Bronsted Henry reactions

In 2007, Ooi and coworkers introduced chiral tetraaminophosphonium salts as a new class of Bronsted acids [166]. Similar to the guanidine/guanidinium case, these tetraaminophosphonium salts act as Bronsted bases in their neutral/ deprotonated (triaminoiminophosphorane) form, while they can also be used as mono-functional Bronsted acids in their protonated, phosphonium form. Phos-phonium salt 67, when neutralized in situ with KO Bu, was shown to be a highly effective catalyst in the enantioselective Henry reaction of nitroalkanes with various aromatic and aliphatic aldehydes (Scheme 10.65). The same strategy was further applied to the catalytic asymmetric Henry reaction of ynals [167] and hydrophosphonylation of ynones (Scheme 10.66) [168]. Brfunctional catalysis using this scaffold were also obtained using the carboxylate salts of tetraaminophosphoniums in the direct Mannich reaction of sulfonyl imines with azlactones (Scheme 10.67) [169]. 

Nitroaldol (Henry) Reactions with Bronsted Base Catalysis... 

Recently, asymmetric reactions catalyzed by metal-free compounds (organoca-talysis) have received increased attention due to the potential advantages of these types of systems [4]. This is primarily due to the operational simpUcity and the obvious industrial applications. This chapter covers recent developments, from 2007 to date, in the area of the enantioselective organocatalytic Henry reaction with an emphasis on catalytic methods based on Bronsted bases, Bronsted acids, and phase-transfer conditions (PTC) approaches. 

The theory of absolute reaction rates has not and probably will not, in the foreseeable future, permit the satisfactory a priori calculation of most reaction rates, particularly in condensed phases. What it does do, instead, is to identify an attainable goal toward which speculation, semiempirical calculation, and experimental work may be directed. That is the elucidation of transition state structure. If transition state structure could be determined, by whatever means, as well as the structure of many stable molecules is now known, it would be possible to calculate reaction rates. Further, even well short of enough information to calculate rates, even very approximate and incomplete transition state structures may often suffice for the calculation of such rate-related quantities as isotope effects,Bronsted a values, and Hammett p values. When successful, such partial calculations tend to confirm the partial transition state structures on which they are based and, thus, to permit further progress on a more secure base. We cannot be certain how well pleased Henry Eyring is with this hybrid of theory and empiricism, but it doesn t seem likely that he is too disturbed. The pragmatist, pleased with what works and ready to take information from anywhere, has always seemed to be as well represented in his work as the rigorous theoretician. 

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