Acetylene alcohols, selective

Selective oxidation of a,p-unsatutrated (allylic, benzylic, acetylenic) alcohols. 

Nickel peroxide is a solid, insoluble oxidant prepared by reaction of nickel (II) salts with hypochlorite or ozone in aqueous alkaline solution. This reagent when used in nonpolar medium is similar to, but more reactive than, activated manganese dioxide in selectively oxidizing allylic or acetylenic alcohols. It also reacts rapidly with amines, phenols, hydrazones and sulfides so that selective oxidation of allylic alcohols in the presence of these functionalities may not be possible. In basic media the oxidizing power of nickel peroxide is increased and saturated primary alcohols can be oxidized directly to carboxylic acids. In the presence of ammonia at —20°, primary allylic alcohols give amides while at elevated temperatures nitriles are formed. At elevated temperatures efficient cleavage of a-glycols, a-ketols... 

Partial hydrogenation of acetylenic compounds bearing a functional group such as a double bond has also been studied in relation to the preparation of important vitamins and fragrances. For example, selective hydrogenation of the triple bond of acetylenic alcohols and the double bond of olefin alcohols (linalol, isophytol) was performed with Pd colloids, as well as with bimetallic nanoparticles Pd/Au, Pd/Pt or Pd/Zn stabilized by a block copolymer (polystyrene-poly-4-vinylpyridine) (Scheme 9.8). The best activity (TOF 49.2 s 1) and selectivity (>99.5%) were obtained in toluene with Pd/Pt bimetallic catalyst due to the influence of the modifying metal [87, 88]. 

Acetylenic alcohols, usually of propargylic type, are frequently intermediates in the synthesis, and selective reduction of the triple bond to a double bond is desirable. This can be accomplished by carefully controlled catalytic hydrogenation over deactivated palladium [56, 364, 365, 366, 368, 370], by reduction with lithium aluminum hydride [555, 384], zinc [384] and chromous sulfate [795], Such partial reductions were carried out frequently in alcohols in which the triple bonds were conjugated with one or more double bonds [56, 368, 384] and even aromatic rings [795]. 

Chiralallenylboronic esters.1 These reagents can react with aldehydes to provide /(-acetylenic alcohols enantioselectivity (60-95% ee). Thus, allenylboronic acid 1, prepared as shown, reacts with aldehydes in the presence of ( + )-DET or ( + )-DlPT to give (S)-alcohols (2), whereas (R)-alcohols (2) are obtained in the presence of (-)-DF.T or ( —) DIPT, both in optical yields of 85-95%. The selectivities are lower in reaction with aryl and z,/3-unsaturated aldehydes. 

Completely stereospecific frans-reduction of acetylenic alcohols to -allyl alcohol is reported with sodium bis(2-methoxyethoxy)aluminium dihydride (SMEAH or Red-Al), where LiAlH4 in various solvents is less selective. 

Under milder conditions, treatment of propargylic alcohols with n-BuLi followed by diisobutylaluminum hydride at -78 °C also affords trans-dlXylic alcohols with excellent stereoselectivity. More recently, Red-Al [Na(AlH2(0CH2CH20CH3)2)], sodium AA(2-methoxyethoxy)aluminum hydride] is the reagent of choice for the reduction of acetylenic alcohols. The reaction proceeds cleanly with high tmns-selectivity. °°... 

Types of reactions we meet elsewhere in the book include the reduction of a,p-unsaturated carbonyl compounds prepared by the aldol or Wittig reactions. The cyclic enone 14, prepared by a Robinson annelation is reduced regio- and stereo-selectively by LiAlH4 to the allylic alcohol2 15. The reduction of acetylenic alcohols 16, prepared by addition of metallo-alkynes to aldehydes, also with LiAlH4 is -selective (chapter 15) giving the allylic alcohol3 -17. 

Recently P. CHABARDES has proposed new titanium / copper based catalysts. [6] which allow a simple, efficient and selective isomerisation of a-acetylenic alcohols in liquid phase. 

Oxammonium salts such as 81 are new and powerful oxidizing agents for the selective oxidation of alcohols to aldehydes or ketones. 28 Such salts can be generated catalytically from small amounts of a nitro-xide in the presence of a secondary oxidation procedure, either chemical or electrochemical,. 29 or with two equivalents of acid and 2 equivalents of a nitroxide. When 81 was mixed with acetylenic alcohol 82 in dichloromethane, aldehyde 83 was isolated in 93% yield. The reaction can be monitored as the initial yellow slurry changes to a white slurry and the presence of unreacted oxidant can be checked with starch. 3l It is not necessary to use anhydrous conditions, and it was discovered that the rate of reaction was enhanced by the presence of silica gel. This reagent is compatible for the mild oxidation of many alcohols, including aliphatic primary and secondary as well as allylic and benzylic alcohols. 

