Di-t-butylmethylsilyl Trifluoromethanesulfonate1


[105866-92-0]  · C10H21F3O3SSi  · Di-t-butylmethylsilyl Trifluoromethanesulfonate  · (MW 306.42)

(bulky trialkylsilylating agent; useful for preparation of hydrolytically stable, sterically congested enol silyl ethers and silyl carboxylic esters; t-Bu2MeSi- also serves as a reactivity control element)

Alternate Name: DTBMSOTf.

Physical Data: colorless, water-sensitive liquid; bp 63-65 °C/1.5 mmHg.

Solubility: sol THF, Et2O.

Form Supplied in: not available commercially.

Preparative Method: addition of Trifluoromethanesulfonic Acid (TfOH) to neat di-t-butylmethylsilane2 at 4 °C, stirring 16 h at 20 °C, followed by distillation in vacuo.1

Purification: vacuum distillation through a Vigreux column.

Handling, Storage, and Precautions: stable in the absence of moisture. Fumes in moist air, producing TfOH by hydrolysis. Owing to the high corrosivity of TfOH, breathing the vapor or the fumes produced by reaction with moist air should be avoided. Use in a fume hood.

Enol Silylation.

Di-t-Butylmethylsilyl triflate converts a hydroxymethylene ketone into the corresponding DTBMS enol silyl ether (eq 1).1 DTBMS ethers were prepared previously by the reaction of alcohols with DTBMS perchlorate.2 The bulky substituents on silicon suppress 1,4-addition of MeLi to the DTBMS ethers of hydroxymethylene ketones. Thus the proportion of 1,2-adduct versus 1,4-adduct improves as the steric bulk of the silyl substituents increases from trimethylsilyl (TMS) to t-butyldimethylsilyl (TBDMS) to DTBMS enol ethers (eq 2).1 Dehydration in conjunction with hydrolysis of the DTBMS enol ether group delivers an a,b-unsaturated aldehyde in excellent yield (eq 3).

TMSCl accelerates the conjugate addition of Grignard reagents catalyzed by copper salts.3 Therefore the influence of silyl substituents on the reaction of 2-cyclohexenone with Ethylmagnesium Bromide in the presence of a chiral nonracemic copper catalyst was examined (eq 4).4 The anticipated silyl enol ether intermediate was rapidly hydrolyzed in the workup, delivering optically active 3-ethylcyclohexanone. The bulky silyl reagent, DTBMSOTf, afforded better enantioselectivities than TMSCl in the addition of EtMgBr to cyclohexenone (eq 4). However, for the reaction with n-BuMgBr, enantioselectivities obtained with TMSCl, DTBMSOTf, and Ph2(t-Bu)SiCl were 51% ee, 40% ee and 78% ee, respectively. The ability of the silyl reagent structure to influence the enantioselectivity suggests that a silylated species is involved in the rate-determining step of the conjugate addition.

Silyl Esterification.

Carboxylic acids are converted into DTBMS esters by treatment with DTBMSOTf and Triethylamine in THF (eq 5). Although hydroxy carboxylic acids can be selectively converted to hydroxy silyl carboxylic esters (eq 5), the extraordinary stability of DTBMS esters allows an alternative route. Thus silyl esterification of a tetrahydropyranyloxy carboxylic acid followed by selective removal of the THP ether protecting group by treatment with Pyridinium p-Toluenesulfonate in ethanol5 delivers a hydroxy DTBMS ester (eq 6).1

The bulky substituents on silicon confer resistance to nucleophilic attack at the carbonyl group of DTBMS esters. Thus selective reduction of a butyrolactone in the presence of a t-butyl or DTBMS ester gives a much better yield than the corresponding methyl ester (eq 7).1,6

Removal of the DTBMS ester protecting group under mild acidic conditions is achieved in better yield than removal of the t-butyl group, but in neither case can deprotection be accomplished without concomitant dehydration (eq 8).1,6

Conversion of DTBMS esters to carboxylic acids can also be achieved by treatment with Tetra-n-butylammonium Fluoride in wet THF. Under these conditions, both TBDMS ether and DTBMS ester protecting groups are removed (eq 9).1

1. Bhide, R. S.; Levison, B. S.; Sharma, R. B.; Ghosh, S.; Salomon, R. G. TL 1986, 27, 671.
2. Barton, T. J.; Tully, C. R. JOC 1978, 43, 3649.
3. Corey, E. J.; Boaz, N. W. TL 1985, 26, 6015.
4. Ahn, K.-H.; Klassen, R. B.; Lippard, S. J. OM 1990, 9, 3178.
5. Miyashita, M.; Yoshikoshi, A.; Grieco, P. A. JOC 1977, 42, 3772.
6. Levison, B. S.; Miller, D. B.; Salomon, R. G. TL 1984, 25, 4633.

Robert G. Salomon

Case Western Reserve University, Cleveland, OH, USA

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