Bis-silylation of terminal alkynes

Bis-silylation of alkynes and alkenes.1 Palladium(ll) acetate in combination with a large excess of a /-alkyl isocyanidc can effect bis-silylation of terminal alkynes by otherwise unreactive disilancs (equation 1). intramolecular bis-silylation of an alkyne is... 

Table 9.7.1 Bis-Silylation of Terminal Alkynes (Conditions In THF, Room Temperature, Isolated...

In a manner similar to that of the bis-silylation of internal alkynes shown above, the use of unsymmetrical disilanes with basic / //-phosphines is effective for intermolecular bis-silylation of terminal alkenes (Equations (32) and (33)).6 Although the regioselectivities need to be improved, clean bis-silylations of 1-octene, styrene, and norbornene have been achieved in high yields. 

The latter isonitrile-palladium complexes effectively catalyze intramolecular bis-silylation of carbon-carbon triple bonds (Eq. 21) [46,47]. Not only terminal alkynes but also internal alkynes including those having aryl, alkenyl, silyl, and... 

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 addition of alkoxycarbonylcarbene derived by catalysed decomposition of methyl diazoacetate to several simple, and in particular terminal, alkynes leads to low yields S7), but the reaction with 1 -trimethylsilylalkynes proceeds reasonably efficiently subsequent removal of the silyl-group either by base or fluoride ion provides a route to l-alkyl-3-cyclopropenecarboxylic acids. In the same way 1,2-bis-trimethylsilyl-ethyne can be converted to cyclopropene-3-carboxylic acid itself58 . The use of rhodium carboxylates instead of copper catalysts also generally leads to reasonable yields of cyclopropenes, even from terminal alkynes 59). 

Addition reactions of the Si-Si bonds across carbon-carbon triple bonds have been most extensively studied since the 1970s by means of palladium catalysts. In the early reports, palladium complexes bearing tertiary phosphine ligands, mostly PPh3, were exclusively employed as effective catalysts, enabling the alkyne bis-silylation with activated disilanes, i.e., disilanes with electronegative elements on the silicon atoms such as hydro [36], fluoro [37], chloro [38], and alkoxy-disilanes [39,40] and those with cyclic structure (Scheme 4) [41-44]. The bis-silylation reactions could be successfully applied to terminal alkynes and acetylenedicarboxylates to give (Z)-l,2-bis(silyl)alkenes, which are otherwise difficult to synthesize. 

Two new classes of ligands, bicyclic phosphate 47 [45] and ferf-alkyl isonitriles [46] on palladium, enable bis-silylation with hexaalkyldisilanes, which have been regarded to be much less reactive than the activated disilanes (Eq. 20). Reactions of the terminal alkynes with hexamethyldisilane in the presence of these palladium catalysts afford (Z)-l,2-bis(trimethylsilyl)alkenes 48 in high yields. 

Only rarely is proton loss observed from the intermediate cation derived from acylation of a terminal alkyne. In contrast, acylation of alkynylsilanes provides an excellent strategy for the synthesis of alkynyl ketones. " Here, it is apparent that elimination of the silyl substituent competes very effectively with capture of a nucleophile. Acylation of bis(trimethylsilyl)ethyne" provides a convenient route to terminal alkynic ketones, particularly since desilylation can be carried out using bases as weak as dilute aqueous borax, conditions mild enough to prevent the ready hydration of the triple bond (Scheme 25)." Silylated diynes are also excellent substrates for Friedel-Crafts acylation. ... 

Interestingly, treatment of Me3SiC = CH under these conditions does not yield the parent iodonium tetrafluoroborate (12, R=H). The parent system can be obtained by reaction of the silyl system with 48 % HF [Eq. (6)] [28], Direct conversion of a terminal alkyne to 12, via treatment with the p-oxo-bis-BF4 13, has also been reported [29] [Eq. (7)]. However, the generality and scope of this interesting and simple reaction was not reported. 

Another example of Pd(0)-catalyzed dimerizations of alkynes was reported by Ishikawa and co-workers. Terminal silylacetylenes 19, containing at least one phenyl group at the silyl moiety, in the presence of Pd(PPh3)4 underwent homodimerization in a head-to-head fashion to produce the corresponding bis-silylenynes 20 in reasonable yields (Scheme 7). The preferred anti-Markovnikov-type regiochemistry in these cases and also in the Pd(+2)-catalyzed homodimerization of trimethylsilylacetylene was explained with steric factors. ... 

Gold-NHC complexes have been shown to be suitable synthons towards the C-H activation of alkynes. The reaction of the air-stable (IPr)Au(OH) with either terminal or silyl-protected alkynes led to the characterisation of three structures , A-bis-, and l,3,5-tr -(IPrAuC=C)benzene. While... 

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