115726-23-3Relevant articles and documents
Homogeneous catalysis. Mechanisms of the catalytic mukaiyama aldol and sakurai allylation reactions
Hollis, T. Keith,Bosnich
, p. 4570 - 4581 (1995)
The mechanisms of a number of Mukaiyama aldol and Sakurai allylation reactions catalyzed by the Lewis acids [Ti(Cp)2(OTf)2], Ph3COTf, and Ph3CClO4 have been investigated. It is found that hydrolysis of the Lewis acid by trace amounts of water in the solvent can lead to the formation of acid. The acid then reacts with the silyl enol ether or allylic silane to generate Me3SiOTf or Me3SiClO4, both of which are powerful catalysts for these reactions. Dehydration of the solvent or addition of a hindered base to quench the acid does not necessarily prevent the formation of these silyl catalysts. In the case of the [Ti(Cp)2(OTf)2] Lewis acid, Me3SiOTf is generated as a consequence of the mechanism, and it is shown that all of the catalysis proceeds by the Me3SiOTf species and that [Ti(Cp)2(OTf)2] acts only as an initiator for the production of Me3SiOTf. For the case of the presumed Ph3COTf catalyst, the Mukaiyama aldol reaction proceeds exclusively by the Me3SiOTf catalyst and Ph3COTf is neither an initiator nor a catalyst. It generates Me3SiOTf by hydrolysis. It is shown that the rate and stereoselectivity of Me3SiOTf catalysis depend on the concentration of triflate ions. The rate slows and the selectivity switches with an increase in triflate concentration. The rate of Sakurai catalysis of allylsilane with an acetal using Ph3CClO4 can be quantitatively accounted for by invoking only Me3SiClO4 catalysis. The Me3SiClO4 can be generated by hydrolysis, but when this is suppressed, Me3SiClO4 can be formed by allylation of Ph3CClO4. It is suggested that many of the reported catalysts may only be agents for the production of the real catalysts, namely, Me3SiX species. The characteristics of Lewis acids which may obviate the intrusion of Me3SiX catalysis are outlined.
Regioselective polymethylation of α-(1 → 4)-linked mannopyranose oligosaccharides
Xia, Li,Lowary, Todd L.
, p. 2863 - 2880 (2013/07/04)
An approach has been developed of the regioselective methylation of α-(1 → 4)-linked mannopyranose oligosaccharides via a four-step methodology. The key reaction involved n-Bu2SnCl2-mediated activation of cis-diols. By tuning protecting groups on the substrates, multiple cis-diols in the substrates were functionalized in a consistent and regioselective manner. Using optimized substrates and reaction conditions, regioselective methylation of di-, tri-, and tetrasaccharide substrates proceeded in isolated yields per cis-diol of 95, 88 and 79%, respectively. The methodology was also applied to functionalize trans-diols in α-cyclodextrin.
Reactions of the Readily Accessible Electrophile, Trifluoroacetyl Triflate: A Very Reactive Agent for Trifluoroacetylations at Oxygen, Nitrogen, Carbon, or Halogen Centers
Forbus, T. R.,Taylor, S. L.,Martin, J. C.
, p. 4156 - 4159 (2007/10/02)
Trifluoroacetyl triflate (TFAT) is readily prepared in 82percent yield by the dehydration (phosphorus pentoxide) of a 2:1 mixture of trifluoroacetic acid and trifluoromethanesulfonic (triflic) acid.Reactions of this highly electrophilic trifluoroacetylating reagent with alcohols, ketones, ethers, amines, and pyridines give esters, enol esters, ether cleavage, amides, and acylpyridinium ions, respectively.Reactions with ionic or easily ionizable alkyl halides give the very volatile trifluoroacetyl halides and the ionic triflate.Triphenylmethyl chloride, for example, is quantitatively converted to triphenylcarbenium triflate in a very convenient synthetic procedure.Trifluoroacetyl triflate is used in the synthesis of the first member of a new class of pyrylium salts, 2,6-dimethoxypyrylium triflate.
