Alkynes siloxy

Silver(I) salts are often utilized as catalysts for addition reactions. Kozmin and Sun have recently shown that AgNTf2 is a catalyst of choice for the hydroamination of siloxy alkynes with either secondary amides or carbamates to give silyl ketene am-inals [34]. The addition occurs in a syn selective manner, for instance, the reaction of siloxy alkyne (24) with carbamate (25) produces silyl ketene aminal (26) in 86% yield at room temperature under the influence of 1 mol% of AgNTf2 (Scheme 18.9). A six-membered chelated transition state is proposed to explain the high syn selectivity. Diastereoselective bromohydroxylation and bromomethoxylation reactions of cinnamoyl compounds possessing a chiral auxiliary are also effectively promoted by silver(I) salts such as AgNOs [35]. The asymmetric halohydrin reaction has been successfully applied into stereoselective syntheses of (-)-chloramphenicol and (+)-thiamphenicol. Csp-H iodination [36], hydrosilylation of aldehydes [37], and deprotection of TMS-alkynes [38] are also catalyzed by silver (I) salts. 

Cycloaddition. A silver-catalyzed [2 + 2] cycloaddition has been described. Although AgOTf is not the most efficient silver salt, it promotes the formation of functionalized cyclobutenes from siloxy alkynes and unsaturated ketones in reasonable yields... 

SCHEME 2.122 Synthesis of a siloxy alkyne starting from a terminal acetylene [191]. 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Intramolecular [3-I-2]-cycloaddition reactions have also been performed using alkyne moieties as reactants. An early example of this methodology is the high-yield formation of 27 from alkyne 26. As in the case of the (Z)-3-benzyloxybut-2-enoate derivatives (vide supra), the presence of the 3-siloxy group is essential for the reaction to proceed. A diastereomeric mixture of 26 leads to the formation of the two diastereomeric bicyclic adducts 27 in a combined yield of 88%. ° Interestingly, albeit with only 60 /o conversion after 100 hours, the same products are also formed at room temperature under the influence of ultrasound and 6 mol /o of tetrakis(triphenylphosphane)palladium(0) as catalyst. In a purely thermal reaction in the absence of a metal catalyst, a totally different reaction, presumably involving a Diels —Alder type addition of the alkyne to a vinylbenzene moiety, takes place. ... 

Rawal et al. have reported that under mild reaction conditions l-amino-3-siloxy-1,3-butadiene 105 a adds smoothly to a variety of dienophiles, for example electron-deficient alkynes and alkenes [290], aldehydes, aldimines [291], and ketones... 

Rh catalyses the hydrosilylation of alkenes and the dehydrogenative silylation of 1,5-dienes and alkynes, and the silylcarbonylation of alkynes to give p-silylacrylaldehydes with exceptional stereocontrol , ketones and enones give silyl enol ethers, 1,3-cyclobutane dione gives 3-hydroxycyclobutanone, and assists in the silylformylation of ethynyl pyirolidone derivatives to give pyrrolizidine alkaloids, and epoxides to give p-siloxy aldehydes, and... 

Scheme 8 anri-Diastereo- and enantioselective siloxy-crotylation in the transfer hydrogenative coupling of primary alcohols with alkynes via hydride-shift enabled ji-aUyl formation. Yields are of material isolated by flash chromatography on silica gel. Diastereoselectivity was determined through NMR analysis of crude... 

Liang T, Zhang W, Chen T-Y, Nguyen KD, Krische MJ (2015) Ruthenium-catalyzed diastereo- and enantioselective coupling of propargyl ethers with alcohols siloxy-crotylation via hydride shift enabled conversion of alkynes to ji-allyls. J Am Chem Soc 137 13066-13071... 

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