Silyl enol ethers trifluoromethanesulfonate

S. Kobayashi, Lanthanide Trifluoromethanesulfonates as Stable Lewis Acids in Aqueous Media. Yb(OTf)3 Catalyzed Hydroxymethylation Reaction of Silyl Enol Ethers with Commercial Formaldehyde Solution Chem Lett. 1991, 2187-2190. 

The reaction of 5-methylene-l,3-dioxepanes 386 with trimethylsilyl trifluoromethanesulfonate in the presence of base forms cyclic silyl enol ethers 387 (Scheme 90) <2000TL2171>. 

In place of dtphenylsilane, 1,2-bis(dimethylsilyl)ethane was applied to the reduction of several ketones by the combination of the Py box-rhodium complex and Silver(I) Trifluoromethanesulfonate to give the corresponding silyl enol ether exclusively (eq 6). ... 

Ketone silyl enol ethers react with derivatives of diacyl azo compounds at room temperature245 or on heating242,243 (see also Eq. 82) as well as enantio-selectively under the influence of silver trifluoromethanesulfonate and BINAP (Eq. 93)244 or copper bis(oxazoline) complexes (Eq. 94). The latter is proposed to proceed via a formal hetero Diels-Alder adduct.252 Ketones themselves react with azodicarboxylic esters either thermally246,389,390 or in the presence of potassium carbonate390 but yields are low. Higher yields can be achieved with LDA,391 394 (see also Eq. 88), LiHMDS,395,396 or KOBu-r325 as the bases. Aryl diazonium... 

Kobayashi, S. (1991) Lanthanide trifluoromethanesulfonates as stable Lewis acids in aqueous media. Yb(OTf)3 catalyzed hydroxymethylation reaction of silyl enol ethers with commercial formaldehyde solution, Chem. Lett., 2187-90. 

Various pH aqueous solutions (pH = 1-6) of trifluoromethanesulfonic acid were prepared independently, and the model reaction of the silyl enol ether derived from cyclohexanone (2) with benzaldehyde was tested. Six independent experiments were performed (pH = 1, 2, 3, 4, 5, 6). In the pH 5 and 6 solutions, only a trace amount of the product was detected on TLC, the yields were less than 5%, and hydrolysis of 2 was also observed. In the pH 1-4 solutions, the silyl enol ether immediately hydrolyzed to give the original ketone, and no aldol adduct was obtained. From these experiments, the protons that may be produced from the hydrolysis of the lanthanide triflates were found not to be an active catalytic species in the present aldol reaction of silyl enol ethers with aldehydes the pH values of Yb(OTf)3 solutions were measured as 5.90(1.6x 10- M,THF-H20,4 l)and6.40(8.0x 10- M, HjO). 

A highly Lewis acidic silyltriflimide can be generated in situ upon treatment of silyl enol ethers with bis(trifluoromethanesulfon)imide (Tf2NH) in a catalytic amount. For example, enol tris(trimethylsilyl)silyl ethers derived from aldehydes react with another aldehyde in the presence of a catalytic amount of TfjNH to give cross-aldols exclusively (Scheme 3-80). ... 

The asymmetric aldol reaction of enol silyl ethers of thioesters with aldehydes is performed in high enantiomeric excess by employing a chiral promoter, tin(II) trifluoromethanesulfonate coordinated with chiral diamine 1 and tri-n-butyltin fluoride (eqs 20 and 21). Highly enantioselective aldol reactions of achiral ketene silyl acetals with achiral aldehydes are carried out by means of the same chiral promoter (eq 22). ... 

Aldol-type condensation of enol silyl ethers and acetals or orthoesters can be accontplished by the use of trimethylsilyl trifluoromethanesulfonate (TMSOTf). In these reactions, TMSOTf acts as a true catalyst and is required in 1-S ntol %. The reactions show interesting chemoselectivity in that acetals are highly reactive receptors of enol silyl ethers but the parent aldehydes and ketones do not react under these conditions (Scheme 37). Similarly, trityl perchlorate is found to be an efficient catalyst to promote the reaction between enol silyl ethers and acetals. 

---