Terminal alkynes hydrosilylation

A separate, quite thorough study of terminal alkyne hydrosilylation with platinum arrived at a similar set of conditions.39 This work utilized a one-pot hydrosilylation with the preformed platinum(O) complex (>Bu3P)Pt[(CH2=CH)Me2Si]20 ([(CH2=CH)Me2Si]20 = DVDS) and subsequent palladium-catalyzed coupling reaction to demonstrate that the platinum catalyst is compatible with cross-coupling conditions, providing a convenient hydrocarbation of terminal alkynes (Table 2). 

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-... 

The use of cationic rhodium complexes such as [ Rh(cod)2 ]" "BF4 or [ Cp Rh(BINAP) (SbF6)2l in terminal alkyne hydrosilylation exhibits high (El-selectivity, whereas the neutral rhodium complex-catalyzed hydrosilylation e.g., [RhCUPPhsls], [ RhCl(cod) 2], and [RhHlCOlPPhals] usually provides (Z)-alkenylsilanes (73-80). A representative example of such a stereodivergent alkyne hydrosilylation with rhodium complexes is presented in (Scheme 8). 

Terminal alkynes can be converted by a process of hydrosilylation followed by oxidative cleavage into carboxylic acids (10). An alternative basic" cleavage yields the corresponding aldehydes. 

Nickel complexes formed in situ by the reaction of NiCl.S-COD) with the iini-dazolium salts IMesHCl or IPrHCl in the presence KO Bu catalyse the hydrosilylation of internal or terminal alkynes with EtjSiH. Interestingly, Ni tri-butylphosphine complexes are inactive in this hydrosilylation reaction. The monosilylated addition products were obtained with slow addition rates of the alkyne in the reaction mixture and were formed with variable degree of stereoselectivity, depending on the type of the alkyne, the silane and the ligand on Ni [50],... 

In the mid 1980s, Voronkov, Lukevics, and coworkers published results demonstrating the strong preference of terminal alkynes to react in platinum catalyzed hydrosilylation in the presence of 1-olefinic groups [10, 11], Enynes were found to give hydrosilylation products predominantly by addition of silanes across the triple bond. To check the versatility of these promising results, 2-allyloxy-3-butyne was chosen as an enyne to be reacted with an a, ui-dihydropoly(dimethylsiloxane) under normal hydrosilylation conditions (10"3 mole percent Pt, 120°C) (Eq. 3). 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

Table 1 Hydrosilylation of terminal alkynes with platinum-phosphine complexes 0.01 mol%...

The robustness and excellent turnover numbers of platinum complexes with terminal alkynes have made it the catalyst of choice for the synthesis of polymers and other macromolecular architechtures. Alkyne hydrosilylation with platinum has also served as a key element in the synthesis of dendrimers. Sequential reaction of an alkyne with HSiMeCl2 and lithiated phenylacetylene afforded the branching unit of a dendrimer synthesis which has been used to afford a large variety of structures at high generation.44,4411 441 ... 

The mechanistic and synthetic puzzle of alkyne hydrosilylation opened more fully with the discovery that rhodium will catalyze the /r.mr-hydrosilylation of terminal alkynes.22 There is much work extant in this area, and good summaries of the various catalytic systems exist.11 A trans-addition process to give (Z)-j3-silane products G is well precedented with trialkylsilanes (Table 3), for both rhodium and mixed rhodium-cobalt complexes (entry 4).22,26 However, the selectivity erodes significantly upon switching to Me2PhSiH (entry 5), and, due to the mechanistic requirements for equilibration of the /3-silyl vinylrhodium intermediate, electron-poor silanes react exclusively to give CE)-/3-silane products B (see entries 6 and 7). 

An alternative disconnection of homopropargylic alcohols substrates for intramolecular hydrosilylation is the opening of an epoxide with an alkynyl anion. This strategy was employed in a total synthesis of the macrolide RK-397 (Scheme 20). Epoxide ring opening serves to establish homopropargylic alcohol C with the appropriate stereochemistry. A hydrosilylation/oxidation protocol affords the diol E after liberation of the terminal alkyne. The... 

Computational and catalytic studies of the hydrosilylation of terminal alkynes have been very recently reported, with the use of [ Ir( r-Cl)(Cl)(Cp ) 2] catalyst to afford highly stereoselectively P-Z-vinylsilanes with high yields (>90%) [35]. B-isomers can be also found among the products, due to subsequent Z —> E isomerization under the conditions employed. The catalytic cycle is based on an lr(lll)-lr(V) oxidahve addition and direct reductive elimination of the P-Z-vinylsilane. Other iridium complexes have been found to be active in the hydrosilylation of phenylacetylene and 1-alkynes for example, when phenylacetylene is used as a substrate, dehydrogenative silylation products are also formed (see Scheme 14.5 and Table 14.3). 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

Several additional points regarding the yttrium-catalyzed cascade cyclization/hydrosilylation of dienynes are worth noting. First, substitution at the 4-position of the 3-(3-butynyl)-l,5-hexadiene and a branched substituent on the terminal alkyne carbon atom were required to achieve high chemo- and regioselectivity. 

Laurel Schafer of the University of British Columbia reports (Organic Lett. 2003,5,4733-4736) that terminal alkynes undergo smooth hydroamination with a Ti catalyst. The intermediate imine 4 can be hydrolyzed to the aldehyde 5 or reduced directly to the amine 6. The alkyne to aldehyde conversion has previously been carried out by hydroboration/oxidation (J. Org. Chem. 1996, 61, 3224), hydrosilylation/oxidation (Tetrahedron Lett. 1984,25, 321), or Ru catalysis (J. Am. Chem. Soc. 2001, 123, 11917). There was no previous general procedure for the anti-Markownikov conversion of a terminal alkyne to the amine. 

Using the same catalyst 66, the hydrosilylation of a terminal alkyne RC=CH with PhSiHa at ambient temperature afforded a mixture of cis- and rrani-RCH=C(H)Si(H)2Ph, CH2= C(R)Si(H)2Ph, RCH=CH2 and RCsCSi(H)2Ph at higher temperatures, the corresponding doubly hydrosilylated compounds RCH=C Si(H)2Ph 2 were also formedThe salt [UH(NEt2)2] [BPh4] (74) was implicated as a crucial intermediate ... 

Hydrosilylation of terminal alkynes may yield three isomeric vinylsilanes ... 

Hydroboration of a variety of alkenes and terminal alkynes with catecholborane in the fluorous solvent perfluoromethylcyclohexane was performed using fluorous analogs of the Wilkinson catalyst.135 136 Recycling of a rhodium-based alkene hydrosilylation catalyst was also successful.137 Activated aromatics and naphthalene showed satisfactory reactivity in Friedel-Crafts acylation with acid anhydrides in the presence of Yb tris(perfluoroalkanesulfonyl)methide catalysts.138... 

Dehydrogenative silylation has also been observed for terminal alkyne substrates. Doyle and co-workers reported in 1991 that a small amount (6%) of alkynylsilane was observed in the product mixture that results from reaction of phenylacetylene and Et3SiH catalyzed by Rh2(pfb)4.41 The remaining components of the product mixture resulted from hydrosilylation. Crabtree and co-workers have found that in reaction of terminal alkynes with various tertiary silanes, dehydrogenative silylation can become the predominant route, depending on reaction conditions [Eq. (7)].42... 

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