Bronsted acid-catalyzed

Bronsted-acid-catalyzed Diels-Alder reactions are not frequent because of the proton sensitivity of many dienes and cycloadducts, especially when long reaction times and high temperatures are required. Examples in aqueous medium involving imines activated by protonation as dienophiles and a proton-promoted Diels-Alder reaction of glyoxylic acid with cyclopentadiene are considered in Section 6.1. 

The fluoboric acid-catalyzed aza-Diels-Alder reaction of aldimine and Danishefsky s diene proceeds smoothly to afford dihydro-4-pyridones in high yields [90] (Equation 4.16). Unstable aldimines generated from aliphatic aldehydes can be prepared in situ and allowed to react under one-pot reaction conditions. This one-pot Bronsted acid-catalyzed three-component aza-Diels-Alder reaction affords the adducts in good to high yields. 

Bronsted acid catalyzed aza-Diels-Alder reaction of Danishefsky s diene with aidimine generated in situ from aldehyde and amine in aqueous media [107]... 

The combinahon of a stronger Bronsted acid with an electrophile bearing a basic electrophile would result in a Bronsted-acid-catalyzed reaction. Use of aldehyde in place of aldimine with neutral acid would lead to a hydrogen-bond-catalyzed reaction. 

In summary, the differences between Br0nsted-acid catalysis and hydrogen-bond catalysis were discussed. Because there is a gradual transition from hydro-gen-bond catalysis to Bronsted-acid catalysis, it is not always easy to differentiate the two modes of catalysis. However, the combination of a stronger acid and an imine will be a Bronsted-acid-catalyzed reachon, while the combination of a neutral acid and an aldehyde will be a hydrogen-bond-catalyzed reaction. 

This novel Bronsted acid catalyzes the Diels-Alder reaction between ethyl vinyl ketone and various acycUc siloxy dienes to furnish adducts in uniformly high yields and ee s. Further, the corresponding chiral phosphoric acid was unable to catalyze this reaction. 

Metal-free reduction of mines enantioselective bronsted acid catalyzed... 

Radioactive tracer experiments reported by Lombardo and Hall (4) showed that each butene isomer can be directly interconverted into the other two. These results are consistent with a common intermediate being in operation in this reaction. In Figure 3 the linear relationship between catalytic activity and percentage of Na+ replaced by H+ strongly favors a Bronsted acid catalyzed mechanism in which the common intermediate could be a secondary carbonium ion. This conclusion is also supported by the tracer experiments. 

The role of pre-association in Bronsted acid-catalyzed decarboxylation and related processes... 

Scheme 5.19 The achiral phosphine and chiral binaphthol-derived Bronsted acid-catalyzed MBH reaction of cyclohexenone and 3-phenyl propionaldehyde.

Table 6.20 Chiral Bronsted acid-catalyzed diastereoselective Mannich reactions.

Table 6.21 Bronsted acid-catalyzed asymmetric Mannich reactions.

Bronsted Acid-Catalyzed Direct Alkylations of a-Diazoesters [21] (p. 228)... 

Asymmetric Bronsted Acid Catalyzed Imino-Azaenamine... 

Classically, the acylzirconocene chlorides can be converted into aldehydes, carboxyhc acids, esters, and acyl halides. Recently, the usefulness of acylzirconocenes has been extended. In fact, these unmasked acyl anions directly add to aldehydes and imines affording a-hydroxy or a-amino ketones, respectively. The reaction of acylzirconocene chlorides with imines also proceeds under Bronsted acid-catalyzed conditions, even with aqneons acids. ... 

The DA dimer obtained in the aminium salt reaction differs from that obtained by the PET method and from that obtained using the aminium salt/hindered base method, but is the same as that obtained by the Bronsted acid catalyzed reaction of this diene. The addition of insoluble bases hke sodium carbonate is not sufficient to suppress the extremely facile, acid-catalyzed cyclodimerization of this particular diene, which yield a highly stabilized tetramethylallyl carbocation intermediate. Nevertheless, this results demonstrates the necessity for caution in assigning a cation radical mechanism to a cyclodimerization reaction observed under aminium salt conditions. 

Bronsted acid catalysis in electron transfer described in Section 1.3.1 has also been effective for redox reactions via the electron transfer step. As shown in the case of metal ion-catalyzed hydride transfer reactions (see above), hydride transfer reactions from an NADH analogue to /7-benzoquinones also proceed via Bronsted acid-catalyzed electron transfer [255, 256]. Since NADH and ordinary NADH model compounds are subjected to the acid-catalyzed hydration [98, 257, 258], an acid-stable NADH model compound, 10-methyl-9,10-dihydroacridine (AcrH2), was used as a hydride donor to / -benzoquinone (Eq. 24) ... 

The kinetic deuterium isotope effects (/th/Zcd) are observed in the Bronsted acid-catalyzed hydride transfer reactions from AcrH2 to the hydride acceptors (A) [259]. The pH dependence of the observed kinetic deuterium isotope effects kn/ko) in-... 

Bronsted acid-catalyzed reactions will be proportional to the proton content of a zeolite as long as the intrinsic acidity of a proton remains unchanged, since the proton concentration is proportional to the Al/Si ratio. Once the Al/Si ratio exceeds a critical value (Al/Si)c, the intrinsic acidity of a zeolitic proton starts to decrease. This effect dominates over the increased number of protons and, as a consequence, the overall rate of the reaction decreases. 

Bronsted acid-catalyzed hydrocarbon activation reactions... 

Products resulting from phenol-tosyl acid reacting with coal have been studied (19). Solubilizing of coal is thought to result from an attack of phenol on aryl-alkyl bonds (19). The reaction is Lewis or Bronsted acid catalyzed. An important feature of this chemistry is the apparent facile attack of phenol at benzylic sites. 

Scheme 3.20 One pot synthesis of enantioenriched isoindolines based on chiral Bronsted acid catalyzed F C reaction/base catalyzed aza Michael addition reaction sequence.

Table 5.6 Bronsted acid catalyzed vinylogous Mannich reaction of vinylketene N,O acetal 36 according to Schneider and coworkers [14b].

In 2004 Terada and his group revealed that 2 methoxy furan (45) that can be considered as a vinylketene 0,0 acetal was sufficiently nucleophilic to engage Boc imines 44 in Bronsted acid catalyzed Friedel Crafts type reaction to furnish y aminoalkyl substituted furans 46 in excellent yields and up to 97% ee (Table 5.9) [ 16]. As already explained, phosphoric add 47 protonates the imine, thus generating a chiral contact ion pair the anionic counterion of which controls the enantioselectivity of the C C bond forming event. Although typically 2 mol% ofthe Bronsted acid were... 

The catalytic, enantioselective, vinylogous Mannich reaction has recently emerged as a very powerful tool in organic synthesis for the assembly of highly functionalized and optically enriched 6 amino carbonyl compounds. Two distinctly different strategies have been developed. The first approach calls for the reaction of preformed silyl dienolates as latent metal dienolates that react in a chiral Lewis acid or Bronsted acid catalyzed Mukaiyama type reaction with imines. Alternatively, unmodified CH acidic substrates such as a,a dicyanoalkenes or 7 butenolides were used in vinylo gous Mannich reactions that upon deprotonation with a basic residue in the catalytic system generate chiral dienolates in situ. 

Scheme 10.41 Bronsted acid catalyzed enantioselective reduction of pyridines.

---