Deprotonation allylic

The transformation of deprotonated allyl ethers into homoallylic alcohols is the [2,3]-sigmatropic version of the [1,2]-Wittig Rearrangement, and is therefore termed [2,3]-Wittig Rearrangement ... 

The violence of superbasic slurries towards functionalised organic molecules means that they are at their most effective with simple hydrocarbons they also tolerate ethers and fluoro substituents. LiCKOR will deprotonate allylic, benzylic, vinylic, aromatic and cyclopropane C-H bonds with no additional assistance. From benzene, for example, it forms a mixture of mono and dimetallated compounds 496 and 497.441 ( Li/K indicates metallation with a structurally ill-defined mixture of lithium and potassium.)... 

In all of the above examples, the polybranching reaction of arene ligands was limited by the steric bulk. In the toluene and mesitylene ligands, however, the deprotonation-allylation re-... 

Scheme 11.5 Decaallylation of 1,2,3,4,5-pentamethylcobaltocenium in a one-pot reaction consisting in 10 deprotonation-allylation sequences (steric constraints inhibit further reaction, and the 10 groups introduced are self-organized according to a single directionality) and follow-up RCM (RCM = ringclosing metathesis) of the deca-allylated complex.21...

The presence of a leaving group adjacent to the enolate diverts the reaction to elimination. 63. (a) Deprotonate allylic to give a conjugate enolatelike anion (see Section 18-11) ... 

Instead of allyl bromide derivative, vinyl diazoacetate in the presence of rhodium acetate also allowed the formation of substituted indolines in excellent yields (Scheme 9.34). N-deprotonated allyl thionium was postulated as intermediate [157], no reaction being observed with an N-methyl indole analogue. 

A review on the reactivity of sugar-derived acyclic sulfones has appeared, and describes conjugate additions, desulfonations, vinyl deprotonation, allylic transposition and dipolar cycloaddition reactions ... 

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafiuoroborate (H. Meerwein, 1960 R.F. Borch, 1968, 1969 from trimethyl orthoformate and BFj, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

The TT-allylpalladium complexes 241 formed from the ally carbonates 240 bearing an anion-stabilizing EWG are converted into the Pd complexes of TMM (trimethylenemethane) as reactive, dipolar intermediates 242 by intramolecular deprotonation with the alkoxide anion, and undergo [3 + 2] cycloaddition to give five-membered ring compounds 244 by Michael addition to an electron-deficient double bond and subsequent intramolecular allylation of the generated carbanion 243. This cycloaddition proceeds under neutral conditions, yielding the functionalized methylenecyclopentanes 244[148], The syn-... 

Geranyl pyrophosphate is an allylic pyrophosphate and like dimethylallyl pyrophosphate can act as an alkylating agent toward a molecule of isopentenyl pyrophosphate A 15 carbon carbocation is formed which on deprotonation gives/ar nesyl pyrophosphate... 

The dianions derived from furan- and thiophene-carboxylic acids by deprotonation with LDA have been reacted with various electrophiles (Scheme 64). The oxygen dianions reacted efficiently with aldehydes and ketones but not so efficiently with alkyl halides or epoxides. The sulfur dianions reacted with allyl bromide, a reaction which failed in the case of the dianions derived from furancarboxylic acids, and are therefore judged to be the softer nucleophiles (81JCS(Pl)1125,80TL505l). 

Metzner and co-workers reported a one-pot epoxidation reaction in which a chiral sulfide, an allyl halide, and an aromatic aldehyde were allowed to react to give a trons-vinylepoxide (Scheme 9.16c) [77]. This is an efficient approach, as the sulfonium salt is formed in situ and deprotonated to afford the corresponding ylide, and then reacts with the aldehyde. The sulfide was still required in stoichiometric amounts, however, as the catalytic process was too slow for synthetic purposes. The yields were good and the transxis ratios were high when Ri H, but the enantioselectivities were lower than with the sulfur ylides discussed above. 

