Deprotonation adjacent

As Beak and coworkers have established several years ago, A-rert-butoxycarbonyla-mines are sufficiently acidic to be deprotonated adjacent to the nitrogen atom . When applying 5-BuLi/(—(-sparteine (11) to A-Boc-pyrrolidine, asymmetric deprotonation (149), onepro-S-H is removed with high selectivity, furnishing the configurationally stable 2-lithio derivative 150 which was trapped with several electrophiles to form the optically active substitution products 151 (equation 33) A prescription in Organic Syntheses is... 

An analogous deprotonation adjacent to sulfur has also been reported, eq. 56 (170) ... 

Conversion of 3 to the corresponding a,/3-enone 16 is accomplished by a sulfenylation-dehydrosulfenylation sequence First, deprotonation adjacent to the carbonyl group of 3 with lithium... 

The thioacetal-/i fT-butylthio(2-pyridylthio)methane is deprotonated adjacent to sulfur on treatment with -BuLi and the anion undergoes diastereoselective and enantioselective addition to a range of aromatic and alkylaldehydes in the presence of the chiral bis(oxazoline) 122. For example, reaction with benzaldehyde proceeds to give anti- and syn-adducts 123 and 124 in 93% yield and 86 14 ratio, respectively <2004JOC1581> (Equation 89). 

This reaction proceeds through initial deprotonation adjacent to the ketone followed by an SN2 -type movement of electrons through the double bond and elimination of a bromide ion. 

Step 1 Deprotonation adjacent to the leaving group gives a carbanion. 

As mentioned above, the synthesis of cyanohydrins from ketones is difficult due to the unfavorable equilibrium. However, it can be achieved. When methyl ketones were treated with only 1.5 equivalents of HCN in an emulsion of citrate buffer (pH 4.0) and MTBE, the S-selective Manihot esculenta HNL catalyzed the synthesis of the corresponding cyanohydrins with good yields (85-97%) and en-antioselectivities (69-98%). The corresponding esters were then used in a second carbon-carbon bond forming reaction. Since cyanohydrins from ketones cannot be deprotonated adjacent to the nitrile group, the esters could be selectively deprotonated and then a ring closing attack on the nitrile function yielded the unsaturated lactones (Scheme 5.6). When the nitrile function was first con-... 

The mechanism of the Shapiro reaction is believed to involve initial deprotonation of the NH proton from tosylhydrazone 5 to generate 6, which undergoes a second deprotonation adjacent to the hydrazone group to afford dianion 7. Elimination of lithium p-toluene-... 

In Sinay s research group, the use of 2-deoxy-D-glucopyranosyl phenyl sul-fones as carbanion precursors has been extensively investigated [38-40].Deprotonation adjacent to the phenylsulfonyl moiety allows for the introduction of carbon electrophiles at the anomeric center. Reductive desulfonylation of 50 (with LN, 42a) leads via the axial lithium compound 51 to the 1-P-substituted deoxyglucopyranose 52 following stereospedfic protonation using methanol [Eq.(18)] [39]. 

Just as deprotonation adjacent to a carbenium ion can form an olefin, similarly removal of H adjacent to a free radical will form an olefin (Eq. 10.95). As we noted in Section 10.10.4, the process of Eq. 10.95 is referred to as radical disproportionation when the radicals are the same. Unimolecular elimination from a radical is the simple reverse of the addition of a radical to an alkene (Eq. 10.96). Since the addition is typically exothermic, it takes heat to reverse the addition. One example is the depolymerization of polystyrene, which will occur at temperatures of 300 °C (Eq. 10.97 see Chapter 13 for a discussion of the polymerization reactions). Strain in an adjacent ring will favor elimination, as shown in Eqs. 10.98 and 10.99. These two examples convert one radical to another, and such reactions will be discussed in more detail in Section 11.11. 

Pentadienyltrimethylsilane may also be deprotonated adjacent to silicon with lithium diisopropylamide, forming the 1-(MesSij-pentadienyl anion. This anion reacts with many electrophiles to give e-substituted dienes (eq 2) which are ideal candidates for protodesilyation or intramolecular Diels-Alder reactions. 

Irradiation of the cyclohexenone 32 with the tertiary a-silylamine EtjNCHjSiMe, affords two adducts 32a and 32b in a product ratio that is solvent dependent (Scheme 20). In acetonitrile solution, the formation of 32a as major product can be attributed to selective a-CH deprotonation adjacent to the silyl substituent, whereas the formation of 32b as the major product in methanol solution can be attributed to nucleophihc displacement of the silyl group followed by addition of the radical EtNCHj to either the neutral enone or its anion radical. Intramolecular analogs of this reaction have also been studied for substrates 33 and 34 and display highly selective a-CH deprotonation adjacent to the silyl substituent in acetonitrile solution to yield 33a and 34a and desdylation in methanol solution to yield 33b and 34b (Scheme 21). Intramolecular addition can also compete with trans,cis isomerization of acychc enones, whereas intermolecular addition is too slow to do so. " ... 

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