Silyl acetylenes

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). 

Trialkylsilanes are usually formed by the addition of a lithium or Grignard reagent to the silyl chloride, and thus, discussions related to the formation of the silyl acetylene bond will be kept to a minimum. Silyl acetylenes are prepared from the alkynylcopper(I) reagents in the presence of PPh3, Zn or TMEDA in CH3CN at 100°, 36-98% yield. It is interesting to note that the... 

Silylated acetylenic alcohols such as 1429 cychze with benzaldehyde in the presence of Bp3-OEt2 (or KF) to give allenyltetrahydrofurans 1430 a or allenyltetrahy-dropyrans 1430b [9] (Scheme 9.7). 

Silylated acetylenic alcohols such as 1500 cyclize on treatment with HMDS-Li to give, via 1501 and 1502, 2-phenylpyrrole 1503 [46] (Scheme 9.27 compare also the formation of 2-pyridyl-2-pyrrole 543 in Chapter 5). 

The silylated acetylene alcohol 1589, however, is converted by ethyllifhium in 87% yield into the 1,2,3-butatriene system 1590 and Me3SiOLi 98 [8] (Scheme 10.2). 

The benzoiodoxol 2035 is converted by Me3SiN3 19 into 2036, which oxidizes cyclohexene to a-azidocyclohexanone 2037 [192], whereas 2035 reacts with bis(trimethyl-silyl)acetylene 2038 to give the iodonium salt 2039 [193] (Scheme 12.57). 

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. 

A much more general and very simple synthesis requiring a minimum of laboratory manipulation uses a Swern oxidation of the corresponding diols to give the a-ketoacyl silanes directly in useful yields (Scheme 38)7,124. Purification in this case was accomplished in the dark, by chromatography at —78°C or by distillation. a-Ketoacyl silanes appear to be intermediates in the oxidation of silyl acetylenes to a-ketoesters by osmium tetroxide125, and indeed have been isolated from the oxidation of silyl acetylenes by dimethyl dioxirane126. 

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). 

Control of the regioselectivity in Co-catalysed cross-cyclization has been solved in an ingenious way utilizing 1,5-hexadiyne (116) as one component and bis(trimethyl-silyl)acetylene (118) as the other. Although bulky bis(trimethylsilyl)acetylene (118) itself cannot cyclotrimerize to hexasilylbenzene due to steric hindrance, it reacts with the cobaltacyclopentadiene 117 formed from the less bulky diyne 116 to produce the... 

Muenter, J. S., and V. W. Laurie Microwave Spectrum, Structure, and Dipole Moment of Silyl Acetylene. J. chem. Physics 39, 1181—1182 (1963). 

Among silicon-containing acetylenes, 3-(trimethylsilyl)-l-octyne and its homologs are converted to high-molecular-weight polymers36 (Table 11). The MoCls-cocatalyst and Mo(CO)6—CC —hv systems afford the highest MW s. On the other hand, 1-silyl-acetylenes such as l-(trimethylsilyl)acetylene and l-(dimethylphenylsilyl)acetylene afford partly toluene-insoluble polymers 34,35), and Mn of the soluble fractions is no more than ca. 1 x 104. 

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... 

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]. 

Stable gem-difluoroallenylindiurri(l) is prepared from bromodifluoromethyl silyl acetylene, which couples with aldehydes to give homopropargylic gem-difluoro alcohols with aqueous formaldehyde allenyl alcohols are obtained (Scheme 8.3) [17]. 

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