The chiral information of stereogenic centers in the allyl moiety of the precursor is destroyed on deprotonation. While an i/3-bound ion pair with a planar carbon frame is a chiral compound, usually rapid racemization takes place by intra- or intermolecular migration of the cation from one face to the opposite one. The sole exceptions known at present are secondary 2-alkenyl carbamates with X = dialkylaminocarbonyloxy21, in which the cation is tied by the chelating ligand, see Section 1.3.3.3.1.2. 

For the deprotonation of less acidic precursors, which do not lead to mesomerically stabilized anions, butyllithium/TMEDA in THF or diethyl ether, or the more reactive, but more expensive,. seobutyllithium under these conditions usually are the most promising bases. Het-eroatomic substitution on the allylic substrate, which docs not contribute to the mesomeric or inductive stabilization often facilitates lithiation dramatically 58. In lithiations, in contrast to most other metalations, the kinetic acidity, caused by complexing heteroatom substituents, may override the thermodynamic acidity, which is estimated from the stabilization of the competing anions. These directed lithiations59 should be performed in the least polar solvent possible, e.g.. diethyl ether, toluene, or even hexane. 

An efficient kinetic resolution of racemic secondary allyl carbamates was accomplished by the jw-butyllithium-(-)-sparteine complex76 131. Whereas the R-enantiomer (80% ee) is recovered unchanged, the 5-enantiomer is deprotonated preferentially. 

In ( )-[2-(l-propenyl)-l, 3-dithian-2-yl]lithium, no problem of EjZ selectivity arises. It is easily prepared by deprotonation of the allylic dithiane87,88 with butyllithium in THF, whereas deprotonation of the 2-propylidene-l, 3-dithiane requires the assistance of HMPA. The addition to saturated aldehydes proceeds with excellent y-regioseleetivity and anti selectivity88,89. As often observed in similar cases, aldehydes which bear an, p2-carbon atom adjacent to the carbonyl group give lower selectivities. The stereoselectivity decreases with ketones (2-bu-tanone y/a 84 16, antiisyn 77 23)88. The reaction with ethyl 2-oxopropanoate is merely nonstereoselective90, but addition of zinc chloride improved the syn/anti ratio to 96 4, leading to an efficient synthesis of ( )-crobarbatic acid. 

Only few allyltitanium reagents bearing a removable chiral auxiliary at the allylic residue are known. The outstanding example is a metalated 1-alkyl-2-imidazolinone14, derived from (—)-ephedrine, representing a valuable homoenolate reagent. After deprotonation by butyllithium, metal exchange with chlorotris(diethylamino)titanium, and aldehyde or ketone addition, the homoaldol adducts are formed with 94 to 98% diastereoselectivity. 

In a conceptually similar fashion, camphor-derived hydroxysulfide 9 is oxidized diastereospecifically into hydroxysulfoxide 10 whose absolute configuration has been determined by X-ray crystallography48. Heating this diastereomer to 145 °C causes complete epimerization at sulfur to form diastereomer 11 in quantitative yield (equation 12). This type of allylic sulfoxide can be deprotonated and then added in a Michael fashion to cycloalkenones (see p. 834). 

When enantiomerically pure allyl p-tolyl sulfoxide is deprotonated and then treated with electrophilic 2-cyclopentenone, a conjugate addition occurs forming a new carbon-carbon bond with very high control of absolute stereochemistry (equation 25)65. See also Reference 48. Similarly, using more substituted enantiomerically pure allylic sulfoxides leads to virtually complete diastereocontrol, as exemplified by equations 26 and 27 the double bond geometry in the initial allylic sulfoxide governs the stereochemistry at the newly allylic carbon atom (compare equations 26 vs. 27)66. Haynes and associates67 rationalize this stereochemical result in terms of frontier molecular orbital considerations... 

Tetraene 4 (Scheme 1.3), when treated with 40 mol % of triflic acid in methylene chloride at -23 °C for 1 h, gives the adducts 5 and 6 in a 1 1 ratio as the main reaction products. The formation of these adducts has been justified [21] by a stepwise mechanism that requires an initial reversible protonation of 4 to produce the allyl cation 7, which then cyclizes to 8 and 9 in a non-reversible process. Deprotonation of 8 and 9 gives 5 and 6, respectively. 

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