- Intrinsic Reactivities of Some Carbanions in ?-Adduct Forming Reactions
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Kinetic and equilibrium data are reported for nucleophilic attack in methanol at unsubstituted ringpositions of 1,3,5-trinitrobenzene, 2,4,6-trinitrotoluene and 1-chloro-2,4,6-trinitrobenzene by carbanions derived from dimethyl malonate, ethyl cyanoacetate and 4-nitro-, 4-cyano- and 2-cyanobenzyl cyanides.The results are used to determine intrinsic reactivities for the carbanions in these ?-adduct forming reactions and are discussed in terms of the electronic and solvent reorganisation occurring during reaction.
- Crampton, Michael R.,Stevens, Andrew J.
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p. 1715 - 1720
(2007/10/02)
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- Nucleophilic Addition to Olefins. 22. Kinetics of Hydrolysis of the Piperidine and Morpholine Adducts of Benzylideneacetylacetone in 50percent Me2SO-50percent Water
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Benzylideneacetylacetone reacts with piperidine and morpholine to form a pH-dependent mixture of anionic adducts, PhCH(R2N)C(COCH3)2- (TA-), and zwitterionic adducts, PhCH(R2NH+)C(COCH3)2- (TA+/-), which slowly hydrolyze to benzaldehyde, acetylacetone (or its anion), and R2NH.A kinetic study of this hydrolysis in 50percent Me2SO-50percent water shows that at high pH carbon protonation of TA-, to form PhCH(R2N)CH(COCH3)2 (TA0), is rate-limiting while at intermediate pH carbon protonation of TA+/-, to form PhCH(R2NH+)CH(COCH3)2 (TA+), is co-rate-limiting with intramolecular proton transfer, TA+/- -> TA0 (Scheme I).At low pH the enol form of TA+ becomes an important intermediate (Scheme II).Rate and equilibrium constants for the various elementary steps leading from TA+/- and TA- to TA0 were determined or estimated and a lower limit for the rate constant of breakdown of TA0 into PhCH=N+R2 and CH(COCH3)2- was estimated.Comparison of the kinetic behavior of benzylideneacetylacetone adducts with that of amine adducts of four other PhCH=CXY-type olefins shows both similarities and important differences which are discussed in detail.It appears that there are two major factors that determine whether formation of TA0 or its breakdown is rate-limiting.One is crowding, which enhances breakdown and slows proton transfer.The other is the sensitivity of the intrinsic barriers to resonance effects in the carbanion that is formed in the breakdown of TA0 on the one hand and that which is formed in the deprotonation of TA0 on the other hand.The smaller sensitivity to this factor in the breakdown reaction tends to make breakdown rate-limiting for systems where resonance in the carbanion is modest and to make proton-transfer rate-limiting where this resonance is strong.
- Bernasconi, Claude F.,Bunnell, Richard D.
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p. 2001 - 2008
(2007/10/02)
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