Hydrogen-bonding activation reactions

What is a carbon-hydrogen bond activation reaction ... 

Another important reaction principle in modem organic synthesis is carbon-hydrogen bond activation [159]. Bergman, Ellman, and coworkers have introduced a protocol that allows otherwise extremely sluggish inter- and intramolecular rhodium-catalyzed C-H bond activation to occur efficiently under microwave heating conditions. In their investigations, these authors found that heating of alkene-tethered benzimidazoles in a mixture of 1,2-dichlorobenzene and acetone in the presence of di-//-... 

Miscellaneous Reactions Berkessel " has identified peptide-like urea-based bifiinctional organocatalysts for the highly efficient dynamic kinetic resolution of azalactones (Scheme 11.14a). Another selective hydrogen-bonding activation mechanism that enables the addition of pyrroles to ketenes using catalytic quantities of azaferrocene 36 has been introduced by Fu and coworkers (Scheme 11.14b). ° ... 

A new cinchona alkaloid-derived catalyst has been developed for the enantioselective Strecker reaction of aryl aldimines via hydrogen-bonding activation. For reference, see Huang, J. Corey, E. J. Org. Lett. 2004, 6, 5027-5029. 

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]. 

In addition to the activation of carbonyl compounds and imines, Schreiner studied on thiourea-catalyzed acetalization reaction, in which ortho esters were activated by hydrogen bond [19]. Jacobsen has utilized the hydrogen-bond catalysis in reactions with acyliminium ions, wherein hydrogen bond activates the acylim-inium salt through complexation with chloride [20]. 

Pyrrole-containing thiourea derivatives 52 and 53 were developed and optimized for hydrogen-bonding activation of N-acyliminium ions [76] in the acyl-Pictet-Spengler [202, 205] (Schemes 6.50 and 52) and acyl-Mannich reaction [204] (Scheme 6.51). List et al. extended the applicability of this thiourea type to... 

Redistribution of electron density in CT complexes results in a modification of the chemical properties of coordinated arenes, and this effect is widely used in organometallic catalysis [2]. To demonstrate the relationship between charge transfer in arene complexes and their reactivity, we focus our attention on carbon-hydrogen bond activation, nucleophilic/ electrophilic umpolung, and the donor/acceptor properties of arenes in a wide variety of organometallic reactions. 

Yang H, Asplund MC, Kotz KT, Wilkens MJ, Frei H, Harris CB. Reaction mechanism of silicon-hydrogen bond activation studied using femtosecond to nanosecond IR spectroscopy and ab initio methods. J Am Chem Soc 1998 120(39) 10154-10165. 

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