Hydrogen-bonding activation aldehydes

Because a comprehensive discussion of the transition state of hydrogen-bond catalysis will be presented by Berkessel in Chapter 3, the hydrogen bond catalyzed hetero Diels-Alder reaction of butadiene with carbonyl compounds will be discussed briefly here. Huang and Rawal reported that the hetero Diels-Alder reaction of aminodiene with aldehyde exhibited significant solvent effects (Scheme 2.7) [15]. The reaction in CHCfi was accelerated 30 times in comparison with that in THF, while that in i-PrOH was accelerated 630 times. They proposed that the Diels-Alder reaction was promoted by the hydrogen-bond activation of aldehyde. This finding resulted in the development of TADDOL catalyst [3]. 

Scheme 6.104 Key intermediates of the proposed catalytic cycle for the 100-catalyzed Michael addition of a,a-disubstituted aldehydes to aliphatic and aromatic nitroalkenes Formation of imine (A) and F-enamine (B), double hydrogen-bonding activation of the nitroalkene and nucleophilic enamine attack (C), zwitterionic structure (D), product-forming proton transfer, and hydrolysis.

Alaimo, P.J.. Arndtsen, B.A. and Bergman. R.G. (2000) Alkylation of iridium via tandem carbon-hydrogen bond activation/decarbonylation of aldehydes. Access to complexes with tertiary and highly hindered metal-carbon bonds. OrganometaUics, 19 (11), 2130-2143. 

As previously mentioned, Rawal and co-workers [35,45] reported the first example of asymmetric catalytic HDA based on hydrogen bonding activation in 2003. They found that the relatively simple diol, TADDOL 52, catalyzed the reaction between diene amine 76 and aldehydes 75 to give hemiaminals 77 (Scheme 6.17). Further treatment of the crude product with acetyl chloride resulted in elimination of the amine to give optically active pyranones 78 in good yields and excellent ee. 

Du HF, Zhao DB, Ding KL (2004) Enantioselective Catalysis of the Hetero-Diels-Alder Reaction Between Brassard s Diene and Aldehydes by Hydrogen-Bonding Activation A One-Step Synthesis of (5)-(-t)-Dihydrokawain. Chem Eur J 10 5964... 

The addition of water across carbon-carbon double bonds, a reaction thoroughly investigated by Lucas and Taft, requires strong activation and is catalyzed by hydrogen ions and hydroxyl ions. Addition of water across the 0= =0 bond of aldehydes has also been studied kinetically. Whereas chloral and formaldehyde are largely hydrated (at equilibrium in dilute aqueous solution), acetaldehyde and other... 

After 19 hours, no reaction between the zinc chelate 2 and benzaldehyde can be detected at 20 °C. However, 10 mol % of the zinc chelate effectively catalyzes theenantioselective addition of diethylzinc to aromatic aldehydes. The predominant formation of the S-configurated products, effected by this conformationally unambiguous catalyst, can be explained by a six-mem-bered cyclic transition state assembly17. The fact that the zinc chelate formed from ligand M is an equally effective catalyst clearly demonstrates that activation of the aldehyde moiety does not occur as a consequence of hydrogen bond formation between the ammonium proton of the pyrrolidine unit and the aldehydic oxygen. 

The modem concept of asymmetric induction is illustrated by the formulas in Fig. 1. As shown, the addition of hydrogen cyanide to the optically active aldehyde can lead to two diastereomers (1 and 2). If the process is under thermodynamic control, the formation of the more stable isomer will be favored that is, that isomer for which the non-bonded interactions between the newly formed cyano and the hydroxyl groups with the dissymmetric R group are weakest. On the other hand, the difference in the yields of 1 and 2 can be the result of kinetic control arising from a difference in the energies of the transition states—that state with the lower energy will form faster and lead to the product of higher yield. It is noteworthy that the tenets... 

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