Allylic protodesilylation

Recently we investigated a new class of silyltriflates and we found that allyl substituted alkynyl-silanes react with triflic acid under protodesilylation of the allyl group. The resulting alkynylsilyl-triflates are interesting starting materials for chain-like oligomeric silylalkynes with various substituents (Eqs.(7,8)). 

Scheme 3. Proposed mechanism for the protodesilylation of the polymer-bound metathesis products formed from allyl ethers or esters...

Products of this type were not isolated, however, when the non-immobilised metathesis substrates were allyl esters or ethers. In these cases, the combined effect of metathesis and TFA cleavage was simply to remove the allyl group. A modified protodesilylation mechanism was proposed to account for these results (Scheme 3). 

Silylformylation and hydroformylation reactions figured prominently in Ojima s approach to isoretronecanol and trachelanthamidine [40]. Thus, silylformylation of alkyne 70 proceeded smoothly to produce aldehyde 71 (Scheme 5.26). Reduction and protodesilylation provided allylic alcohol 72, which was protected to give 73. Hydroformylation in the presence of HC(OEt)3 led to a 2 1 mixture of 74 and 75. Deprotection and amide and amidal reduction then provided the target compounds. 

The regiochemistry of the reaction of unsymmetrical allylic esters was addressed by the introduction of a silyl group, equivalent to H, on the carbon of the allylic terminus. Eor example, 25 bearing a BuMe2Si group afforded 26 as the sole product in the three-component coupling. Since the subsequent protodesilylation of 26 selectively formed 27 (Scheme 6.6), this sequential procedure provides a practical method to control the regiochemistry in the substitution of unsymmetrical allylic substrates [13 c]. 

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

The proton serves as the simplest electrophile to displace the silyl group stereo- and regioselectively. Numerous conditions have been used for the electrophilic protodesilylation of allyl- and vinylsilanes19. ( )-vinyl sulfones 2 are prepared from the silylallylic sulfones 1 in the presence of protic acids in high yield (equation l)34. Diastereoselective protodesilylation of allylsilane 3 gives 4 with excellent control of the geometry of the double bond exocyclic to the ring (equation 2)35. 

Sharpless epoxidation of the alkenylsilanol 214 gave, after protodesilylation of the silyl epoxide 215, styrene epoxide 216 in 95% e.e. the stereochemical course of the reaction follows that predicted by Sharpless for allylic alcohols341. 

Protodesilylation, A new preparation of -unsaturated carboxylic acids involves the amide acetal Claisen rearrangement of a 3-(trimethylsilyl)allyl alcohol such as 1 to allylsilanes (2). Desilylation of 2 by usual methods results in formation of stereoisomers. However, use of liquid HF at low temperatures results only in (E)-3 in 88% yield. The HF-pyridine complex is not so stereoselective. Amides such as 3 can be converted into carboxylic acid esters by Meerwein s trialkyloxonium salts (—70% yield). 

Protodesilylation of allyl- or aryl-substituted trialkylsilanes with an acid partner is a cleaner method, producing propene or an aromatic hydrocarbon as a volatile inert byproduct. Although some methods of isolation have been reported, they can usually be prepared in situ in aprotic media and employed without further purification. Methods of preparation of representative organosilicon Lewis acids, la, trimethylsilyl bis(trifluoro-methanesulfone)imide (7) [9], trimethylsilyl perchlorate (6a) [10], iodotrimethylsilane, and trimethylsilyl tetrakis(trifluoromethanesulfonyl)borate [11] are described below. In principle, these analogs are also accessible in a similar manner (Table 1) [2d]. 

An (allyl)silylcuprate, [(2-methyl-2-butenyl)diphenylsilyl]cuprate, is also a hydroxy anion equivalent. The 2-methyl-2-butenyl group can easily be removed selectively in the presence of other allyl or vinyl groups by protodesilylation under mild conditions (Scheme 31) (13). 

Allylic or propargylic metal compounds are often intermediates in isomerization reactions of unsaturated systems (see Section 4.5.5) the product distribution on protonation depends very much on the substitution and the conditions of hydrolysis and most often mixtures of isomers are obtained. In contrast double bond shift generally occurs on protodesilylation of allylic silanes by acids, a reaction which takes place stereospecifically anti. Only when intramolecular delivery of the proton in a chair-like transition state is possible may a different kind of stereoselectivity be effective and good 1,3- or 1,4-induction in hydrogen transfer be observed (Scheme 83). ° ... 

For sulfoxides with P-silyl substituents there is some preference for elimination to take place towards the silyl group. On protodesilylation both the allyl- and vinyl-silanes formed give the same alkene, so providing a regioselective and high yielding alkene synthesis (Scheme 5). If there is no P-hydrogen,... 

The dimethylcyclohexenyl cation has been used to prepare o-homosteroid (example 1, Scheme 31) and steroid skeletons (example 2, Scheme 31). One solution to the problem of preparing steroids without an alkyl substituent at C-4 is the use of a phenyldimethylsilyl substituent at one terminus of the allyl cation. Carbon bond formation occurs remote from the silyl substituent, which is then eliminated by protodesilylation in situ (Scheme 32). Reduction of the number of alkyl substituents on the allylic cation has a dramatic effect on yield (example 4, Scheme 31), suggesting that useful yields are obtained only when the allylic cations have three or four carbon substituents. 

Allyl sulfones vinyl sulfones. a- lowed by protodesilylation with TfOH or major products have an ( ) configuration Intramolecular Schmidt reactions. from unsaturated azides. 

Allyl sulfones - vinyl sulfones. a-Trimethylsilylation (BuLi/TMSCl) followed by protodesilylation with TfOH or TsOH achieves the transformation. The major products have an (E) configuration. 

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