Trimethylsilyl acetylene derivatives, with

A variety of alkynyl derivatives, including trimethylsilyl acetylene and terminal acetylene derivatives, have also been prepared by delithi-ation reactions with N3P3F6 (77, 79). The terminal silyl group of the trimethylsilyl acetylene derivatives has been substituted by H using KF and EtOH. The alkynyl groups of some of these substituted phos-phazenes have been found to react with Co2(CO)8, forming Co2(CO)6 complexes (77). 

Electron-rich alkynes, including ethoxy(trimethylsilyl)-acetylene, reacted with chromium alkenylcarbene conplexes at — 10°C in acetonitrile in the presence of a stoichiometric amount of Ni(cod)2 with warming to 20 °C over 2 h to afford moderate yields (40-49%) of cyclopentenones (eq 10). This completely stereoselective nickel(0)-mediated [3 + 2] cyclization reaction of chromium alkenyl(methoxy)carbene complexes with both electron-withdrawing and electron-donating substituted alkynes provides a general synthesis of substituted 2-cyclo-pentenone derivatives, important synthons for the construction of more complex molecules. ... 

Oxidations. Aikynes of high nucleophilicity such as ethoxy-(trimethylsilyl)acetylene react with electrophilic O3 to give vicinal dicarbonyl derivatives. In contrast to alkylated or ary-lated acetylenes, neither products of complete C-C cleavage nor peroxidic materials were detected as primary products. Ethoxy-(trimethylsilyl)acetylene reacted with ozone to yield a mixture of ethyl 2-oxo-2-(trimethyl)acetate (13) and ethyl trimethylsUyl oxalate (14) (eq 11). The mechanism of this reaction was also discussed. ... 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-1 -cyclohexenyl t-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide417. When the nucleophile was an sp3-hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

Reactions of salts of 1,2,3-triazole with electrophiles provide an easy access to 1,2,3-triazol-jV-yl derivatives although, usually mixtures of N-l and N-2 substituted triazoles are obtained that have to be separated (see Section 5.01.5). Another simple method for synthesis of such derivatives is addition of 1,2,3-triazole to carbon-carbon multiple bonds (Section 5.01.5). N-l Substituted 1,2,3-triazoles can be selectively prepared by 1,3-dipolar cycloaddition of acetylene or (trimethylsilyl)acetylene to alkyl or aryl azides (Section 5.01.9). 

Alkynyl-substituted sydnones 106 are prepared from the new trimethylsilylethynyl derivative 105. 4-Cuprio-3-phenylsydnone 104 <1996CHEC-II(4)165> reacts with l-bromo-2-trimethylsilyl acetylene to give product 105,... 

Sonogashira reactions of both a-halothiophenes [117] and P-halothiophenes [118] proceed smoothly even for fairly complicated molecules as illustrated by the transformation of brotizolam (134) to alkyne 135 [119]. Interestingly, 3,4-bis(trimethylsilyl)thiophene (137), derived from the intermolecular cyclization of 4-phenylthiazole (136) and bis(trimethylsilyl)acetylene, underwent consecutive iodination and Sonogashira reaction to make 3,4-bisalkynylthiophenes [120], Therefore, a regiospecific mono-i/wo-iodination of 137 gave iodothiophene 138, which was coupled with phenylacetylene to afford alkynylthiophene 139. A second iodination and a Sonogashira reaction then provided the unsymmetrically substituted 3,4-bisalkynylthiophene 140. 

The reaction of trimethylsilylated terminal alkynes with iodoarenes can be performed under 1 atm CO pressure in the presence of dppf complex of palladium, and BU4NF at room temperature (Equation (23)). As trimethylsilyl derivatives of terminal acetylenes are known to undergo facile cleavage by fluoride ions, this reaction actually involves not the organosilicon compound, but acetylenide nucleophile. The method has been successfully applied to the modification of uracyl deoxynucleosides. 

