Surface neutral alumina

A brief exposure of diacetate derivatives of aromatic aldehydes to MW irradiation on neutral alumina surface rapidly regenerates aldehydes (Scheme 6.5) [36], The selectivity in these deprotection reactions is achievable by merely adjusting the time of irradiation. As an example, for molecules bearing acetoxy functionality (R = OAc), the aldehyde diacetate is selectively removed in 30 s, whereas an extended period of 2 min is required to cleave both the diacetate and ester groups. The yields obtained are better than those possible by conventional heating methods and the procedure is applicable to compounds bearing olefmic moieties such as cinnamaldehyde diacetate [36],... 

Keywords ketone, aldehyde, ethane-1,2-dithiol, neutral alumina surface, dithio-acetal, 1,3-dithiolane... 

The utility of recyclable alumina as a viable support surface for deacylation reactions is described by Varma and his colleagues [83] wherein the orthogonal deprotection of alcohols is possible under solvent-free conditions on a neutral alumina surface using MW irradiation (Scheme 2.2-23). Interestingly, chemoselectivity between alcoholic and phenolic groups in the same molecule has been achieved simply by varying the reaction time phenolic acetates are deacetylated faster than alcoholic analogues. 

Such a product was indeed used in logarithmic form as DPK [22-24], but in contrast to water, which mainly consists of neutral water species even in acidic and basic solution (i.e., [H20] = const), the neutral alumina surface phase is only stable near pH 7 [25]. This shown in Fig. 4, in addition to Fig. 1, for systems differing mainly in the BET surfaces. So DPK is a constant for pH = pzc only. But, as even under these conditions it is not certain that Q ioh = it is more reasonable to describe the system by using the equilibrium constants. 

Structure and mobility of adsorbed molecules may exhibit a wide spectrum of features. As a general view it is commonly accepted that layers are formed on increasing abundance. The mobility of molecules in the first layer depends strongly on interactions present in a given case. Local domains of adsorbed molecules may be formed in the case of non-uniform surface. Analysis of deuteron spectra for a series of molecules adsorbed on neutral alumina led to the conclusion that while the exchange of molecules between layers in a given domain is fast, the diffusion rate between domains is slow [5],... 

Alumina has been widely used in normal phase chromatography. Its surface exhibits amphoteric properties (available as basic, acidic, and neutral alumina) and can react as a cation or an anion exchanger. The surface area is in the usual range around lOOm /g, however, the pore structure is not very favorable as a relatively large portion of narrow pores (<2nm) is present. 

Clayfen (1.13 g, 1.2 mmol of iron(III) nitrate) is thoroughly mixed with neat thioacetal lb (0.227 g, 1 mmol) in the solid state. The material is transferred in a test tube and placed in an alumina bath inside the microwave oven and irradiated (40 s). Upon completion of the reaction, monitored on TLC (hexane-EtOAc, 8 2, v/v), the product was extracted into ethylene chloride. The resulting solution is passed through a small bed of neutral alumina. Evaporation of the solvent delivers pure p-nitrobenzaldehyde 2b in 97% yield. In the case of cyclic thio acetals and ketals, the liberated dithiols bind to the clay surface rather tightly and a simple washing of the clayfen affords clean products. 

Solvent-free conditions can also be applied without microwave assistance. In [24] reactions of o-PDA with several substituted chalcones were reported to have been carried out on neutral A1203 at 80°C with high yields (60-80%) of the target dihydrobenzodiazepines. Besides diazepines, some samples of benzothia-zepines 21 were also synthesized by the treatment of o/Y/70-aminothiophenol 19 with oc,(3-unsaturated ketones 20 on a silica gel surface (Scheme 4.6). The application of other solid supports like acidic, basic or neutral alumina,... 

Alternative approaches have been proposed over the years. In none of these cases is there sufficient experimental evidence for truly heterogeneous catalysis. Frechet et al. (55) used a polyvinylpyridinium (PVP) material for supporting chlorochromate [Cr(IV)02Cl ] or dichromate [CrCVI Oy-]. Cr3+ can be immobilized by simple ion exchange on polymers such as Nation or on a Y zeolite (59, 60). However, it is doubtful whether these methods ensure complete Cr anchoring when the material is brought into contact with oxidants. Clark et al. (61) advocated the use of alumina-anchored dichromate. Particularly when a neutral alumina is used, surface-anchored species are formed ... 

The total surface concentration and intensity distribution of acidic and basic active sites are presented in Fig. 7.10. The total height of the stacked bars represents the total surface concentration of the acidic and basic active sites in millimoles per gram. The individual parts of the stacked bar correspond to the intensity distribution. As shown in Fig. 7.10, these data indicate that magnesium silicate has a total acidic and basic site concentration of 1.8 and 2.3 mM/g, respectively [17]. In comparison with other types of adsorbents used in frying oil (activated carbon, alumna [basic], alumina [neutral], alumina [acidic], bleaching earth, dia-tomaceous earth, and silica), magnesium silicate shows the highest values of total acidic and basic sites. 

A solution of ethyl diazoacetate (10.41 g, 0.100 mol) in anhyd EtjO (30 mL) was slowly added with a syringe pump to a stirred solution of the catalyst (< 1.0 mmol) and the alkene (0.100 mol) in Et O (50 mL). Reactions were generally performed at 25 °C in air (reactions performed under did not noticeably result in improved yields). The needle of the addition syringe was placed just below the surface of the reaction solution, and Nj evolution commenced with each fractional addition of diazo compound. After addition of the first 15 mL of ethyl diazoacetate solution, the rate of addition was slowed to approximately one half of the initial rate. When addition was complete, the mixture was filtered through active, neutral alumina. The alumina was washed with EtjO (200 mL), and the EtjO in the combined filtrate was evaporated to a residual oil which was distilled under reduced pressure. The cyclopropanes so obtained contain < 3% of the formal carbene dimers, diethyl ( )- and (Z)-but-2-enedioate. Analytically pure samples were obtained by redistillation or by preparative GC. Cyclopropane isomers can be separated by GC on Car-bowax 20 M columns (Table 9). 

The use of alumina and silica to promote Diels-Alder reactions was first reported by Hudlicky [24]. The reactions of 1-acetoxy-l,3-butadiene with tetrafluoro-p-benzoquinone or / -benzoquinone were considerably accelerated when they occurred inside the pores of the adsorbents. Several other silica- or alumina-catalyzed Diels-Alder reactions, dealing either with investigation of standard model reactions [25] or of reactions of more synthetic utility, were subsequently described. For example, neutral alumina catalyzed the intramolecular reaction of a tetraene to the verrucarol skeleton [26] (Scheme 3). The catalytic action was observed in an attempt to isolate the tetraene by column chromatography. A variety of Diels-Alder reactions were accelerated by silica or the more basic Florisil (Mg0.Si02) in the absence of solvent [27-30]. Table 3 gives some examples of these reactions, which are thought to be catalyzed via multicentered donor-acceptor interactions of the substrates with the active sites of the surface of the adsorbents. 

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