Proto-desilylation

The spread of rates in the mercuridesilylation reaction is close to that obtained in bromo- and proto-desilylation suggesting a similar transition state, but nothing... 

Several diynyl complexes of Group 6 metals have been prepared directly from LiC=CCsCSiMe3. In the cases of WCl(CO)3Cp, >4 MoCl(CO)(dppe)Cp,i and MoBr(CO)2(/ -C7H7) the silyl-diynyl complexes could be isolated in low to moderate yields. In all cases, proto-desilylation was achieved using KF in MeOH/thf. The terminal diynyl complexes were also obtained from MoBr(CO)(L) r -C H ) (L = CO, PMes) and Li2C4 in low yield. 

Tributylsilyl)trifluoroethene has been used as a precursor to the vinylic fragment.1516 This variation of the original methodology10 avoids the need for butyllithium, since proto-desilylation to give allyl difluorovinyl ether intermediates 34 is achieved with tetrabutylam-monium fluoride. The 2-fluoroalk-4-enoie acid derivatives 35 are produced in a one-pot procedure (Table 11). Various mechanistic possibilities have been discussed in order to account for the stereochemical outcome.16 The need for two equivalents of allylic alcohol limits the utility of this method. 

CM has been reported to provide a synthetic tool for immobilization of reagents. Polymer-supported synthesis with an allylsilyl unit as a linker was developed. Divinylbenzene cross-linked allyldimethylsilylpolystyrene has been reported to undergo highly efficient ruthenium-catalyzed CM with functionalized terminal alkenes (Eq. 45) [78]. Products have been liberated by proto-desilylation with trifluoroacetic acid. 

Proto-desilylation (KOH) of Co3(p3-CSiMe3)( i3-CC=CSiMe3)Cp3 affords Co3(p3-CSiMe3)(p3-CC=CH)Cp3 (243).443 No reaction occurs between this complex and Co2(CO)8. 

An impressive synthesis of estrone (51) was reported Vrdlhardt and cowoikers. G>bait-catalyzed cooligomerization of the diyne (49) with bis(trimethylsilyl)acelylene gave the estratrienone (50) in 71% yield. Introduction of the hydroxy substituent at C-3 was then cleverly achieved by selective proto-desilylation at C-2, followed by oxidation of the carbon-silicon bond using LTFA (Scheme 19). 

Nitrosium tetrafiuoroborate (NOBF.,) also undergoes [3+2] cycloaddition to a-substituted allylsilanes without use of a promoter (Scheme 10.176) [490]. The A -isoxazoline products would be formed by deprotonation and the subsequent proto-desilylation of fhe initial [3+2] adducts, Abisoxazolines. A -Isoxazolines serve as precursors of /i-hydroxy ketones and y-amino alcohols. 

The synthesis and reactivity of the six membered ring, potentially aromatic, phosphinine system has received further attention. The first examples of a new class of axially chiral phosphorus ligands, the atropisomeric phosphinines (204), have been prepared and isolated in an enantiopure state." Also reported are syntheses of the wide-bite-angle diphosphinine (205)" and the diphosphinine analogue of terpyridine (206), a new class of neutral, 7i-acceptor PNP-pincer ligand." Studies of the reactivity of phosphinines include the first observations of protonation, alkylation and silylation in systems usually viewed as extremely weak bases," " the proto-desilylation of 2,6-disubstituted silylphosphinines," a theoretical study of... 

Further control experiments gave some insights into the mechanism of this organocataly tic process. Informatively, the reaction did not occur in the absence of the phenol, suggesting that the active catalytic species would result from a preassociation between the A -ttiflyl thiophosphoramide 82a and phenol. The authors speculated that the formation of a chiral oxonium ion pair would promote the proto-desilylation of the silyl enolate 80 through a two-step mechanism. Initial enantioselective protonation of the silyl enolate 80 would then be followed by desilylation of the resulting intermediate to furnish the desired ketone 81 along with silylated phenol (Scheme 3.43). 

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