Propargylic alcohols, hydrosilylation

This disadvantage can be ruled out by spacers between the allylic and the acrylic group [15], but the selectivity in favor of the allylic group is not improved. An acetylenic triple bond instead helps to clarify the situation. 2-Propynoxyethyl acrylate, available in 90 % yield from ethoxylated propargylic alcohol by esterification, is hydrosilylated very smoothly only at the triple bond, leaving the acrylic side virtually untouched (Eq. 5). 

Denmark pursued intramolecular alkyne hydrosilylation in the context of generating stereodefined vinylsilanes for cross-coupling chemistry (Scheme 21). Cyclic siloxanes from platinum-catalyzed hydrosilylation were used in a coupling reaction, affording good yields with a variety of aryl iodides.84 The three steps are mutually compatible and can be carried out as a one-pot hydro-arylation of propargylic alcohols. The isomeric trans-exo-dig addition was also achieved. Despite the fact that many catalysts for terminal alkyne hydrosilylation react poorly with internal alkynes, the group found that ruthenium(n) chloride arene complexes—which provide complete selectivity for trans-... 

Analogously silylated cyclohexadiene 25 having propargyl alcohol as a pendant was used in the radical intramolecular hydrosilylation followed by ionic ring opening to provide alcohol 26 in 55 % yield (Scheme 6.6) [6]. The cyclization of silyl radical 27 to radical 28 represents an example of a 5-endo-dig process. 

The tandem hydrosilylation-isomerization process of sec-propargyl alcohols 140 provides an easy access to /J-silylketones 142 via silylallylic alcohols 141 (equation 57)136. 

Intramolecular hydrosilylation. The hydrodimethylsilyl ether (2) of a homo-propargyl alcohol (1) undergoes intramolecular hydrosilylation catalyzed by H2PtCl6 to give a cyclic (E)-vinylsilane (3). This can be converted to a 0-hydroxy ketone (4) by hydrogen peroxide oxidation or into (Z)-3-bromo-3-decene-l-ol (5) by brom-ination/bromodesilylation with inversion of geometry. 

The cationic [Cp Ru(MeCN)3]+(PF6) complex, reported as a stereoselective catalyst for trans hydrosilylation of internal alkynes, has been successfully used in intermolecular endo-dig hydrosilylation of propargyloxyhydrosilanes synthesized in situ via silylation of propargylic alcohols by tetramethyldisilazane [116]. 

The hydrosilylation of propargyl alcohol gives a mixture of y- and /3-adducts and their dehydrogenatively silylated products although earlier publications claimed only the formation of y-adduct. ... 

For the last two decades, the use of the platinum- or rhodium-catalyzed intramolecular hydrosilylation/Tamao-Fleming oxidation sequence has been well recognized as a powerful method for the stereoselective synthesis of various structurally diverse alcohols (1,3-diols, 2-alkoxy-l,3-diols, and 2-aminoalcohols) and ketone derivatives (/3-hydroxyketones, y-hydroxyketones, o, /3-dihydroxyketones, and a,y-dihydroxyketones) from simple and readily available starting materials such as substituted allyl or propargyl alcohols and their homologues (4,177). Selected applications are presented in equations (24-26). 

The synthetic potential of this strategy based on consecutive Pt- or Ru-catalyzed intramolecular hydrosilylation of propargyl and homopropargyl alcohols Pd-catalyzed cross-coupling reactions have been proved in the stereoselective synthesis of a wide variety of aryl-substituted (E)- and (Z)-allyl and homoallyl alcohols (Scheme 25) (178-180) ... 

The hydrosilylation of terminal alkynes disclosed by Trost can be applied to internal alkynes as well. i Remarkably, the (Z)-isomer is generated in this process, resulting from trans addition during hydrosilylation. The protodesilylation of these sily-lated products in the presence of copper(I) iodide and tetrabuty-lammonium fluoride (TBAF) or silver(I) fluoride (eq 15) leads to internal fraws-olefins. This two-step method is a useful synthetic transformation to access ( j-alkenes from internal alkynes. In contrast, the chemoselective reduction of alkynes to the corresponding ( -alkenes is conventionally accomplished readily with Lindlar s catalyst. The complementary process to afford ( )-olefins has proven much more difficult. Methods involving metal hydrides, dissolving metal reductions, low-valent chromium salts provide the desired chemical conversion, albeit with certain limitations. For example, functional substitution at the propargylic position (alcohols, amines, and carbonyl units) is often necessary to achieve selectivity in these transformations. Conversely, the hydrosilylation/protodesilyla-tion protocol is a mild method for the reduction of alkynes to ( )-alkenes. 

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