Probably the most commonly encountered example of the E1cB mechanism in introductory organic chemistry is in the aldol condensation reaction, specifically the last step where the new C-C double bond is formed.(See post: The Aldol Addition and Condensation Reactions]
organic reaction mechanism ppt download
When alkenyl halides are treated with NaNH2 in NH3 as solvent, elimination to give the alkyne occurs. This reaction works even if the H and Cl are cis to each other, which would seem to rule out the E2 mechanism. [The elimination when H and the halide are trans is much faster, though! [Note 3]
A key strategy for studying organic reaction mechanisms is to measure kinetic isotope effects, which take advantage of the subtle differences in bond strengths between isotopes. For example C-D (deuterium) bonds are slightly shorter (and stronger) than comparable C-H bonds. If C-H bond breaking occurs in the rate determining step, then the rate for the reaction of the C-H compound (kH) should be slightly faster than the rate of the deuterium labelled analog with a C-D bond (kD). This difference, termed the primary kinetic isotope effect, has been experimentally determined to have a range kH/kD of about 2-8 in most cases for the E2.
In the case of the reaction above, the kinetic isotope effect is about 1.0 . That is a clear indication that the C-H bond is not broken in the rate determining step, which rules out the E2 (concerted) mechanism.
Certainly, it is important and valuable that the reaction could be carried out under mild conditions, avoiding some side reactions. Ji et al. [21] studied the reaction of the alcohols oxidized to aldehydes under mild conditions, choosing the effective and inexpensive reactants (Scheme 2a). Because of the ability to form inclusion complexes between cyclodextrins and the reactants, the reaction solution was homogeneous, which was beneficial to the progress of the reaction. The substrate scope of the alcohols had been widely expanded to include aromatic alcohols in good yield. The advantages of these reactions consisted in inexpensive reactants and recycled catalysts of cyclodextrins. On the other hand, the mechanism of the oxidation of alcohols was proposed (Scheme 2b).
In general, the aldehyde was directly oxidized to the carboxylic acid in the heterogeneous system. However, Shi et al. [24] studied the oxidation reaction with β-cyclodextrin in the homogeneous system. Benzaldehyde could be accommodated in the β-cyclodextrin cavity to form a complex, which was dissolved in the aqueous phase. Using the mild oxidizing agents (NaClO/HCl), benzaldehyde could be oxidized to benzoic acid, and the optimum reaction conditions were explored (Scheme 4a). Under the optimized condition, the yield was up to 98%. In this reaction, the reactants were inexpensive and β-cyclodextrin could be recycled. In addition, the corresponding mechanism was that the oxidizing agents oxidized benzaldehyde to carboxylic acid when the acid provided protons (Scheme 4b).
Under non-aqueous conditions, Pospisil et al. [29] studied the reduction reaction of 3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dione using catalyzed by β-CD using the electrochemical method (Scheme 6a). The mechanism of the reduction of vinclozoline through the inclusion complex with β-CD was proposed (Scheme 6b).
Guo et al. [30] modified cyclodextrin with palladium to synthesize a new material (DACH-Pd-β-CD) for the reduction reaction of a serial of nitrobenzene to aromatic amine compounds in excellent yield (Scheme 7a). On the other hand, the catalyst could be easily separated from the reaction solution, and still maintained high catalytic activity after five cycles. The mechanism of the reduction of nitrobenzene through the inclusion complex with DACH-Pd-β-CD was proposed (Scheme 7b).
Krishnaveni et al. [33,34] studied the addition reaction of thiols with olefins in the presence of β-cyclodextrin under room temperature (Scheme 10a), and the yield was higher (up to 98%) than that in the absence of β-cyclodextrin. In addition, the advantages of this method consisted in a lower reaction time and the recycling of the catalyst. The mechanism of the addition of thiols with olefins through the inclusion complex with β-cyclodextrin was proposed (Scheme 10b). Similarly, Srinivas et al. [35] studied the Michael addition reaction of benzeneselenol with olefins, and the yield could achieve up to 88% under the optimal condition. In those addition reactions mediated by β-CD, the process was economical and eco-friendly.
In the aspect of the Aza-Michael addition reaction, Surendra et al. [36] studied the addition reaction of amines with alkenes in the presence of β-cyclodextrin (Scheme 11a). When the contents of β-cyclodextrin increased, the yields were improved. More interestingly, β-cyclodextrin (as the catalyst) could be recycled. The mechanism of the addition of amines with alkenes mediated by β-cyclodextrin was proposed (Scheme 11b).
Condensation polymerization was a simple method for the synthesis of the sequence chain. Tao et al. [38] synthesized a hydrophilic periodic polymer using the nitroso benzene/cyclodextrin complex (Scheme 13a). Firstly, the water-soluble inclusion complexes of CD/NB were formed and then polymerized with poly (ethylene glycol) bis (α-bromoisobutyrate) to produce macromolecular polymers in good yield. The mechanism of this reaction was proposed (Scheme 13b). The process provided a simple method to synthesize the sequence chain and was beneficial to the application in industry.
Girish et al. [39] reported the synthesis of 2-phenyl imidazole derivatives by the addition reaction using nano ZrO2-β-cyclodextrin as a supramolecular catalyst in good yield (Scheme 14a). In addition, the catalyst can be recycled to facilitate resource conservation. The mechanism of the addition reaction of 2-phenyl imidazole derivatives was proposed (Scheme 14b).
(a) The reduction reaction of 3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dione catalyzed by β-CD; (b) the reduction mechanism of 3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dione catalyzed by β-CD.
(a) The addition reaction of 2-phenyl imidazole derivatives catalyzed by ZrO2-β-cyclodextrin; (b) the mechanism of addition reaction of 2-phenyl imidazole derivatives. 2ff7e9595c
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