1-alkynylsilane

This section describes Michael-analogous processes in which, mostly under electrophilic conditions, ally - or alkynylsilanes undergo addition to enones or dienones (Sakurai reactions). The intramolecular addition of allylsilanes is an extremely useful reaction especially for the construction of carbocyclic ring systems, which occurs in a diastereoselective manner, in many cases with complete asymmetric induction. 

Analogous to the allylsilane cyclizations, alkynylsilanes can also be used to synthesize 1,2-fused bicyclic compounds. The intramolecular addition of 4-[5-(trimethylsilyl)-3-pentynyl]-2-cyclo-hexenone proceeded smoothly in the presence of various Lewis acids, yielding functionalized cw-fused octahydro-5/f-inden-5-ones containing the synthetically useful terminal allene unit45. 

Further functionalized compounds, such as 4-hydroxy-2-cyclohexenones, were found to be very useful substrates for both allylsilane, and alkynylsilane cyclizations. More importantly, these cyclizations can be run in high chemical yields under mild acid catalysis. 

The starting materials may easily be obtained from a commercially available ketone, which thus can be transformed in a short sequence into the cyclization substrates41. In addition, dienones containing the alkynylsilane terminator can also easily be transformed to m-fused cyclopcn-tanes with the synthetically useful angular alkenyl side chain41,45. 

Regiospecificity of addition becomes reversed, for obvious steric reasons, when alkynylsilanes (Chapter 7) are employed as substrates the product 1,2-disilylalkynes undergo regiospecific protiodesilylation, and so provide a good route to 2-trimethylsilylalk-l-enes, as is discussed later. 

Hydrometallation and carbometallation of alkynylsilanes proceeds regio-and stereospecifically, the metal becoming attached to the silicon-bearing carbon atom in what is normally a co-addition process (hydrostannylation, however, shows the opposite regioselectivity). Electrophilic cleavage, with retention, of the carbon-metal bond then leads to vinylsilanes of various types. 

A solution of hex-l-yne (0.3 mol) in ether (100 ml) was treated consecutively at -78 °C with a solution of n-butyl lithium (0.306 mol) in hexane and with TMSC1 (0.306 mol). The reaction mixture was brought to ambient temperature, stirred for 2h, and then quenched with ice-water. The layers were separated, and the aqueous layer was re-extracted with pentane. The combined organic extracts were washed with water and brine, and dried. Concentration and distillation gave the alkynylsilane (0.267mol, 89%), b.p. 71-73 C/36mmHg. 

To a solution of the alkynylsilane (60 mmol) in EtOH (125 ml) was added a solution of silver nitrate (160 mmol) in water (60 ml) and EtOH over 25 min... 

A convenient route to both saturated and unsaturated acylsilanes lies in the hydroboration-oxidation of alkynylsilanes (Chapter 7). Recent improvements (6) to this method involve the use of the borane-dimethyl sulphide complex for hydroboration, and of anhydrous trimethylamine A-oxide for the oxidation of the intermediate vinyl boranes. 

A subsidiary approach involves nuclear modification of the arylsilanes so obtained. The requisite organometallics can be prepared from aryl halides, or by deprotonation of a suitably activated (c.g. methoxy-substituted) arene. A more specialized route involves cycloaddition between alkynylsilanes and diynes. 

Terminal alkynes can be converted readily into alkynylsilanes by reaction of the corresponding alkyne anion or its metalloid equivalent with a suitable chlorosilane (/). The reverse reaction, that of liberation of the alkyne, is quite facile, being effected by several reagent combinations, including hydroxide ion, methanolysis, fluoride anion, silver(i) followed by cyanide anion, and methyl lithium-lithium bromide (2). 

To a cooled (2-8 °C) solution of A1C13 in carbon disulphide or nitrobenzene was added dropwise an equimolar mixture of the alkynylsilane and the acid chloride (or anhydride), dissolved in a little of the same solvent. After being stirred for 30 min, the reaction mixture was poured onto dilute sulphuric acid/ice. Normal isolation procedures gave the alkynone (50-90%). 

To a stirred solution of the alkynylsilane (20 mmol) and triethylbenzyl-ammonium chloride (0.7 mmol) in MeCN (15 ml) cooled to 0°C was added aqueous sodium hydroxide (15 ml, 12m). After 5-10min, the mixture was diluted with ether and extracted with ether/dichloromethane. Drying, concentration and suitable purification gave the free alkyne (80-90%). 

