Di-ferf-butylsilylene

Transition metal complexes were known to facilitate the addition of silylene to acetylenes from a variety of different sources.60,61,90,91 These conditions, however, often required heating, and the initially formed silacyclopropene often incorporated a second molecule of the acetylene to afford a silole.92,93 With their discovery of low-temperature silver-mediated di-ferf-butylsilylene transfer conditions from cyclohexene silacyclopropane 58 to olefins, Woerpel and coworkers set out to investigate the... 

Woerpel and Clark identified silver phosphate as the optimal catalyst to promote di-ferf-butylsilylene transfer from cyclohexene silacyclopropane to a variety of substituted alkynes (Scheme 7.25).95 While this silver salt exhibited attenuated reactivity as compared to silver triflate or silver trifluoroacetate, it exhibited greater functional group tolerance. Both di- and monosubstituted silacyclopropenes were easily accessed. Terminal alkynes are traditionally difficult substrates for silylene transfer and typically insert a second molecule of the starting acetylene.61,90 93 Consequently, the discovery of silver-mediated silylene transfer represents a significant advance as it enables further manipulation of monosubstituted silacyclopropenes. For enyne substrates, silylene transfer the alkynyl group was solely observed. The chemoselectivity of the formation of 99f was attributed to ring strain as theoretical calculations suggest that silacyclopropenes are less strained than silacyclopropanes.96 97... 

SILVER-MEDIATED TRANSFER OF DI-ferf-BUTYLSILYLENE TO IMINES... 

Scheme 7.56. Transition metal-catalyzed insertion of di-ferf-butylsilylene into C-O bonds.

Scheme 7.59. Silver-catalyzed di-ferf-butylsilylene transfer. REFERENCES...

Trost first introduced the di-fe/7-butylsilylene derivative as a means for protecting 1,2- and 1,3-diols during a synthesis of PiUaromycinone derivatives.213 Di-ferf-butylsilylene derivatives are not as robust as isopropylidene or benzylidene acetals and their use is best reserved for systems requiring deprotection under very mild conditions. Di-isopropylsiiylene derivatives are occasionally used but they usually only survive in highly crowded environments.214 Di-feri-butylsily-lene derivatives survive hydroboration with 9-BBN, mild oxidation (e g the Dess-Martin, ozone), Lewis acids such as trifluoroborane e the rate and titanium tetrachloride, mild acids (pyridinium p-toluenesulfonate). camphorsulfonic acid, strong bases such as feri-butyllithium (THF, -50 °C), DDQ, and sodium meth-oxide in methanol at 0 C — conditions used to cleave acetate esters. 

Brooks, C.J.W. and W.J. Cole. 1988. Cyclic di-ferf-butylsilylene derivatives of substituted salicylic acids and related compounds. A study by gas chromatography-mass spectrometry. J. Chromatogr. 441 13-29. 

The final step in a recent synthesis of the antifungal agent Papulacandin D involved deprotection of five O-silyl groups including a di-ferf-butylsilylene group, a triethylsilyl ether and two phenolic tri-isopropylsilyl ethers [Scheme 3.115]. Acid conditions were precluded by the acid lability of the side chain. Use of TBAF was complicated by problems in separating the product from tetrabuty-lammonium salts. The desired global deprotection was accomplished with tris-(dimethylamino)sulfonium difluorotrimethylsilicate (TAS-F). 

Alkyl-boronic acids, such as n-butylboronic acid, react with 1,2- or 1,3-diols or with a- or p-hydroxy acids to form 5- or 6-membered ring non-polar boronate derivatives (Figure 4.1(e)). They are prepared simply by adding n-butylboronic acid to a solution of the hydroxy-compound in dimethylformamide. The reaction is complete in 10-20 minutes at room temperature and the reaction mixture can be injected directly into a gas chromatographic column for analysis [55,133]. As alternatives, cyclic di-ferf-butylsilylene derivatives have been shown to be of value in the analysis of diols and hydroxy acids [132]. The preparation and use of cyclic derivatives for the analysis of bifunctional compounds have been reviewed [730]. 

The reaction of a diol with di-terf-butyldichlorosilane [Scheme 3.118]223 in the presence of 1-hydroxy-benzotriazole (HOST) was the first method used to prepare di-fert-butylsilylene derivatives. Di-ferf-butylsilyl bisftrifluoromethanesulfo-nate),224 225 in the presence of a base such as 2,6-lutidine, effects silylene formation more rapidly and under milder conditions than the less reactive dichloride [Scheme 3.1I9].110... 

Di-tert-butylsilylene (43) (generated by photolysis of hexa-rert-butylcyclotrisilane) reacts with N-methylpyrrole (44) possibly via an intermediate [2+1] cycloadduct 45 to furnish 3,3-di-ferf-butyl-2-methyl-2-aza-3-silabicyclo[2.2.0]hex-5-ene (46) <970M3080>. On heating, 46 undergoes electrocyclic rearrangement to afford the 1 -aza-2-silacyclohexa-3,5-diene 47. 

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