Silylated terminal alkynes

In cases where stable carbocations can be generated, e.g., tertiary propargylic cations, the rather low nucleophilicity of silylated terminal alkynes is still sufHcient for a coupling reaction. Ring closures have been carried out in this fashion to give the permethylated pericyclynes, which are discussed by L. T. Scott and M. J. Cooney in Chapter 9 of this monograph [8]. 

Fleming has shown (2) that the cuprate reagent (Chapter 8) derived from dimethylphenylsilyl lithium and copper(t) cyanide (molar ratio 2 1) adds regioselectively in an overall syn manner to terminal alkynes, the silyl moiety becoming attached to the terminal carbon atom (variation in reagent... 

Terminal silylation of alkynes affords two main benefits the triple bond is protected against chemical attack, either for steric reasons or because the potentially acidic proton is masked, and, perhaps paradoxically, the bond is activated towards regioselective electrophilic attack under certain conditions. 

SN displacement reactions, 27-9 a-Selenocyclohexanones, 77 Senecioyl chloride, 33 Silmagnesiation, platinum-catalysed, 8 Silametallation of terminal alkynes, 7-9 Silver acetylide, 49 Silver trifluoroacctate, 42,127 Silyl cuprates, 7... 

Carboxylic acids can be protected as oxazolines [96, 105-107, 186, 191] or as ester functions. Alkynic esters such as silyl esters [153, 211], tert-butyl esters [216], and even benzyl esters [153, 211] have been successfully hydrozirconated when the reactive site was a terminal alkyne or vinyl group (Scheme 8-27). 

In Entry 4 the silyl group appears to introduce a controlling steric factor, leading to the observed stereoisomer. The unsubstituted terminal alkyne, which reacts through the dianion, gives the alternate isomer. 

Addition of diphenyl disulfide (PhS)2 to terminal alkynes is catalyzed by palladium complexes to give l,2-bis(phe-nylthio)alkenes (Table 3)168-172 The reaction is stereoselective, affording the (Z)-adducts as the major isomer. A rhodium(i) catalyst system works well for less reactive aliphatic disulfides.173 Bis(triisopropylsilyl) disulfide adds to alkynes to give (Z)-l,2-bis(silylsulfanyl)alkenes, which allows further transformations of the silyl group to occur with various electrophiles.174,175 Diphenyl diselenide also undergoes the 1,2-addition to terminal alkynes in the presence of palladium catalysts.176... 

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). 

Tris(trimethylsilyl)silane [20,21], thiols [22], germanes [23-25] and gallium hydride [26] can be added easily to terminal alkynes in the presence of Et3B/02. This process was extended to internal alkenes (Scheme 8, Eq. 8a) as well as silyl enol ethers (Eq. 8b) by using tri-2-furylgermane. In this last case, basic or acidic treatment of the main syn /J-siloxygcrmanc furnishes the corresponding E- or Z-alkene, respectively [24],... 

When the terminal alkynes 96 are treated with the trimethylsilylalkyne 97 in the presence of HfCl4 as a Lewis acid, the silylated vinylallenes 98 are produced in acceptable yields. In an intramolecular variant of this process, 100 was obtained from the diyne 99 [32]. Vinylallenes, incorporated into a cyclic framework and hence of restricted conformational mobility, are of interest for photochemical studies [33] and are among the photoproducts in ring-enlargement reactions of polycyclic allenes [34]. 

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). 

The development of the first alkyne silylformylation reaction was reported in 1989 by Matsuda [27]. Alkynes were treated with Me2PhSiH and Et3N with 1 mol% Rh4(CO)i2 under CO pressure to produce yS-silyl-a,/ -unsaturated aldehydes (Scheme 5.20). A second report from Ojima detailed the development of rhodium-cobalt mixed metal clusters as effective catalysts for alkyne silylformylation [28]. Shortly thereafter, Doyle reported that rhodium(II) perfluorobutyrate was a highly efficient and selective catalyst for alkyne silylformylation under remarkably mild reaction conditions (0°C, 1 atm CO) [29]. In all these reports, terminal alkynes react regiospedfically with attachment of the silane to the unsubstituted end of the alkyne. The reaction is often (but not always) stereospecific, producing the cis-product preferentially. 

Blechert et al. succeeded in intermolecular CM of terminal alkyne and terminal alkene. A reaction carried out in CH2CI2 at RT in the presence of 5-7mol% Ic gives a mixture of ( )- and (Z)-isomers (Table 2). Because of the nonselective stereochemical course, a silyl-protected ally alcohol is employed and the resulting metathesis product is deprotected and oxidized to afford the desired diene having an -configuration (Equation (13)). 

Intramolecular cyclization is particularly effective to terminal alkynes containing three or four methylene units between acetylenic and silyl moieties such as 158, 160, and 162. But-3-ynylmethylphenylsilane does not give any positive result for CO incorporation. Regioselectivity for the silylformylation is completely reversed from the one in the standard silylformylation discussed in Section 11.14.2.2. A bulky /ft7-butyl group in 162 (R = Bu ) plays an... 

The reaction worked with both internal and terminal alkynes (except silylated alkynes) and in many solvents, even in the neat alcohol added [105]. The mechanism proposed involved two catalytic cycles first, gold catalysis would lead to dihydro-furan by a fast intramolecular reaction then, the subsequent slower intermolecular reaction would be produced by the addition of alcohol to the enol ether to deliver a ketal (Scheme 8.18). 

Eastmond, R. Johnson, T. R. Walton, D. R. M. Silylation as a Protective Method for Terminal Alkynes in Oxidative Couplings, Tetrahedron 1972,28, 4601. 

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... 

Similar results are observed in the intramolecular double silylation of alkynes.59cd Both terminal and internal alkynes lead to exocyclic olefin formation. Interestingly, the reaction is not successful for internal alkynes tethered to a disilanyl functionality by a four atom chain, but is accomplished when four atoms link a disilanyl to a terminal alkyne. Internal alkynes with ester or olefin in conjugation with the C-C triple bond undergo chemoselective double silylation, and alkynes with substituents in the tether are also good substrates for the reaction. 

Allyl cyanides can be added across alkynes in the presence of a nickel catalyst prepared from (COD)2Ni and (4-CF3CeH4)3P in situ to give functionalized di- or tri-substituted acrylonitriles in a highly stereoselective manner, presumably via n-allylnickel intermediates. a-Siloxyallyl cyanides also react at the y -position of a cyano group with both internal and terminal alkynes to give silyl enol ethers, which can be converted into the corresponding aldehydes or ketones upon hydrolysis.70... 

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