Alkylidene-l-phenylazetidin-2-ones, 71 Alkylideneazetidin-2-ones, 70-1 Alkynylsilanes, 7, 10, 39, 45-9, 84-5, 131 (Z)-2-Alkynylvinylsilancs, 11... 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

We do not list the many hydrolyses of sodium or potassium enolates, and so on, found in Organic Syntheses. The hydrolysis of a Grignard reagent to give an alkane is found at OS II, 478 the reduction of a vinylic tin compound at OS VIII, 381 and the reduction of an alkynylsilane at OS VIII, 281. 

To see if silacyclobutenes (28) would react in the same manner as the alkynyl-polysilanes (1) and respond similarly to steric differences, compounds 28a and b were heated in the presence of NiCl2(PEt3)2 and PTMSA (Scheme 9). " Compound 28a gave silole 20a in 94% yield,but the only isolable products from the thermolysis of 28b were 26 (51%) and 27b (36%). The products from the thermolysis of silacyclopropene 28b were very similar to that for alkynylsilane lb, but for some reason there were many fewer products for the thermolysis of 28a than for alkynylsilane la. These results suggest that the more sterically hindered lb... 

Numerous other studies " by Ishikawa and co-workers, with or without nickel catalysts, have reinforced the importance of l-silaallenes and nickel-complexed 1-silaallenes as intermediates in the pathways of the photolyses and thermolyses of alkynylsilanes. 

In 1997, the intermolecular addition of organolithium reagents to fluoro-alkynylsilanes was used to synthesize three novel, stable 1-silaallenes. In this... 

An interesting X-ray structure of the highly strained, sp hybridized vinyl cation 8 was recently reported by Muller et al. The synthesis of 8 was accomplished by the reaction of alkynylsilane 7 and triphenylmethyl (trityl) cation. 

As was mentioned above (see Section 12.13.4.2), the 1,1-organoboration of alkynylsilanes, -stannanes, and -germans with trialkylboranes and boracyclanes occurs under very mild (Sn), mild (Ge), or harsh (Si, > 100 °C) reaction conditions <1995CCR125>. Similar reactions with triallylboranes proceed in general under milder conditions in comparison with trialkylboranes and furthermore a competition between 1,1- and 1,2-allylboration often takes place... 

Table 9 Bicyclic compounds with bridgehead boron atom via allylboration of 1 -alkynylsilanes and -tins...

Cyclization reactions of vinyl- and alkynylsilanes have been reviewed100. The course of the reaction of the cyclohexenone derivative 184 depends on the catalyst employed ethylaluminium dichloride gives solely the product 185 of 1,6-addition, whereas tetrabuty-lammonium fluoride yields a mixture containing 69% of the 1,4-adduct 186 and 31% of the bridged compound 187 (equation 89)101. Intramolecular addition reactions of allylic silanes102 may also be catalysed by Lewis acids (equation 90) or fluoride ions, and in this case an allyl anion or a pentavalent silicon intermediate may be involved (equation 91). Such reactions are exemplified by the formation of a 1 5 mixture of the diastereomers 189 and 190 when the cyclohexenone derivative 188 is treated with ethylaluminium dichloride (equation 92). In the presence of fluoride anion the ratio of the isomers is reversed103. 

Alkynylsilanes C=C—SiH Acetylene exchange in complexes 1 and 3 by the alkynylsilanes RC=CSiMe2H (R = 7Bu, Ph, SiMe3, SiMe2H) yields complexes in which an agostic interaction between the Si—H bond and the metal center is indicated for complex 100, but not for 101 [53],... 

Figure 10.15. Products obtained with alkynylsilanes containing Si—H groups.

Another application of [Bu4N][Ph3SiF2] 826 involves the silicon-carbon bond cleavage of allyl-, benzyl-, and alkynylsilane derivatives 827-829.826 Subsequent reactions of the generated carbanions with electrophiles (Scheme 112) and alkyl halides (Scheme 113) provide high yields of carbon-carbon coupled products. 

Alkynylsilanes will react with bis(tributyltin) oxide in the presence of fluoride ion as catalyst to give the stannylalkynes in excellent yield and hexamethyldisiloxane, which can be removed under reduced pressure (Equation (82)).68 244... 

Electrophilic substitution reactions on alkynylsilanes are also known. In 1978 K. Ultimootd, M. Tanaka and coworkers by an intramolecular reaction carried out the synthesis of a macrocyclic ketone. 

A number of additional methods involve the addition of alkynylsilanes to electrophiles with concomitant 1,3-isomerization to afford allenylsilanes geminally substituted with the electrophile moiety. The first of these methods employed a trimethyl-silyl-substituted propargylic silane as the alkynylsilane and various acetals as the electrophile precursors (Table 9.29) [53], The allenylsilanes are formed without contamination by alkynyl isomers. 

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