Silyl acetylenes, reactions

Simple cyclobutanes do not readily undergo such reactions, but cyclobutenes do. Ben-zocyclobutene derivatives tend to open to give extremely reactive dienes, namely ortho-c]uin(xlimethanes (examples of syntheses see on p. 280, 281, and 297). Benzocyclobutenes and related compounds are obtained by high-temperature elimination reactions of bicyclic benzene derivatives such as 3-isochromanone (C.W. Spangler, 1973, 1976, 1977), or more conveniently in the laboratory, by Diels-Alder reactions (R.P. Thummel, 1974) or by cycliza-tions of silylated acetylenes with 1,5-hexadiynes in the presence of (cyclopentadienyl)dicarbo-nylcobalt (W.G, Aalbersberg, 1975 R.P. Thummel, 1980). 

The reactions of complex 2a with ketones and aldehydes show a strong dependence on the substituents. With benzophenone, substitution of the silyl-substituted acetylene leads to the r]2-complex 58, which is additionally stabilized by a THF ligand. This complex can serve as an interesting starting material for other reactions. With benzaldehyde and acetophenone, the typical zirconadihydrofuran 59, akin to 2c, is obtained from a coupling reaction. This complex is unstable in the case of benzaldehyde and dimerizes, after elimination of bis(trimethylsilyl)acetylene, to yield 60. In this respect, it is similar to the above discussed complex 2c, since both of them show a tendency to eliminate the bis(trimethyl-silyl)acetylene. The reaction of methacrolein with complex 2a depends strongly on the solvent used [40]. 

Vollhardt118) employed the organocobalt-promoted reaction between Wjftrimethyl-silyl)acetylene (322) and the diyne (338) to construct the steroid estrone 118). 

In 1999, Sekiguchi et al. prepared the first silyl acetylene dendrimers 255 and 256 with up to 22 Si atoms and 21 acetylene units (Schemes 35 and 36).358 The preparation of 255 and 256 has been achieved by the reaction of the triple Grignard reagent 257 with the trisila diyne 258 and the heptasila hexayne 259, respectively. 

These compounds have been obtained indirectly by reactions of silylated acetylenes with metal carbonyls or olefin complexes. Thus, trimethylsilylphenylacetylene reacts with rj5-cyclopentadienylcobalt dicarbonyl, cobaltocene, or rjs-cyclopentadienyl-(l,3-cyclooctadiene) cobalt, in refluxing xylene, to give a mixture of cis- and trans-bis-(trimethylsilyl)cyclobutadiene complexes (R = Me, R = Ph) 68, 127, 137) ... 

Perhaps the best general method to date for preparing a-haloacyl silanes involves bromi-nation of silyl enol borinates (9) at 0 °C, a reaction which proceeds in good yield and involves no sensitive intermediates. This route offers a most convenient one-pot synthesis of a-haloacyl silanes from readily available starting materials, as the intermediate enol borinates are very easily prepared from silyl acetylenes (Scheme 35)7,117,118. 

Another very elegant reaction is the synthesis of carbon-silylated isoxazoles213 by means of a 1,3-dipolarophilic cycloaddition of nitrile oxide with silylated acetylenes (7), (12) (Scheme 52). If mesitylnitrile oxide (361) and 3,3-dimethyl-3-sila-l, 4-pen-... 

In a different pattern, by using silylated acetylenes, substituted pyridazines are obtainable217 from the tetrazine derivative 401 in a diene-type reaction, first introduced by Carboni and Lindsey218. Via this reaction 4-TMS- (402) and 4,5-bis(TMS)-3,6-bis(methoxycarbonyl)pyridazine (403) can be achieved in very high yield, being inert against acid catalyzed desilylation (Scheme 59). 

Silylene extrusion from siliranes in the presence of alkynes, notably bis(trimethyl-silyl)acetylene, gives the silirene (35) in good yield (Scheme 41) (76JA6382). Compound (35) is more stable thermally than hexamethylsilirane and shows Si NMR absorptions for the ring atom at 5 = 106.2 p.p.m., some 50 p.p.m. downfield from those of silacyclopropanes, and about 100 p.p.m. downfield from normal cyclic and acyclic tetraalkylsilanes. Notable reactions include alcoholysis and the insertion of aldehydes and ketones, dimethylsilylene... 

Allylic phosphates and allylic phosphinates (114) have been used as electrophiles in efficient silyl cupration reactions of a variety of acetylenes. This method provides an easy access to substituted 1,4-diene systems (115) (Scheme 25). "... 

Under extremely high pressure, the isonitrile-palladium catalyst promotes intramolecular bis-silylation of bis(silyl)acetylenes 51 to give tetrakis(silyl) alkenes 52, which are otherwise difficult to synthesize the reaction under atmospheric pressure hardly proceeds even at 200°C (Eq. 22) [48,49]. 

Further examples of the utility of 2-acetoxyglycals as C-glycosidation substrates were reported by Tsukiyama, et al.,68 and are illustrated in Scheme 2.3.42. This study demonstrated the highly stereospecific high yielding nature of these reactions. Furthermore, the generality of these reactions was demonstrated in their application to a variety of silylated acetylenes. 

A common approach is the reaction of halogenoalkylsilanes with alkali metal (lithium, sodium)110,111 or magnesium (Grignard)112 derivatives of acetylene to form silylated acetylenes which can be illustrated by the synthesis of TMS-acetylenes (182/57a) (equation 84). The reaction can be slightly modified by using bis(TMS)-sulfate (184) as silylating agent (equation 85)113. 

The classical route to bisilylacetylenes is the reaction of the di-Grignard derivative of acetylene (188) (equation 88)116. Also, the reaction of tetrachloroethene (53) with lithium and Me3SiCl in THF at — 78 °C gives rise to 189 (equation 89)114. Alternatively, the application of methyllithium to halogenated alkanes or alkenes with subsequent employment of bromo- or fluorosilanes affords bis-silylated acetylenes (equation 90)115. 

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