed on MgO surfaces is at 3320 cm, those zeolites for which the... 

Lewis acid catalysts increase the reactivity of dienophiles in Diels-Alder reactions by complexing to basic sites on the dienophile. ° The Lewis acid lowers the LUMO of the adjacent ir-system, which strengthens the overlap between the LUMO of the dienophile and the HOMO of the diene. In 1979 Scheeren reported that ZnCh catalyzes the cyclocondensation reaction of unactivated aldehydes with l-methoxy-3-(trimethylsilyloxy)-l,3-butadiene. Experimental details of this reaction, however, were not fully documented. In 1982 Scheeren also reported the use of aluminum alkoxydichlorides as catalysts... 

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