Deprotonation Desilylation

Due to the fact that the removal of a bonding electron from the HOMO of the substrate RH leads to a radical cation with enhanced reactivity with respect to fragmentation reactions, the pathway often employed in radical cation chemistry results in the separation of charge and spin by dissociative processes, such as deprotonation, desilylation or cleavage of a stable cationic leaving group... 

When the catalyst coordinates to the pyrazoline nitrogen and carbonyl oxygen at the step of 1-pyrazoline formation, desilylation or deprotonation takes place at the same position to give either Na or Nb, respectively. On the other hand, when the catalyst coordinates to the two carbonyl oxygens, the methine hydrogen derived from the acceptor molecule is deprotonated to give Nc. In the reaction using a Le-... 

As the intermediate formed in a polyene cyclization is a carbocation, the isolated product is often found to be a mixture of closely related compounds resulting from competing modes of reaction. The products result from capture of the carbocation by solvent or other nucleophile or by deprotonation to form an alkene. Polyene cyclizations can be carried out on reactants that have structural features that facilitate transformation of the carbocation to a stable product. Allylic silanes, for example, are stabilized by desilylation.12... 

A general route to allylanilines, reported by Harmata and co-workers <95TL4769> could be of value in organic synthesis. Deprotonation of 2,1-benzothiazines 193 with n-BuLi followed by alkylation with iodomethyltrimethylsilane, and subsequent desilylation with fluoride followed by hydrolysis led to allyl aniline 194 in good yields (Scheme 54). 

In 1998, Harmata and co-workers <98T9995> published a new synthesis of 2-alkenyl anilines. The silylated 2,1-benzothiazines 187 could be deprotonated by n-BuLi and alkylated by different electrophiles. The corresponding products could be desilylated by fluoride with concomitant cleavage of the carbon-sulfur bond to give 2-alkenylsulfinanilides that can then be hydrolyzed by base to the anilines 195 in good yields (Scheme 55). 

The attack of a cation on the oxygen atom of SENA should give rise to iminium cations, which can be stabilized by deprotonation or desilylation. (These possibilities will be considered in detail in Section 3.5.)... 

The synthetic usefulness of reactions of lithiated methoxyallene 42 with suitable electrophiles was demonstrated by several syntheses of bioactive natural products or substructures thereof [52-58]. An interesting application was described by Fall et al. [52] after addition of alkyl iodide 55 to lithiated methoxyallene 42, deprotonation by tert-butyllithium and addition of carbon dioxide occurred at the terminal y-carbon and thus provided butenolide 57 after acidic workup. Desilylation of this intermediate with TBAF finally gave bicyclic oxepane derivative 58 in good overall yield (Scheme 8.14). 

Product 34 predominates in the polar aprotic solvent (acetonitrile), while in the polar protic solvent (methanol) products 35 are formed preferentially. The different products are caused by the relative rate of deprotonation against desilylation of the aminium radical, that is in turn governed by the action of enone anion radical in acetonitrile as opposed to that of nucleophilic attack by methanol. In an aprotic, less silophilic solvent (acetonitrile), where the enone anion radical should be a strong base, the proton transfer is favoured and leads to the formation of product 34. In aprotic solvents or when a lithium cation is present, the enone anion radical basicity is reduced by hydrogen bonding or coordination by lithium cation, and the major product is the desilylated 35 (Scheme 4). 

Consequently, matched/mismatched cases [25] can result, and indeed our investigations on cooperative effects of stereogenic elements in such systems revealed 9 be the matched case and 23 (which is also easily prepared by following a directed deprotonation-silylation-deprotonation-trapping-desilylation sequence [11]) to be the mismatched case in diethylzinc additions to aldehydes [26]. Later, these investigations were extended to more complex systems such as 24 [27], but ferrocene 9 still remains superior to all other compounds. 

Finally, the LDA deprotonation of amine N-oxides has been reported to generate azomethine ylides that can be trapped in [2 + 3] cycloadditions with simple alkenes.126 For example, N-methylpyrrolidine N-oxide (137) reacts with LDA in the presence of cyclopentene to give adduct (139 Scheme 31). A variety of other N-oxides behave similarly. Interestingly, there are no examples published to date where nonstabilized azomethine ylides generated by the desilylation procedure can be trapped by simple, unactivated alkenes. It is not clear whether these discrepancies are due to some fundamental difference in the reactive intermediate being generated, or whether the differences in environment are responsible for differing behavior. Further work is needed to establish this point. 

Radiolytic studies have now demonstrated that the Wheland intermediate type 1 does in fact form and that earlier work had neglected the possibility that rapid desilylation by bases present in the mass spectrometric study would compete effectively with, and prevent, deprotonation, i.e. that reaction 2 is faster than reaction 3. This would therefore give the appearance that an Si—C bond was not formed62 - 65. If a nitrogen-centred base... 

Desilylation or destannylation of the corresponding cyclopentadiene derivatives can be easily performed in addition to deprotonation, as shown schematically in Scheme 6. The possibility for desilylation indicates the pronounced reactivity of the silicon-carbon bond, which is described in more detail in Section III. Nucleophiles as well as... 

The key point is the deprotonation rather than the desilylation of the MSMA s cation radical, an unprecedented feature. Aimed at giving some support to this mechanism, the amine-flavine electron transfer photochemistry has been studied.493 Later, a polar mechanistic pathway was established in place of the SET mechanism mentioned above.494... 

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