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Lithium conjugate substitution reaction

The formation of the above anions ("enolate type) depend on equilibria between the carbon compounds, the base, and the solvent. To ensure a substantial concentration of the anionic synthons in solution the pA" of both the conjugated acid of the base and of the solvent must be higher than the pAT -value of the carbon compound. Alkali hydroxides in water (p/T, 16), alkoxides in the corresponding alcohols (pAT, 20), sodium amide in liquid ammonia (pATj 35), dimsyl sodium in dimethyl sulfoxide (pAT, = 35), sodium hydride, lithium amides, or lithium alkyls in ether or hydrocarbon solvents (pAT, > 40) are common combinations used in synthesis. Sometimes the bases (e.g. methoxides, amides, lithium alkyls) react as nucleophiles, in other words they do not abstract a proton, but their anion undergoes addition and substitution reactions with the carbon compound. If such is the case, sterically hindered bases are employed. A few examples are given below (H.O. House, 1972 I. Kuwajima, 1976). [Pg.10]

The conjugate addition reactions probably occur by a mechanism similar to that for substitution on allylic halides. There is probably an initial complex between the cuprate and enone.44 The key intermediate for formation of the new carbon-carbon bond is an adduct formed between the enone and the organocopper reagent. The adduct is formulated as a Cu(III) species which then undergoes reductive elimination. The lithium ion also plays a key role, presumably by Lewis acid coordination at the carbonyl oxygen.45 Solvent... [Pg.488]

Due to their high conformational flexibility, chiral acyclic Michael acceptors often display low diastereofacial selectivities in copper-mediated conjugate addition reactions.80 Nevertheless, acceptable levels of diastereoselection can be obtained, in particular with heteroatom-substituted Michael acceptors. For example, treatment of the y-benzyloxymethyl-substituted enoate 100 with lithium dimethylcuprate in the presence of chlorotrimethylsilane gives the anti-adduct 101 with excellent diastereoselectivity (Scheme 26). a Products of this type can be easily... [Pg.515]

The mechanistic picture for addition of organocuprates to a,P-unsaturated carbonyl compounds is no less complex than that for substitution reactions. On the basis of current information, conjugate addition of lithiocuprates to a, P-unsaturated ketones and esters may proceed via a initial reversible copper(I)-olefin-lithium association, which then undergoes oxidative addition followed by reductive elimination. ... [Pg.293]

The application of the aliphatic Friedel-Crafts reaction (chapter 5) to alkynes is a stereoselective approach to vinyl-lithiums that forms a bridge between the Shapiro reaction and the next section-hydrometallation. Acid chlorides 64 react with alkynes under Lewis acid catalysis to give E-P-chloroenones -65 stereoselectively. The chlorine can be replaced by more reactive iodine by conjugate substitution.11... [Pg.261]

Conjugate Addition Reactions.—The use of polymer-bound dialkylcuprates gives comparable or higher than normal yields in the conjugate addition to enones the major advantage lies in the ease of work-up. The mechanism of conjugate addition of cuprates and the mechanism and stereochemistry of the addition of lithium dimethylcuprate to jS-cyclopropyl-substituted enones have been reported. [Pg.81]

Thus, the unsubstituted starting compound 69 was treated with resorcinol in the presence of trifluoroacetic acid (TFA) to yield 70. Then, reaction of 69 with the cyclic a,/3-unsaturated ketone in the presence of lithium hydride gave the 7-substituted heteroaromatic compound 71, and ethyl cyanoacetate afforded the cross-conjugated product 72, whereas reaction with pyrimidine-2,4,6-trione in the presence of triethylamine yielded the addition product 73. Indole also been reacted with 69, and heating of the dichloromethane solution for 90 min in the presence of TFA yielded the addition product 74 in excellent yield (95%) <1998ZOR450> (Scheme 12). [Pg.854]

Complex hydrides have been used rather frequently for the conjugate reduction of activated dienes92-95. Just and coworkers92 found that the reduction of a,ft-unsaturated ketene 5,5-acetals with lithium triethylborohydride provided mixtures of 1,4- and 1,6-reduction products which were transformed into enals by treatment with mercuric salts (equation 27). Likewise, tetrahydro-3//-naphthalen-2-ones can be reduced with L-Selectride to the 1,6-reduction products93 -95 this reaction has been utilized in the stereoselective synthesis of several terpenes, e.g. of (/ )-(—)-ligularenolide (equation 28)95. Other methods for the conjugate reduction of acceptor-substituted dienes involve the use of methylcopper/diisobutylaluminum hydride96 and of the Hantzsch ester... [Pg.658]


See other pages where Lithium conjugate substitution reaction is mentioned: [Pg.316]    [Pg.124]    [Pg.471]    [Pg.79]    [Pg.101]    [Pg.124]    [Pg.145]    [Pg.202]    [Pg.316]    [Pg.79]    [Pg.101]    [Pg.124]    [Pg.145]    [Pg.202]    [Pg.316]    [Pg.262]    [Pg.565]    [Pg.508]    [Pg.79]    [Pg.101]    [Pg.124]    [Pg.145]    [Pg.202]    [Pg.316]    [Pg.1448]    [Pg.217]    [Pg.421]    [Pg.626]    [Pg.541]    [Pg.962]    [Pg.74]    [Pg.210]    [Pg.443]    [Pg.108]    [Pg.1027]    [Pg.1230]    [Pg.235]    [Pg.780]   
See also in sourсe #XX -- [ Pg.316 ]




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