Diels-Alder reactions of bis(trimethylsilyl)acetylene.1 A catalyst obtained from TiCl4 and (C2H5)2A1C1 (1 20) effects Diels-Alder reactions of this acetylene with butadiene and methyl-substituted derivatives to form l,2-bis(trimethylsilyl)-cyclohexa- 1,4-dienes in 70-78% yield (equation I). The yield is low (15%) only when R, R4 = CH3,R2,R3 = H because of polymerization of the diene. The products undergo thermal dehydrogenation at 240° to form l,2-bis(trimethylsilyl)ben-zenes in almost quantitative yield. This cycloaddition has been effected in low yield with an iron-based catalyst. 

The ethoxycarbocation intermediate (363) produced by the action of acid on the cyclobutenedione monoacetal (362) has been found to react with bis(trimethylsilyl)-acetylene to afford a 2-methylenecyclopent-4-ene-l,3-dione derivative (365). The authors426 proposed that the rearrangement results from an unprecedented cationic 1,2-silyl migration on the alkynylsilane, subsequent ring expansion via a vinyl cation intermediate (364), and re-closure by intramolecular addition of an acyl cation to a silylallene in a 5-exo-trig mode (see Scheme 90). 

Additionally, acetylene itself is a useful two-carbon building block but is not very convenient to handle as it is an explosive gas. Trimethylsilyl acetylene is a distillable liquid that is a convenient substitute for acetylene in reactions involving the lithium derivative as it has only one acidic proton. The synthesis of this alkynyl ketone is an example. Deprotonation with butyl lithium provides the alkynyl lithium that reacted with the alkyl chloride in the presence of iodide as nucleophilic catalyst (see Chapter 17). Removal of the trimethylsilyl group with potassium carbonate in methanol allowed further reaction on the other end of the alkyne. 

Cycloaddition of thiazolium azomethine ylides with dialkyl acetylenedicarboxylates 61 provides another approach to pyrrolo[2,1 -bjthiazoles 64 <070L4099>. Quatemization of 2-methylthiothiazole with trimethylsilylmethyl trifluoromethanesulfonate (TMSChkOTf) and subsequent fluoride-induced desilylation of the resulting (trimethylsilyl)methylammonium salt generate the acyclic azomethine ylide 62. This ylide readily participates in 1,3-dipolar cycloadditions with acetylene derivatives 61 to give adducts 63, which undergo spontaneous elimination of methylmercaptan to give the A-fuse cl thiazoles 64. ... 

The next three reactions (Scheme 3.38), cotrimerization of enediyne 146 with bis(trimethylsilyl)acetylene 150, thermolysis of the benzocyclobutene derivative 151, and the intramolecular Diels-Alder reaction of the intermediate 152, were... 

A major initial limitation of the benzocyclobutene approach to o-quinodimethanes was the lack of efficient, large-scale syntheses for many specifically substituted derivatives. Fortunately, recent developments have lemov much of this impediment. Q>nceptually, the synthesis of benzocyclobutenes from aromatic precursors can be envisaged in only a limited number of ways. These include [2 -i- 2] cycloadditions involving benzynes and alkenes, intramolecular cyclization on to a benzyne, cyclizations involving arene anions, and electrocyclic closure of o-quinodimethanes. Benzocyclobutene derivatives can also be prepared by aromatization of bicyclo[4.2.0]octanes. Detailed discussion of variations to these approaches can be found in the cited reviews. The cobalt catalyzed co-oligomerization of 1,5-hexadiynes with al-kynes, especially bis(trimethylsilyl)acetylene, has also been employed for the preparation of specifically substituted benzocyclobutenes. In the latter case the cyclobutenes are often not isolated but converted directly to o-quinodimethanes and subsequent products. ... 

To determine the aetivated faee of a carbonyl group in an acetylenic aldehyde-CAB 2 complex, an aldol reaction of acetylenic aldehydes with the trimethylsilyl enol ether derived from acetophenone was performed in the presence of 20 mol % 2 under conditions similar to those in the Diels-Alder reaction (Eq. 32). Good enantioselec-tivity was, with the predominant enantiomer corresponding to attack on the re face, as expected. Although it is essential to stress that the results of an aldol reaction cannot be directly used to explain the transition state in cycloaddition, the effective steric shielding of the si face of the coordinated aldehyde is consistent with cycloaddition via the proposed transition-state model 16. 

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