Aldol reactions pdf

Clicking on the donut icon will load a page at altmetric. Find more information on the Altmetric Attention Score and how the score is calculated. Aldol condensation and esterification reactions provide paths to upgrade ethanol and acetaldehyde to higher-value molecules useful as fuels or intermediates for the synthesis of polymers. Transition-metal-substituted BEA zeolites M-BEA catalyze these reactions; however, the mechanisms for these processes in M-BEA and the effects of incidental or purposefully included silanol groups are not reported.

Here, we combine kinetic and spectroscopic measurements obtained during catalytic reactions of acetaldehyde CH 3 CHO , ethanol C 2 H 5 OH , and hydrogen H 2 mixtures over a series of Ti-BEA catalysts that possess a known range of silanol group densities to examine the kinetic relevance of intervening steps and the impact of silanol groups on catalytic rates.

The kinetic isotope effects together with the measured dependence of rates on reactant pressures suggest that aldol condensation and esterification occur on unoccupied Ti sites and involve multiple kinetically relevant steps.

During catalysis, in situ infrared spectroscopy demonstrates that these silanol groups react with C 2 H 5 OH to form ethoxysilane groups i. As initial silanol densities increase, steady-state turnover rates for aldol condensation and esterification increase by factors of 5 and 2, respectively. The changes in rates and selectivities among Ti-BEA catalysts likely reflect changes in excess free energies of transition states for enolization and nucleophilic attack of the enolate to adsorbed coreactants.

The differences in excess stability report on the interactions among reactive intermediates at framework Ti atoms and the ethoxysilane and remaining silanol groups present. The in situ modification of these pore environments confers changes in the stability of reactive species in a manner that contradicts intuition when considering the initial state of the catalyst but can be reconciled after accounting for the formation of persistent alkoxy surface moieties in the pores.

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More by Zhongyao Zhang. More by Claudia E. More by Daniel T. More by Hongbo Zhang. More by David W. Addition of diazoacetatate to acetophenone. The reaction was quite sluggish at lower temperatures without the admission of additives. It is worth adding that a catalytic enantioselective addition of an enolate to a-halo-substituted ketones is realized for the first time, giving access to highly functionalized building blocks.

Transformation of the diazo adduct 2 to 3 was used to establish the absolute configuration of prod- ucts Scheme 2. Tentatively we suggest for the reaction a mechanism 1 Et 0 6 — — 44 in which the diazoacetate is deprotonated by R2Zn. In 2 Et 25 48 — 25 78 fact, the treatment of ethyl diazoacetate with Me2Zn 3 Et 35 24 — 25 78 4 Me r. Unfortunately, the adducts are quite sensitive to [a] basic conditions. It is well known that a-diazo-b-hy- All the reactions were performed in DCM at the indicat- ed temperature.

To a solution of R2Zn 2 equiv. Acetophenone 1 equiv. The re- azoacetate 2 equiv. The reaction was quenched with hydrides provided a complete reduction of ester and water after the indicated hours, and the product isolated diazo groups, in low yield. The adducts derived from after usual work-up. In order to stabilize the adducts we analysis see supporting information for details.

Abso- lute R configuration was established by correlation investigated the possibility of protecting the hindered after reduction. The additive was added working in excess of TMSOTf trimethylsilyl triflate at room temperature to the reaction before the addition in the presence of imidazole as base alternative of the diazoacetate and the ketone.

KGaA, Weinheim Adv. Addition of ethyl diazoacetate to aromatic and aliphatic ketones. Unfortunately, all attempts to per- Scheme 2, a. Reduction of the diazo group of 4 was form the reduction of 5 with SmI2 as described for the carried out with LiEt3BH[20] or with phosphines,[21] diazo derivatives of aldehydes[20] gave complex mix- and the derivative was isolated as a single diastereo- tures of products, in which the cleavage of the N N isomer after reaction with benzoyl chloride.

We found group was not observed. Treatment with Raney nickel that the reductive cleavage of the diazo group can be gave extensive decomposition. Other tailored re- carried out also by PEt3. The product is unstable towards mild acids and it was In summary, we report the first enantioselective ad- stabilized by the preparation of the corresponding dition of ethyl diazoacetate to ketones, that gives Adv.

KGaA, Weinheim asc. Scheme 2. Transformation of the diazo adducts occurred with no racemization. For Acknowledgements the first time, the addition of enolate to a-halo ke- tones is realized in a highly enantioselective manner. OR, PRIN Progetto Nazionale Stereoselezioni in the facile generation of zinc enolates in the presence Chimica Organica: Metodologie ed Applicazioni and Bolo- gna University are acknowledged for financial support for of norephedrine derivatives for the controlled forma- this research.

These and fellowship grant. References Experimental Section [1] For reviews, see: a M. Shibasaki, M. Kanai, Chem. Cozzi, R. Hilgraf, N. Ketones [2] a M. Minnaard, B. Feringa, Chem.

To a solution of Me2Zn 2 equiv. Cozzi, Angew. The flask was cooled to 25 8C Chem. Cozzi, P. Mignogna, Adv. Mileo, F. Benfatti, added by syringe. The flask was kept at 25 8C for the indi- P. Cozzi, M. Lucarini, Chem. After [4] P. Ocampo, W. Dolbier, Jr. Denmark, Y. Fan, J. General Procedure for the Addition to Methyl , , Ketones [7] a S.

Kanai, K. Funabashi, Chem. Shibasaki, N. Yoshikawa, To a solution of Et2Zn 2 equiv. Trost, H. Ito, J. Ito, E. Silcoff, J. The flask was cooled to 25 8C and the ketone 1 equiv. The [9] S. Saito, S. Kobayashi, J. After usual work-up the re- [10] D. Alonso, S. Kitagaki, N. Utsumi, C. Barbas III, sulting oil was purified by chromatography n-hexane:ace- Angew. Zhang, J.