Alcohols model substrates

Fungal cutinase catalyzes hydrolysis of model substrates and in particular p-nitrophenyl esters of short chain fatty acids, providing a convenient spectro-photometric assay for this enzyme activity [101,102,116]. Hydrolysis of model esters by this cutinase showed the high degree of preference of this enzyme for primary alcohol ester hydrolysis. Wax esters and methyl esters of fatty acids were hydrolyzed at low rates. Alkane-2-ol esters were hydrolyzed much more slowly than wax esters and esters of mid-chain secondary alcohols were not... 

The chemical diversity of carboxylic acid esters (R-CO-O-R ) originates in both moieties, i.e., the acyl group (R-CO-) and the alkoxy or aryloxy group (-OR7). Thus, the acyl group can be made up of aliphatic or aromatic carboxylic acids, carbamic acids, or carbonic acids, and the -OR7 moiety may be derived from an alcohol, an enol, or a phenol. When a thiol is involved, a thioester R-CO-S-R7 is formed. The model substrates to be discussed in Sect. 7.3 will, thus, be classified according to the chemical nature of the -OR7 (or -SR7) moiety, i.e., the alcohol, phenol, or thiol that is the first product to be released during the hydrolase-catalyzed reaction (see Chapt. 3). Diesters represent substrates of special interest and will be presented separately. 

Two of the commonest models are benzyl and cinnamyl alcohols - the former because it is easily oxidised beyond benzaldehde to benzoic acid and the latter because its double bond is often attacked, so that oxidation to cinnamaldehyde would show that the oxidant is mild enough to avoid competing double-bond attack. Geraniol is also included as a model substrate as it is in the same category as cinnamyl alcohol. Since there are so many examples of smdies on their oxidations a limited selection only is given. 

Our initial work on the TEMPO / Mg(N03)2 / NBS system was inspired by the work reported by Yamaguchi and Mizuno (20) on the aerobic oxidation of the alcohols over aluminum supported ruthenium catalyst and by our own work on a highly efficient TEMP0-[Fe(N03)2/ bipyridine] / KBr system, reported earlier (22). On the basis of these two systems, we reasoned that a supported ruthenium catalyst combined with either TEMPO alone or promoted by some less elaborate nitrate and bromide source would produce a more powerful and partially recyclable catalyst composition. The initial screening was done using hexan-l-ol as a model substrate with MeO-TEMPO as a catalyst (T.lmol %) and 5%Ru/C as a co-catalyst (0.3 mol% Ru) in acetic acid solvent. As shown in Table 1, the binary composition under the standard test conditions did not show any activity (entry 1). When either N-bromosuccinimide (NBS) or Mg(N03)2 (MNT) was added, a moderate increase in the rate of oxidation was seen especially with the addition of MNT (entries 2 and 3). 

Therefore we used 4-androsten-3,17-dione 1 and 5a-androstan-3,17-dione 2 as model substrates to investigate the chemo- regio- and stereoselectivity of hydrogen transfer from different secondary alcohols, 2-propanol, 2-octanol, cyclohexanol, 1-phenyl-ethanol and diphenylmethanol in the presence of CU/AI2O3. In particular, hydrogenation of 1 allowed to determine the selectivity towards 5p isomers, whereas the percent of axial alcohol was derived from the hydrogenation of 2. These results can be compared with those obtained with the same catalyst in the presence of molecular hydrogen. 

In 1965, Albright and Goldman3 demonstrated that alcohols are oxidized to aldehydes and ketones by the action of a mixture of DMSO and acetic anhydride at room temperature. Two years later,56 they presented a full paper, in which optimized conditions for this oxidation were established using yohimbine (16) as a model substrate. Thus, it was found that treatment of yohimbine with a mixture of DMSO and AC2O produces the desired oxidation to yohim-binone (17), accompanied by formation of the methylthiomethyl ether 18. 

The course of addition reactions of ROH-XeF2 to alkenes has been elucidated using norbomene, 2-methylpent-2-ene and hex-l-ene as model substrates. It turned out that the alkoxyxenon fluoride intermediates (ROXeF) can react either as oxygen electrophiles (initially adding alkoxy substituent) or as apparent fluorine electrophiles (initially adding fluorine), depending on the reaction conditions. Simple addition of poorly nucleophilic alcohols to norbomene was also observed in certain instances. Selectivity between the various reaction pathways (simple fluorination, alkoxyfluorina-tion, or alcohol addition) proved to be sensitive to various reactions conditions, especially solvent, temperature, and catalyst.27... 

To date, mechanistic studies into the carbonylations of secondary alcohols with the same type of rhodium/RI catalyst system have used 2-propanol as a model substrate. At least part of the reason for this has been to minimize the expected complexities of the product analyses. The carbonylation of 2-propanol gives mixtures of n- and isobutyric acids. Two studies have been (24b, 32) reported with this system. The first of these (32) concluded that the reactivity could be described in terms of the same nucleophilic mechanism as has been described above, despite the fact that the reaction rates at 200°C were approximately 140 times faster than predicted by this type of chemistry (24b). Other data also indicated that this SN2-type reactivity was probably not the sole contributor to the reaction scheme. For example, the authors were not able to adequately explain either the effect of reaction conditions on product distribution or the activation parameters. They also did not consider the possible contribution of a hydrocarboxylation pathway, which is known to be extremely efficient in analogous systems (55). For these reasons, a second study into the carbonylation of 2-propanol was initiated (24b, 57). 

