Alcohols, benzylic examples

Leuco naphthazarins have been well studied as hair dyes.25 Human hair has been colored purplish red from dye solutions in aqueous benzyl alcohol. For example, 33 has been oxidized on hair during the drying process. 

Ring-substituted benzyl alcohols sometimes undergo such reduction more effectively than unsubstituted alcohols. For example, treatment of a dichloromethane solution of 2,4,6-trimethylbenzyl alcohol with trifluoroacetic acid and triphenylsilane produces a 41% isolated (89% by GLC) yield of isodurene.26 Treatment of 2-me(hyl-4,6-di-/m-buty I benzyl alcohol with a three-fold excess of triethylsilane and trifluoroacetic acid in dichloromethane at room temperature gives an 85% yield of 2-mclhyl-4,6-di-/m-butyltoluene together with 15% of 3,5-di-ferf-butyltoluene. The latter is presumably formed by loss of protonated formaldehyde from the Cl ring-protonated substrate.128 Similar treatment of 2,4,6-tri-ferf-butylbenzyl alcohol produces a 90% yield of 2,4,6-tri-tert-butyltoluene within one hour (Eq. 21).128... 

The availability of oxepins that bear a side chain containing a Lewis basic oxygen atom (entry 2, Table 6.4) has further important implications in enantioselective synthesis. The derived alcohol, benzyl ether, or methoxyethoxymethyl (MEM) ethers, in which resident Lewis basic heteroatoms are less sterically hindered, readily undergo diastereoselective uncatalyzed alkylation reactions when treated with a variety of Grignard reagents [17]. The examples shown below (Scheme 6.7) demonstrate the excellent synthetic potential of these stereoselective alkylations. 

Examination of the scope of the individual kinetic resolution systems leads to some general trends. Typically, oxidation proceeds most rapidly with activated alcohols, for example benzylic, allylic, and a-cyclopropyl alcohols. Occasionally, saturated alkyl alcohols can be oxidized, although reaction times are usually longer. Furthermore, sterically encumbered alcohols tend to be difficult to oxidize. Selectivity in the kinetic resolutions presumably relies on steric differences between the two alcohol substituents. 

This C - H activation event is reversible, and is required to achieve catalytic turnover [62], A series of alcohols, mostly secondary benzylic examples, have been oxidized using this catalyst. The catalytic activity does not match that of the Ir examples described above, but it has been used in several tandem reactions that feature both dehydrogenation and hydrogenation steps to achieve interesting transformations. One example is a tandem alcohol oxidation/Wittig reaction/alkene hydrogenation sequence (Scheme 9) [61,62],... 

Organoselenium reagents have been observed to exhibit selectivity for the oxidation of allylic alcohols, for example a catalytic amount of dimesityl diselenide with r-butyl hydroperoxide as cooxidant will oxidize benzylic and allylic alcohols in the presence of saturated alcohols, as in the case of the diol (7 equation 4). ... 

Urethanes. The reagent (I) reacts with primary alcohols, for example 1-hexanol (2), to form the salt (3), which when heated at 95° for 1 hr. and then treated with water is converted into methyl-N-hexylcarbamate (4) in 55 % yield. Benzyl alcohol is converted in Ihc same way into methyl-N-bcnzylcarbamate (80% yield). 

One reaction, oxidation, which directly involves the hydrogen atoms attached to the carbon bearing the —OH group, takes an entirely different course for each class of alcohol. Usually, however, alcohols of different classes differ only in rate ot mechanism of reaction, and in a way consistent with their structures. Certain substituents may atfect reactivity in such a way as to make an alcohol of one class resemble the members of a different class benzyl alcohol, for example, though formally a primary alcohol, often acts like a tertiary alcohol. We shall find that these variations, too, are consistent with the structures involved. 

Tertiary alcohols tend to react without rearrangement while secondary alcohols are liable to do so, equations (14) and (15), as discussed in Section 1.9.1.2. Primary alcohols normally fail to react with nitriles even under severe conditions, but this restriction does not apply to benzylic examples (equation 16). In appropriate cases," the Ritter reaction can be stereoselective (Scheme 10). Either alcohol isomer, separately or as a mixture, gave identical mixtures of the two amine products, showing that axial attack on the cation was predominant. 

Using aliphatic and benzyl alcohols as examples, Dess and Martin demonstrated that oxidation occurred smoothly at room temperature, in methylene chloride, using a slight excess of 2 (1.05-1.1 eq). Although strong acid (i.e., TFA) was found to catalyze the reaction, it is usually not necessary. Below are a few examples of oxidations listed in the first disclosure.1... 

The LiBCCfjFjla-LiOTf/MgO combination catalyzes benzyl ether formation from alcohols (10 examples, 72-100%). The method is valuable for dealing with substrates containing base sensitive funetionalities. ... 

Benzyl alcohol may be prepared by the methods used in the preparation of aliphatic alcohols. For example, it is formed when benzyl chloride is heated with water —... 

Carbinol kar-bo- n61, - nol [ISV, fir. obs. Gr Karbin methyl, fir. Gr karb-] (ca. 1885) n. (1) -CH2OH. Monovalent primary alcohol radical. It may be part ofi an aliphatic or an aromatic alcohol. For example, ethyl alcohol (C2H5OH) can be described as methyl carbinol, or benzyl alcohol (C6H5CH2OH) as phenyl carbinol. (2) It is sometimes used as a Syn methanol. 

