Proton abstractions parallel

Fl-bonds in, 138, 140 Proton abstractions parallel to Sn2, 141 Proton affinities, 97 Proton affinity (PA) acetaldehyde, 123 acetone, 123 acrolein, 123 butenone, 123 dimethyl ether, 123 dime thy lacrolein, 123 formaldehyde, 123 methanol, 123 methyl acetate, 123 methyl acrylate, 123 ( )-rnethylacrolein, 123 oxetane, 123 table of, 123 tetrahydrofuran, 123 water, 123... 

One more example of metal ion catalysis will be considered briefly. In a now classic paper, Cox (1974) showed that the enolization of 2-acetylpyri-dine (but not 4-acetylpyridine) is catalysed by divalent transition metal ions. Proton abstraction by acetate ions is strongly accelerated by Zn2+, Ni2+ and Cu2+ ions and the transition state stabilization by these ions roughly parallels their abilities to bind to the substrate (Table A6.5). The three metal ions are significantly superior to the proton as electrophilic catalysts, no doubt because they can chelate to both the pyridine nitrogen and the... 

The rate-pD profile for the 2-position could be accounted for by parallel rate-determining proton abstraction from the conjugate acid of imidazole by OD and by DgO leading to an ylide intermediate (Scheme 2). These pathways together with OD -catalyzed proton abstraction from the neutral imidazole molecule accounted for the 4(5)-... 

On the other hand, at high concentrations of acetaldehyde, when the intermediate enolate carbanion is rapidly captured by another molecule of aldehyde, reverse of the initial parallel proton-abstraction steps is prevented (k3[CH3CHO] k-1 and k 2[BH 1 ]), and the rate of the overall reaction is effectively limited by the initial proton abstractions these then constitute (parallel) rate-limiting steps. The overall process is now first order in acetaldehyde and shows general-base catalysis [5], i.e. the rate law is given by Equation 3.13 ... 

The alkylation reaction is very selective. It yields quantitatively pure B-monoalkyldi-carba-nn/o-undecaborate, independent from the excess of alkylating agent. The most active alkyl halides are alkyl iodides and then - bromides. For the alkylation to be complete, 5-10 percent excess of alkyl halide is usually used. And even greater excess makes no difference, since a monoanion formed does not react with RX, and dialkylation does not occur. As a parallel process there could be the proton abstraction from the formed alkylated monoanion with the dicarbollide that has not yet entered the alkylation reaction ... 

Two parallel processes appear to be occurring rate-determining proton abstraction from the imidazolium ion by D2O and by OD to give the ylide at C-2 (137) followed by deuteration there (Scheme 66). The pD profile for 4-substitution can be accounted for by an additional path involving proton abstraction from the imidazole neutral molecule. In strongly alkaline medium imidazoles with no nitrogen substituent exchange more readily... 

The reason for this increased acidity is best seen by looking at an orbital picture of the enolate ion (Figure 22.5). Proton abstraction from a carbonyl compound occurs when the C-H bond is oriented roughly parallel to the... 

The attack of the acid (154) on the readily polarizable 1,2-dithiafulvene (155) corresponds to the extremely ready addition of electrophilic reagents to the simple and vinylogous heptafulvene derivatives, which are iso-n-electronic with 155. The opening of the dithiole rings in 156 and 158 under the pressure of the carbanionoid electron pair liberated by the proton abstraction and of the free electron pair on the sulfur, as well as the elimination of elementary sulfur and the intramolecular electrophilic attack of the mercaptide ion (157) on the 5-position to form 158, are simply the typical reactions of 1,2-dithioles that have already been discussed (Section II, B, 3). The reactivity of the 3-methyl group in 154 finds many parallels in the ease of condensation of the methyl-substituted pyridinium, pyrylium, thiopyrylium, and tropylium salts, and particularly... 

Rates of potassium / r/.-butoxide-catalyzed isomerization of l-alkenes and methylenecycloalkanes in dimethylsulfoxide at 55°C are summarized in Table 2. For the acyclic as well as exocyclic alkenes, isomerization rates parallel rates of base-catalyzed enolization of structurally analogous ketones. This is further evidence that the rate-limiting step in the alkene isomerizations is proton abstraction to form the allylic carbanion. 

It is well known that in many brominations and protonations of cyclohexenols (91) axial entry is favored (Eliel et al., 1965). This is attributed to the parallel alignment of the v orbitals on the three centers. The overlap preference is well illustrated in the oxidation of allyl vs. saturated alcohols. Normally, axial alcohols are oxidized more rapidly by chromic acid than equatorial alcohols. In the absence of large strain factors, equatorial allyl alcohols are oxidized faster than axial alcohols by chromic acid hydrogen is abstracted in the rate-determining step. The contribution of a-j8 ketonic resonance lowers the activation energy,... 

Extensive studies of the sensitizer dependence and the solvent dependence of the polarization patterns led to the identihcation of two parallel pathways of that deprotonation. One is a proton transfer within the spin-correlated radical pairs, with the radical anion A acting as the base. The other is a deprotonation of free radicals, in which case the proton is taken up by surplus starting amine DH. Furthermore, evidence was obtained from these experiments that even in those situations where the polarization pattern suggests a direct hydrogen abstraction according to Equation 9.6 these reactions proceed as two-step processes, electron transfer (Eq. 9.7) followed by deprotonation of the radical cation by either of the described two routes. The whole mechanism is summarized by Chart 9.3 for triethylamine as the substrate. Best suited for an analysis is the product V. 

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