A quite similar behavior was observed in the reaction of DF.C with alcohols to give unsymmetrical alkyl carbonates. By using ben/yl alcohol as model substrate, the reaction earned out with both III-TBD and MCM-41-TBD was faster (96 and 93% yield, respectively, after X hours) than the similar reaction for the preparation of carbamates (92 and 99% yield, respectively, after 15 hours). However, at shorter reaction times (3 hours), the homogeneous... 

Aluminium alkoxides were anchored in the pores of siliceous MCM-41 type materials. The resulting catalysts were used in the hydrogen transfer reduction of a,p-unsaturated ketones to the corresponding allylic alcohols. The most active material is obtained by exposure of MCM-41 to a toluene solution of Al(OPr )3. With benzalacetone as a model substrate, optimum reaction conditions are cyclopentanol (hydride donor), toluene (solvent), and addition of 5A molecular sieve (water trapping). 

We therefore conducted a study on the homoallylazide hydroformylation. Various homoallylazides (44a-h) were synthesized from the corresponding alcohol by a mesylation/nucleophilic substitution sequence. (l-Azidobut-3-enyl)benzene 44a served as a model substrate to develop the reaction conditions. Whatever the conditions used, the azide function was stable to the hydroformylation reaction. Various azidoaldehydes (45a-h) were obtained with yields ranging from 83% to 92% (Table 2). 

Hydrosilylation reactions are formal additions of Si-H units across multiple bonds. They are fundamental reactions in organosilicon chemistry. Despite early reports of Marinetti and Ricard on Pd-catalysed hydrosilylation of alkenes with phosphetanes (up to 65% ee with styrene) and of Zhang and co-workers on Ru-catalysed hydrosilylation of ketones (up to 54% ee with acetophenone), most of the work on enantioselective hydrosilylation with P-stereogenic ligands has been carried out with Rh(I) complexes and prochiral ketones as substrates. Initially, silyl ethers are formed but they are usually cleaved under acidic conditions affording alcohols. As a result, hydrosilylations of ketones are formally identical to hydrogenations but do not involve the manipulation of dihydrogen. The model substrate for enantioselective hydrosilylation is acetophenone (Scheme 7.17). 

Recently, we have developed a versatile aqueous medium self-assembly method for the generation of diverse multicopper(II) complexes and coordination polymers derived from cheap and commercially available ligands such as aminoalcohols and benzenecarboxylates [6-15]. The obtained compounds were applied as highly efficient and versatile catalysts or catalyst precursors in two different alkane functionalization reactions. These include the mild oxidation of alkanes (typically cyclohexane as a model substrate) by hydrogen peroxide into alkyl hydroperoxides, alcohols, and ketones [6-9, 11, 16, 17], as well as the hydrocarboxylation of gaseous and liquid C ( = 2 - 9) alkanes, by carbon monoxide, water, and potassium peroxodisulfate into the corresponding carboxylic acids [12-15, 18-22]. 

C02EtH3Ll and C02EtHL2 only differ in R" and allow comparison of the phosphoesterase-like activity in the presence and absence of alcohol nucleophiles. The model substrate BDNPP will be used. Also the ability of the complexes derived from these two ligands to hydrolyze substrates other than organophosphates will be investigated using the p-lactam substrates nitrocefin and penicillin G. 

Nolan and coworkers have recently reported the gold-catalysed transformation of allqmes and allylic alcohols to homoallylic ketones. This reaction involved the hydroalkojqrlation of the allq ne followed by Claisen rearrangement. It proceeded in solvent-free conditions, at veiy low catalyst loading. Internal and terminal allq nes were successfully used as well as different allylic alcohols. The recyclability of the catalyst was studied by conducting the reaction with model substrates (diphenyalcetylene and allyl... 

In 1988, Walde and coworkers studied the kinetic and structural properties of another serine protease, namely trypsin, in two reverse micellar systems, AOT/ isooctane and CTAB/chloroform/isooctane, employing three different model substrates, an amide and two esters [71], The main aim of this work was to compare the behavior of trypsin in reverse micelles with that of a-chymotrypsin. In the case of trypsin, superactivity was not observed and in general no obvious similarities between the two enzymes were recorded. Some years later, reverse micelles formulated with biocompatible surfactants such as lecithin of variable chain lengths in isooctane/alcohol were studied in relation to their capacity to solubilize a-chymotrypsin and trypsin [72]. The hydrolytic behavior of the same serine proteases, namely a-chymotrypsin and trypsin, in both AOT and CTAB microemulsions was studied and related to the polarity of the reaction medium as expressed by the logP value and measured by the hydrophilic probe 1-methyl-8-oxyquinolinium betaine [39], In this study a remarkable superactivity of trypsin in reverse micelles formed with the cationic surfactant CTAB was reported. 

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