We studied the behavior of catalytic nanoparticles formed in a nanostructured polymeric environment in the hydrogenation of long chain acetylene alcohols and the direct oxidation of a monosaccharide (L-sorbose). These reactions were chosen because of their industrial relevance and also because of the special importance of high selectivity, which can be achieved using a polymeric environment in mild... 

It is well established that low molecular weight modifiers such as quinoline, pyridine, etc. [51] increase the selectivity of the hydrogenation of acetylene alcohols, but often the modifiers leach and selectivity deteriorates. In the case of pyridine units of the P4VP block, the modification is fairly permanent. The stability of modification, which governs the stability of catalytic properties and high selectivity, is one of the important advantages of catalytic nanoparticles stabilized in the polymeric media [47]. 

Table 3.1 Catalytic properties and kinetic parameters of selective hydrogenation of acetylene alcohols with micellar catalysts based on PS-b-P4VPl l... 

In Table 3.1 C is the initial acetylene alcohol concentration, Q is the catalyst concentration, S is the selectivity (%), A is the acetylene alcohol conversion (%), turnover frequency (TOP) is the mole of substrate converted over a mole (Pd) of the catalyst per second, Xt is the relative concentration Xi= Q /Q (where Q is the current concentration of the substrate at i= 1 and product at i=2). Strictly speaking TOP should be calculated per Pd atoms participating in the catalytic reaction (available surface atoms), but for the sake of comparison with hterature data, in this chapter we will use the TOP definition given above. To find the kinetic relationships, we have studied the reaction kinetics at different substrate-to-catalyst ratio SCR=Co /Q. Kinetic curves for DHL hydrogenation with Pd and bimetallic catalysts are presented in Pig. 3.4. 

The catalytic properties of Pd-containing aluminas were studied in the selective hydrogenation of aU three long chain acetylene alcohols DMEC, DHL and DHIP (Scheme 3.1, Table 3.3). Because the structure of aluminum sites in both A-Pd-... 

Table 3.3 Catalytic activity and selectivity of microgel-templated mesoporous alumina in the hydrogenation of acetylene alcohols. ...

In the area of 2, 3 -didehydro-2, 3 -dideoxynucleosides, a new route to compounds of this type in the pyrimidine series is outlined in Scheme 4. The thioglycoside 54 was produced directly from deoxyribose and thiophenol in acidic conditions, and the condensations to form the nucleoside derivatives were P-selective by about 2 l/ A full account has been given of the formation of 2, 3 -didehydro-2, 3 -dideoxy systems from 2, 3 -dimesylates, protected at 0-5, by treatment with telluride anion (see Vol. 27, p. 247)7 Treatment of the furanoid glycal 55, made by cyclization of an acetylenic alcohol (Chapter 13), with silylated thymine in the presence of iodine, followed by sodium methoxide, provides a new route to d4T (56)7 A new synthesis of d4T (56) from 5-methyluridine has also been described, as has a route to d4T labelled with at C-1, which starts from [l- C]-ribose and proceeds via [r- C]-5-methyluridine, convertible in very high yield to [l - C]-d4T. ... 

By the judicious choice of reaction conditions, it is possible to control the regioselectivity and stereoselectivity of acetylide addition to a keto group. For instance, the reaction of the diketone 14 with lithium acetylide in THF at low temperatures gives the C(9)-acetylenic alcohol 75 (Scheme 4) [10], and a stereospecific synthesis of the acetylenic triol 16 is achieved by the condensation of the lithium reagent 77 derived from the isopropenylmethyl (IPM) ether of ( j-3-methylpent-2-en-4-yn-l-ol (18) with the optically active ketone 19, followed by acid-catalysed removal of the protecting groups [11]. Only 3% of the C(6)-diastereoisomer of 16 was detected (Scheme 5). The preparation of 16 is described in Worked Example 2. Table 1 lists a selection of a-hydroxyalkynes that have been prepared from metal acetylides. 

Addition of 9-BBN to alkynes is sufficiently slower than to similar alkenes to allow selective hydroboration-oxidation of skipped enynes (45) to produce 5,e-acetylenic alcohols (Scheme 22) without protection of the triple bond. ... 

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