Sila-acetylene

Novel photochemical (and thermal) reactions of macrocyclic oxa-sila-acetylenic ring systems (expected to show unusual optical properties because of electronic effects arising from orbital overlap of the acetylenic n system with the silicon a bonds and the oxygen lone-pair electrons) were described. While thermolysis in the presence of a transition metal carbonyl compound gave cyclization to both benzenoid and fulvene species, photolysis in the presence of the transition metal carbonyl compound (which catalyzes 1,2-silyl shifts across a carbon-carbon triple bond) gave fulvene and vinylidene products, the latter being readily photolyzed to the fulvene 159 (equation 101). 

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-... 

Both ring-expansion reactions proceed via a putative pallada(sila)cyclopentane intermediate. In the course of the mechanistic elucidation studies, Tanaka succeeded in preparing a l-pallada-2-silacyclopentane complex with quantitative conversion (Scheme 51) <19970M3246>. Formation of the complex is reversible, and the starting SCB can be released by addition of an acetylene, which acts as a ligand for palladium and displaces the SCB. 

There is a significant decrease of about 10 Hz in the 13C—13C-coupling constants on going from the open-chain reference compound to the seven-membered sila-cycloheptyne system however, considering a difference of 83 Hz between acetylene... 

Addition Reactions. All the metallaaromatic species kinetically stabilized by a Tbt group showed considerably high reactivity toward the reagents such as water, methanol, styrene, phenyl acetylene, mesitonitrile oxide, benzophenone, and 2,3-dimethyl-1,3-butadiene. Some of them were allowed to react with elemental chalcogen such as sulfur and selenium to give several kinds of unique cyclic chalcogenides as the cycloadducts. In Scheme 60 were summarized the reactions of the sila- and germabenzenes as the representative. 

The overwhelming majoritiy of currently known alkynyliodonium species are prepared by interaction of a terminal, sila- or tin-acetylene with an electrophilic X -iodane, also referred to as a 10-1-3 hypervalent species [14]. Key reagents are iodosobenzene (4), [hydroxyftosyl-oxy)iodo]benzene (5, HTIB) [15] the -oxo-X -iodane 6 [16], and cyano(phenyl)iodonium triflate 7 [17]. 

A more general, simpler procedure [20] takes advantage of the in situ formation of the p-oxo-bis-triflate (6 R=CF3) and its interaction with a sila- or tin-acetylene [Eq. (2)]. This methodology affords a wide variety of stable, alkynyl(phenyl)iodonium triflates 10 in good to excellent yields and is applicable to the synthesis of the parent [21] ethynyl(phenyl)iodonium triflate (10 R=H) from n-Bu3SnC=CH. 

While 1-sila- and 1-germa cyclopent-3-enes undergo addition and ring isomerization with PhSOaNSO to give the readily reducible cyclopent-2-enes (50), allenylsilenes give the acetylene [equation (60)]. ... 

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