Electrooxidation of aromatic compounds has been intensively investigated, and many useful fine chemicals have been prepared by both side-chain and aromatic nucleus oxidation. Side-chain oxidation of alkylbenzenes may furnish benzyl alcohols, benzyl acetates, benzyl methyl ethers, Af-benzyl acetamides, benzaldehydes, benzoic acids, and so on. For instance, electrooxidation of p-methoxytoluene affords p-methoxybenzyl methyl ether, p-methoxybenzaldehyde, and/or its dimethylacetal depending on the choice of electrolysis media [3]. Many examples of electrooxidation of aromatic nucleus have been also reported. p-Quinones and their methyl acetals and semiquinones are prepared by electrooxidation of phenol derivatives and hydroquinones [3]. Nucleus-nucleus coupling of methoxybenzene derivatives... 

Furthermore, the same catalytic system was successfully applied to the alkylation reaction of secondary and primary alcohols. For example, the reaction of 1-phenylethanol (1.0 mmol) with benzyl alcohol (1.1 mmol) under the conditions described above gave 1,3-diphenyl-1-propanone in 93 % yield (Scheme 8, route B). 

In 1998, Peterson and Larock showed that Pd(OAc)2 in combination with NaHCOs as a base in DMSO as solvent catalyzed the aerobic oxidation of primary and secondary allylic and benzylic alcohols to the corresponding aldehydes and ketones, respectively, in fairly good yields [70]. In both cases, ethylene carbonate and DMSO acted both as the solvent and as the ligand necessary for a smooth reoxidation [71]. Similarly, PdCl2, in combination with sodium carbonate and a tetraalkylammonium salt, Adogen 464, as a phase transfer catalyst, catalyzed the aerobic oxidation of alcohols for example, 1,4- and 1,5-diols afforded the corresponding lactones (Eq. (5.11)) [72, 73]. 

However, these methods suffer from low activities and/or narrow scope. Uemura and coworkers [74,7 5] reported an improved procedure involving the use of Pd(OAc) 2 (5 mol%) in combination with pyridine (20 mol%) and 3 A molecular sieves (500 mg per mmol of substrate) in toluene at 80 °C. This system smoothly catalyzed the aerobic oxidation of primary and secondary aliphatic alcohols to the corresponding aldehydes and ketones, respectively, in addition to benzylic and allylic alcohols. Representative examples are summarized in Table 5.7. The corresponding lactones were afforded by 1,4- and 1,5-diols. This approach could also be employed under fluorous biphasic conditions [76]. 

A hydroxyl first overtone peak may also be split due to interactions with the electrons of phenyl rings or halogens, as in benzyl alcohol, for example. 

Benzenoids contribute to characteristic fragrance of many flowers. Methyl benzoate, for example, is a major scent constituent of Petunia flowers. Other benzenoids that frequently contribute to floral scents are benzal-dehyde, benzyl alcohol, benzyl acetate, and methyl salicylate (Fig. 18) (Knudsen et al., 1993). The latter compound is responsible for the characteristic smell and the analgesic effect of wintergreen Gaultheria procumbens, Ericaceae) (Dewick, 2002). 

Tungsten oxide is not the only nanomaterial that forms unusual morphologies in benzyl alcohol. Another example is ZnO that grows into fan-like nanorod bundles [168]. The chemical formation mechanism, elaborated from the analysis of the organic compounds detected by GC coupled with mass spectrometry, involved a nucleophilic attack of the hydroxyl function of benzyl alcohol on one of the carbonyl groups of the acetylacetonate ligand of the precursor molecule (Scheme 2.4). Release of acetone (in its enol form) and benzyl acetate resulted in the formation of a zinc hydroxyl species, which then underwent condensation to a Zn—O—Zn bridge. 

Alcohols that contain one to four carbon atoms have common names consisting of the name of the alkyl group followed by the term alcohol. For example, CH CH OFl is ethyl alcohol and CH CH(OH) CHj is isopropyl alcohol. Other common names are aUyl alcohol and benzyl alcohol, whose structures are shown below. 

LiCl04 and LiNTf2 are known to be effective additives in the stereocontrolled synthesis of a-D-ribofuranosides from 2,3,5-tri-O-benzyl-D-ribofuranose and several alcohols. Some examples are shown below in which eff ects of Li salts are notable (Scheme 3.15) [37-41]. 

Esterification and etherification are the two most commonly used methods to modify HA to form a scaffold while maintaining its biocompatibility [39, 40]. The -COOH group of the glucuronic acid subunit can be reacted with an alcohol, for example benzyl alcohol, to form an ester, in this case henzyl ester (Figure 2.1a) [40]. This modification also increases the hydrophobicity of the HA macromer, thus prolonging its degradation time. These hydrophobic modified HA compounds can be made into scaffolds via electrospinning, for example, into fibres, meshes, and membranes. In particular, this chemistry is used to prepare the commercially available HYAFF 11 scaffold [39-42]. 

Example. Add a solution of 0 5 ml. of benzyl alcohol in 5 ml. of petroleum (b.p. 100-120 ) to a similar solution of 0 5 ml. of phenylisocyanate, and boil the mixture gently under reflux for 20 minutes. Filter hot if necessary from any insoluble diphenylurea, and cool. Filter off the crystalline urethane, and recrystallise from the petroleum colourless crystals, m.p. 76 . 

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